Ca2+-Dependent and Independent Mechanisms of Sustained Contraction in Vascular Smooth Muscle of Rat Aorta

Potentiation by endothelin-1 of Ca2+ sensitivity of contractile elements depends on Ca2+ influx through L-type Ca2+ channels in the canine cerebral artery

1. Endothelin-1 (ET-1) contracted canine cerebral artery in a concentration-dependent manner with an increase in intracellular Ca2+ concentration ([Ca2+]i), and at higher concentrations it produced a greater contraction with a smaller increase in [Ca2+]i. 2. Ca2+ channel antagonist such as d-cis-diltiazem inhibited the tension more effectively than the [Ca2+]i increased by ET-1. 3. In Ca(2+)-free solution containing 0.2 mM EGTA, ET-1 elicited a transient increase in [Ca2+]i and tension. 4. In the Staphylococcus aureus alpha-toxin-permeabilized artery, ET-1 shifted the pCa-tension relationship leftwards in the presence of GTP. 5. These findings suggest that ET-1 contracts the canine cerebral artery by increasing not only the Ca2+ influx through L-type Ca2+ channels, but also Ca2+ release from the intracellular storage sites, and also Ca2+ sensitivity of contractile elements. The degree of Ca2+ sensitivity is strongly affected by [Ca2+]i which is increased by the Ca2+ influx through L-type Ca2+ channels.

Effect of endothelin-3 on cytosolic calcium level in vascular endothelium and on smooth muscle contraction

In isolated rat aorta, endothelin (ET)-3 increased cytosolic Ca2+ ([Ca2+]i) in the endothelium at a concentration (100 nM) which had little effect on muscle resting tone. In the absence of external Ca2+, ET-3 still transiently increased endothelial [Ca2+]i. Verapamil (10 microM) did not change the effects of ET-3. In aortas stimulated with 100 nM norepinephrine, 100 nM ET-3 relaxed the muscle with an increase in endothelial [Ca2+]i. An inhibitor of nitric oxide synthase, 100 microM NG-monomethyl-L-arginine, inhibited the relaxant effect of ET-3 but not the increase in endothelial [Ca2+]i. In the absence of the endothelium or in the presence of an antagonist of ETB receptors, 3 microM IRL 1038, the ET-3-induced increase in endothelial [Ca2+]i and relaxation of norepinephrine-induced contraction were inhibited. Under these conditions, ET-3 increased smooth muscle [Ca2+]i and induced contraction, both of which were inhibited by an inhibitor of ETA receptors, 3 microM BQ123. These results suggest that ET-3 acts on ETB receptors in the vascular endothelium to increase [Ca2+]i by releasing Ca2+ from storage sites and by opening non-L type Ca2+ channels, activates nitric oxide synthase, releases nitric oxide and relaxes vascular smooth muscle. Although ET-3 also activates ETA receptors in smooth muscle to induce contraction, this effect is overcome by the relaxant effect mediated by ETB receptors.