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Am J Physiol Regul Integr Comp Physiol 290: R1601-R1607, 2006. First published January 26, 2006; doi:10.1152/ajpregu.00877.2005
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RENAL HEMODYNAMICS AND CARDIORENAL INTEGRATION

Vasa recta pericytes express a strong inward rectifier K+ conductance

Chunhua Cao,1 Jae Hwan Goo,1 Whaseon Lee-Kwon,1 and Thomas L. Pallone1,2

1Division of Nephrology, Department of Medicine, and 2Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland

Submitted 14 December 2005 ; accepted in final form 23 January 2006


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Strong inward rectifier potassium channels are expressed by some vascular smooth muscle cells and facilitate K+-induced hyperpolarization. Using whole cell patch clamp of isolated descending vasa recta (DVR), we tested whether strong inward rectifier K+ currents are present in smooth muscle and pericytes. Increasing extracellular K+ from 5 to 50 and 140 mmol/l induced inward rectifying currents. Those currents were Ba2+ sensitive and reversed at the K+ equilibrium potential imposed by the electrode and extracellular buffers. Ba2+ binding constants in symmetrical K+ varied between 0.24 and 24 µmol/l at –150 and –20 mV, respectively. Ba2+ blockade was time and voltage dependent. Extracellular Cs+ also blocked the inward currents with binding constants between 268 and 4,938 µmol/l at –150 and –50 mV, respectively. Ba2+ (30 µmol/l) and ouabain (1 mmol/l) depolarized pericytes by an average of 11 and 24 mV, respectively. Elevation of extracellular K+ from 5 to 10 mmol/l hyperpolarized pericytes by 6 mV. That hyperpolarization was reversed by Ba2+ (30 µmol/l). We conclude that strong inward rectifier K+ channels and Na+-K+-ATPase contribute to resting potential and that KIR channels can mediate K+-induced hyperpolarization of DVR pericytes.

kidney; medulla; microcirculation; electrophysiology; potassium channel

INWARD RECTIFIER POTASSIUM channels (KIR) are named for their characteristic ability to conduct K+ into cells (inward current) more readily than out of cells (outward current) (31). Ca2+-dependent (KCa) and ATP-dependent K+ channels (KATP) are ubiquitously found in vascular smooth muscle, but expression of strong inward rectifier K+ channels is less common (14, 23). Strong inward rectifier K+ currents were described in cerebral arterioles by Edwards and coworkers (8, 9). Subsequently, they have been identified in other small-generation resistance vessels (2, 3, 5, 16–18, 21, 23). Characteristics of KIR currents include enhancement of K+ conductance at membrane potentials that lie near or below the K+ equilibrium potential and potent blockade by the Ba2+ ion that is both voltage and time dependent (31, 34). Functional investigation of KIR expression is somewhat hampered by the lack of a completely specific antagonist. The Ba2+ ion blocks broad classes of K+ channels, but the marked inhibition of K+ current at low micromolar concentrations is generally accepted as characteristic of strong KIR isoforms (23). Weakly inward rectifying KATP channels, comprising KIR6.x subunits coupled to sulfonurea receptors are also sensitive to Ba2+ but require a higher concentration to achieve inhibition. The KIR2.x subfamily has been associated with vascular smooth muscle inwardly rectifying K+ currents (3).

