Endothelin-1 Receptors in Cultured Vascular Smooth Muscle Cells and Cardiocytes of Rats

Summary 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 (K_d = 2–6 $$ 10–¹⁰M) 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⁺.