MECHANISMS OF CALCIUM MOBILIZATION AND PHOSPHOINOSITIDE HYDROLYSIS IN HUMAN BRONCHIAL SMOOTH-MUSCLE CELLS BY ENDOTHELIN-1

We have previously demonstrated that human bronchial smooth muscle cells possess a single class of high-affinity binding sites for endothelin 1. In this study, we further characterized the receptor for endothelin 1 and evaluated the signal transduction mechanisms of this peptide. Stimulation of cultured human bronchial smooth muscle cells with endothelin 1 induced mobilization of Ca2+ from both intracellular and extracellular pools with a biphasic increase in cytoplasmic free Ca2+ concentration. Endothelin 1 increased cellular levels of inositol phosphates and diacylglycerol, indicating activation of phospholipase C, but induced production of inositol phosphates in smooth muscle cell membranes only in the presence of guanosine 5'-O-(thiotriphosphate) (GTP-gamma-S). Treatment of smooth muscle cells with pertussis toxin failed to block the endothelin 1-induced increase in inositol phosphate production and Ca2+ mobilization. These results suggest that the receptor for endothelin 1 in bronchial smooth muscle is coupled to phospholipase C through a pertussis toxin-insensitive G protein. Affinity crosslinking experiments identified the endothelin 1 receptor as a single band with an apparent molecular weight of approximately 70,000 on sodium dodecyl sulfate polyacrylamide gel electrophoresis, further supporting the functional evidence that endothelin 1 receptor belongs to the G protein-linked rhodopsin type of receptor superfamily.