Small increases of extracellular K+ concentration can induce smooth muscle cell hyperpolarization, leading to vasodilation, by either activating KIR channels or by enhancing the electrogenic exchange of 3Na+ for 2K+ by Na+-K+-ATPase. Through those mechanisms, K+ release from endothelial cells has been hypothesized to function as an endothelium-dependent hyperpolarizing factor (EDHF) and vasodilator (2, 10, 14, 18, 31). Apart from the elegant demonstration that an increase in extracellular K+ concentration can dilate renal afferent arterioles in a Ba2+-sensitive manner (5) and the documented expression of KIR2.1 by renin-producing juxtaglomerular cells of the afferent arteriole (20), there is little definitive information concerning KIR activity in the renal microvasculature. The evidence favoring expression of strong KIR isoforms in the efferent microcirculation of the kidney is mixed (22, 37). Descending vasa recta (DVR) are ~13-µm diameter vessels that arise from juxtamedullary efferent arterioles to supply blood flow to the renal medulla. Pericytes are smooth muscle-like cells that surround the DVR endothelial monolayer to impart contractility. DVR contract and dilate in response to a variety of agonists (27, 29). In this study, we used methods adapted to patch clamp of isolated DVR (25, 33) to test whether strong KIR currents exist in the pericytes of this small-generation microvessel. The studies show that elevation of extracellular K+ concentration induces a strongly inward rectifying current that is inhibited by Ba2+ and Cs+. Micromolar concentrations of Ba2+ depolarize DVR pericytes and reverse the hyperpolarization induced by elevation of extracellular K+ from 5 to 10 mmol/l. Taken together, these data strongly support a role for KIR channels to regulate DVR pericyte membrane potential, resting K+ conductance, and play a role in modulation of renal medullary perfusion. To our knowledge, this is the first electrophysiological identification of KIR current in the efferent microcirculation of the kidney.


   METHODS
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 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
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Isolation of DVR. Investigations involving animal use were performed according to protocols approved by the Institutional Animal Care and Use Committee of the University of Maryland. Kidneys were harvested from Sprague-Dawley rats (100–150 g; Harlan, Indianapolis, IN) that had been anesthetized by an intraperitoneal injection of ketamine/xylazine (80 mg/kg; 10 mg/kg). Tissue slices were stored at 4°C in a physiological saline solution (PSS; in mmol/l: 155 NaCl, 5 KCl, 1 MgCl2, 1 CaCl2, 10 HEPES, and 10 glucose, pH 7.4). Small wedges of renal medulla were dissected and transferred to Blendzyme 1 (Roche Diagnostics, Munich, Germany) at 0.27 mg/ml in high-glucose DMEM media (Invitrogen, Carlsbad, CA), incubated at 37°C for 45 min, transferred to PSS, and stored at 4°C. At intervals, DVR were isolated from the enzyme-digested renal tissue by microdissection and transferred to a perfusion chamber on an inverted microscope (Nikon TE300) for patch-clamp recording.

Whole cell patch clamp current recording. Whole cell currents were acquired by conventional ruptured patch recording (25, 33). For ruptured patches, the pipette solution contained (in mmol/l): 120 K-aspartate, 20 KCl, 10 NaCl, 1 CaCl2, 5 EGTA, 3 Mg ATP, 7.5 HEPES, 2.5 Na HEPES, pH 7.2 in ultrapure water. The extracellular solution was PSS (in mmol/l): 155 NaCl, 5 KCl, 1 MgCl2, 1 CaCl2, 10 HEPES, and 10 glucose, pH 7.4. In most experiments designed to elicit inward K+ currents, the KCl concentration was increased to 50 or 140 mmol/l by isosmotic substitution of NaCl for KCl. Patch pipettes were made from borosilicate glass capillaries (PG52151–4, external diameter 1.5 mm, internal diameter 1.0 mm; World Precision Instruments, Sarasota, FL), using a two-stage vertical pipette puller (Narshige PP-830) and heat polished. Whole cell currents were sampled at 10 kHz. Measurements were obtained using a CV201AU headstage and Axopatch 200B amplifier (Axon Instruments, Foster City, CA) and Clampex as previously described (45, 46). To minimize other K+ currents, the intracellular solution contained 3 mM ATP (to inhibit KATP channels) and a low concentration of Ca2+ (EGTA, 5 mmol/l, to minimize the activity of KCa channels). The extracellular solution also contained glybenclamide (10 µmol/l) and niflumic acid (100 µmol/l) to inhibit prominent conductances by KATP and Ca2+-dependent Cl– channels, respectively (4, 25).

