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(Hypertension. 1997;30:1403-1409.)
© 1997 American Heart Association, Inc.

Articles

Increased Expression of Ca²⁺-Sensitive K⁺ Channels in Aorta of Hypertensive Rats

Yanping Liu; Kay Pleyte; Hans-Guenther Knaus; ; Nancy J. Rusch

From the Department of Physiology (Y.L., K.P., N.J.R.), Medical College of Wisconsin (Milwaukee), and Institute for Biochemical Pharmacology (H.-G.K.), Innsbruck, Austria.

Correspondence to Nancy J. Rusch, PhD, Associate Professor, Department of Physiology, Medical College of Wisconsin, 8701 Watertown Plank Rd, Milwaukee, WI 53226. E-mail nrusch{at}post.its.mcw.edu

	Abstract

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Abstract Potassium efflux through Ca²⁺-sensitive K⁺ channels (K_Ca channels) is increased in arterial smooth muscle cells from hypertensive rats, but the molecular mechanism is unknown. The goal of this study was to compare the levels of K_Ca channel current between aortic smooth muscle cells from adult Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) and then use Western blot methods and ribonuclease protection assays to examine the expression and mRNA levels for the K_Ca channel in these same vascular tissues. Whole-cell patch-clamp methods indicated a larger component of K_Ca channel current, sensitive to block by iberiotoxin (100 nmol/L), in single aortic smooth muscle cells from SHR compared with WKY. Subsequent Western blot analysis using a site-specific antibody (anti–_913–926) directed against the S9/S10 linker of the -subunit of the K_Ca channel revealed a 125-kD immunoreactive band in lanes loaded with either WKY or SHR aortic muscle membranes. The immunoreactive density of this band, which corresponded to the known molecular size of the -subunit, was 2.2-fold greater in lanes loaded with aortic smooth muscle membranes from the hypertensive animals. However, despite this evidence for an increased expression and functional enhancement of K_Ca channels in aortic smooth muscle membranes of SHR, ribonuclease protection assays with a ³²P-labeled riboprobe targeted against the S9/S10 linker of the K_Ca channel -subunit revealed no difference in mRNA levels for the -subunit between WKY and SHR aortic tissue. These findings provide initial evidence that (1) an increased expression of K_Ca channels may be a mechanism for the enhanced K_Ca current in aortic smooth muscle membranes of SHR, and (2) the upregulation of K_Ca channels in arterial muscle membranes during hypertension, which is regarded as a homeostatic mechanism for buffering vascular excitability, may rely on post-transcriptional events.

Key Words: vascular smooth muscle • K⁺ channel • ion channel expression • iberiotoxin

	Introduction

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Patch-clamped arterial smooth muscle membranes from rats with genetic, renal, and salt-induced forms of hypertension show an increased K_Ca current.^1–6 The amplitude of this ionic current appears to be positively correlated to the blood pressure level of the host animal, suggesting that K_Ca channels in vascular smooth muscle membranes may be dynamically regulated by the in situ blood pressure profile.² Notably, arteries obtained for tension recording from hypertensive rats show pronounced constrictions in response to pharmacological block of K_Ca channels, whereas arteries from normotensive rats show less contraction.^1,2,7–10 Thus, the enhanced K_Ca current observed in arterial myocytes from hypertensive rats may represent a cellular compensatory mechanism to limit vascular reactivity in this disease.¹⁰

The molecular mechanism by which K⁺ current through K_Ca channels is increased in arterial smooth muscle membranes of hypertensive animals is unknown. Single-channel analysis has indicated a similar unitary conductance of 230 pS for K_Ca channels in aortic muscle membranes from normotensive and hypertensive rats.^3,5 However, an increased number of single-channel events and transient outward currents were observed in the smooth muscle membranes from the hypertensive animals.^3,5 These data imply that the increased K_Ca current in aortic muscle membranes from hypertensive rats is not caused by the expression of a K_Ca channel subtype with a higher single-channel conductance but may relate to (1) an increased expression level of the -subunit (pore-forming subunit) of the K_Ca channel, which would provide more unitary membrane pathways for K⁺ efflux in hypertension, and/or (2) a change in the regulation of the K_Ca channel -subunit, resulting in an enhanced open-state probability of these channels in vascular muscle membranes exposed to high blood pressure.

