Endothelin receptor binding and cellular signal transduction in neurohybrid NG108-15 cells

Endothelins are a novel group of potent vasoconstrictor peptides originally isolated from cultured porcine endothelial cells. We and others have previously reported the presence of endothelin receptors in the central nervous system, and this study was designed to further characterize endothelin receptors and their transduction mechanism in cultured neurohybrid NG108-15 cells. Specific binding of [125I]endothelin-1 to NG108-15 cells reached saturation within 60 min at 22 degrees C and was only partially reversible. Scatchard analysis of the saturation binding revealed the presence of one class of high-affinity binding sites with an apparent dissociation constant of 160 pM and a maximal binding capacity of 3.3 x 10(4) sites/cell. Unlabeled endothelin analogues competitively inhibited [125I]endothelin-1 binding to NG108-15 cells and the apparent dissociation constant values obtained from the competition curves correlated well with the EC50 values obtained for inducing elevation of intracellular free Ca2+ level. Endothelin stimulated phosphoinositide metabolism in a dose-dependent manner with an EC50 value of 5.4 nM for inositol trisphosphate formation. The protein kinase C-activator phorbol ester dose-dependently inhibited endothelin-induced phosphoinositide turnover and intracellular free Ca2+ increase, suggesting the involvement of protein kinase C in the regulation of endothelin-induced responses. Neither endothelin-induced phosphoinositide hydrolysis nor endothelin-induced increase in intracellular free Ca2+ were affected by pertussis toxin. These data indicate that endothelin receptors are present on NG108-15 cells and the G protein coupled to endothelin receptor for inducing activation of phospholipase C and increase of free intracellular Ca2+ is insensitive to pertussis toxin.