Whole cell membrane potential recording. To obtain electrical access to the pericytes for membrane potential recording, we used perforated, rather than ruptured patches (24, 25). For those experiments, the electrode solution was (in mmol/l): 120 K-aspartate, 20 KCl, 10 NaCl, 10 HEPES, pH 7.2 and nystatin (100 µg/ml, 0.1% DMSO). The extracellular solution was PSS without niflumic acid or glybenclamide. Membrane potential recordings were performed in current clamp mode (I = 0) at a sampling rate of 10 Hz using 8- to 10-M{Omega} pipettes. Whereever needed, ouabain (1 mmol/l) was included in the bath to inhibit Na+-K+-ATPase. The data have been corrected for junction potentials, as previously described (25).

Reagents. Glybenclamide, niflumic acid, ouabain, nystatin, and other chemicals were from Sigma (St. Louis, MO). Liberase Blenzyme 1 was from Roche Applied Science. Glybenclamide and niflumic acid were dissolved in DMSO. Reagents were thawed and diluted 1:1,000 on the day of the experiment. Excess was discarded daily. Blendzyme was stored in 40-µl aliquots of 4.5 mg/ml in water and diluted into high glucose DMEM on the day of the experiment.

Statistics. Data in the text and figures are reported as means ± SE. The significance of differences was evaluated with SigmaStat 3.11 (Systat Software, Point Richmond, CA) using parametric or nonparametric tests as appropriate for the data. Comparisons between two groups were performed with Student's t-test (paired or unpaired, as appropriate) or the rank sum (Mann-Whitney) test (nonparametric). Comparisons between multiple groups used repeated-measures ANOVA, or repeated-measures ANOVA on ranks (nonparametric). Post hoc comparisons were performed using Tukey's or Holm-Sidak tests. P < 0.05 was used to reject the null hypothesis. Curve fits to data points were performed using the Levenberg-Marquardt algorithm in Clampfit 9.2 (Axon Instruments).


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
Identification of KIR currents in DVR pericytes. We first tested whether inward rectifying K+ currents are present in DVR pericytes. Pericytes were held at –60 mV, and membrane current was recorded as extracellular K+ was increased from 5 to 50 and 140 mmol/l (Fig. 1A). At 5-s intervals, cells were subjected to ramp depolarizations from –150 to 40 mV over 100 ms (Figs. 1, A and B). Elevation of extracellular K+ markedly increased the inward holding current (Fig. 1A). Examples of the membrane currents elicited by ramp depolarizations are illustrated in Fig. 1B and summarized in Fig. 1C. The times at which the ramps in Fig. 1B were obtained are labeled as numbers 1 and 2 in Fig. 1A. The strongly inward rectifying current elicited during the hyperpolarizing phase of the ramps was markedly inhibited by Ba2+ (30 µmol/l). The Ba2+-sensitive currents reversed sign at the K+ equilibrium potential (EK), verifying selectivity for K+ ion (Fig. 1D).


Figure 1
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  		Fig. 1. Inward rectifier K+ currents in descending vasa recta (DVR) pericytes. A: whole cell current measured in DVR pericytes, held at –60 mV, as extracellular KCl was increased from 5 to 50 and 140 mmol/l. At each K+ concentration, BaCl2 (30 µmol/l) was added to the extracellular buffer to test its ability to block the inward currents. B: examples of ramp currents in the presence and absence of Ba2+ when external K+ concentration is 140 mmol/l. Numbers 1 and 2 correspond to the time of execution of the ramps in A. The inset illustrates the 100-ms ramp protocols used to elicit currents. The arrow shows the zero current level. C: summary of Ba2+-sensitive currents, calculated by subtracting ramp currents in Ba2+ from those in its absence. Data are expressed as means ± SE; most error bars are suppressed for clarity (n = 8). D: means ± SE of the reversal potential of K+ currents in n = 8 cells is plotted vs. equilibrium potential for K+ ion (EK, abscissa) in 5, 50, and 140 mmol/l external KCl. The dashed line is identity.

