ET(A) AND ET(B) RECEPTORS ON SINGLE SMOOTH-MUSCLE CELLS COOPERATE IN MEDIATING GUINEA-PIG TRACHEAL CONTRACTION

We investigated the distribution of endothelin A (ET(A)) and ET(B) receptors in single smooth muscle cells and their contribution to ET-induced contractions of guinea pig trachea. ET(A) and ET(B) receptors were detected in smooth muscle membranes (maximum binding capacities of 810 and 360 fmol/mg protein and dissociation constants of 38 and 5.1 pM for I-125-labeled ET-1 and I-125-ET-3, respectively) and visualized autoradiographically in primary cultured cells. ET-1 and ET-3 evoked concentration-dependent increases in intracellular Ca2+ concentration and smooth muscle tension. The half-maximally effective concentrations of ET-1 and ET-3 at inducing contractions were 1.9 and 2.7 nM, respectively. The Ca2+ responses showed tachyphylaxis to both ETs after stimulation with ET-1, but only to ET-3 after stimulation with ET-3. Consecutive applications of ET-3 and ET-1 (10 nM each) classified the cells into ET(A) dominant (similar to 30%) responding to only ET-1, ET(B) dominant (similar to 20%) responding to only ET-3, and ET(A)- and ET(B)-possessing (similar to 50%) cells responding to both. The ET(A) antagonist, 10 mu M BQ-123, attenuated ET-1-induced contractions but did not affect the ET-3-induced contractions. The results indicate that both receptors coexist in a major population of smooth muscle cells and cooperate in mediating ET-1-induced contractions.