Of these two potential mechanisms, several factors make the potential link between high blood pressure and an increased expression of K_Ca channels in arterial smooth muscle membranes appealing to investigate. First, the expression levels of other membrane proteins involved in vascular excitation, such as membrane receptors, GTP-binding proteins, and ion exchangers, reportedly are altered in hypertension, suggesting that blood pressure may influence membrane protein expression.^11–15 Second, other forms of mechanosensitive stimuli have been reported to modulate protein expression in other vascular cell types. For example, shear stress regulates the expression of several distinct genes at the transcriptional level in vascular endothelial cells.¹⁶ Hence, physical forces apparently may act as induction stimuli for protein expression. Third, the recent cloning of the K_Ca channel from vascular smooth muscle cells and the subsequent development of site-specific antibodies to assess its relative expression levels in different cytoplasmic membranes have provided the first opportunity to explore the possibility that K_Ca channel expression is altered in disease states.^17,18 In application of these techniques, the goal of this study was to combine patch-clamp techniques with Western methods and ribonuclease protection assays to compare the levels of K_Ca channel current, protein expression, and transcript between aortic smooth muscle cells from SHR and WKY.

	Methods

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Experimental Animals
Adult (12 to 17 weeks old) WKY and SHR were obtained from Taconic Farms. In preliminary experiments, verification of antibody specificity and design of cDNA probes were based on aortic tissues from control Sprague-Dawley rats (Sasco/Charles River Laboratories). On the day of experiments, SHR and WKY were anesthetized with sodium pentobarbital (60 mg/kg IP), and the carotid artery was cannulated with polyethylene tubing to measure mean arterial blood pressure. Mean arterial pressure as measured by carotid catheterization was 120±5 mm Hg in WKY and 183±4 mm Hg in SHR. Rats were killed with an overdose of sodium pentobarbital (45 mg/kg IP), and the aortas were immediately removed. In several WKY, a piece of the left ventricle also was rapidly removed to serve as a negative control for K_Ca channel transcript. Tissues used for Western immunoblot experiments and RNAase protection assays were frozen immediately in liquid nitrogen and stored at -80°C until extraction. On other days, aortas were cleaned of loose connective tissue and exposed to enzymatic dissociation to obtain single aortic smooth muscle cells for patch-clamp recording of macroscopic K⁺ current. Protocols for animal use were approved by the Animal Care and Use Committee at our institution.

Cell Isolation and Patch-Clamp Recording
Enzymatic isolation of vascular smooth muscle cells was performed as described recently in detail.¹⁹ Briefly, aortas were cut into 1-mm segments and placed for 10 minutes in a 1-mL aliquot of PSS containing 100 µM Ca²⁺ and 1 mg/mL bovine serum albumin for 10 minutes. Vascular segments were then transferred to a fresh 1-mL aliquot of the same solution containing 1.5 mg/mL papain and 1 mg/mL dithioerythreitol (Sigma Chemical), which was warmed to 37°C for 20 minutes. Segments were then incubated for 20 to 30 minutes in 1 mL of PSS containing (in mg/mL) collagenase 2, elastase 0.5, and soybean trypsin inhibitor 1 (Sigma). Single smooth muscle cells were released from the vessels by gentle trituration, and the resulting cell suspensions were stored at 4°C for up to 6 hours. Only long, smooth, optically refractive cells were used for patch-clamp measurements.