 
Concentration and voltage dependence of Ba2+ blockade. In other smooth muscle cells, block of KIR currents by Ba2+ has been reported to be both voltage and time dependent (1, 6, 8, 12, 16, 19, 20, 30, 32, 34, 36). Because ramp depolarizations change membrane potential too rapidly to quantify such effects, we performed a series of experiments in which DVR pericytes were depolarized from –80 mV to membrane potentials between –150 to +30 mV, in 20-mV increments, for 4 s (Fig. 2). Whole cell currents in the absence and presence of Ba2+ are illustrated in Fig. 2A. Some inactivation was observed during hyperpolarizing pulses, even in the absence of Ba2+. Ba2+-sensitive currents showed time dependence at 0.1, 1, and 10 µmol/l but not at 100 µmol/l (not shown). End-pulse currents for Ba2+ concentrations of 0, 0.1, 1.0, 10, and 100 µmol/l are summarized in Fig. 2B. These are displayed as the Ba2+-sensitive currents obtained by subtracting records of individual cells in Ba2+ from the currents that were present before addition of Ba2+ to the extracellular buffer. Inward currents at negative potentials were very sensitive to Ba2+. The normalized Ba2+ currents were fit to the equation IBa/I0 = 1/(1 + [Ba2+]/k), where IBa and I0 are whole cell current in the presence and absence of Ba2+, respectively, and k is the binding constant. Results are shown in Fig. 2C. The binding constant for Ba2+ was voltage dependent; k = 0.24 ± 0.03, 0.25 ± 0.03, 0.30 ± 0.02, 0.53 ± 0.13, 1.04 ± 0.43, 8.2 ± 3.3, and 23.7 ± 15.5 µM at –150, –130, –110, –90, –70, –50, and –30 mV, respectively (Fig. 2D).


Figure 2
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  		Fig. 2. Voltage-dependent inhibition of inward rectifier current by Ba2+. A: examples of pericyte whole cell current in 140 mmol/l KCl at baseline and after inhibition by Ba2+ (100 µmol/l). The arrow indicates the zero current level, and the inset shows the protocol used to depolarize the cells. B: summary of means ± SE of Ba2+-sensitive end pulse currents of eight cells. With 4-s depolarizations, 1 µmol/l Ba2+ achieved near-complete inhibition of inward currents, verifying exquisite sensitivity. Little additional inhibition was observed when Ba2+ concentration was increased from 10 to 100 µmol/l. C: current in Ba2+, normalized to that in its absence vs. extracellular Ba2+ concentration. Lines are the least squares fit of the data to the equation, IBa/I0 = 1/(1 + [Ba2+]/k). D: binding constant k vs. pulse potentials. The values are strongly voltage dependent. Data are means ± SE (n = 8).

 
Blockade of pericyte KIR currents by Cs+. KIR currents are generally sensitive to inhibition by Cs+ ion, albeit at higher concentrations than Ba2+ (16, 30, 31). We investigated this characteristic using an approach similar to that shown in Fig. 2 for Ba2+ except that pericytes were depolarized in 140 mM extracellular KCl using 500-ms steps from a holding potential of –80 mV to pulse potentials between –160 and 40 mV (10-mV increments). Examples of the currents elicited are shown in Fig. 3A. The currents are summarized as the Cs+-sensitive current calculated by subtracting records in Cs+ from those elicited before the introduction of Cs+ into the extracellular buffer (Fig. 3B). Cs+ strongly inhibited inward currents at the most negative pulse potentials but was less effective above –50 mV (Figs. 3, B and C). Blockade by Cs+ was rapid at all concentrations tested, implying lack of significant time dependence. The ratio of the membrane current in Cs+ (ICs) is normalized to that in its absence (I0) in Fig. 3C. As in other smooth muscle preparations, blockade of DVR pericyte KIR current by Cs+ was voltage dependent (Fig. 3D). The binding constant for Cs+, fit to ICs/I0 = 1/(1 + [Cs+]/k), yielded; k = 0.27 ± 0.11, 0.33 ± 0.09, 0.40 ± 0.10, 0.86 ± 0.28, 2.62 ± 1.5, and 4.9 ± 4.0 mmol/l at –140, –120, –100, –80, –60, and –50 mV, respectively.


