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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

Effect of temperature and prolonged anoxia exposure on electrophysiological properties of the turtle (Trachemys scripta) heart

¹Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada; ²Department of Biology, University of Joensuu, Joensuu, Finland; ³Faculty of Land and Food Systems and Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada

Submitted 12 February 2007 ; accepted in final form 16 April 2007

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION GRANTS REFERENCES

 
Cardiac activity of the turtle (Trachemys scripta) is greatly depressed with cold acclimation and anoxia. We examined what electrophysiological modifications accompany and perhaps facilitate this depression of cardiac activity. Turtles were first acclimated to 21°C or 5°C and held under either normoxic or anoxic (6 h at 21°C; 14 days at 5°C) conditions. We then measured cardiac action potentials (APs) using spontaneously contracting whole heart preparations and whole cell current densities of sarcolemmal ion channels using isolated ventricular myocytes under appropriate normoxic and anoxic conditions. Compared with 21°C-acclimated turtles, 5°C-acclimated turtles exhibited a less negative resting membrane potential (by 18–29 mV), a 4.7- to 6.8-fold slower AP upstroke rate, and a 4.2- to 4.9-fold greater AP duration. Correspondingly, peak densities of ventricular voltage-gated Na⁺ (I_Na) and L-type Ca²⁺ currents and inward slope conductances of inward rectifier K⁺ (I_K1) channel current were 1/7th (Q₁₀ = 3.4), 1/13th (Q₁₀ = 5.0), and one-half (Q₁₀ = 1.4) of those of 21°C-acclimated ventricular myocytes, respectively. With anoxia at 21°C, peak I_Na density doubled and ventricular AP duration increased by 47%, a change proportional to the reported 30% reduction of intrinsic heart rate. In contrast, with anoxia at 5°C, ventricular AP characteristics were unaffected; of the ion currents investigated, only the inward conductance via I_K1 changed significantly (reduced by 46%). The present findings indicate that cold temperature, more so than prolonged anoxia, results in substantial modifications of cardiac APs and reduction of ventricular ion current densities. These changes likely prepare cardiac muscle for winter anoxia conditions.

action potential; delayed-rectifier potassium channel current; voltage-gated sodium channel current; L-type calcium channel current; inward-rectifier potassium channel current; red-eared slider turtle; thermal acclimation

The mechanisms underlying the reduction of cardiac activity during prolonged anoxia are not fully understood. In warm-acclimated turtles, cholinergic cardiac inhibition contributes to 36–48% of the anoxic bradycardia (20, 21), but -adrenergic (58) and adenosinergic (59) cardiac inhibitory mechanisms do not. However, in cold-acclimated turtles, autonomic cardiovascular control is blunted during anoxia and does not account for the anoxic bradycardia (20, 57). Similarly, adenosinergic cardiac inhibition is not involved either (59). Instead, the suggestion has been made that intrinsic electrophysiological changes account for the anoxic depression of cardiac activity in cold-acclimated turtles (20, 41, 56). Certainly, an increased prevalence of atrial-ventricular blocks, a phenomenon in which ventricular contraction rate is less than the atrial contraction rate, in isolated turtle hearts during anoxia exposure (25) suggests a reduced ventricular excitability and/or a delay or blockage of electrical impulses through the atrial-ventricular node. Also, intrinsic electrophysiological modifications are likely involved in the profound depression of cardiac activity with cold acclimation, since cholinergic cardiac inhibition is known to be suppressed (20) and rates of contraction and relaxation are decreased (41).

As a first step to understanding these potential electrophysiological modifications with both cold acclimation and prolonged anoxia exposure, the present study examined hearts from the red-eared slider turtle before and after cold acclimation and before and after prolonged anoxia exposure. Specifically, measurements were made of cardiac action potentials (APs) and current densities of four ventricular sarcolemmal ion channels involved in generating cardiac APs, namely the voltage-gated Na⁺ (I_Na), L-type Ca²⁺ (I_Ca), inward-rectifier K⁺ (I_K1), and delayed-rectifier K⁺ (I_Kr) channel currents (46). Recordings were acquired from four acclimation groups of turtles under conditions similar to those experienced by the turtle. The four acclimation groups were 1) 21°C-acclimated, normoxic; 2) 21°C-acclimated, 6-h anoxia exposure; 3) 5°C-acclimated, normoxic; and 4) 5°C-acclimated, 14-day anoxia exposure. This experimental design allowed comparisons for the effects of temperature alone in normoxic turtles and the effects of temperature alone in anoxic turtles, as well as the effects of anoxia with and across the two temperature acclimation groups. In addition to measuring electrophysiological characteristics under conditions similar to those in which the turtle itself had been acclimated and exposed to, we also made some electrophysiological recordings after an acute change in either the experimental temperature or the perfusate pH. Cardiac APs and ventricular sarcolemmal ion channel currents were rerecorded after an acute temperature change in both 21- and 5°C-acclimated, normoxic hearts. This acute procedure was done to explicitly distinguish the effects of cold acclimation from acute and perhaps direct effects of ambient temperature on the rate of these physiological processes (termed here direct temperature effects). Specifically, the acute temperature change was from either 5 to 21°C or from 21 to 5°C for the AP recordings, and the acute temperature change was to the common temperature of 11°C for the ion channel current recordings. To mimic in vivo blood plasma pH of anoxia-exposed turtles, cardiac APs were rerecorded for anoxia-acclimated hearts after a switch to a combined acidotic and anoxic saline. This was done to investigate the effect of acidosis on turtle cardiac APs given the reported temperature dependency of its negative inotropic and chronotropic effects on the turtle myocardium (41, 49, 56, 71, 73). For sarcolemmal ion channel current recordings, we focused on ventricular myocytes because the ventricle is the power-generating tissue of the turtle heart. Our prediction was that changes in AP shape and duration induced by cold acclimation and/or prolonged anoxia exposure would be reflected in changes in ion current densities. Also, because the duration of cardiac contraction and AP duration (APD) are closely correlated in other ectothermic vertebrate species (e.g., Refs. 43, 55), we reasoned that changes in cardiac APD accompany the known decreases in f_H associated with both cold acclimation and with prolonged anoxia exposure.