Macroscopic K_Ca current was recorded in single aortic myocytes using standard pulse protocols and a patch-clamp station previously described in detail.^1–3 Cells were dialyzed with pipette solution containing 1 µmol/L ionized calcium to amplify the component of Ca²⁺-dependent K⁺ current in whole-cell recordings.² The pipette solution contained (in mmol/L) K⁺ glutamate 130, MgCl₂ 1, EGTA 0.1, and HEPES 10 and 1 µmol/L ionized Ca²⁺ (pH 7.2), and the bath solution composition was (in mmol/L) NaCl 135, KCl 4.7, MgCl₂ 1, CaCl₂ 2, HEPES 5, and glucose 10 (pH 7.4). Families of whole-cell K⁺ currents were generated by progressive 10-mV depolarizing steps (400-msec duration; 5-sec intervals) from a constant holding potential of -60 mV to a peak command potential of +60 mV. Currents were recorded 2 to 5 minutes after conversion to the whole-cell recording mode to permit stable amplitudes in drug-free bath solution, and then currents in the same cell were subsequently recorded after superfusion with 100 nmol/L IBTX (Sigma) to provide pharmacological block of K_Ca channels.²⁰ Trials in drug-free and IBTX-containing bath solution were performed in triplicate and averaged together to estimate peak current amplitudes. Membrane capacitance was estimated in each cell by integrating capacitative currents generated by 10-mV hyperpolarizing pulses after electronic cancellation of the pipette-patch capacitance, and peak K⁺ current amplitudes were expressed in picoamperes per picofarad (pA/pF) to normalize for differences in cell membrane area between isolated myocytes.²

Western Immunoblotting
Aortas from SHR and WKY stored at -80°C were minced into small pieces and homogenized on ice in a glass tissue grinder containing 250 mmol/L sucrose, 50 mmol/L MOPS, 0.1 mmol/L phenylmethylsulfonyl fluoride, 5 mg/mL leupeptin, 5 g/mL antipapain, and 5 mg/mL aprotinin A. The pH was titrated to 7.4 using NaOH as a base. All chemicals were obtained from Sigma. The volume of homogenizing solution containing the initial tissue was 200 µL, with incremental addition of 100-µL aliquots of solution resulting in an end volume of 1 mL. Large tissue debris and nuclear fragments were removed by two low centrifuge spins (1000g for 10 minutes; 14 000g for 15 minutes) at 4°C, and the pellet of membrane protein was obtained after a subsequent centrifugation at 100 000g for 1 hour. The protein concentration was determined by the BioRad method using bovine serum albumin as a standard.

Protein samples were electrophoretically size-separated using a discontinuous system consisting of a 10% polyacrylamide resolving gel and a 5% polyacrylamide stacking gel. High range molecular weight markers (40 to 200 kD) were loaded into one lane as a size standard. Equivalent amounts (3 to 7 µg) of total protein from the aortas of WKY and SHR were added to adjacent duplicate lanes, and the samples were run at 200 V for 1 hour on an 8x10-cm electrophoresis cell (BioRad). After separation, the proteins were electrophoretically transferred to a nitrocellulose membrane at 100 V for 1.5 hours. The membranes were washed in Tris-buffered saline-Tween 20 (TBS-T), and blocked with 10% nonfat dried milk in TBS-T overnight at 4°C. Subsequently, membranes were incubated for 3 hours with a 1:1000 dilution of polyclonal rabbit anti-_913–926, which is a sequence-directed antibody raised against amino acids 913 to 926 of the -subunit of the K_Ca channel. A detailed description of the preparation and use of this antibody, which recognizes residues located on the S9/S10 linker (Fig 1), has been published previously.¹⁸ Aortic membranes were then incubated for 2 hours with horseradish peroxidase-labeled goat anti-rabbit IgG in TBS-T containing 2% nonfat dried milk. In some experiments, the specificity of anti–_913–926 for its putative residues was determined by coincubation of anti–_913–926 with its corresponding antigenic peptide (1 µmol/L) to determine whether this competition abolished the immunostaining reaction.¹⁸ A monoclonal mouse antibody raised against the structural protein ß-actin (Sigma) was used as a lane-loading control.²¹ Membranes were stripped between incubations with different antibodies in a Tris-buffered solution containing 2% SDS and 100 mmol/L ß-mercaptoethanol at 50°C. The bound antibody was detected by enhanced chemiluminescence (ECL, Amersham) on radiograph film. The density of immunoreactive bands associated with anti–_913–926 was determined by a microcomputer imaging system and expressed as a percent of the ß-actin density for each lane.²¹

View larger version (37K): [in this window] [in a new window]   Figure 1. Binding sites on the -subunit of the KCa channel for anti–913–926, the site-directed antibody used in Western immunoblots, and for the riboprobe used in ribonuclease protection assays. The 10 transmembrane segments of the -subunit are labeled 1 to 10, and the pore region is denoted by the region P. The antibody epitope (open oval) and the binding site for the riboprobe (thickened sequence) were both located on the S9/S10 linker of the -subunit.