Figure 3
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  		Fig. 3. Voltage-dependent inhibition of inward rectifier current by Cs+. A: examples of pericyte whole cell current in 140 mmol/l KCl at baseline and after inhibition by Cs+ (1,000 µmol/l). The arrow indicates the zero current level, and the inset shows the protocol used to depolarize the cells from –80 mV to pulse potentials ranging from –160 to +40 mV (500 ms, 10-mV increments, inset). Time dependence of inhibition was not observed. B: summary of Cs+-sensitive end pulse currents determined by subtracting records in Cs+ from those in its absence. Data are expressed as the means ± SE . C: current in Cs+, normalized to that in its absence vs. pulse potential. D: binding constant k vs. pulse potentials. Binding constants are strongly voltage dependent. Data are means ± SE (n = 6).

 
Contribution of KIR conductance to resting membrane potential. The experiments in Figs. 1–3 show that KIR conductance is observed in DVR pericytes using conventional ruptured patch whole cell recording. To examine the contribution of KIR channels to overall resting conductance, we also tested the ability of low concentrations of Ba2+ to depolarize pericytes when bathed in PSS (5 mmol/l KCl). To minimize perturbation of the intracellular environment, we used nystatin-perforated patch recording for the experiments (25, 33). To obtain a relevant measurement of the KIR contribution to membrane potential, we did not include any blockers (i.e., niflumic acid or glybenclamide) in the bath. As illustrated in Fig. 4A, low concentrations of Ba2+, between 0 and 30 µmol/l, depolarized DVR pericytes. Resting potential increased from –68 ± 3.9 mV to –63 ± 3.4 and –57 ± 2.8 mV in 10 and 30 µmol/l Ba2+, respectively (Fig. 4B, P < 0.05 vs. 0 µmol/l Ba2+). The depolarization was incompletely reversed upon washout of Ba2+, possibly due to accumulation of Ba2+ within the pericyte cytoplasm.


Figure 4
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  		Fig. 4. Depolarization of DVR pericytes by Ba2+. A: example of the effect of Ba2+ on DVR pericyte membrane potential. The resting membrane potential for the cell is indicated at the beginning of the recording. B: summary of membrane potential measurements at baseline and in Ba2+ at 1, 10, 30, and 100 µmol/l. Low Ba2+ concentrations significantly depolarize DVR pericytes. (n = 10; *P < 0.05 vs. 0 µmol/l Ba2+; #P < 0.05, washout vs. 100 µmol/l Ba2+).

 
KIR channels can respond to a small increase in extracellular K+ concentration by increasing the conductance of K+ outward current, resulting in hyperpolarization. Through that mechanism, K+ has been hypothesized to act as an EDHF (31, 35, 38). We tested for K+-induced hyperpolarization in DVR pericytes by recording membrane potential, as extracellular K+ was increased from 5 to 10 mmol/l. Because such increases in extracellular K+ can also induce hyperpolarization by activating exchange of 3 Na+ for 2 K+ by Na+-K+-ATPase, we concomitantly tested the effects of ouabain (1 mmol/l). As illustrated in Fig. 5A and summarized in Fig. 5B, ouabain induced a substantial DVR pericyte depolarization from –55 ± 3.4 to –31 ± 2.8 mV (P < 0.05). Subsequent elevation of extracellular K+ concentration to 10 mmol/l, in the presence of ouabain, repolarized membrane potential to –46 ± 4.2 mV (P < 0.05). The repolarization by 10 mmol/l KCl was reversed by Ba2+ (30 µmol/l) to –38 ± 2.9 mV. Taken together, these results favor a role for both Na+-K+-ATPase and KIR channels in the setting of pericyte membrane potential. They also support a possible role for K+ ion to act as a renal medullary EDHF.


