title: Blockade of endothelium-dependent relaxation by the amiloride analog dichlorobenzamil: possible role of Na+/Ca++ exchange in the release of endothelium-derived relaxant factor.
abstract: The importance of extracellular calcium for the expression of endothelium-dependent relaxation was examined in isolated rat aortic rings contracted by methoxamine. The endothelium-dependent relaxation generated by acetylcholine or the calcium ionophore A23187 was eliminated when rings were placed in physiological buffer to which calcium had not been added. The endothelium-independent relaxation to sodium nitroprusside was still elicited in the presence of this "low calcium" buffer. Pretreatment of aortic rings with high concentrations of nifedipine (5 X 10(-7) M) or verapamil (10(-5) M) caused a comparable displacement to the right (2-3 times) in the relaxant dose-response curve for acetylcholine, A23187 and sodium nitroprusside with little or no changes in the maximal relaxation obtained with these vasodilators. Increasing concentrations of dichlorobenzamil, an analog of amiloride and a recently described inhibitor of calcium influx via sodium-calcium exchange, functionally antagonized and abolished the relaxations elicited by acetylcholine and A23187, but had no appreciable effect on the relaxations to sodium nitroprusside or atrial natriuretic factor (an endothelium-independent vasodilator). Similar results were obtained using isolated rabbit aortic rings. Thus, although the presence of extracellular calcium is critically required for the expression of endothelium-dependent relaxation, the associated calcium translocation is not blocked by the organic calcium entry blockers. The results with dichlorobenzamil suggest that sodium-calcium exchange may be an important mechanistic step in the release of endothelium-derived relaxant factor.
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology; D000584:Amiloride / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000502:physiology; D009543:Nifedipine / Q000494:pharmacology; D009569:Nitric Oxide; D009599:Nitroprusside / Q000494:pharmacology; D011817:Rabbits; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism*; D014664:Vasodilation / Q000187:drug effects; D014665:Vasodilator Agents / Q000378:metabolism*; D014700:Verapamil / Q000494:pharmacology
chemical_list: D014665:Vasodilator Agents; C041470:3',4'-dichlorobenzamil; D009599:Nitroprusside; D009569:Nitric Oxide; D000001:Calcimycin; D000584:Amiloride; D012964:Sodium; D014700:Verapamil; D009543:Nifedipine; D000109:Acetylcholine; D002118:Calcium