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(Circulation Research. 1996;79:295-301.)
© 1996 American Heart Association, Inc.

Articles

Alterations in Rat Interlobar Artery Membrane Potential and K⁺ Channels in Genetic and Nongenetic Hypertension

Jeffrey R. Martens, Craig H. Gelband

the Department of Physiology, University of Florida College of Medicine, Gainesville.

Correspondence to Craig H. Gelband, PhD, University of Florida College of Medicine, PO Box 100274, Gainesville, FL 32610-0274. E-mail gelband@phys.med.ufl.edu.

	Abstract

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The renal vasculature plays an important role in the control of blood pressure. K⁺ channels have been demonstrated to regulate smooth muscle membrane potential and thereby control smooth muscle tone. However, few data are available on K⁺ channel function in the renal vasculature of hypertensive animals. This study details changes in K⁺ currents and membrane potential in genetic and nongenetic models of hypertension. The patch-clamp technique and Ca²⁺-imaging fluorescence were used to examine the differences in Wistar-Kyoto (WKY), Sprague-Dawley (SD), spontaneously hypertensive (SHR), and deoxycorticosterone acetate (DOCA) hypertensive single cells of rat kidney interlobar arteries. In current-clamp experiments, SHR and DOCA hypertensive cells were 20 mV more depolarized than the control cells. In voltage-clamp experiments with 4-aminopyridine and niflumic acid present to inhibit voltage-dependent K⁺ (K_(v)) and Ca²⁺-activated Cl^- (Cl_(Ca)) currents, SHR and DOCA hypertensive Ca²⁺-activated K⁺ (K_(Ca)) currents were significantly larger and activated at more negative potentials than the control. Conversely, with charybdotoxin and niflumic acid present to inhibit K_(Ca) and Cl_(Ca) currents, SHR and DOCA hypertensive K_(v) current was significantly smaller than the control. Finally, basal and angiotensin II–stimulated peak intracellular free [Ca²⁺] was greater in the SHR and DOCA hypertensive cells compared with control cells. These results suggest that membrane potential and the activity of K_(Ca) and K_(v) channels are altered in hypertensive rat renal interlobar arteries and may play a role in the regulation of renal blood flow under physiological and pathophysiological conditions.

Key Words: vascular smooth muscle • hypertension • K⁺ channel • membrane potential • [Ca²⁺]_i

	Introduction

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The etiology of systemic hypertension involves a gradual and sustained increase in total peripheral resistance.¹ A number of anatomic, biochemical, or biophysical mechanisms may underlie this phenomenon.² Anatomically, rarefaction of the vascular beds and blood vessel hypertrophy occur in many hypertensive models. Biochemically, alterations in Na⁺, K⁺-ATPase, Ca²⁺-ATPase, and Na⁺-Ca²⁺ exchange activity have been reported in blood vessels and isolated cell preparations from various models of hypertension. Biophysical changes that may occur in hypertension involve changes in ion channel function, thereby causing changes in vascular reactivity. Vascular smooth muscle contains at least two different Ca²⁺ channels, at least four different K⁺ channels, nonselective cation channels, and Ca²⁺-activated Cl^- channels.^{3 4 5 6} Therefore, alterations in ion channel function of any of these channels could potentially change the contractile state of vascular smooth muscle, thereby altering blood pressure.