Figure 5
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  		Fig. 5. Effects of extracellular K+ and ouabain on DVR pericyte membrane potential. A: example of the effect of ouabain (1 mmol/l) and extracellular K+ elevation (5 to 10 mmol/l) on DVR pericyte membrane potential. Ouabain depolarized the cell, following which an increase in K+ concentration, in the presence of ouabain, induced repolarization. The repolarization by 10 mmol/l K+ was reversed by Ba2+ (30 µmol/l). The effects of ouabain, K+ elevation and Ba2+ were reversible upon washout. B: summary of membrane potential measurements at baseline, in ouabain, in 10 mmol/l K+ + ouabain, and in 10 mmol/l K+ + ouabain + Ba2+. Open circles show results for individual cells, and the solid circles show means ± SE. (n = 8; *P, #P, and &P < 0.05 for the comparisons indicated on the graph).

 

   DISCUSSION
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 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
Descending vasa recta are the final resistance vessels in the microvascular circuit that supplies blood flow to the renal medulla. They have dual, interdependent functions to regulate medullary perfusion and trap solute to maintain corticomedullary gradients of NaCl and urea needed for urinary concentration (28). The vessels are of small diameter (typically, 12–15 µm). They traverse the renal outer medulla in vascular bundles and comprise a continuous endothelial monolayer surrounded by contractile pericytes. Early evidence favored a dominant role for receptor-operated Ca2+ entry into smooth muscle of efferent arterioles; however, it has recently been confirmed that voltage-gated Ca2+ entry is important in juxtamedullary efferent arterioles and DVR pericytes (13, 41). It has been confirmed that vasoconstriction of DVR by ANG II and endothelin I is accompanied by depolarization of the pericyte cell membrane through dual actions involving activation of a Ca2+-dependent Cl– channels (13, 25, 40) and inhibition of K+ conductance (4, 24). The resulting shift in membrane potential toward the equilibrium potential of Cl– ion activates voltage-gated Ca2+ entry that can be inhibited by diltiazem (41) or nifedipine (39).

Motivated by the importance of membrane potential in the control of cytosolic Ca2+ concentration ([Ca2+]CYT), and contractility in the DVR pericyte, we have undertaken studies to define the presence and contributions of various classes of K+ channels to membrane conductance. Figures 1 and 2 show the presence of a strong inwardly rectifying K+ conductance that is exquisitely sensitive to inhibition by Ba2+. The KIR currents reverse sign at the K+ equilibrium potential (Figs. 1C, 1D), verifying the selectivity of the Ba2+-sensitive conductance for K+. The associated inward currents are easily observed in 50 and 100 mmol/l extracellular K+ but are less prominent in 5 mmol/l K+ (Fig. 1C). The latter is likely to have several explanations. First, KIR conductance increases with extracellular K+ concentration. Second, for reasons that are not entirely clear to us, depolarizations in 5 mmol/l K+ give rise to noisier current recordings than those in 50 or 140 mmol/l K+. Finally, the experimental conditions in Fig. 1 are not optimized to observe Ba2+-sensitive currents. Block of KIR by Ba2+ is time dependent, and the 100-ms ramp depolarizations used to elicit the currents illustrated in Fig. 1C may be too rapid for full Ba2+ inhibition to develop. In that case, the inwardly rectifying currents, calculated from their Ba2+ sensitivity, may be somewhat underestimated. The prolonged depolarizations (4 s) used in Fig. 2 were performed to examine that issue and lend confidence to the definition of the concentration dependence of Ba2+ inhibition.

