Endothelin-1 receptors in cultured vascular smooth muscle cells and cardiocytes of rats.

Specific binding sites for endothelin-1 (ET-1; formerly, porcine and human ET), a novel endothelium-derived vasoconstrictor peptide, and its effect on cytosolic free Ca2+ concentrations ([Ca2+]i) were studied in cultured vascular smooth muscle cells (VSMCs) and cardiocytes of rats. Radioligand binding studies revealed the presence of a single class of high-affinity (Kd = 2-6 X 10(-10) M) binding sites for ET-1 in both cells, although the maximal binding capacity of cardiac receptor was about 6- to 12-fold greater than that of vascular receptor. Neither the well-recognized vasoactive substances nor Ca2+-channel blockers affected the binding. ET-1 dose-dependently induced increases in [Ca2+]i of both cells loaded with the fluorescent Ca2+ indicator fura-2. The second sustained plateau phase, but not the initial transient phase, of [Ca2+]i increases stimulated by ET-1 was abolished by removal of extracellular Ca2+. These data suggest that ET-1 exerts its direct action on the cardiovascular system through receptor-mediated Ca2+ influx and mobilization of intracellular Ca2+.