Arterial smooth muscle from hypertensive rats shows an increased membrane permeability to K+ that depends on Ca2+ influx. To define the mechanism of this membrane alteration, we tested the hypothesis that Ca(2+)-dependent K+ current (IK(Ca)) is increased in arterial muscle membranes from genetically hypertensive rats. Single-channel K+ currents measured in cell-attached and inside-out aortic membrane patches from spontaneously hypertensive rats (SHR) were compared with those from normotensive Wistar-Kyoto rats (WKY). Inside-out patches from both rat strains showed a predominant 225 pS, Ca(2+)- and voltage-dependent K+ channel in symmetrical 145 mM KCl solutions, which was blocked by tetraethylammonium [concentration for half-maximal block (IC50)< or = 0.3 mM]. In cell-attached patches of aortic muscle cells bathed in physiological salt solution, this channel [IK(Ca) channel] showed a fivefold higher open-state probability (NPo) in SHR as compared with WKY. This increased NPo of SHR IK(Ca) channels in membranes of intact aortic muscle cells was not correlated with an altered membrane potential in current-clamped SHR myocytes or with changes in cytosolic free Ca2+ concentration in fura-2-loaded aortic muscle cells. However, inside-out aortic membrane patches from SHR showed more detected IK(Ca) channels per patch, a higher IK(Ca) channel NPo, and a greater total patch current than their WKY counterparts. Further analysis revealed a greater Ca2+ sensitivity of SHR than WKY IK(Ca) channels. These results suggest that IK(Ca) channel function is altered in isolated membrane patches of arterial muscle from genetically hypertensive rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Femoral vein pressure in adult male Sprague-Dawley rats kept in specially designed tubelike cages rose immediately from a control value of 2.9 +/- 0.2 (SE) mmHg to a gravity-induced sustained value of 5.9 +/- 0.2 mmHg on initiation of a 2-wk 45 degrees head-up tilt period. Femoral arterial pressure was not altered by tilting. In 2 wk mean external diameter, but not total wall thickness, of in vitro distal saphenous vein segments from tilted rats was increased approximately 30% above that of segments from nontilted controls at each of four successive 5-mmHg intralumenal pressure (IP) increments applied between 0 and 20 mmHg. Consequently, in tilted rats isobaric stress was increased 38% at low and 24% at high IP, whereas incremental distensibility was decreased at mid IP. Vascular smooth muscle (VSM) in tilted rat vein, but not artery, was hyperpolarized relative to controls both in vitro at normal physiological pressures [membrane potential (Em) = -58.2 +/- 0.8 vs. -52.4 +/- 0.8 mV, respectively] and in situ during local neural blockade (Em = -61.3 +/- 2.3 vs. -53.5 +/- 0.5 mV, respectively). The conclusion is that a moderate chronic elevation of IP in a vein results in hyperpolarization of its VSM and an elevation of its total capacity due to an as yet unexplained mechanism of physiological adaptation.

Comparative measurements of transmembrane potential (Em) were made in situ in vascular smooth muscle cells (VSM) of mesenteric small principal arteries and veins with innervation and circulation intact. Vessels were in an externalized, topically suffused jejunal loop in 4- to 5-wk-old (initial hypertension) and 12- to 15-wk-old (established hypertension) anesthetized, spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive control rats. Comparable in vitro measurements of Em were also made in VSM of isolated intact small mesenteric vessel segments (from the 12- to 15-wk-old animals) maintained at their in situ lengths and suffused with physiological salt solution (PSS). During suffusion in situ with control PSS, VSM of both small veins and arteries in older (but not younger)SHR were less polarized than in WKY. Local chemical sympathetic denervation in situ (with 6-hydroxydopamine) hyperpolarized VSM of both vessel types in older (but not younger) SHR to the same Em levels measured in situ in respective WKY vessels. After local denervation, VSM of small arteries (but not veins) of both SHR and WKY remained less polarized in situ than in vitro, suggesting the presence of one or more circulating factors with a specific depolarizing action on the arterial side in both animal types. In vitro, VSM of both small arteries and veins from WKY but not SHR were depolarized immediately by 10(-3) M ouabain. In contrast, reduction of the PSS suffusate temperature to 16 degrees C caused a significantly greater depolarization in VSM of SHR vessels.(ABSTRACT TRUNCATED AT 250 WORDS)