The use of fluorescent Ca²⁺ indicators and electrophysiological techniques has enabled studies of hypertension at the single-cell level. However, to date, the data from various models have led to differing hypotheses regarding the roles of ion channels in the regulation of [Ca²⁺]_i and in the etiology of hypertension. For example, when SHR aortic smooth muscle cells were loaded with fura 2, basal [Ca²⁺]_i was increased when compared with WKY cells.⁷ Furthermore, vasopressin-stimulated and Ang II–stimulated increases in [Ca²⁺]_i were greater in SHR aortic cells when compared with WKY cells.⁸ Conversely, Storm et al⁹ and England et al¹⁰ showed no change in basal [Ca²⁺]_i levels in hypertension. Using conventional microelectrodes, it was reported that the ouabain-insensitive component of resting membrane potential is 10 to 15 mV more depolarized in SHR blood vessels than in WKY vessels.¹¹ However, other laboratories report that no change in resting membrane potential exists in hypertensive models.^{12 13 14} Recently, Rusch et al¹³ and England et al¹⁰ demonstrated that SHR aortic cells have more K_(Ca) current than do WKY cells. However, no studies to date have shown alterations in K_(v) current in hypertensive models. To date, the fluorescence and electrophysiological studies performed in various models of hypertension have been obtained from larger conduit arteries, which play a less significant role in setting peripheral vascular resistance than blood vessels from the kidney or mesentery. Studies of clinically relevant vessels from the renal vasculature should provide useful information concerning the electrophysiological differences in hypertension at the single-cell level. Here, we present data that for the first time demonstrate that alterations in K_(v) channel activity, membrane potential, and [Ca²⁺]_i exist in small vessels of the genetic and nongenetic hypertensive renal vasculature.

	Materials and Methods

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Animal Preparation
SHR, WKY, and SD rats were bought from Charles River Breeders (Boston, Mass). Control SD rats were made hypertensive by implanting one 50-mm-long silastic tube (Dow Corning), containing DOCA (100 mg/kg), subcutaneously on the dorsal side of the rats. Rats were unilaterally nephrectomized and given 0.15 mmol/L NaCl to drink as their sole drinking fluid. Control blood pressures were taken 2 weeks before the above procedure and weekly thereafter. Hypertension developed in 4 to 6 weeks after the DOCA/salt/nephrectomy regime. Adult male rats were used at the age of 15 weeks. The mean systolic blood pressure of WKY rats was 140±4 mm Hg (n=15) and was significantly greater in the SHR (195±11 mm Hg) (n=15, P<.01). Similar results were obtained for the SD and DOCA hypertensive rats (SD, 115±7 mm Hg; DOCA, 165±10 mm Hg; n=16, P<.01). WKY and SHR strains are used as the control and experimental models for genetic hypertension, respectively; the SD and DOCA hypertensive rats are used as the control and experimental models for nongenetic hypertension, respectively.

Electrophysiological Measurements
Single smooth muscle cells from WKY, SHR, SD, and DOCA hypertensive interlobar arteries were enzymatically dissociated using previously described methods.^{15 16} Care was taken to minimize trauma to either arterial preparation. Cells from normotensive and hypertensive animals were made on the same day using the same procedure, thereby ruling out nonspecific membrane effects that were due to the cell isolation technique. All experiments were conducted blind, thereby minimizing bias in the results. However, there is a possibility that the connective tissue of the arterial wall is altered in hypertension,² causing the effectiveness of the digestive enzymes to be altered. Thus, smooth muscle cells isolated from the vessels of hypertensive rats may have been in the presence of the enzymatic digestion cocktail for longer periods of time than control cells, possibly altering sarcolemma proteins. Although difficult to test, efforts were made to reduce the possible effects of variable digestion in the tissues by examining reversibility of an Ang II (100 nmol/L)–induced contraction of isolated single smooth muscle cells. Only batches of cells that showed contraction and relaxation to Ang II were used.

Single cells were voltage-clamped, and membrane currents were measured using the whole-cell configuration of the patch-clamp technique.¹⁷ Patch pipettes for whole-cell patch-clamp recordings were made from borosilicate glass capillaries and had resistances of 1 to 3 M. Voltage-clamp command potentials were applied to the cells, and membrane currents were recorded using an Axopatch-1D patch-clamp amplifier. Membrane current was monitored on a digital oscilloscope, digitized on-line (0.5 to 2.0 kHz), and stored on a computer. Resting membrane potentials were measured in the I=0 position of the Axopatch-1D. Input resistances were determined in current clamp by applying a 1- to 5-pA hyperpolarizing current pulse and examining the change in membrane potential. Any cell that did not have a seal resistance of >5 G was not used. Series resistance was in the range of 2 to 6 M and was compensated for by 80% using the Axopatch-1D. Junction potentials were zeroed at the beginning of each experiment with Axopatch-1D. At the end of each experiment, no change in the nulled junction potential occurred. Data analysis was performed with pCLAMP 5.5.1 and 6.0 software (Axon Instruments). All experiments were performed at room temperature, and data illustrated in the figures were from different batches of cells isolated from different animals.

