title: Smooth muscle membrane potential modulates endothelium-dependent relaxation of rat basilar artery via myo-endothelial gap junctions.
abstract: The release of endothelium-derived relaxing factors, such as nitric oxide (NO), is dependent on an increase in intracellular calcium levels ([Ca(2+)](i)) within endothelial cells. Endothelial cell membrane potential plays a critical role in the regulation of [Ca(2+)](i) in that calcium influx from the extracellular space is dependent on membrane hyperpolarization. In this study, the effect of inhibition of vascular smooth muscle delayed rectifier K(+) (K(DR)) channels by 4-aminopyridine (4-AP) on endothelium-dependent relaxation of rat basilar artery to acetylcholine (ACh) was assessed. ACh-evoked endothelium-dependent relaxations were inhibited by N-(Omega)-nitro-L-arginine (L-NNA) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), confirming a role for NO and guanylyl cyclase. 4-AP (300 microM) also suppressed ACh-induced relaxation, with the maximal response reduced from approximately 92 to approximately 33 % (n = 11; P < 0.01). However, relaxations in response to exogenous NO, applied in the form of authentic NO, sodium nitroprusside or diethylamineNONOate (DEANONOate), were not affected by 4-AP treatment (n = 3-11). These data are not consistent with the view that 4-AP-sensitive K(DR) channels are mediators of vascular hyperpolarization and relaxation in response to endothelium-derived NO. Inhibition of ACh-evoked relaxation by 4-AP was reversed by pinacidil (0.5-1 microM; n = 5) or 18beta-glycyrrhetinic acid (18betaGA; 5 microM; n = 5), indicating that depolarization and electrical coupling of the smooth muscle to the endothelium were involved. 4-AP caused depolarization of both endothelial and vascular smooth muscle cells of isolated segments of basilar artery (mean change 11 +/- 1 and 9 +/- 2 mV, respectively; n = 15). Significantly, 18betaGA almost completely prevented the depolarization of endothelial cells (n = 6), but not smooth muscle cells (n = 6) by 4-AP. ACh-induced hyperpolarization of endothelium and smooth muscle cells was also reduced by 4-AP, but this inhibition was not observed in the combined presence of 4-AP and 18betaGA. These data indicate that 4-AP can induce an indirect inhibition of endothelium-dependent relaxation in the rat basilar artery by electrical coupling of smooth muscle membrane depolarization to the endothelium via myo-endothelial gap junctions.
authors.affiliation: The Smooth Muscle Research Group, Canadian Institutes of Health Research Group in Regulation of Vascular Contractility, University of Calgary, 3330 Hospital Drive NW, Calgary, Alberta, Canada T2N 4N1.
authors.forename: Tracy
authors.initials: T
authors.lastname: Allen
authors.forename: Mircea
authors.initials: M
authors.lastname: Iftinca
authors.forename: William C
authors.initials: WC
authors.lastname: Cole
authors.forename: Frances
authors.initials: F
authors.lastname: Plane
mesh_terms: D015761:4-Aminopyridine / Q000037:antagonists & inhibitors / Q000494:pharmacology; D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001488:Basilar Artery / Q000502:physiology*; D004730:Endothelium, Vascular / Q000502:physiology*; D017629:Gap Junctions / Q000502:physiology*; D006034:Glycyrrhetinic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009131:Muscle, Smooth, Vascular / Q000502:physiology*; D009569:Nitric Oxide / Q000494:pharmacology; D020110:Pinacidil / Q000494:pharmacology; D026902:Potassium Channel Blockers / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D014664:Vasodilation / Q000187:drug effects / Q000502:physiology*; D014665:Vasodilator Agents / Q000494:pharmacology
chemical_list: D026902:Potassium Channel Blockers; D014665:Vasodilator Agents; D009569:Nitric Oxide; D020110:Pinacidil; D015761:4-Aminopyridine; D000109:Acetylcholine; D006034:Glycyrrhetinic Acid