	MATERIALS AND METHODS

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Experimental Animals

Fifty-six red-eared sliders (Trachemys scripta, Gray) with body masses ranging between 124 and 518 g [235 (SD 77) g] were used in this study. Turtles were obtained from Lemberger (Oshkosh, WI) and The Charles D. Sullivan (Nashville, TN) and shipped by air to the University of Joensuu, Joensuu, Finland, or the University of British Columbia, Vancouver, Canada. The exposure design for the turtles was normoxia and anoxia exposure at 21 and 5°C acclimation. In vitro measurements were made at the same temperature as the acclimation temperature (unless stated otherwise; i.e., an acute temperature change) and in the appropriate normoxic or anoxic saline. Turtles studied at 21°C were held indoors in aquaria under a 12:12-h light-dark photoperiod, had free access to basking platforms and diving water, and were fed several times a week with commercial turtle food pellets. The turtles studied at 5°C were kept in aquaria with shallow water (3–4 cm) under a 12:12-h light-dark photoperiod in a temperature-controlled room set to 5°C for 5 wk before experimentation to allow adequate time for cold acclimation (19). Acclimation to 5°C occurred during winter months, and turtles were fasted during the acclimation period. Normoxic turtles were sampled from these conditions. For prolonged anoxia, 21°C turtles were exposed to anoxia for 6 h and 5°C turtles for 14 days. These anoxia durations were utilized to be consistent with previous examinations of the anoxic turtle heart and because anoxic cardiovascular status is relatively stable at these times (19, 20, 56, 57). The anoxic conditions were achieved by individually placing turtles into an enclosed, water-containing plastic chamber that still allowed access to air for 24 h, after which the plastic chamber was completely filled with water, continuously bubbled with N₂ (water PO₂ of <0.3 kPa), and access to the water surface denied by means of mesh suspended below the surface of the water. Turtles acclimated at 21°C were not comatose after the anoxia exposure, whereas 5°C-acclimated turtles were found to be unresponsive to tactile stimulation.

All experiments were conducted with the consent of the University of British Columbia or University of Joensuu committee for animal experimentation.

AP Recordings From Intact Cardiac Tissue

Intracellular APs were measured from all three cardiac chambers (right atrium, left atrium, and ventricle) of a spontaneously beating whole heart preparation. The heart was accessed through removal of a 2 cm x 2 cm piece of the plastron using a bone saw after death by decapitation, which for anoxia-acclimated turtles occurred underwater in the plastic containers. The chambers of the excised heart were then medially opened, spread, and gently fixed with insect pins to the Sylgard-coated bottom of a 10-ml, water-jacketed tissue chamber filled with physiological saline containing (in mmol/l) 125 NaCl, 2.5 KCl, 2 CaCl₂, 1 MgSO₄, 1 NaH₂PO₄, 10 HEPES, and 5 glucose, as well as a physiologically relevant tonic (1 nmol/l) epinephrine concentration (glucose and epinephrine were added immediately before use). The desired temperature of the saline (see below) was maintained with a circulating water bath, and saline was bubbled continuously with either O₂ (for normoxia-acclimated hearts) or N₂ (for anoxia-acclimated hearts; saline PO₂ was 2 kPa). Saline pH was adjusted (Teopal P600, Teo-Pal, Espoo, Finland) to 7.75 with NaOH at 21°C and allowed to change with temperature. Thus pH was 7.95 at 5°C. Saline was refreshed every 30 min throughout the 4- to 6-h recording period to avoid potential buildup of anaerobic waste products with anoxia-acclimated hearts and epinephrine degradation. Saline for anoxia-acclimated hearts was prebubbled with N₂.

Hearts were allowed to stabilize to the experimental conditions for 30–45 min before recordings were made. For normoxia-acclimated hearts, APs and spontaneous f_H were recorded first at the acclimation temperature of the animal (i.e., 21 or 5°C), and then the saline temperature acutely changed either from 5 to 21°C or from 21 to 5°C; hearts were allowed 45–60 min to stabilize before APs and spontaneous f_H were rerecorded for that preparation. For anoxia-acclimated hearts, APs and spontaneous f_H were recorded only at the acclimation temperature of the animal but were also recorded after a switch to acidotic saline (pH 7.25 at 21°C and pH 7.55 at 5°C, still continuously bubbled with N₂), which mimicked in vivo blood plasma pH of turtles exposed to anoxia for 6 h (warm acclimated) or 14 days (cold acclimated) (63, 72). A 25- to 30-min stabilization period was allowed after the switch to the acidotic saline before APs and spontaneous f_H were rerecorded for that preparation.

Cardiac APs were recorded with high-resistance, sharp microelectrodes (6–30 M when filled with either 3.0 or 0.3 mol/l KCl), fabricated from borosilicate glass with an internal filament (World Precision Instruments, Sarasota, FL) and using an L/M-3P-A vertical puller (List Medical, Darmstadt, Germany). KCl (0.3 mol/l) was utilized in some experiments to ensure that observed changes in resting membrane potential (RMP) with temperature were not an artifact of the 3.0 mol/l KCl. Microelectrode signals were amplified by a high-impedance amplifier (KS-700, World Precision Instruments), digitized at a sampling rate of 2 kHz (Digidata 1200, Axon Instruments, Union City, CA), and recorded to computer using Axotape 2.2 acquisition software. Spontaneous f_H was calculated from the peak-to-peak intervals of right atria contraction force, which was obtained via attachment of one edge of the right atria to a force transducer (FT03, Grass Instruments, West Warwick, RI) by a small metal hook and braided silk thread. The contraction force signal was amplified (7D, Grass Instruments), routed through the digitizer, and stored to computer at a sampling rate of 200 Hz. APs and contraction force recordings were analyzed off-line using Clampfit 9.2 (Axon Instruments, Foster City, CA).

Whole Cell Voltage Clamp From Isolated Myocytes

Whole cell voltage-clamp experiments were performed on individual ventricular myocytes to assess the effect of cold temperature acclimation and prolonged anoxia acclimation on ion current density of I_Na, I_Ca, I_K1, and I_Kr.

Myocyte isolation. Single ventricular myocytes were enzymatically isolated by adapting an established isolation protocol for teleost fish (51, 65). The turtle heart was accessed as describe above, excised, and cannulated through the left aortic arch into the ventricle. The heart was then perfused retrograde at room temperature (21 ± 1°C) from a height of 50 cm, first with a nominally Ca²⁺-free, low-Na⁺ isolation saline solution (containing, in mmol/l, 100 NaCl, 10 KCl, 4 MgSO₄, 1 NaH₂PO₄, 1.2 KH₂PO₄, 50 taurine, 20 glucose, and 10 HEPES, with pH adjusted to 6.9 at 21°C with KOH) for 10 min and then for 20 min with fresh isolation solution supplemented with the proteolytic enzymes collagenase (1.5 mg/ml; type IA) and trypsin (1 mg/ml; type IX) and with 1.5 mg/ml of fatty acid-free BSA. All chemicals were purchased from Sigma-Aldrich (St. Louis, MO). The isolation solutions were continuously bubbled with O₂ for normoxia-acclimated hearts or N₂ for anoxia-acclimated hearts, and the enzyme-supplemented isolation solution was recycled with the use of a peristaltic pump and retained after perfusion. The ventricle was then dissected from the atria and the sinus venosus, minced with scissors in fresh isolation solution, and transferred to the retained enzyme-supplemented isolation solution. Ventricular tissue was gently stirred with a small magnetic bar at room temperature with periodic trituration through the opening of a Pasteur pipette for 10–20 min or until individual viable myocytes were observed by light microscopy. The solution was then left to settle, and myocytes were resuspended in fresh isolation solution and stored at 6°C. Cells were usually recorded from within 4–6 h, as done previously for recordings from anoxia-acclimated crucian carp (Carassius carassius) cardiomyocytes (42, 67). However, in some instances, cells were stored for up to 12 h because of the extreme difficulty in obtaining reliable measurements from viable cells.