Measurement of Intracellular Ca²⁺
The Ca²⁺ indicator indo 1 (pentapotassium salt, 100 mmol/L) was included in the patch pipette solution and dialyzed into the cell as previously described.¹⁵ Background autofluorescence from the same cell was measured before gaining access to the cell interior and subtracted from all fluorescence measurements. Sufficient loading had taken place 5 to 10 minutes after access to the cell interior. A 10-mm diameter of the cell was irradiated with ultraviolet light at a wavelength of 340 nm with a mercury lamp. The light emitted from the cell was collected using an epifluorescence microscope (Nikon) and measured at 400 and 500 nm by means of a microfluorometer and matched photomultiplier tubes. Changes in [Ca²⁺]_i were calibrated using the equation of Grynkiewicz et al¹⁸ :

where K_d is the dissociation constant of the Ca²⁺–indo 1 complex; R is the F₄₀₀/F₅₀₀ fluorescence ratio; R_min and R_max are the ratios measured by the addition of the Ca²⁺ ionophore ionomycin (10 mmol/L) to Ca²⁺-free (10 mmol/L EGTA) solution and Ca²⁺-containing solution, respectively; and S_f2/S_b2 is the ratio of the 500-nm fluorescence signal in Ca²⁺-free and Ca²⁺-containing solutions.

Solutions
The Krebs' solution contained (mmol/L) NaCl 120, NaHCO₃ 25, KCl 4.8, MgCl₂ 1.2, CaCl₂ 2.5, and D-glucose 10. For all whole-cell voltage-clamp experiments examining outward currents, the bath solution contained (mmol/L) NaCl 130, NaHCO₃ 10, KCl 4.2, KH₂PO₄ 1.2, MgCl₂ 0.5, CaCl₂ 1.5, D-glucose 5.5, and HEPES 10 (pH 7.4 with NaOH). The pipette solution contained (mmol/L) KCl 140, ATP (potassium salt) 5, and HEPES 10 (pH 7.2 with KOH). Charybdotoxin was obtained from Peninsula Laboratories, Inc, and the stock was 0.1 mol/L in 150 mmol/L NaCl. All other chemicals were from Sigma Chemical Co.

Statistics
Results are expressed as mean±SEM. Statistical significance was evaluated using Student's t test for unpaired observations. Differences were considered significant at P<.05, and n corresponds to the number of cells examined. Membrane currents were measured from the zero current level and normalized to cell capacitance.

	Results

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Electrical Properties of Single Rat Interlobar Artery Cells
Table 1 illustrates the electrical properties of isolated WKY, SD, SHR, and DOCA hypertensive interlobar artery cells. In 15 to 16 cells from nine different preparations, a significant difference was observed in the resting membrane potential of the cells. WKY, SD, SHR, and DOCA hypertensive cells had an average resting membrane potential of -52±2.3, -50±3.8, -32±3.8 (P<.01), and -36±2.5 (P<.01) mV, respectively (n=15 or 16). All cells had similar input resistances and membrane capacitances (Table 1). One explanation of these results is that the altered contractile state of the hypertensive interlobar artery may be due to alterations in K⁺ permeability of the hypertensive single smooth muscle cell.

