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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

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