Experimental procedures. Electrophysiological measurements and analysis of sarcolemmal current densities were achieved with the use of established methods and solutions for teleost fish that were adapted for the turtle (14, 15, 42, 51, 55, 65, 66). Specific details for each current measured are given below; however, in all instances, an aliquot of dissociated myocytes was placed into a recording chamber mounted on the stage of an inverted microscope and left to adhere to the bottom of the chamber. Cells were then superfused at a rate of 1–2 ml/min with an extracellular saline solution. Temperature of the extracellular solution was regulated either by water bath circuits that chilled or heated the inflow tube carrying the extracellular solution to the recording chamber or a Peltier device. Thermocouples positioned no less than 5 mm from the cell under investigation were used to continuously monitor temperature. For cells from normoxia-acclimated animals, current density was recorded in the same cell first at the acclimation temperature of the animal (i.e., 21 or 5°C) and then after an acute exposure to the common experimental temperature of 11°C (temperature change was accomplished within 3–5 min). This was done to distinguish cold-acclimation effects from direct temperature effects. For cells from anoxia-acclimated animals, current density was recorded only at the acclimation temperature of the animal, and the extracellular solution was continuously bubbled with N₂ (PO₂ of the extracellular solution in the recording chamber was 5 kPa).

Patch pipettes were pulled from borosilicate glass without an internal filament and had a resistance of 2–4 M when filled with pipette solution. Offset potentials were zeroed just before formation of the gigaohm seal, and pipette capacitance was compensated after formation of the gigaohm seal. The patch was ruptured by delivering a short voltage pulse (zap) to the cell, and capacitive transients were eliminated by iterative adjustments of series resistance and cell capacitance circuits. Mean series resistance was 5.3 M (SD 4.9) (n = 165), and mean cell capacitance was 50.2 ± 1.0 pF (n = 165; mean ± SE).

Voltage-gated I_Na. I_Na recordings were made (at the University of Joensuu) with an EPC-9 amplifier in conjunction with Pulse v8.65 software (HEKA, Lambrecht, Germany) and a temperature-controlled 500-µl recording chamber (RCP-10T; Dagan, Minneapolis, MN). Patch pipettes were pulled from borosilicate glass (Garner F-78045, Claremont, CA) using a two-stage vertical puller [either a L/M-3P-A (List Medical) or PP-83 (Narishige, Tokyo, Japan)]. Cells were first superfused with normal K⁺-based extracellular solution containing (in mmol/l) 125 NaCl, 2.5 KCl, 2 CaCl₂, 1 MgSO₄, 1 NaH₂PO₄, 10 HEPES, 5 glucose, and 0.01 nifedipine (to block L-type Ca²⁺ channels; Ref. 12), where gigaohm seal and whole cell patch-clamp recordings of the myocytes were established. Internal perfusion of the myocytes with pipette solution (containing, in mmol/l, 5 NaCl, 130 CsCl, 1 MgCl₂, 5 EGTA, 5 MgATP, and 5 HEPES; pH adjusted to 7.4 at 21°C with CsOH) continued for at least 3 min to allow buffering of intracellular Ca²⁺ with EGTA. The extracellular solution was then switched to a low-Na⁺ solution (containing, in mmol/l, 19 NaCl, 108.5 CsCl, 1 MgSO4, 1 NaH₂PO₄, 2 CaCl₂, 10 HEPES, 5 glucose, and 0.01 nifedipine; pH adjusted to 7.75 at 21°C with CsOH) without inducing contracture in the patched myocyte, as previously accomplished in teleost fish myocytes (15, 16). I_Na was elicited in the low-Na⁺ extracellular solution from the holding potential of –80 mV by 10-ms (21°C) or 30-ms (5°C) depolarizing square pulses to voltages between –100 and 60 mV in 10-mV steps that were preceded by a 20-ms prepulse to –120 mV to remove inactivation. Sampling rate was 20 kHz, and the signal was filtered on-line with a 10-kHz Bessel filter. Voltage off-set caused by series resistance was compensated (70%; 10 µs). Leak current was estimated from current at end of depolarizing pulses at –100, –90, –80, and –70 mV and subtracted off-line. The amplitude of I_Na was calculated as the peak inward current during the depolarizing pulses.

I_Ca. I_Ca recordings were made (at the University of British Columbia) with an Axopatch 200B amplifier, a CV 203BU headstage, and ClampEx v9.2 software (Axon Instruments). An RC-26GLP recording chamber (234 µl; Warner Instruments, Hamden, CT) was used, and temperature was regulated with a PHC-2 heater/cooler jacket in conjunction with a SC-20 dual in-line solution heater/cooler and a CL-100 bipolar temperature controller (Warner Instruments). Pipettes were pulled from borosilicate glass (GC150T-7.5; Harvard Apparatus, St. Laurent, QC, Canada) with a Sutter P-97 puller (Sutter Instruments, Novato, CA). Myocytes were superfused with a Cs⁺-based extracellular solution to eliminate contaminating K⁺ currents. Extracellular solution contained (in mmol/l) 125 NaCl, 2.5 CsCl, 2 CaCl₂, 1 MgSO₄, 1 NaH₂PO₄, 10 HEPES, and 5 glucose; pH was adjusted to 7.75 at 21°C with CsOH. The pipette solution contained (in mmol/l) 130 CsCl, 1 MgCl₂, 5 Na₂-phosphocreatine, 4 MgATP, 0.03 Na₂GTP, 5 EGTA, 15 tetraethylammonium chloride (to block K⁺ currents; Ref. 23), and 10 HEPES; pH was adjusted to 7.4 at 21°C with CsOH. I_Ca was elicited from the holding potential of –70 mV by 500-ms depolarizing square pulses to voltages between –70 and +70 mV in 10-mV steps. A preceding 50- or 100-ms prepulse to –40 mV was used to inactivate voltage-gated Na⁺ channels and eliminate fast Na⁺ currents, as the turtle ventricular myocytes are relatively insensitive to the specific Na⁺ blocker TTX (12). Sampling rate was 10 kHz, and signals were low-pass filtered at 2 kHz on-line with the Axopatch amplifier. Signals were analyzed off-line with Clampfit 9.2 software (Axon Instruments). The amplitude of I_Ca was calculated as the difference between peak inward current and the current at the end of the depolarizing pulse.

Additionally, because I_Ca can run down over time when measured in whole cell configuration and the experimental protocol utilized here required repeated measurements of I_Ca, the magnitude of I_Ca rundown was assessed in a separate group of myocytes. This was accomplished by repeatedly measuring I_Ca at 3-min intervals over a period of 15 min (Fig. 1).

