Reduced functional expression of K⁺channels in vascular smooth muscle cells from rats made hypertensive withN^ω-nitro-l-arginine

Abstract

, we demonstrated that superior mesenteric arteries from rats made hypertensive with N -nitro-L-arginine (L-NNA) are depolarized and express less K ϩ channel protein compared with those from normotensive rats. In the present study, we used patch-clamp techniques to test the hypothesis that L-NNA-induced hypertension reduces the functional expression of K ϩ channels in smooth muscle. In whole cell experiments using a Ca 2ϩ -free pipette solution, current at 0 mV, largely due to voltage-dependent K ϩ (KV) channels, was reduced ϳ60% by hypertension (2.7 Ϯ 0.4 vs. 1.1 Ϯ 0.2 pA/pF). Current at ϩ100 mV with 300 nM free Ca 2ϩ , largely due to large-conductance Ca 2ϩ -activated K ϩ (BKCa) channels, was reduced ϳ40% by hypertension (181 Ϯ 24 vs. 101 Ϯ 28 pA/pF). Current blocked by 3 mM 4-aminopyridine, an inhibitor of many K V channel types, was reduced ϳ50% by hypertension (1.0 Ϯ 0.4 vs. 0.5 Ϯ 0.2 pA/pF). Current blocked by 1 mM tetraethylammonium, an inhibitor of BK Ca channels, was reduced ϳ40% by hypertension (86 Ϯ 14 vs. 53 Ϯ 19 pA/pF). Differences in BK Ca current magnitude are not attributable to changes in single-channel conductance or Ca 2ϩ /voltage sensitivity. The data support the hypothesis that L-NNAinduced hypertension reduces K ϩ current in vascular smooth muscle. Reduced molecular and functional expression of K ϩ channels may partly explain the depolarization and augmented contractile sensitivity of smooth muscle from L-NNA-treated rats. , and contraction are intimately intertwined in a phenomenon referred to as electromechanical coupling (20,30). K ϩ channels play an important role in electromechanical coupling, functioning to set a negative membrane potential and limit the activation of L-type Ca 2ϩ channels. The loss of proper K ϩ channel function results in altered vascular reactivity and hypertension (5, 33). Conversely, hypertension alters the expression of ion channels involved in the electromechanical coupling of smooth muscle (11,20). Recently, interesting light has been shed on concepts of cause and effect between hypertension and alterations in smooth muscle K ϩ channels (1, 2), and it remains to be determined in what scenarios ion channel changes precede hypertension and vice versa.We previously demonstrated a variety of vascular changes in male rats made hypertensive with the nitric oxide synthase inhibitor N -nitro-L-arginine (L-NNA). Smooth muscle changes associated with L-NNA-induced hypertension include increased contractility (22), enhanced Ca 2ϩ sensitivity (6), and augmented responses to dihydropyridines (29). In the companion paper to this article (3), we demonstrated that L-NNAinduced hypertension depolarizes the smooth muscle membrane potential and diminishes expression of K ϩ channel proteins. In the present study, we tested the hypothesis that L-NNA-induced hypertension reduces the functional expression of K ϩ channels in vascular myocytes from hypertensive rats. Smooth muscle cells were isolated from the superior mesenteric artery of normotensive and hypertensive rats and studied...

Exercise Training Reverses Alterations in K_v and BK_Ca Channel Molecular Expression in Thoracic Aorta Smooth Muscle Cells from Spontaneously Hypertensive Rats

“…The mechanism that regulates K v channel expression and BK Ca channel expression in SHRs is very complicated and is poorly understood because the differential expression of these channels has been observed in various arteries [6,16]. The loss of K v channels can be considered a factor that contributes to augmented vessel force [23]. β1-subunits of BK Ca promote the function of the channels through increasing Ca 2± and voltage sensitivity; thus, the upregulation of β1-subunit in hypertension could be considered a mechanism that suppresses augmented vessel force in hypertension.…”

“…In order to determine the role of the K v and BK Ca channel on modulating vascular tone, the ring-like aortas were treated with 4-aminopyridine (4-AP, 3 m M ), tetraethylammonium (TEA, 1 m M ), Maurotoxin (MTX, 30 n M ), and Mephetyl tetrazole (MEP, 10 μ M ). 4-AP is a potent K v channel blocker [4] and TEA at ≤1 m M can inhibit BK Ca channel but has little effect on K v channels [23,24,25]. In order to specifically target K v 1.2 and K v 1.5, the ring segments were treated with their respective blockers, MTX and MEP [26,27].…”

Activation of Potassium Channels by Tempol in Arterial Smooth Muscle Cells From Normotensive and Deoxycorticosterone Acetate-Salt Hypertensive Rats

“…Recently, a reduction in BK currents has been reported by others in myocytes from SHRs 32 and angiotensin II-induced, 33 and N -nitro-L-arginine-induced hypertensive rats 30,31 with or without the reduction of expression of BK channel ␣-subunits. The studies of Amberg and colleagues 32,33 demonstrate a reduc- tion in BK current secondary to diminished expression of the regulatory ␤1-subunit in SHR and angiotensin II-induced hypertension without a change in expression of ␣-subunits.…”

“…Other studies in N -nitro-L-arginine-induced hypertensive rats demonstrated that there was a reduced BK channel ␣-subunit expression and BK current but no difference in the Ca 2ϩ /voltage sensitivity of BK channels. 30,31 These data suggest that the effect of hypertension on BK channel expression may be model dependent. .…”

“…For example, decreased voltage-gated K ϩ currents occur in MA from SHR, 21,23 DOCA-salt, 29 and N -nitro-L-arginine-induced hypertensive rats. 30,31 However, BK channel currents in arterial myocytes are known to increase as a compensatory response to increased vascular pressure. 20,22 In the present study, we did not assess the role of voltage-gated K ϩ channel function in DOCA-salt myocytes, but we did find that tempol increased I o even in the presence of 4-AP.…”