Vascular endothelium and Ca2+ antagonists

Abstract

Endothelial cells can release both relaxing and contracting factors. Since the release of endothelium-dependent relaxing factor is prevented by incubation of Ca2+-free solution and can be triggered by the Ca2+ ionophore A 23187, it must require an increase in cytoplasmic Ca2+ concentration in the endothelial cells. Ca2+-channel agonists also evoke the release of endothelium-dependent relaxing factor, an effect prevented by dihydropyridines; thus, the endothelial cell membrane must contain voltage-operated Ca2+ channels. However, since verapamil and dihydropyridines do not prevent the release of endothelium-dependent relaxing factor evoked by acetylcholine, the increase in cytoplasmic Ca2+ concentration leading to the release of the mediator is not due to activation of these channels. Diltiazem has an inhibitory effect on the release of endothelium-dependent relaxing factor, which cannot be attributed to the action of the compound at Ca2+ channels. In the presence of functional endothelium, endothelium-dependent relaxing factor and Ca2+ antagonist such as nisoldipine are synergistic in inhibiting contractions of vascular smooth muscle. In a variety of arteries and veins, endothelium-dependent contractions can be evoked by anoxia; these contractions, as well as the endothelium-dependent increases in tension evoked by stretch in cerebral arteries, can be inhibited by Ca2+ antagonists. In the aorta of the spontaneously hypertensive rat, acetylcholine causes endothelium-dependent contractions, an effect that is inhibited by diltiazem. To judge from experiments in anoxic coronary arteries, the inhibitory effect of Ca2+ antagonists on endothelium-dependent contractions is at the level of the vascular smooth muscle, and not the endothelium. Thus, Ca2+ antagonists do not prevent the release of either endothelium-dependent relaxing factor or endothelium-dependent contracting factor; by facilitating the inhibition exerted by the former, and preventing the activation of vascular smooth muscle by the latter, they favor the occurrence of dilatation.