View larger version (10K): [in this window] [in a new window]   Fig. 1. Time-dependent changes in peak amplitude of L-type Ca2+ current (ICa) of 21°C-acclimated (A) and 5°C-acclimated (B) turtle cardiac myocytes. EGTA (5 mmol/l) was included in the pipette solution. Values are means ± SE. For 21°C experiments, n = 7 (normoxia-acclimated), 12 (anoxia-acclimated), and 7 (11°C acute exposure) turtles unless otherwise indicated in parentheses. For 5°C experiments, n = 5 (normoxia-acclimated), 8 (anoxia-acclimated), and 3 (11°C acute exposure) turtles unless otherwise indicated in parentheses. *Statistically significant changes (P < 0.05; Friedman repeated-measures ANOVA on ranks, Dunn's posttest) from control (t = 0) for each experimental group.

Data and Statistical Analysis

All results are expressed as means ± SE. The number of observations (n) for cardiac AP data was number of turtles (i.e., 4 or 5 for each of the 4 acclimation groups), with AP characteristics from one to six cells per tissue per animal averaged for each individual. AP shape and duration were quantified by measuring RMP and peak potential and calculating duration to 0 mV (APD₀), 50% (APD₅₀), 90% (APD₉₀), and 100% (APD₁₀₀) repolarization. AP upstroke rate was calculated by dividing the difference in RMP and peak potentials by upstroke duration. Number of myocytes constitutes n for whole cell voltage-clamp experiments. Cells were obtained from 2–10 animals for each exposure condition. Densities of I_Na, I_Ca, I_K1, and I_Kr (expressed as pA/pF) were calculated by dividing measured currents by the cell capacitance. Slope conductance (pS/pF) of inward-rectifier K⁺ channel was calculated for the linear region of the I_K1 current-voltage plot by dividing the difference in I_K1 between –120 and –100 mV by the change in voltage (i.e., 20 mV). Statistically significant differences in AP characteristics and ion current densities between either 21- and 5°C-acclimated turtles or normoxia- and anoxia-exposed turtles of the same acclimation temperature were determined with a two-way ANOVA or a t-test where appropriate. Two-way repeated measures ANOVA tests, or paired t-tests where appropriate, were used to compare AP characteristics or ion current densities after an acute temperature change, and introduction of acidic saline (i.e., AP characteristics). In all instances, P < 0.05 was used as the level of significance. Where appropriate, multiple comparisons were performed with Student-Newman-Keuls tests.

	RESULTS

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Effect of Temperature on Spontaneous Normoxic f_H

The initial spontaneous f_H of 21°C- and 5°C-acclimated heart preparations were 35.3 ± 1.3 beats/min (n = 5) and 5.5 ± 0.7 beats/min (n = 5), respectively. This sixfold difference in spontaneous f_H with acclimation to 5°C corresponds to a Q₁₀ value of 3.2. Acute exposure of 21°C-acclimated hearts to 5°C decreased spontaneous f_H to a rate (3.0 ± 0.7 beats/min; n = 3) statistically similar to the 5°C-acclimated f_H. Conversely, acute exposure of 5°C-acclimated hearts to 21°C significantly increased spontaneous f_H to 27.3 ± 0.2 beats/min (n = 3), but this rate was significantly lower than the 21°C-acclimated f_H. These results suggest that direct temperature effects rather than temperature acclimation were predominant in setting the spontaneous f_H under normoxic conditions.

Effect of Temperature on Cardiac Action Potentials in Normoxia

Like spontaneous f_H, the shape and duration of the cardiac AP were considerably modified by temperature. Primarily, after acclimation to 5°C, the RMP was significantly less polarized (by 18–26 mV) in all chambers of the heart (Figs. 2, 3, A and C, 4, A and C, and 5, A and C). Also, acclimation to 5°C significantly decreased AP upstroke rate by 4.7- to 6.8-fold in all cardiac chambers, which corresponded to Q₁₀ values of 3.3 for the right atria, 2.8 for the left atria, and 2.6 for the ventricle (Figs. 2, 3, A and C, 4, A and C, and 5, A and C; Table 1). Furthermore, APD was prolonged at least fourfold in all cardiac chambers compared with 21°C-acclimated hearts (Figs. 2, 3, A and C, 4, A and C, 5, A and C). Specifically, APD₁₀₀ of 5°C-acclimated hearts was increased by 4.4-fold in the right atria (614.0 ± 39.0 to 2,680.5 ± 113.5 ms; Fig. 2, A and C), 4.9-fold in the left atria (590.0 ± 50.5 to 2,894.0 ± 198.5 ms; Fig. 3, A and C), and 4.2-fold in the ventricle (850.0 ± 38.5 to 3,559.0 ± 529.5 ms; Figs. 2 and 5, A and C). Q₁₀ values for the prolongation of APD at 5°C (calculated from reciprocal values of APD₉₀) were 2.4, 2.6, and 2.6 for the right atria, left atria, and ventricle, respectively. These Q₁₀ values point to direct temperature effects playing a predominant role in contributing to the prolongation of APD with cold acclimation, a conclusion that was confirmed by the results for the acute temperature change.

View larger version (14K): [in this window] [in a new window]   Fig. 2. Representative recordings of ventricular action potentials from 21°C normoxia- and anoxia-acclimated turtles (A) and 5°C normoxia- and anoxia-acclimated turtles (B).

View larger version (26K): [in this window] [in a new window]   Fig. 3. Graphical representation of right atrial action potentials from 21°C normoxia-acclimated (A), 21°C anoxia-acclimated (B), 5°C normoxia-acclimated (C), and 5°C anoxia-acclimated (D) turtles. The effect of acute exposure to combined anoxia with acidosis on action potential shape is also depicted in B and D. Values are means ± SE; n = 4 or 5 turtles. Statistically significant changes (P < 0.05) in action potential shape after acclimation to 5°C (C). Statistically significant changes (P < 0.05) in action potential shape between anoxia acclimation and acute exposure to combined anoxia with acidosis (B).

View larger version (26K): [in this window] [in a new window]   Fig. 4. Graphical representation of left atrial action potentials from 21°C normoxia-acclimated (A), 21°C anoxia-acclimated (B), 5°C normoxia-acclimated (C), and 5°C anoxia-acclimated (D) turtles. The effect of acute exposure to combined anoxia with acidosis on action potential shape is also depicted in B and D. Values are means ± SE; n = 4 or 5 turtles. Statistically significant changes (P < 0.05) in action potential shape after acclimation to 5°C (C). Statistically significant changes (P < 0.05) in action potential shape between anoxia acclimation and acute exposure to combined anoxia with acidosis (D).

View larger version (27K): [in this window] [in a new window]   Fig. 5. Graphical representation of ventricular action potentials from 21°C normoxia-acclimated (A), 21°C anoxia-acclimated (B), 5°C normoxia-acclimated (C), and 5°C anoxia-acclimated (D) turtles. The effect of acute exposure to combined anoxia with acidosis on action potential shape is also depicted in B and D. Values are means ± SE; n = 4 or 5 turtles. Statistically significant changes (P < 0.05) in action potential shape after acclimation to 5°C (C). *Statistically significant changes (P < 0.05) in action potential shape after anoxia acclimation (B and D; 6 h at 21°C and 14 days at 5°C).