Ribonuclease Protection Assay
For each experiment, total RNA was extracted and pooled from three aortas of a single rat strain using the Trizol method (GIBCO BRL). Using the known DNA sequence corresponding to the -subunit of the K_Ca channel of rat aortic smooth muscle, which we recently deposited in GenBank (accession no. U93052), a cloned segment of cDNA containing the corresponding sequence for the S10 region of the -subunit of the K_Ca channel was generated by RT-PCR amplification of total RNA from aortic smooth muscle of Sprague-Dawley rats. The forward and reverse primer sequences were 5'-GATTGACAACATGGACTCC-3' and 5'-GACGGCAAATGCTGTCC-3', respectively. The amplified cDNA was inserted into the pCR2.1 cloning vector (InVitrogen), linearized by cutting with SspI (Pharmacia), and in vitro transcribed using T7 polymerase in the presence of [³²P]UTP (800 Ci/mmol; Amersham). From 2.5 to 10 µg of total RNA was cohybridized to the labeled riboprobe of 390 nt and to a commercially available riboprobe for rat ß-actin (Ambion), which was used as an internal standard to verify similar amounts of RNA in each sample.²² Hybridization was performed according to the manufacturer's instructions (Ambion RPA II). Protected fragments of 323 nt (-subunit) and 126 nt (ß-actin) were resolved in a 5% polyacrylamide/7 mol/L urea gel and visualized after autoradiographic exposure.

Statistical Analysis
All averaged data are expressed as mean±SEM. Statistical comparisons between groups were made with one-way repeated-measure analysis of variance with a subsequent Newman-Keuls test. Significance was accepted at P<.05.

	Results

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Comparison of IBTX-Sensitive K⁺ Current Between WKY and SHR
Whole-cell K⁺ currents were elicited to compare the membrane density of IBTX-sensitive K_Ca current between aortic smooth muscle cells from SHR and WKY. These SHR and WKY generally were littermates to those used subsequently to compare K_Ca channel expression and transcript levels. Fig 2A and 2B shows that macroscopic K⁺ currents generated by incremental 10-mV depolarizing steps from -60 to +60 mV elicited families of outward currents in single WKY and SHR cells, respectively. The maximum current generated at +60 mV under our recording conditions was 673±80 pA in WKY cells (n=19) and 927±147 pA in SHR cells (n=20). Fig 2C and 2D shows that superfusion with 100 nmol/L IBTX, a specific blocker of K_Ca channels, reduced maximum current amplitude by 78% in WKY cells and 85% in SHR cells, respectively, suggesting that IBTX-sensitive K_Ca current was the predominant contributor to voltage-elicited outward current under these conditions. Fig 3A and 3B shows the current-voltage relationships of average K⁺ current densities (pA/pF) for all WKY and SHR cells studied (n=19 and 20). Plots of K⁺ current densities as a function of membrane potential show that an IBTX-sensitive current was the predominant current component observed. Maximum current density at +60 mV was 1.6-fold higher in aortic smooth muscle cells from SHR than those from WKY (45.1±4.1 and 28.9±3.1 pA/pF, respectively). This increased current in vascular muscle cells of SHR was reflected by a upward shift in the current-voltage curve, which was significant at higher activating voltages. Membrane capacitance, an indicator of cell membrane area, was not significantly different between WKY and SHR cells, averaging 20±1 and 17±1 pF, respectively. These values were similar to those reported previously for these same preparations.²

View larger version (19K): [in this window] [in a new window]   Figure 2. Original traces of whole-cell K+ currents in WKY and SHR aortic smooth muscle cells. Currents were elicited by 10-mV depolarizing steps from a constant holding potential of -60 mV to membrane potentials as positive as 0 mV. IBTX (100 nmol/L) blocked most of the outward current in WKY (A and C) and SHR (B and D) cells, with a larger component of IBTX-sensitive current observed in SHR.

