KINETICS OF VASOCONSTRICTOR ACTION OF ENDOTHELINS

Endothelin peptides (Et) induce slowly developing and long-lasting contractions of rat aortic strips with a rank order of potency (Et-1 = Et-2 > sarafotoxin S6b > Et-3) consistent with the involvement of an Et(A)-like receptor subtype. A similar profile of action is observed for Et-induced intracellular [Ca2+]i mobilization in cultured aortic myocytes. Modeling the association of Et-1 to its receptor shows that, at concentrations which produce large increases in tension, Et-1 associates rapidly to its receptors and that a slow rate of association is not responsible for the slow rate of tension development. Action of endothelins on [Ca2+]i was studied using isolated cultured aortic myocytes and compared with that of angiotensin II and vasopressin. Results show that three vasoconstrictors produce similar and rapid changes in [Ca2+]i. The rate-limiting step for the contractile action of Et is a postreceptor event probably distal to early changes in [Ca2+]i. Biological responses to Et are usually characterized by a relative insensitivity to the peptide as compared with K(d) values determined in binding experiments. Data presented show that insensitivity of the early [Ca2+]i responses to Et could be accounted for by the fact that the responses develop under nonequilibrium conditions. Tension amplitude seems also to be determined by nonequilibrium binding conditions. It correlates with the fraction of the Et-1 binding sites occupied 20 s after addition of the peptide and not to the fractional site occupancy at the time of maximum tension development. In conclusion, kinetic parameters of the interaction of Et with their receptors do not determine the slow development of the contractile responses. They may, however, be responsible for the relative insensitivity of the biological responses to Et.