View larger version (13K): [in this window] [in a new window]   Fig. 6. Graphical representation of action potentials from right atria (A and B), left atria (C and D), and ventricle (E and F) of 5°C (left) and 21°C (right) normoxia-acclimated turtles recorded at the acclimation temperature of the animal and after acute exposure to 5°C (for 21°C-acclimated hearts) or 21°C (for 5°C-acclimated hearts). Values are means ± SE; n = 4 or 5 turtles. *Statistically significant changes (P < 0.05) in action potential shape between acclimated and acutely exposed hearts.

Effect of Temperature on Ventricular Sarcolemmal Ion Channel Current Densities

Consistent with the changes in AP characteristics with cold acclimation, current densities of ventricular sarcolemmal ion channels involved in generating APs, as elicited from square voltage-clamp pulse protocols, were drastically reduced in 5°C-acclimated ventricular myocytes compared with 21°C-acclimated cells (Fig. 7). However, the manner through which current density is depressed with cold acclimation differed with each channel type (Figs. 8–10).

View larger version (32K): [in this window] [in a new window]   Fig. 7. Mean current-voltage relationships and representative original recordings (insets) of voltage-gated Na+ (INa; A), L-type Ca2+ (TCa; B), inward-rectifier K+ (IK1; C), and delayed-rectifier K+ (IKr; D) channel currents for ventricular myocytes isolated from 21°C and 5°C normoxia-acclimated turtles. Values are means ± SE. *Significant differences (P < 0.05).

View larger version (14K): [in this window] [in a new window]   Fig. 8. Effect of temperature on peak INa density (A), peak ICa density (B), and inward slope conductance of IK1 (C) in ventricular myocytes isolated from 21°C and 5°C normoxia-acclimated turtles. Recordings were first made at the acclimation temperature of the animal and second at a common temperature of 11°C. Values are means ± SE. *Significant differences (P < 0.05) between 21°C- and 5°C-acclimated turtles at 11°C. Q10 for acute and acclimation-induced changes are indicated by black and grey arrows/text, respectively.

View larger version (8K): [in this window] [in a new window]   Fig. 10. Mean current-voltage relationship of IK1 of 21°C and 5°C normoxia-acclimated ventricular myocytes recorded at the common temperature of 11°C. Values are means ± SE; n = 8 (21°C-acclimated) and 15 (5°C-acclimated) turtles. *Significant difference (P 0.05).

View larger version (7K): [in this window] [in a new window]   Fig. 9. Inactivation time (A) and time to peak current (B) of INa recorded from ventricular myocytes isolated from 21°C- and 5°C-acclimated, normoxia- and anoxia-exposed turtles. Values are means ± SE; n = 11 (21°C normoxia-acclimated), 6 (21°C anoxia-acclimated), 9 (5°C normoxia-acclimated), and 5 (5°C anoxia-acclimated) turtles.

The inward I_K1 density and the inward slope conductance of inward-rectifier K⁺ channels were significantly reduced with acclimation to 5°C (Fig. 7C), findings consistent with the depolarized RMP and prolonged APD of 5°C-acclimated ventricular tissue. At –120 mV, inward I_K1 density of 5°C-acclimated myocytes (–4.6 ± 0.3 pA/pF) was 26% less than the inward I_K1 density of 21°C-acclimated myocytes (–6.2 ± 0.7 pA/pF). Inward slope conductance of inward-rectifier K⁺ channels between –120 and –100 mV was reduced by almost 50% from 214.7 ± 16.8 pS/pF at 21°C to 125.3 ± 9.6 pS/pF at 5°C. When measured at a common temperature of 11°C, 5°C-acclimated ventricular myocytes displayed greater inward I_K1 density at –120 mV (–5.5 ± 0.3 pA/pF compared with –3.3 ± 0.4 pA/pF; Fig. 10) and greater inward rectifier K⁺ channel slope conductance (191 ± 8.6 pS/pF compared with 89.8 ± 11.0 pS/pF; Fig. 8C) than 21°C-acclimated ventricular myocytes. The fact that the Q₁₀ for the reductions in slope conductance was 1.4 when comparing acclimated hearts but around 2 for an acute temperature change lends support to the notion that the density of functional inward-rectifier K⁺ channels was upregulated with cold acclimation (Fig. 8C). In contrast to inward I_K1 density, no significant difference in outward I_K1 density (i.e., at –80 and –60 mV) existed between 5°C-acclimated ventricular myocytes and 21°C-acclimated ventricular myocytes (Fig. 7C). This finding indicates that temperature did not affect I_K1 equally at all voltages. However, like inward I_K1 and inward slope conductance, outward I_K1 density of 5°C-acclimated myocytes was significantly greater than 21°C-acclimated myocytes at –80 and –60 mV when measured at the common temperature of 11°C (Fig. 10). This finding further supports the notion that the density of functional inward-rectifier K⁺ channels was upregulated with acclimation to 5°C to partially compensate for the negative effect of cold temperature on I_K1.

I_Kr was minor and no different for 21- and 5°C-acclimated ventricular myocytes (Fig. 7D), and this situation was unchanged after acute exposure to 11°C (data not shown).

Effect of Prolonged Anoxia on Spontaneous f_H

Spontaneous f_H of 5°C anoxic turtles (3.0 ± 0.7 beats/min; n = 5) was significantly 45% lower than the spontaneous f_H (5.5 ± 0.7 beats/min; n = 5) of 5°C normoxic turtles under normoxic conditions. At 21°C, spontaneous f_H of anoxic preparations was 27.4 ± 4.6 beats/min (n = 5); although this was 32% lower than the 21°C spontaneous f_H (35.3 ± 1.3 beats/min; n = 5) under normoxic conditions, there was no statistically significant difference. These findings are consistent with our other work (56) demonstrating that prolonged anoxia at both 21 and 5°C resets intrinsic f_H to a rate 25–53% lower than during normoxia.

Effect of Prolonged Anoxia on Cardiac Action Potentials

In contrast to the large effects of temperature on AP shape and duration, prolonged anoxia exposure caused few and only small changes in cardiac APs. Furthermore, the effect of anoxia on cardiac APs was cardiac chamber specific unlike the chamber-independent effects of temperature on AP shape and duration. At 21°C, APs for the right and left atria were not significantly modified after 6 h of anoxic exposure (Figs. 3, A and B, 4, A and B). Likewise, AP shape or duration of all cardiac chambers remained unchanged after 14 days of anoxia at 5°C (Figs. 2, 3, C and D, 4, C and D, and 5, C and D). In contrast, ventricular APD₅₀, APD₉₀, and APD₁₀₀ were increased significantly by 39%, 49%, and 47%, respectively, after anoxia exposure at 21°C (Figs. 2 and 5, A and B). Therefore, the prolongation of the ventricular APD after prolonged anoxia at 21°C is of the same magnitude as the reduction in spontaneous f_H.