View larger version (22K): [in this window] [in a new window]   Figure 3. Current-voltage relationships showing the averaged effect of 100 nmol/L IBTX on macroscopic K+ current in 19 aortic smooth muscle cells from WKY (left) and 20 similar cells from SHR. Currents were elicited from a holding potential of -60 mV. Shaded areas depict the current component sensitive to block by IBTX. *Control current was significantly higher at the indicated membrane potential than current remaining after IBTX. Component of IBTX-sensitive current was greater in SHR than in WKY at the indicated membrane potential.

Comparison of -Subunit Expression Between WKY and SHR Aortic Smooth Muscle Membranes
In initial control studies, immunoblotting reactions were performed to determine the specificity of the primary antibody (AB, anti–_913–926) for its recognition site on the -subunit of the K_Ca channel. The immunoblot containing the left two lanes in Fig 4 shows that the -subunit, which represents an immunoreactive protein with an apparent molecular mass of 125 kD,¹⁸ was readily detected in lanes containing aortic smooth muscle membranes from Spague-Dawley rats. A second immunoblot run in parallel on the right demonstrates that the immunoreactive band at 125 kD was abolished by coincubation of anti–_913–926 with 1 µmol/L of the antigenic competing peptide (AB+CP), confirming the specificity of the primary antibody for its recognition sequence. Stripping and rehybridization of the membranes with the monoclonal antibody for the 42-kD protein, ß-actin, showed the same signal density for this internal standard, demonstrating uniformity of lane loading.

View larger version (77K): [in this window] [in a new window]   Figure 4. Immunoblot analysis of the KCa channel -subunit in aortic smooth muscle membranes of control Sprague-Dawley rats. Top left, -subunit was recognized by the antibody (AB) as a 125-kD diffuse band. Top right, No band was detected when the antibody was coincubated with the competing antigenic peptide (AB+CP). The CP concentration was 1 µmol/L. Bottom, ß-actin, used as an internal standard, was detected as a 42-kD band in all four lanes. Each lane was loaded with 7 µg of protein.

Subsequently, the anti–_913–926 antibody was used to compare the expression levels of the K_Ca channel -subunit between aortic smooth muscle membranes from SHR and WKY. Fig 5 shows a representative experiment in which adjacent lanes were loaded in duplicate with either WKY (left) or SHR (right) membrane proteins and illustrates that the density of the 125-kD immunoreactive band was higher in aortic smooth muscle membranes from SHR. In the same blot, the 42-kD immunoreactive band corresponding to the ß-actin internal standard was similar between lanes. In eight Western blot experiments using membranes obtained from different rats, the 125-kD immunoreactive signal was 2.2-fold greater in lanes loaded with SHR than with WKY membranes, averaging 31±5% and 14±3% of the signal density of the ß-actin standards, respectively. The density of the ß-actin signal was not statistically different between lanes loaded with the SHR and WKY membrane preparations.

View larger version (89K): [in this window] [in a new window]   Figure 5. Immunoblot analysis of the expression of the KCa channel -subunit in aortic smooth muscle membranes from WKY (lanes 1 and 2) and SHR (lanes 3 and 4). The density of the 125-kD immunoreactive band, corresponding to the known molecular size of the -subunit, was greater in SHR than in WKY membranes. ß-actin, used as an internal standard, showed similar expression between lanes.

