Endothelin-induced increases in vascular smooth muscle Ca2+ do not depend on dihydropyridine-sensitive Ca2+ channels

Abstract

Endothelin is a potent mammalian vasoconstrictive peptide with structural homology to cation channel-binding insect toxins. We tested the proposal that this peptide directly activates dihydropyridine-sensitive Ca2+ channels in cultured vascular smooth muscle (VSM) cells. First, we found that cell Ca2+ can be altered in VSM by activation of voltage-operated Ca2+ channels. KCl-induced depolarization and the dihydropyridine Ca2+ channel agonist (-) Bay K 8644 (10 μM) both raised cell Ca2+ more than twofold; the effect of KCl was blocked by the inhibitory enantiomer, (+) Bay K 8644 (40 μM). Similar responses were observed in Chinese hamster ovary (CHO) cells. Synthetic endothelin (4 x 10-8 M) raised CA2+ in VSM but not CHO cells from 100 ± 17 to 561 ± 34 nM within 12 s. Ca2+ subsequently fell to basal levels after 30 min. Half maximal Ca2+ response was at 4 x 10-9 M endothelin. Unlike endothelin, thrombin raised Ca2+ in both VSM and CHO cells. The Ca2+ responses to endothelin and thrombin were not affected by nicardipine (1 μM), (+) Bay K 8644, or Ca2+-free solutions, Lastly, both hormones caused release of inositol phosphates in VSM cells. However, the response to thrombin was more than 10-fold larger and was more rapid than the response to endothelin; the thrombin response was sensitive to pertussis toxin, while the response to endothelin was not. Thus endothelin, like thrombin, raises cell Ca2+ in VSM by mobilization of intracellular stores and not by activation of dihydropyridine-sensitive Ca2+ channels. However, their receptors are distinct and they exhibit important differences in signal transduction.