Binding data point to the coexistence of three endothelin receptors (ET-R) in rat heart myocytes. Induction of phosphoinositide hydrolysis in this preparation by endothelins (ET-1 and ET-3) or sarafotoxins (SRTX-b and SRTX-c) was demonstrated by measurement of labeled inositol phosphate generation. Pertussis toxin (PT) enhanced the induction of phosphoinositide hydrolysis by all four peptides. The process seems to be mediated by at least two heterotrimeric G-proteins, the one sensitive and the other insensitive to PT. Measurement of GTPase activity induced in rat myocytes clearly indicates for the first time the direct functional coupling between ET-R and a G-protein. These GTPase activity experiments provide evidence that phosphoinositide hydrolysis is stimulated via functional coupling between the endothelin receptor of the ETA-R subtype and a PT-insensitive G-protein, Gq/11. The involvement of PT-sensitive G-proteins, ie Gi-like and/or Go-like proteins, in the signal transduction pathways of ETs and SRTXs is discussed.