Comparison of Transcript Levels for the -Subunit Between WKY and SHR Aortae
Initial control experiments using total RNA from aortas of Sprague-Dawley rats were performed to assess the sensitivity of the ribonuclease protection assay for detecting changes in transcript expression corresponding to the -subunit of the K_Ca channel. The left lane of the autoradiograph in Fig 6 shows the undigested riboprobe of 390 nt. Adjacently, a lane loaded with total RNA from the left ventricle of rat heart shows an absence of the 323-nt fragment protected by the K_Ca channel riboprobe after processing for RNase protection. These findings concur with Western blot studies indicating the absence of the K_Ca channel protein in cardiac myocytes.¹⁸ In the same lane, cohybridization with the riboprobe for the ß-actin transcript showed mRNA for this internal standard at the expected protected fragment size of 126 nt. To assess the sensitivity of the assay for detecting changes in mRNA levels, four adjacent lanes were loaded with 2.5, 5, 7.5, and 10 µg of total RNA from rat aorta, which were cohybridized with the riboprobes for the rat K_Ca channel -subunit and the ß-actin standard. The four right lanes in Fig 6A demonstrate that the expression intensity of the 323-nt fragment increased progressively with RNA loading between 2.5 and 7.5 µg but showed saturation at the higher RNA concentration of 10 µg. In the same lanes, the density of the 126-nt fragment corresponding to the ß-actin standard showed a similar increase in signal intensity. The lower plot of expression density as a function of total RNA indicates that density values (normalized to the 2.5-µg sample) of 1 (2.5 µg), 1.9 (5 µg), 3.5 (7.5 µg), and 3.7 (10 µg) were obtained, suggesting that hybridization using 5 µg of total rat aortic RNA may permit the detection of bidirectional changes in expression of the 323-nt fragment.

View larger version (44K): [in this window] [in a new window]   Figure 6. Top, Ribonuclease protection assay using total RNA from Sprague-Dawley aorta. Lane 1, Undigested 390-nt probe for the -subunit gene. Lane 2, Rat heart, which served as a negative control for KCa channel gene expression, showed only the ß-actin internal standard at 126 nt. Lanes 3 to 6, Lanes loaded with increasing concentrations (2.5, 5, 7.5, and 10 µg) of total aortic RNA showed an increase in the expression density of the 323-nt signal, corresponding to the protected fragment of the -subunit mRNA. The value at 126 nt, representing the ß-actin internal standard, showed a similar increase. Bottom, Expression density was plotted as a function of total RNA. Data were normalized to the signal observed at 2.5 µg. The proportional relationship between 2.5 and 7.5 µg of RNA suggested a quantitative ability of the assay within this expression range.

Ribonuclease protection assays were subsequently performed to compare transcript levels between the aorta of SHR and WKY (n=3). Aortic tissues from three animals from the same strain were pooled for total RNA extraction. Duplicate adjacent lanes were loaded with 5 µg of total aortic RNA from either SHR or WKY and cohybridized with the riboprobes for the K_Ca channel -subunit and the ß-actin internal control. Comparison of expression levels for the 323-nt signal indicated similar transcript levels for the K_Ca channel -subunit in aortas from WKY and SHR (n=3), as shown in Fig 7. Average band intensities for WKY (lanes 1 and 2) and SHR (lanes 3 and 4), expressed as percent of signal density of the 126-nt ß-actin internal control, were 0.687 and 0.629, respectively. Two additional assays using total RNA from pooled aortic tissue of three other SHR or WKY also showed no difference in the 323-nt signal between SHR and WKY.

View larger version (122K): [in this window] [in a new window]   Figure 7. Ribonuclease protection assay of aortic tissue from WKY rat (lanes 1 and 2) and SHR (lanes 3 and 4). Each lane contained 5 µg of total RNA. No difference in the expression of the 323-nt protected fragment, corresponding to expected size for the protected mRNA of the -subunit of the KCa channel, was detected between SHR and WKY rat aortas. The 126-nt band represents mRNA expression for the internal standard, ß-actin.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
The goal of this study was to begin to define the molecular pathways by which K_Ca channel current is increased in arterial smooth muscle cells of hypertensive rats, and the results of our study may provide two new findings helpful in this regard. First, in experiments using Western methods, an antibody specific for a binding epitope on the -subunit of the K_Ca channel indicated an increased expression of K_Ca channels in arterial smooth muscle membranes from SHR, which also showed a higher K_Ca current density than similar membranes from WKY. Second, the higher levels of K_Ca channel current and protein expression in aortic smooth muscle membranes of SHR were not associated with a detectable change in mRNA level, implying that the increased expression of vascular K_Ca channels may by mediated primarily by post-transcriptional events.

