Sympathetic Control of Arterial Membrane Potential by ATP-Sensitive K ⁺ -Channels

Abstract

Abstract-Stimulation of perivascular nerve terminals leads to a release of various neurotransmitters such as norepinephrine, epinephrine, acetylcholine, nitric oxide, and calcitonin gene-related peptide (CGRP). Because some of these substances have been shown to cause smooth muscle hyperpolarization by direct or endothelium-dependent mechanisms, we hypothesized that the liberation of 1 or more of these transmitters may lead to neurogenic hyperpolarization in arterial muscle cells. The present study was designed to determine the presence or absence of neurogenic hyperpolarization and, if present, its underlying mechanisms in isolated rat mesenteric resistance arteries, through the use of conventional microelectrode techniques. The experiments were performed under the combined blockade of ␣-adrenoceptors and purinoceptors with phentolamine and suramin to eliminate depolarizing responses to nerve stimulation. Under these conditions, perivascular nerve stimulation (5 Hz, 30 seconds) evoked smooth muscle hyperpolarization (Ϫ3.3Ϯ0.3 mV, nϭ15), which was abolished by tetrodotoxin, indicating the neurogenic origin of the response. This neurogenic hyperpolarization was resistant to atropine, nitro-L-arginine, or CGRP8-37, a CGRP antagonist, but was abolished by guanethidine and ␤-blocker propranolol. This hyperpolarization was also abolished by glibenclamide, an ATP-sensitive K ϩ channel (K ATP ) blocker, but was unaffected by apamin, a Ca 2ϩ -activated K ϩ channel blocker. In separate experiments, exogenous norepinephrine caused glibenclamide-sensitive hyperpolarization in the presence of phentolamine. On the other hand, norepinephrine-induced depolarization in the absence of phentolamine was enhanced by propranolol. These findings suggest that neurally released catecholamines cause membrane hyperpolarization through the activation of K ATP by ␤-adrenoceptors. Such hyperpolarization may play an important role in the control of arterial membrane potential by opposing ␣-adrenergic depolarization.

Impaired β-Adrenergic Hyperpolarization in Arteries From Prehypertensive Spontaneously Hypertensive Rats

“…[1][2][3] Although endothelium-derived nitric oxide has been suggested to account in part for ␤-adrenoceptor-mediated relaxation in certain blood vessels, 15 isoproterenol appears to act directly on smooth muscle cells to elicit hyperpolarization in the rat mesenteric arteries. 2,4 In the present study, hyperpolarization to forskolin, a direct activator of adenylate cyclase, 13 and to levcromakalim, a direct opener of KATP, 14 did not differ between prehypertensive SHR and age-matched WKY, suggesting that the main defect responsible for the impaired ␤-adrenoceptormediated hyperpolarization in prehypertensive SHR may lie upstream of adenylate cyclase. Recently, increased activity of G-protein-coupled receptor kinase, which phosphorylates and downregulates G-proteinlinked receptors, has been documented in lymphocytes from hypertensive subjects.…”

“…2,4 The resting membrane potentials in this study are rather negative, and the magnitude of the hyperpolarizations that occur in response to ␤-agonists is relatively small. However, if the membrane potentials were measured in situ, the resting membrane potentials would be much less negative because of neuronal and hormonal influences 1 and possibly the influence of a higher pressure in the vessel.…”

“…After an equilibration of Ն60 minutes, the membrane potentials of vascular smooth muscle cells were recorded with conventional glass capillary microelectrodes filled with 3 mol/L KCl, with tip resistances of 50 to 80 M⍀. 2,4 Microelectrodes were impaled into smooth muscle cells from the adventitial side. Criteria for successful impalement included the following: an abrupt drop in voltage on penetration of the microelectrode into the vascular smooth muscle cell, a stable membrane potential for Ն2 minutes, and a sharp return to zero potential on withdrawal of the electrode.…”

“…1,2 We recently demonstrated that isoproterenol-induced hyperpolarization is mediated by the opening of ATP-sensitive K ϩ channels (KATP) in rat mesenteric resistance arteries, 2 which parallels the case of the canine saphenous veins, 3 and that this ␤-adrenergic hyperpolarization plays an important role in the sympathetic control of membrane potential by opposing ␣-adrenoceptor-mediated depolarization. 4 In view of the importance of membrane potential as a determinant of smooth muscle tone, 5 ␤-adrenoceptor-mediated hyperpolarization may play an important role in the control of vascular tone.␤-Adrenoceptor-mediated relaxation is impaired in arteries from hypertensive rats, 6 -11 and in some models, such an abnormality has been shown to precede the development of hypertension. 6,7 On the other hand, information is limited regarding ␤-adrenoceptor-mediated hyperpolarization in hypertension.…”

“…1,2 We recently demonstrated that isoproterenol-induced hyperpolarization is mediated by the opening of ATP-sensitive K ϩ channels (KATP) in rat mesenteric resistance arteries, 2 which parallels the case of the canine saphenous veins, 3 and that this ␤-adrenergic hyperpolarization plays an important role in the sympathetic control of membrane potential by opposing ␣-adrenoceptor-mediated depolarization. 4 In view of the importance of membrane potential as a determinant of smooth muscle tone, 5 ␤-adrenoceptor-mediated hyperpolarization may play an important role in the control of vascular tone.…”