Effect of Prolonged Anoxia on Ventricular Sarcolemmal Ion Channel Current Densities

Of the four ventricular sarcolemmal membrane currents examined, prolonged anoxia at 21°C only significantly altered I_Na. Specifically, peak I_Na density doubled from –8.7 ± 0.9 to –16.1 ± 1.7 pA/pF with 6 h of anoxia at 21°C (Fig. 11A), although without affecting activation and inactivation kinetics of I_Na (Fig. 9). No significant changes in I_Ca, I_K1, or I_Kr density occurred as a result of 6 h of anoxia exposure at 21°C (Fig. 11, B–D).

View larger version (31K): [in this window] [in a new window]   Fig. 11. Mean current-voltage relationships and representative original recordings (insets) of INa (A), ICa (B), IK1 (C), and IKr (D) for ventricular myocytes isolated from 21°C normoxia- and anoxia-acclimated turtles. Values are means ± SE. *Significant difference (P < 0.05).

View larger version (34K): [in this window] [in a new window]   Fig. 12. Mean current-voltage relationships and representative original recordings (insets) of INa (A), ICa (B), IK1 (C), and IKr (D) for ventricular myocytes isolated from 5°C normoxia- and anoxia-acclimated turtles. Values are means ± SE. *Significant difference (P < 0.05).

The Combined Effect of Acidosis and Anoxia on Spontaneous f_H and Cardiac Action Potentials

Spontaneous f_H of anoxia-acclimated spontaneously beating whole heart preparations was unaffected by acute acidosis exposure. At 21°C, spontaneous f_H of anoxia-acclimated heart preparations before acidosis was 21.6 ± 5.2 beats/min (n = 5) and 19.3 ± 3.5 beats/min (n = 5) with acidosis. Similarly, at 5°C, spontaneous f_H was unchanged before (3.8 ± 0.7 beats/min; n = 5) and with acidosis (4.3 ± 0.3 beats/min; n = 5).

In contrast, acute acidosis exposure altered the AP shape and duration depending on the acclimation temperature and cardiac chamber. At 21°C, right atrial APD₀, APD₅₀, APD₉₀, and APD₁₀₀ were increased significantly by 18–20% with acidosis (Fig. 3B), but left atria and ventricular AP shape and duration were unaffected (Figs. 4B and 5B). At 5°C, APD₅₀ increased by 18% in the left atria with acidosis (Fig. 4D), but right atria and ventricular AP shape and duration were unaffected (Figs. 3D and 5D).

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION GRANTS REFERENCES

 
The present study examined whether modification of electrophysiological properties of the turtle heart facilitates the downregulation of cardiac activity that accompanies cold acclimation and prolonged anoxia exposure. To this end, we compared cardiac APs of spontaneously contracting turtle heart preparations and whole cell current densities of key sarcolemmal ion channels of ventricular myocytes isolated from turtles that had been acclimated to either 21 or 5°C and exposed to either normoxia or prolonged anoxia. Our findings revealed that the substantial modifications of cardiac APs and reduction of ion current densities that likely contribute significantly to decreased cardiac activity were more evident with cold temperature, both direct and acclimation effects, than with prolonged anoxia exposure. In view of this finding, we suggest that the reduction in electrophysiological activity of the turtle heart associated with cold exposure is a critical component in the preparation of cardiac muscle so that turtles can successfully overwinter under anoxia conditions.

Critique of Methods

AP recordings from intact cardiac tissue in vitro. A difficulty with in vitro work is that both the isolation procedure and the test conditions may not exactly mimic the in vivo situation, especially when trying as we did to maintain the anoxic condition of the tissue. Clearly, a caveat with our AP results for anoxia-acclimated turtles is that the recording chamber saline PO₂ could not be completely depleted of oxygen due to practical limitations of the experimental setup. However, we were greatly encouraged by spontaneous f_H for the normoxic 21- and 5°C-acclimated heart preparations being close to previously reported in vivo and in vitro intrinsic rates (8, 20, 72). Similarly, the reduced spontaneous f_H of anoxic 21- and 5°C-acclimated heart preparations is qualitatively and quantitatively comparable with previous in vivo and in vitro work (20, 55). Thus, if there had been a problem with anoxia-acclimated hearts reverting to their normoxic state during isolation and measurement, we would not have detected the decreased intrinsic f_H or seen the similarity to previous studies. Furthermore, the clear, chamber-specific effect of anoxia acclimation on APs at 21°C despite similar measurement conditions argues against the possibility that the lack of effects of anoxia acclimation on cardiac APs at 5°C were a measurement artifact. Therefore, we are confident that our novel recordings of turtle APs for all three cardiac chambers and under all four acclimation conditions were from viable cardiac tissues and representative for the prior exposure history of the animal. Nevertheless, our methods could have underestimated some effects of anoxia.

Whole cell patch clamp in isolated ventricular myocytes. For turtles, we have provided the first measurements of ventricular sarcolemmal I_Na, I_K1, and I_Kr at any acclimation temperature or condition (i.e., normoxia or anoxia exposed). Our novel measurements of ventricular I_Ca of warm-acclimated, anoxia-exposed turtles and cold-acclimated, normoxia- and anoxia-exposed turtles add to a very recent report of I_Ca for 20- to 21°C-acclimated yellow-bellied turtles (Trachemys scripta scripta) (12).

Like the anoxia-acclimated turtle AP data, a caveat with our recordings of I_Na, I_Ca, I_K1, and I_Kr from anoxia-acclimated turtles is that cardiomyocytes could not be maintained anoxic throughout all steps of the isolation and recording protocols. In particular, cells were not continuously bubbled with N₂ during storage at 6°C and were exposed to air when left to settle in the recording chamber, a step that lasted only 1–2 min at 21°C and up to 5 min at 5°C, and recording chamber saline PO₂ could not be completely depleted of oxygen. However, we are confident that our ion-channel current recordings from anoxia-acclimated turtles faithfully represent the prior anoxia exposure history of the animal. First, this is because differences in ion channel current densities were indeed found for cardiomyocytes before and after anoxia acclimation, the currents affected by anoxia acclimation were temperature dependent (i.e., I_Na at 21°C and I_K1 at 5°C), and the directional changes in current amplitude with anoxia exposure were channel dependent (Figs. 11 and 12). Furthermore, the finding that peak I_Ca remained unchanged with anoxia exposure appears legitimate, at least for 5°C-acclimated turtles, since twitch force and time-to-peak force of ventricular tissue from 5°C-acclimated turtles does not differ between normoxia- and anoxia-acclimated animals (41). Finally, given that at the organismal level the changes in cardiovascular status with anoxia take 1 h and 24 h at 21 and 5°C, respectively (18–21, 57, 59), it seems highly unlikely that these changes would all be reversed during the relatively short (i.e., 9 min) recording periods when perfusate oxygen levels were not zero.