View this table: [in this window] [in a new window]   Table 1. Electrical Properties of WKY, SD, SHR, and DOCA Rat Interlobar Artery Cells

Ionic Currents in WKY, SD, SHR, and DOCA Hypertensive Cells: K_(v) and K_(Ca) Currents
It has been reported that K_(Ca) current was increased in hypertension,^{10 13} but to date no evidence for changes in K_(v) current has been demonstrated in hypertensive models. Fig 1 shows pharmacological isolation of K_(v) and K_(Ca) currents from WKY and SHR cells. The use of pharmacological agents to isolate these two components of K⁺ current was based on experimental results obtained from the canine renal artery.¹⁵ Charybdotoxin (100 nmol/L) and niflumic acid (100 µmol/L) were used to isolate K_(v) current, 4-AP (10 mmol/L) and niflumic acid (100 µmol/L) were used to isolate K_(Ca) current, and 5 mmol/L ATP was present in the recording pipette to inhibit ATP-sensitive K⁺ current. Fig 1A (top) shows representative ramp depolarizations from a WKY and an SHR cell in the presence of 4-AP and niflumic acid. Under these conditions, K_(v) current was significantly reduced, yet K_(Ca) current was present. Fig 1A (bottom) shows the mean current-voltage relationship obtained for K_(Ca) current during voltage-step depolarizations (n=7). In seven experiments, K_(Ca) current in SHR cells was significantly increased when compared with control (P<.01, Table 2). Fig 1B illustrates the nature of K_(v) current in WKY and SHR cells in the presence of charybdotoxin and niflumic acid. The top of Fig 1B shows representative ramp depolarizations, and under these conditions, K_(Ca) current was nearly abolished, yet K_(v) current was present. The bottom of Fig 1B shows the mean current-voltage relationship obtained for K_(v) current during voltage-step depolarizations (n=7). In seven cells tested, K_(v) current was decreased in SHR cells compared with WKY cells during voltage-step depolarizations (P<.01). Table 2 illustrates mean current density values for K_(Ca) and K_(v) currents in WKY and SHR cells at membrane potentials of 0 and 40 mV.

View larger version (27K): [in this window] [in a new window]   Figure 1. Isolation of K(v) and K(Ca) current in WKY and SHR interlobar artery cells. A, In the presence of 4-AP (10 mmol/L) and niflumic acid (100 µmol/L), SHR K(Ca) current was significantly greater than WKY K(Ca) current. Similar results were obtained in seven cells. Voltage-ramp and -step depolarizations are shown at the top and bottom of the panel, respectively. B, In the presence of charybdotoxin (100 nmol/L) and niflumic acid (100 µmol/L), WKY K(v) current was significantly greater than SHR K(v) current. Similar results were obtained in seven cells. Voltage-ramp and -step depolarizations are shown at the top and bottom of the panel, respectively. These two cells had similar membrane capacitances (WKY, 26 pF; SHR, 27 pF).

View this table: [in this window] [in a new window]   Table 2. Mean Alterations in K+ Current at 0 and +40 mV

Similar results were obtained when K_(v) and K_(Ca) currents were isolated in control and nongenetic hypertensive cells. Fig 2 illustrates the characteristics of K_(v) current in SD and DOCA hypertensive cells. During voltage-step depolarizations in the presence of charybdotoxin and niflumic acid (Fig 2A), K_(v) current in DOCA hypertensive cells is less than in control cells. During a voltage-ramp depolarization (Fig 2B), the DOCA hypertensive cells possessed a decreased K_(v) current when compared with control cells. In another set of experiments, the nature of K_(Ca) current in SD and DOCA hypertensive cells was investigated (Fig 3). Fig 3A shows representative traces of step depolarizations from an SD and a DOCA hypertensive cell in the presence of 4-AP and niflumic acid. K_(Ca) current was significantly increased in DOCA hypertensive cells compared with control cells (n=7, P<.01, Table 2). Fig 3B is a representative trace of K_(Ca) current during voltage-ramp depolarizations in both cell types. Again, it shows that the DOCA hypertensive K_(Ca) current was greater when compared with control. Fig 4 illustrates mean current-voltage relationships for K_(v) and K_(Ca) currents during voltage-step depolarizations in SD and DOCA hypertensive cells (n=7, P<.01). The DOCA hypertensive mean K_(v) current-voltage relationship was shifted right and activated at more positive potentials than control (Fig 4A). However, the DOCA hypertensive mean K_(Ca) current-voltage relationship was shifted left and activated at more negative potentials than control (Fig 4B). Table 2 illustrates mean current density values at 0 and 40 mV for K_(Ca) and K_(v) currents in SD and DOCA cells. These biophysical characteristics may be of some importance, since K⁺ current has been suggested to play a role in regulating resting membrane potential of vascular smooth muscle cells.^{15 19}

