Both endothelin-A and endothelin-B receptors are present on adult rat cardiac ventricular myocytes

Endothelin-A (ETA) and endothelin-B (ETB) receptors have been demonstrated in intact heart and cardiac membranes. ETA receptors have been demonstrated on adult ventricular myocytes. The aim of the present study was to determine the presence of ETB and the relative contribution of this receptor subtype to total endothelin-1 (ET-1) binding on adult ventricular myocytes. Saturation binding experiments indicated that ET-1 bound to a single population of receptors ( K-d = 0.52 +/- 0.13 nM, n = 4) with an apparent maximum binding ( B-max) of 2.10 +/- 0.25 sites ( x 10(5))/ cell (n = 4). Competition experiments using 40 pM [I-125] ET-1 and nonradioactive ET- 1 revealed a K-i of 660 +/- 71 pM (n = 10) and a Hill coefficient ( n(H)) of 0.99 +/- 0.10 (n = 10). A selective ETA antagonist, BQ610, displaced 80% of the bound [I-125] ET-1. No displacement was observed by concentrations of an ETB-selective antagonist, BQ788, up to 1.0 muM. However, in the presence of 1.0 muM BQ610, BQ788 inhibited the remaining [I-125] ET-1 binding. Similarly, in the presence of 1.0 muM BQ788, BQ610 inhibited the remaining specific [I-125] ET-1 binding. Binding of an ETB1-selective agonist, [I-125] IRL-1620, confirmed the presence of ETB. ETB bound to ET-1 irreversibly, whereas binding to ETA demonstrated both reversible and irreversible components, and BQ610 and BQ788 bound reversibly. Reducing the incubation temperature to 0degreesC did not alter the irreversible component of ET-1 binding. Hence, both ETA and ETB receptors are present on intact adult rat ventricular myocytes, and the ratio of ETA:ETB binding sites is 4:1. Both receptor subtypes bind to ET-1 by a two-step association involving the formation of a tight receptor-ligand complex; however, the kinetics of ET-1 binding to ETA versus ETB differ.