Beyond the issues noted above for the isolation and test conditions, a limitation of the whole cell patch-clamp technique is the disruption of the native intracellular milieu by the pipette solution. This disruption affects intracellular ion balance and buffering capacity, interferes with normal cellular signaling by intracellular pH, second messengers, and covalent modification (47), and leads to deterioration (i.e., rundown) of currents over time (34). At the outset of this study, turtle cardiac myocytes had never been investigated with the whole cell patch-clamp technique. Thus our intracellular and extracellular solutions were modified from established whole cell patch-clamp methodologies for teleost fish (15, 16, 42, 51, 55, 65, 66) but were relevant to the freshwater turtle in terms of ionic composition and pH (see Refs. 17, 27, 28 for detailed description of turtle blood ionic composition and pH). Specifically, pipette pH was set to 7.4 at 21°C and was 7.6 at 5°C (49, 71) (Stecyk JAW, Bock C, Overgaard J, Wang T, Farrell AP, Pörtner HO, unpublished observations). Furthermore, to minimize changes in the amplitude of I_Ca, a current particularly sensitive to rundown, throughout the duration of the experimental protocol 5 mmol/l EGTA was included in the pipette solution (23). In the present study, I_Ca was –5.7 ± 0.5 pA/pF for 21°C-acclimated normoxic turtles, and in all experimental groups, this current remained unchanged for at least 9 min with repeated measurements of I_Ca (Fig. 1). The magnitude of I_Ca and the repeatability of its measurement are comparable to recent reports for cardiomyocytes of yellow-bellied turtles using the perforated-patch technique (12), a technique that overcomes some of the concerns with whole cell patch clamp. Therefore, we are confident of our measured currents and that our reported alterations in current densities with acute temperature change, which were all completed within 9 min, are not due to variation in current density over time.

Nevertheless, the possibility exists that our reported current densities differ from the in vivo condition. Clearly, the square voltage-clamp pulses used to characterize I_Na, I_Ca, I_K1, and I_Kr do not emulate the change in membrane potential that occurs with an AP. Furthermore, we did not mimic all the changes in intracellular and extracellular environment that accompany prolonged anoxia exposure (for detailed descriptions, see Refs. 17, 27–29, 49, 71) (Stecyk et al., unpublished observations). For instance, adrenergic stimulation of cardiomyocytes likely exists at a tonic level under normoxia and perhaps at an increased level under anoxia because circulating catecholamines are greatly elevated (31, 32, 70). Epinephrine increases the open probability of L-type Ca²⁺ channels (45) but was not utilized during measurements of I_Ca. Because this study was a first step in comparing electrophysiological properties of 21- and 5°C-acclimated turtles exposed to normoxia and anoxia, future studies, some of which have already commenced, utilizing physiologically relevant AP pulse-protocols and faithfully mimicking the extracellular changes in pH and epinephrine that accompany prolonged anoxia are logical next steps for research. Finally, it should be noted that the 20-ms prepulse to –120 mV from the holding potential of –80 mV utilized to remove steady-state inactivation of voltage-gated Na⁺ channels for measurement of I_Na may not have been sufficient to fully recover channels from inactivation at 5°C (15). This could partially contribute to the small I_Na reported for cold-acclimated turtles.

Modification of Turtle Heart Electrophysiology by Temperature and Anoxia

The coordinated pumping action of the turtle heart first involves production of APs in pacemaker cells, which set the spontaneous rhythm of cardiac contraction through a synchronized propagation of excitation throughout the atria and ventricle. The generation of the AP and its expression throughout the heart require the integrated activities of a number of sarcolemmal ionic currents. Thus the potential exists for numerous types of cardiac control mechanisms that are intrinsic to the sarcolemma and that could modify cardiac performance in response to a change in whole body blood flow demand, extracellular conditions, and ambient temperature.

The present study focused on the effects of cold temperature acclimation and prolonged anoxia exposure on turtle cardiac APs, and our goal was to elucidate the contribution of sarcolemmal ion channels to the known reduction in cardiac activity. Four key sarcolemmal ion channel currents were studied in ventricular myocytes. I_Na, a fast inward Na⁺ current via voltage-gated Na⁺ channels, is the first current to be activated in atrial and ventricular cells, determines the amplitude and slope of the AP upstroke (10), is linked to excitation-contraction coupling of cardiac myocytes via the sarcolemmal Na⁺/Ca²⁺ exchange (3), and allows for subsequent activation of other ion channels involved in AP generation (33). I_Ca, the inward Ca⁺ current via voltage-gated L-type Ca²⁺ channels, is responsible for the plateau phase of the AP (2) and is the predominant source of free intracellular Ca²⁺ needed to bind to the myofilaments and trigger contraction in the turtle heart (11, 12). I_K1, an outward K⁺ current via inward-rectifier K⁺ channels, is primarily responsible for maintaining a stable RMP and terminal repolarization of the AP but not thought to be present during AP plateau (35, 48). I_Kr, a repolarizing K⁺ current, gradually develops during the plateau phase of the AP, conducts outward current at more positive voltages than the I_K1, and thus is important in balancing I_Ca and contributing to the plateau of the cardiac AP (46).

Modification of Turtle Heart Electrophysiology With Cold Temperature

In vivo cardiac activity of cold-acclimated freshwater turtles is substantially lower than that of warm-acclimated turtles, with a 5- to 15-fold decrease in f_H after acclimation to 5°C from 21–22°C driving similar reductions in Q_sys and PO_sys (19, 57, 59). Similarly, spontaneous f_H is six to seven times lower in in vitro heart preparations from 5°C-acclimated turtles than in 21°C-acclimated turtles (56). The present findings not only confirm these findings for spontaneous f_H but also indicate that, at least for ventricular tissue, cold temperature can induce changes in the density of sarcolemmal ionic currents in response to direct temperature effects and acclimation. These changes are presumed to serve to alter cardiac AP characteristics and contribute to the depression of cardiac activity in vivo.

To summarize these temperature effects for normoxic turtles, 5°C-acclimated turtle heart exhibited depolarization of RMP by 18–26 mV, 4.7- to 6.8-fold decreases in AP upstroke rate and a prolongation of APD by 4.2- to 4.9-fold in all cardiac tissues examined (Figs. 2, 3, A and C, 4, A and C, 5, A and C). In the ventricle, the increased RMP and APD are consistent with the 50% reduction of I_K1 conductance (Fig. 7C), whereas the decreased AP upstroke rate is consistent with the sevenfold reduction in peak density of I_Na (Fig. 7A). The 13-fold reduction in ventricular I_Ca of 5°C-acclimated turtles (Fig. 7B) is consistent with reductions in twitch force and time to peak force of 5°C-acclimated turtle ventricular tissue compared with that shown in ventricular tissue from 21°C-acclimated turtles (41).