When exposed to an increasing transmural pressure, middle cerebral arteries of tbe cat exhibit reduction of internal diameter which is mediated by vascular muscle cell depolarization. This laboratory has recently demonstrated that this "pressure-induced" activation is dependent upon the presence of an intact endothelium. The present studies were undertaken to determine if this phenomenon is due to inhibition of tonically released endothelium-derived relaxing factors (EDRF) or release of a contractile substance. When cerebral arterial segments were pressurized to between 40 and 160 mm Hg there was 13.2% reduction in internal diameter accompanied by significant muscle cell depolarization from -53±2.7 to -22±1.4 mV. There was a significant positive correlation between the A E,, and step increases in transmural pressure. These excitatory responses were lost and vessels dilated to pressure when the endothelium was removed. Upon exposing the denuded vessel to a pressurized intact donor, the denuded vessel recovered its ability to contract and depolarize suggesting that a contractile substance might be released from the vascular endothelium upon pressurization. The EDRF antagonist oxyhemoglobin did not alter the excitatory response to pressure in these isolated cerebral arteries further suggesting that the reduction in diameter and muscle cell depolarization results from the release of a contractile substance from the vascular endothelium and not inhibition of EDRF. {Circulation Research 1989;65:193-198) R ecent reports have demonstrated that the vascular endothelium of various arterial beds may release factors that are contractile in nature. 1 -7 In the cerebral circulation, this apparent endothelium-dependent contraction is observed on elevation of transmural pressure and has been speculated to be partially responsible for blood flow autoregulation. 4 -8 One of the primary problems in defining a contractile factor thought to be released from the vascular endothelium is the ubiquitous presence of endothelium-derived relaxing factor(s) (EDRF). 9 -12 It is possible that the contractile response observed upon various maneuvers (e.g., hypoxia, pressure, and stretch) may be the result of inhibition of tonic release of EDRF. The present studies were designed to determine whether elevation of transmural pressure triggers endothelium-dependent contraction of cat cerebral arteries by effecting the synthesis and release of diffusible endothelial contractile factor(s) or, alternatively, by depressing the release of EDRF. The experiments were carried out using a novel bioassay system in which segments of intact and denuded cerebral arteries are pressurized and perfused in series.14 The results suggest that elevation of transmural pressure induces the release of a transferable contractile factor released from the endothelium that acts via muscle cell membrane depolarization. Materials and Methods GeneralAdult mongrel cats were anesthetized with sodium pentobarbital (60 mg/kg) and their brains removed. Middle cerebral art...

Sympathetic neural activation of vascular smooth muscle ^-receptors induces membrane hyperpolarization and arterial relaxation. This response, which likely is mediated by the G, protein-adenylyl cyclase-cyclic AMP signaling cascade, is reduced in some hypertensive animal models and in human essential hypertension. Since reduced preceptor-mediated vasodilation is a potential mechanism for enhanced arterial resistance, this study was designed to identify which step (or steps) in the /3-receptor signaling cascade is altered in hypertension. Transmembrane potentials were recorded in situ in small first-order arterioles and venules of cremaster muscle from hypertensive, reduced renal mass rats and normotensive, sham-operated controls. Vascular muscle cells in arterioles and venules of hypertensive rats were 5-7 mV more depolarized than in respective vessels of control rats during supervision with physiological salt solution. Hyperpolarization and depolarization responses were reduced in hypertensive rats during superfusion with a ^-receptor agonist and antagonist, respectively, suggesting attenuated -receptor responsiveness compared with normotensive rats. Furthermore, direct activation of G, protein by 10 ng/mL cholera toxin did not affect arterial or venous transmembrane potential in hypertensive rats, but hyperpolarized arterial and venous vascular muscle in normotensive controls by 17 mV. However, when the G, protein-adenylate cyclase coupling step of the /3-receptor cascade was bypassed by using 10"' M forskolin to directly activate adenylate cyclase, arterial and venous vascular muscle of hypertensive rats hyperpolarized by 25-27 mV. These data strongly suggest that the ^-receptor-mediated cascade regulating arterial and venous vascular muscle hyperpolarization and vasodilation is reduced in this rat model of volume-expanded hypertension and implicate the G, protein-adenylate cyclase coupling step as the likely abnormal cellular event and sodium-dependent renal models of hypertension in the rat (reduced renal mass [RRM] 9 and one-kidney, renal wrap models 10 ). However, despite its prevalence in the pathophysiology of hypertension, the abnormal cellular events mediating this enhanced sympathetic efferent regulation of VSM tone are still obscure. 3At the level of the VSM cell, one potential determinant of elevated sympathetically mediated VSM tone in hypertension may be a reduced /3-receptor-mediated membrane hyperpolarization and vasodilation. The latter has been reported in VSM of SHR 11 and saltsensitive human hypertensive patients.12 -13 This alteration may coexist with an unchanged or elevated a-adrenergic receptor function. 3 In view of this, it is possible that a reduced ^-receptor-mediated hyperpo- larization also may favor an increased sympathetic excitatory control of in situ membrane potential (En,) in VSM of RRM rats, as suggested recently by our laboratory. 9 However, to define possible cellular sites of impaired /3-adrenergic receptor response in small arteries and veins of RRM, one must consider...