View larger version (17K): [in this window] [in a new window]   Figure 2. Isolation of K(v) current in SD and DOCA hypertensive interlobar artery cells. In the presence of charybdotoxin (100 nmol/L) and niflumic acid (100 µmol/L), SD K(v) current was significantly greater than DOCA hypertensive K(v) current during step depolarizations (holding potential, -80 mV; test potential, -20 mV to 80 mV; 20-mV steps) and ramp depolarizations (-80 to 80 mV, 4 seconds). The membrane capacitances of the two cells were similar (SD, 30 pF; DOCA, 29 pF). Similar results were obtained in seven cells.

View larger version (24K): [in this window] [in a new window]   Figure 3. Isolation of K(Ca) current in SD and DOCA hypertensive interlobar artery cells. In the presence of 4-AP (10 mmol/L) and niflumic acid (100 µmol/L), DOCA hypertensive K(Ca) current was significantly greater than SD K(Ca) current during step depolarizations (A; holding potential, -80 mV; test potential, -20 to 70 mV; 10-mV steps) and ramp depolarizations (B, -80 to 80 mV, 4 seconds). These two cells had similar membrane capacitances (SD, 29 pF; DOCA, 30 pF). Similar results were obtained in seven cells.

View larger version (16K): [in this window] [in a new window]   Figure 4. Mean current-voltage relationships for K(v) and K(Ca) currents in SD and DOCA hypertensive cells. DOCA hypertensive K(v) current (A) and K(Ca) current (B) are significantly altered compared with control (n=7). Error bars are the same size as the symbol and therefore are not visible.

Regulation of [Ca²⁺]_i in WKY, SD, SHR, and DOCA Hypertensive Cells
As illustrated in Table 1, the resting membrane potential for SHR and DOCA hypertensive artery cells was more depolarized than it was for control cells. Since [Ca²⁺]_i has been suggested to inhibit K_(v) channels and regulate membrane potential,¹⁵ a separate group of experiments was performed to investigate the regulation of [Ca²⁺]_i in single WKY, SD, SHR, and DOCA hypertensive interlobar artery cells. Fig 5 shows that resting levels of [Ca²⁺]_i were significantly elevated in the hypertensive cells compared with control cells (WKY, 92±7 nmol/L; SHR, 125±9 nmol/L [P<.05]; SD, 89±7 nmol/L; and DOCA, 113±11 nmol/L [P<.05]; n=6). Since circulating Ang II is a potent renal vasoconstrictor and its circulating concentration is increased in various forms of hypertension,² we investigated the effects of Ang II on peak [Ca²⁺]_i. Application of Ang II (100 nmol/L) significantly increased peak [Ca²⁺]_i over basal levels in each cell type (n=6). This occurred within 5 seconds of Ang II application. A significant difference was also observed in the Ang II–stimulated increase in peak [Ca²⁺]_i when hypertensive cells were compared with control cells (WKY, 450±25 nmol/L; SHR, 665±30 nmol/L [P<.05]; SD, 369±31 nmol/L; and DOCA, 595±20 nmol/L [P<.05]; n=6). No difference was observed in the time course of the Ang II effect in any cell type. When combined with the results of Figs 1 through 4, these data support the hypothesis that [Ca²⁺]_i may modulate resting membrane potential by altering K_(v)¹⁵ and/or K_(Ca)²⁰ current.

View larger version (17K): [in this window] [in a new window]   Figure 5. Basal and Ang II–stimulated [Ca2+]i. Both basal and Ang II–stimulated [Ca2+]i are significantly different from control (n=7, P<.05).