Increased K_Ca Channel Current in SHR Aortic Muscle Membranes
Original reports by Jones and Friedman²³ described an enhanced ⁴²K turnover in arteries from hypertensive rats, which was reduced by pharmacological block of Ca²⁺ influx. More recently, workers in our laboratory verified an increased density of K_Ca current in aortic smooth muscle membranes exposed to high levels of in situ blood pressure in SHR and in aortic-coarcted hypertensive rats, and we identified the high-conductance K_Ca channel as the single-channel pathway of this current.^1–3 Subsequently, Liu and colleagues^4,5 observed an increased whole-cell and single-channel K_Ca current in aortic smooth muscle cells from renal- and salt-sensitive hypertensive rats, and Martens and Gelband⁶ also measured an enhanced component of K_Ca current in renal arterial muscle cells of SHR. Notably, in aortic muscle membranes of SHR with established hypertension, the high level of K_Ca current was reduced after blood pressure was lowered for 2 weeks by antihypertensive drug therapy, implying that the membrane density of vascular K_Ca current may be dynamically regulated by the arterial blood pressure level of the host animal.² The patch-clamp findings of this study, showing a higher density of IBTX-sensitive K⁺ current in SHR compared with WKY vascular smooth muscle cells, confirm these previous observations that K_Ca current density is 2-fold higher in aortic muscle membranes from SHR compared with control cells and lend further support for the use of the SHR aorta as one model of K_Ca channel upregulation in hypertension. However, it should be noted that patch-clamp results from dialyzed cells do not assess the gating influences of cytosolic factors on K_Ca channel activity and hence may not reflect the true open-state probability of K_Ca channels in their native environment. This may be one reason why we observed low levels of K_Ca current at negative membrane potentials in the vascular smooth muscle cells used in this study.

Increased K_Ca Channel Expression in SHR Aortic Muscle Membranes
With a site-directed antibody targeted against the S9/S10 linker of the K_Ca channel, our Western blots showed an increased density of a 125-kD immunoreactive band in SHR compared with WKY aortic smooth muscle membranes, which corresponded to the known molecular size of the -subunit of the K_Ca channel.¹⁸ This result provides the first direct evidence for an increased expression of the pore-forming subunit of the K_Ca channel in arterial smooth muscle membranes of hypertensive rats and further raises the possibility that an elevated in situ blood pressure may result in the induction of the expression of K_Ca channels in vascular smooth muscle membranes. This finding also may begin to clarify the mechanism underlying the increased whole-cell K_Ca current observed in aortic smooth muscle cells of hypertensive rats^{1,2 4–6} and may help to explain why an increased number of single-channel events have been observed in detached patches of arterial smooth muscle membranes from hypertensive animals.³

However, it is important to acknowledge that Western analyses were performed on whole-tissue homogenates from WKY and SHR aortas. Thus, the finding of an increased expression of K_Ca channel -subunits in SHR aorta may not be representative of the individual responses of all vascular smooth muscle cell subtypes within the aortic media to increased blood pressure. Rather, patch-clamp experiments indicate that K_Ca current density varies considerably between vascular muscle cells isolated from the same SHR aorta,² implying that different cell populations may respond heterogeneously to the elevated blood pressure signal.

In addition, an increased expression of K_Ca channels in aortic smooth muscle membranes of SHR may not represent the sole mechanism for enhanced K_Ca current. Rather, total K_Ca current (I_Kca) will be the product of (n)(i)(p), where n is channel number, i is unitary current amplitude, and p is channel open-state probability. Because the unitary conductance (i) of K_Ca channels in aortic smooth muscle cells is not altered in hypertension,^3,5 the enhanced vascular K_Ca current in SHR must reflect changes in n or p. Although our study supports the concept of an increased channel number (n) as one mechanism for enhanced vascular K_Ca current in hypertension, an altered regulation of the K_Ca channel resulting in an increased open-state probability (p) also represents a second possible factor. In this regard, alterations of multiple and diverse cellular pathways may favor activation of the K_Ca channel, including expression of alternative subunit isoforms, enhanced association of the -subunit with regulatory proteins, or changes in the composition of the lipid bilayer.^24–28 Thus, the observation of an increased expression of -subunits of the K_Ca channel in aortic muscle membranes of SHR represents only one of several molecular mechanisms available to the vascular smooth muscle cell for increasing compensatory K_Ca channel current during conditions of high blood pressure.