By making additional measurements of ventricular sarcolemmal current densities after acutely switching the temperature to a common temperature of 11°C, we found that direct temperature effects could be clearly dissected from those due to cold temperature acclimation. Specifically, we found that I_Na decreased (also reflected in the decreased AP upstroke rate of 5°C-acclimated turtles) predominantly as a result of a direct temperature effect (Figs. 5, A and C, and 8A), whereas density of functional L-type Ca²⁺ channels was downregulated as part of the cold acclimation (Fig. 8B). These findings contrast in one respect with the effect of cold acclimation in anoxia-tolerant crucian carp (Carassius carassius), where functional Na⁺ channels were reduced (15), but share a common response to cold acclimation of active downregulation of functional L-type Ca²⁺ channels (67). It remains to be clarified whether the cold acclimation-induced change in ventricular I_Ca for turtles is due to temperature-dependent changes in channel phosphorylation, transcription, translation, rate of protein degradation, or trafficking of channels to the sarcolemmal membrane. Furthermore, we found that density of functional inward rectifier K⁺ channels was upregulated with cold acclimation to partially compensate the negative direct effect of cold temperature on I_K1, despite an overall decrease in I_K1 conductance (Fig. 8C). This active regulation of inward-rectifier K⁺ channel density with cold temperature acclimation is consistent with the indication from AP recordings that the increase in RMP to less negative values with cold acclimation is not solely a result of direct temperature effects (Table 1). Again, the possible mechanisms underlying the cold acclimation-induced change in ventricular I_K1 and subsequently RMP, such as changes in channel phosphorylation, transcription, translation, rate of protein degradation, or trafficking of channels to the sarcolemmal membrane, remain to be clarified. However, it is tempting to speculate that the known suppression of turtle cholinergic cardiac tone with cold acclimation (20) plays an important role. In rainbow trout atrial myocytes, acetylcholine activates inwardly rectifying K⁺ currents and stabilizes RMP at more negative voltages (38). Therefore, if a similar phenomenon exists for turtle cardiomyocytes, it is conceivable that the absence of turtle cardiac cholinergic tone with cold acclimation allows for the decreased I_K1, the less negative RMP, and the potential for control of I_K1 by other mechanisms.

The reduced ventricular sarcolemmal ion currents and the prolongation of cardiac APs of 5°C-acclimated turtles compared with 21°C-acclimated turtles are consistent with the concept of inverse thermal compensation as a strategy to cope with cold temperature. Because of the inhibitory effect of cold temperature on rates of physiological processes, ectothermic vertebrates exhibit a variety of strategies to cope with the cold. Some ectotherms show physiological compensation that allows for the continuation of an active lifestyle at cold temperature. At the cardiac level, established compensatory changes include increased relative ventricular mass (14), increased myofibrillar ATPase activity and decreased refractoriness of the heart (1), proliferation of the sarcoplasmic reticulum (5), modulation of Ca²⁺ cycling (50–53), increased I_Na (15), and alterations in K⁺ conductances that shorten APD (42, 66). However, for a freshwater turtle that becomes inactive during a prolonged period of winter anoxia, physiological processes must be primed to conserve fuel and make compensatory changes maladaptive (22). Accordingly, these organisms reduce activity, routine metabolic rate, and subsequently cardiac activity with cold exposure in anticipation of winter anoxia conditions (18, 26). Similarly, cold-acclimated crucian carp reduced f_H (37, 64), reduced the rate of cardiac contraction (61), decreased the maximal conductance of I_Na by 4.4-fold (15), and decreased the peak I_Ca density by 6.1-fold (67). Therefore, similar to the findings for crucian carp, the reduced peak density of 5°C-acclimated turtle ventricular I_Na and I_Ca as well as inward slope conductance of inward-rectifier K⁺ channels suggest inverse thermal compensation at the electrophysiological level, which would provide for energetic savings through reducing the cost of ion pumping, one of the largest energy-consuming processes of cells (22). In principle, decreased I_Na and I_Ca would reduce demands on the Na⁺-K⁺-ATPase, which extrudes a proportion of the Na⁺ that enters the myocyte during AP upstroke and also via the sarcolemmal Na⁺/Ca²⁺ exchange. Furthermore, the current required to trigger an AP is likely to be less with a smaller I_K1 and less negative RMP, making myocytes more readily excitability at cold temperature.

Nevertheless, still needed to confirm these suggestions are investigations of direct and acclimatory temperature effects on I_Na and I_Ca with pulse protocols representative of physiological APs and in-depth examination of Na⁺ and Ca²⁺ channel activation/inactivation kinetics. This is because of the possibility that the prolonged AP and slower activation/inactivation kinetics at cold temperature could allow channels to be open longer, resulting in an increase in total charge transferred despite the decreased peak densities. The possibility is certainly in evidence for atrial myocytes of rainbow trout (Oncorhynchus mykiss), a cold-active species that exhibits positive thermal compensation; the charge carried by I_Ca, although temperature dependent with square pulses, becomes temperature independent when stimulated with physiologically relevant AP pulse protocols (51). Of course, the reductions in peak I_Na and I_Ca densities with cold acclimation in the turtle are much larger than the proportional prolongation of APD, which suggests that these reductions should indeed reduce total charge transferred and contribute to energy conservation. Furthermore, tonic adrenergic stimulation, which likely occurs in vivo, influences I_Ca (45), and certainly plays a role in cold rainbow trout (54), could be important during cold acclimation in turtles. Future studies investigating the role of epinephrine on I_Ca would thus be very insightful. Similarly, future investigation on the effect on temperature on Na⁺/Ca²⁺ exchanger current is needed to clarify the apparent juxtaposition of decreased I_Ca and prolonged ventricular APD at 5°C and to determine whether reversed Na⁺/Ca²⁺ exchanger current maintains the long AP in the cold.

A surprising discovery in the present study is that I_Kr was not a predominant current in turtle ventricular myocytes under any experimental condition (Figs. 7D, 11D, and 12D). In contrast, I_Kr is the predominant repolarizing current in mammalian (46) and fish cardiomyocytes. In fact, cold acclimation markedly increases I_Kr in rainbow trout (66), and in the burbot (Loto lota), a cold stenothermic fish, I_Kr is much larger than I_K1 (55). This interspecific difference in I_Kr does not appear to be related to the different cold survival strategies among these species because I_Kr is present in the crucian carp and is upregulated with cold acclimation (M. Vornanen, personal communication). Whether this phenomenon is unique to turtles remains to be determined.

Finally, the longer APD₉₀ and APD₁₀₀ of atrial tissue, but not ventricular tissue, of 5°C-acclimated turtles when acutely warmed (Fig. 6, B and D) suggest that the density of K⁺ currents and/or the effect of cold acclimation on K⁺ currents differs among cardiac tissue types. A similar phenomenon was reported for rainbow trout where substantial differences in I_K1 exist between atrial and ventricular myocytes (66). Again, future studies are needed to clarify this possibility.

Given these important direct and acclimation temperature effects, we can return to the quantitative issue of how these effects might account for the depression of cardiac activity in cold-acclimated turtles. Even the longest tissue APD₁₀₀, i.e., 3,559.0 ms measured in the ventricle, would allow a f_H of 17 beats/min if no refractory period existed. Therefore, APD is unlikely to represent a major restriction on the spontaneous f_H of 5 beats/min. Because cholinergic cardiac inhibition is not involved in the depression of cardiac activity with cold acclimation (20), the decrease in contraction frequency must be due to changes in cardiac refractoriness and other mechanisms such as intercellular electrical coupling and/or pacemaker mechanisms. In ventricular tissue of the anoxia-tolerant<