	Discussion

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Alterations in ion channel activity have been associated with an increased vascular reactivity in hypertension.^{2 14} Specifically, depolarization of the resting membrane potential^{11 21 22} and increases in Ca²⁺ current,^{23 24 25} K_(Ca) current,^{10 13} and [Ca²⁺]_i^{7 9 26 27 28} have been demonstrated in various forms of hypertension. The present study describes the electrophysiological properties of a physiologically relevant vascular preparation, the interlobar artery of the kidney, in hypertension. Our results suggest that there are changes observed in both K⁺ current and Ca²⁺ homeostasis in a genetic and nongenetic model of hypertension. These effects may contribute to the increase in vascular reactivity observed in hypertension.

Resting Membrane Potential and Hypertension
Normotensive vascular smooth muscle cells, including those isolated from the renal vasculature, have resting membrane potentials between -50 and -60 mV.^{13 15 16 19 29} In the present study, it was shown that SHR and DOCA hypertensive interlobar artery cells have a resting membrane potential of -30 mV (Table 1). This was significantly depolarized when compared with the control cells (-50 mV). In a variety of conduit artery vascular preparations, significant changes in passive membrane electrical properties have been reported in SHR cells compared with control cells.^{11 12 21 29 30} However, there have been some reports of an unaltered resting membrane potential in vascular smooth muscle cells from the SHR and other models of hypertension.^{10 14 31} One reason for the discrepancy in resting membrane potentials may be the site and size of the vessel itself. There is ample evidence that larger capacitance vessels, like the aorta and first-branch coronary arteries, do not have similar excitation-contraction coupling properties compared with smaller resistance vessels.³² This suggests that small arteries (<200 µm in diameter) from physiologically relevant vascular beds (eg, renal, mesenteric, and cerebral) are altered in hypertension and play a significant role in the regulation of peripheral resistance.

K⁺ Channels and Hypertension
Membrane K⁺ permeability is the primary initiator of vascular smooth muscle contractility.^{4 5 45}K⁺ efflux was increased in conduit artery hypertensive vascular smooth muscle compared with normotensive vascular smooth muscle.³³ To date, there is no evidence that alterations in K_(v) channels exist in any model of hypertension. Our results are the first to show that K_(v) current was significantly reduced in SHR and DOCA hypertensive interlobar artery cells compared with control cells (Figs 1 and 3). A significant decrease of K_(v) current would cause membrane depolarization and an increase in vascular smooth muscle tone. Indeed, a significant depolarization of the resting membrane potential of isolated cells from SHR and DOCA hypertensive animals was observed when compared with control cells (Table 1). This would suggest that K_(v) channels play a major role in the regulation of membrane potential and therefore tone in rat interlobar arteries.

It was previously demonstrated there was an increased K_(Ca) channel and open probability of large-conductance K_(Ca) channels in SHR cells isolated from the aorta.^{10 13} It was suggested that this enhanced K⁺ permeability might provide a negative-feedback mechanism by which an increased arterial contractility may be limited in hypertension.¹³ The data described in the present study are consistent with these conclusions. Basal [Ca²⁺]_i is increased in the SHR and DOCA hypertensive cells (Fig 5), and this would lead to a greater K_(Ca) current density in the SHR and DOCA hypertensive cells (Figs 1 and 2). However, based on the voltage-dependent properties of this channel (ie, extremely low open probability at more negative membrane potentials), it would be difficult for this channel to be the sole channel protein that causes the increased ⁴⁵K⁺ efflux observed in SHR arterial smooth muscle.