The current records in Figs. 1B, 2A, and 3A show Ba2+ and Cs+ insensitive outward currents at positive membrane potentials. Those currents are present despite the use of glybenclamide and niflumic acid to inhibit KATP and Ca2+-dependent Cl– channels, respectively. The conductance(s) that carry those outward currents is uncertain. They are likely to be due to K+ efflux from, or Cl– influx into the pericytes. In the case of K+ efflux, the current could be conducted by voltage-gated K+ channels (KV) or incomplete inhibition of Ca2+-dependent K+ channels by Ca2+ chelation in the electrode. In the case of Cl– influx, it is possible that incomplete inhibition of Ca2+-dependent Cl– channels by niflumic acid exists or that volume-dependent ClC type Cl– channels are present to conduct the current. Given that Ba2+ and Cs+ sensitivity define the inwardly rectifying currents in Figs. 2 and 3, the outward currents are unlikely to obscure the characterization of KIR that is the focus of this report.

In a prior study, we found a mean resting pericyte membrane conductance of 2.25 ± 0.32 nS at –60 mV in symmetrical K+. That conductance was reduced to 1.47 ± 0.27 nS by the specific KATP channel blocker glybenclamide (4). In this study, under similar conditions, at –60 mV, a Ba2+-sensitive current of –81 ± 16 pA was found measured, representing a chord conductance of 1.35 ± 0.3 nS. Taken together, we conclude that the combination of strong KIR and KATP channel activity accounts for the majority of pericyte conductance in symmetrical K+ at negative membrane potentials. Whereas those calculations clearly demonstrate the presence of KIR channels, they do not define their contribution to resting pericyte conductance in physiological saline. At typical resting potentials (–70 to –50 mV) in 5 mmol/l KCl, the conductance of KIR channels is expected to be lower than that observed when membrane potential is below EK in symmetrical K+. For example, the Ba2+-sensitive outward current in 5 mmol/l KCl was 16.5 ± 13 pA at –60 mV (Fig. 1), corresponding to a KIR chord conductance of 0.55 ± 0.44 nS, that is, not significantly different from zero. This illustrates the inherent difficulty involved in quantifying the relative contributions of various K+ channel classes to resting membrane potential; those conductances are generally low. To address this, we also examined the contribution of KIR to membrane potential, quantified by the responses to low concentrations of Ba2+. At 30 µmol/l, a concentration that is expected to be specific for KIR blockade, Ba+ depolarized pericytes from –68 ± 3.4 to –57 ± 2.6 mV (Fig. 4). Thus we conclude that KIR channels contribute to resting K+ conductance but do not dominate it. In a recent study, under identical experimental conditions, we found that the specific KATP blocker, glybenclamide, depolarized pericytes by a mean of only 4 mV (–60.4 to –56.4 mV) (4). An interesting finding of the present study is that ouabain depolarized pericytes by 24 mV (Fig. 5), implying that the electrogenic activity of Na+-K+-ATPase may play a greater role to set membrane potential than KATP and KIR channel conductances combined.

The ability of external K+ to increase KIR conductance near EK may be as physiologically important as the KIR contribution to resting conductance. The presence of KIR expression in DVR pericytes raises the possibility that elevation of extracellular K+ might induce pericyte hyperpolarization to favor DVR vasodilation. The results in Fig. 5 verify that such K+-induced hyperpolarization can occur, thereby supporting the possibility that extracellular K+ regulates DVR tone in vivo. Those findings also focus attention on factors that could alter K+ ion concentration in the vicinity of DVR pericytes. An established hypothesis has been that stimulation of endothelial cells of resistance vessels by vasodilator agonists induces endothelial hyperpolarization and [Ca2+]CYT elevation to stimulate KCa channels and drive K+ efflux into the perivascular interstitium. Endothelial K+ release has been proposed to create a "K+ cloud" that hyperpolarizes and relaxes adjacent smooth muscle (4, 7, 10, 11). DVR endothelia have been shown to respond to endothelium-dependent hyperpolarizing factors such as ACh and bradykinin by hyperpolarizing and increasing [Ca2+]CYT (27, 29, 33). Thus, along with KIR expression by pericytes, the appropriate endothelial responses exist for such a putative response mechanism to operate.