Enhanced -Subunit Expression Is Not Associated With Increased Transcript Levels
Using a riboprobe directed against the -subunit of the K_Ca channel, we did not detect a difference in transcript levels between lanes loaded with 5 µg of total RNA from SHR or WKY aortas. The same riboprobe detected a 1.5-fold change in transcript expression level as evaluated in studies using incremental lane loading of total aortic RNA (Fig 6). These findings infer that the 2.2-fold increase in K_Ca channel expression observed in SHR aortic smooth muscle membranes in this study may not result from altered gene regulation but rather may be mediated by posttranscriptional events, including translational and translocational processes, or channel stability in the cytoplasmic membrane.

Notably, recent attention has focused on the trafficking and turnover of membrane-associated, transport proteins as key factors that regulate the excitation level of many cell types. These processes profoundly influence the expression levels of such critical transport proteins as glucose and H⁺-ATPase transporters, renal water channels, and cAMP-sensitive Cl^- channels in the cytoplasmic membrane.²⁹ For example, antidiuretic hormone primarily increases water reabsorption in the kidney by enhancing the insertion of water channels into the cytoplasmic membranes of renal epithelial cells, thereby increasing the level of channel expression. This overexpression is readily reversed by endocytotic processes when the level of antidiuretic hormone returns toward normal.²⁹ Similarly, altered channel expression occurs in disease states such as cystic fibrosis, in which the most common gene mutation results in an inability of the cAMP-sensitive Cl^- channel to translocate to the cytoplasmic membrane and results in the hallmark finding of insufficient Cl^- current in epithelial cells.³⁰ Thus, in addition to changes in gene regulation, multiple post-transcriptional mechanisms, including alteration of protein translocation and turnover, may be primary determinants of the level of ion channel expression in cytoplasmic membranes. In the present study, our findings raise the further possibility that changes in post-transcriptional processes also may be instrumental in altering ion channel expression in arterial smooth muscle membranes during hypertension. Although the precise mechanisms by which vascular smooth muscle cells transduce the signal of high blood pressure into increased K_Ca channel expression remain to be investigated, the increased expression of K_Ca channels during hypertension may provide a mechanism for the arterial smooth muscle cells to enhance K⁺ efflux and hence provide a long-term, homeostatic mechanism for counteracting vascular excitability during hypertension.

	Selected Abbreviations and Acronyms

 

IBTX = iberiotoxin KCa = current Ca+-sensitive K+ nt = nucleotide SHR = spontaneously hypertensive rats WKY = Wistar-Kyoto rats

	Acknowledgments

 
This work was supported by the APART program of the Austrian Academy of Sciences and grants (S6611-MED, P-11187, 6239) from the Austrian Research Foundation and the Austrian National Bank Foundation (Dr Knaus) and by National Heart, Lung and Blood Institute grant P01-HL-29587 from the National Institutes of Health (Dr Rusch).

Received May 28, 1997; first decision June 18, 1997; accepted July 25, 1997.

	References

Top Abstract Introduction Methods Results Discussion References

 
1. Rusch NJ, De Lucena RG, Wooldridge TA, England SK, Cowley AW. A Ca²⁺-dependent K⁺ current is enhanced in arterial membranes of hypertensive rats. Hypertension. 1992;19:301–307.[Abstract/Free Full Text]

2. Rusch NJ, Runnells AM. Remission of high blood pressure reverses arterial potassium channel alterations. Hypertension. 1994;23:941–945.[Abstract/Free Full Text]

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