[Ca²⁺]_i and Hypertension
A number of studies have attempted to compare resting levels of [Ca²⁺]_i in SHR and WKY artery preparations.^{2 5 7 8 10 13 27 34} These studies have shown differing results depending mainly on the age of the animal used. In preparations of SHR arteries from young animals (3 days to 5 weeks) that have not developed hypertension, resting [Ca²⁺]_i levels did not differ from the WKY arteries.^{2 5 9} However, Ang II–stimulated or vasopressin-stimulated changes in [Ca²⁺]_i did show significant differences in young SHR, suggesting a change in Ca²⁺ homeostasis at an early age in development before the onset of hypertension.⁸ This is consistent with our results (Fig 5). In older rats (>8 weeks), an increase in resting levels on [Ca²⁺]_i between control and hypertensive cells was observed.^{7 27} However, there are studies stating that resting levels of [Ca²⁺]_i were not altered in adult SHR vascular smooth muscle cells from conduit arteries.^{9 10} The present data using indo 1 fluorescence suggest that [Ca²⁺]_i was significantly greater in SHR and DOCA hypertensive interlobar artery cells than in control cells. This increase in resting [Ca²⁺]_i levels can be directly correlated to the changes observed in membrane potential and K_(v) current. Previous results in the canine renal artery¹⁵ demonstrated the sensitivity of K_(v) channels to increased [Ca²⁺]_i, which in turn caused membrane depolarization. These data are consistent with our observations that cells from resistance blood vessels have different excitation-contraction coupling pathways and Ca²⁺ homeostatic mechanisms than cells from conduit blood vessels.

Ion Channel Control of Vascular Smooth Muscle Tone in Hypertension
Recently, a number of K⁺ channels have been suggested to regulate membrane potential and tone of smooth muscle cells. It has been demonstrated that voltage-dependent delayed rectifier K⁺ channels are important regulators of smooth muscle resting membrane potential.^{15 19} This is consistent with our observations in the interlobar artery smooth muscle cells. Our results in clinically relevant resistance vessels suggest that the inhibition of the K_(v) channel and the associated membrane depolarization would be of physiological importance in the regulation of tone in normotensive and hypertensive states.¹⁵ However, other K⁺ channels have also been suggested to play a role in the regulation of smooth muscle tone. Large-conductance K_(Ca) channels may represent a negative-feedback mechanism for cerebral arteries when pressurized.³⁴ ATP-sensitive³⁵ and inwardly rectifying K⁺ channels³⁶ also have been implicated in regulating tone in the small vessels of cerebral, pulmonary, and coronary circulation. Therefore, a combination of K⁺ channels or variation in K⁺ channel distribution may explain why different K⁺ channels are observed to regulate membrane potential in various vascular beds.

Significance of the Kidney in Hypertension
Despite years of intensive investigation, the etiology of hypertension remains unknown. Elevated blood pressure and total peripheral resistance associated with hypertension may result from increases in neural, humoral, or vasoconstrictor substances, passive or genetic structural alterations in blood vessels, or intrinsic changes in the control of vascular tone. The interlobar artery of the kidney is a significant blood vessel to study, since alterations in renal blood flow are known to influence fluid volume regulation, the secretion of a number of important neurohumoral substances, and arteriolar resistance, which may be involved in the etiology or maintenance of hypertension. However, direct measurements of the major ionic conductances that may underlie contraction of hypertensive rat interlobar arterial smooth muscle are lacking. This is rather remarkable given the results of earlier renal transplant studies in humans^{37 38} and animals,³⁹ which suggested that the genetic defect in a proportion of the cases with essential hypertension is expressed in the kidney. In these studies, kidney transplants from normotensive donors into hypertensive recipients effectively reversed many of the abnormalities associated with hypertension. In summary, the data demonstrate that the alterations in resting membrane potential observed in the SHR and DOCA hypertensive interlobar artery could be manifested through alterations in one or a combination of K_(v) channels or K_(Ca) channels. These alterations in ionic conductances may play a role in the etiology and/or maintenance of the hypertensive state.

	Selected Abbreviations and Acronyms

 

4-AP = 4-aminopyridine Ang II = angiotensin II DOCA = deoxycorticosterone acetate K(Ca) = Ca2+-activated K+ K(v) = voltage-dependent K+ SD = Sprague-Dawley SHR = spontaneously hypertensive rat(s) WKY = Wistar-Kyoto

	Acknowledgments

 
This study was supported by a Young Investigator Grant from the National Kidney Foundation (YIG-33) and an Initial Investigator Award from the American Heart Association, Florida Affiliate, Inc.

Received February 2, 1996; accepted May 6, 1996.

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