In addition to release of K+ from endothelia, medullary K+ recycling might regulate extracellular K+ concentration near vasa recta. Outer medullary DVR (the vessels studied herein) cannot be directly observed or sampled by micropuncture in vivo, because they are enveloped by overlying renal cortex. Exposure of the deep inner medulla (the papilla) by ureteral excision in young rats, however, makes inner medullary vasa recta accessible. Plasma sampled from vasa recta at the papillary tip has a high K+ concentration ~30 mmol/l (26), possibly resulting from K+ recycling between collecting duct and pars recta (15). On the basis of those observations, it seems possible that changes in the rate of K+ recycling could regulate vascular bundle K+ concentration to modulate DVR vasoactivity through KIR and Na+-K+-ATPase-mediated mechanisms (Fig. 5).

There is a relative paucity of information regarding expression and function of KIR channels in the renal microvasculature. Chilton and Loutzenhiser (5) showed that increasing K+ from 5 to 15 mmol/l dilates afferent arterioles in the split hydronephrotic kidney preparation. In a separate study, efferent arteriolar vasodilation by ACh could not be traced to KIR channel activity, providing evidence against KIR expression in the glomerular efferent microcirculation (37). In contrast to the latter study, K+ elevation caused a reduction of efferent arteriolar [Ca2+]CYT that was blocked by low concentration of Ba2+ or ouabain. Those results favor a role for KIR in efferent arterioles (22). The apparent disparity between the conclusions of Wang and Loutzenhiser (37) and Marchetti et al. (22) might be due to the use of juxtamedullary efferent arterioles in the latter study. Superficial efferent arterioles are likely to have been studied in the split hydronephrotic kidney model used by Wang and Loutzenhiser (37), raising the possibility that KIR are absent in superficial efferent arterioles but present in juxtamedullary efferent arterioles. Such cortical efferent arteriolar heterogeneity has been described for expression of voltage-gated Ca2+ channels; superficial efferent arterioles lack them, whereas juxtamedullary efferent arterioles express them (13). DVR are branches of juxtamedullary efferent arterioles, and our demonstration that DVR pericytes clearly do express KIR supports the possibility that they may be present in juxtamedullary efferents. Rigorous electrophysiological and immunochemical studies will be needed to resolve that issue.

The molecular identity of the KIR subtype responsible for the Ba2+- and Cs+-sensitive currents shown in Figs. 1–3 is unknown, however, it is likely to be one of the classes of KIR capable of strong inward rectification, likely KIR2.x or KIR4.x (34). Strong inward rectification in smooth muscle has generally been traced to KIR2.1 (3, 14, 35, 38). The characteristics of KIR in DVR pericytes are similar to those identified in other vascular myocytes. Quayle and colleagues (30, 31) found Kd for inhibition by Ba2+ to be 8.0 and 0.6 µmol/l at –40 and –100 mV, respectively, in cerebral arteriolar myocytes. Those values are similar to our measurements, 8.2 and 0.3 µmol/l at –50 and –90 mV, respectively, (Fig. 2).

In summary, DVR pericytes express a strong KIR current that is highly sensitive to inhibition by Ba2+ and Cs+. The KIR channels make a significant contribution to resting membrane potential. To our knowledge, these are the first direct measurements of KIR currents in the efferent microcirculation of the kidney. In addition to the activity of KIR and KATP channels, Na+-K+-ATPase activity makes a surprisingly large hyperpolarizing contribution to resting membrane potential, documented during ouabain inhibition. A small increase in extracellular K+ hyperpolarized pericytes in a Ba2+-sensitive manner, and this is consistent with a putative role for K+ to act as a renal medullary vasodilator.


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REFERENCES
 
Studies in our laboratory are supported by National Institutes of Health Grants R37-DK-42495, R01-DK-67621, and P01-HL-78870.


   FOOTNOTES
 

Address for reprint requests and other correspondence: T. L. Pallone, Div. of Nephrology, N3W143, 22 S. Greene St., Univ. of Maryland, School of Medicine, Baltimore, MD 21201 (E-mail: tpallone{at}medicine.umaryland.edu)

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   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

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