Pseudo-Noncompetitive Antagonism by BQ123 of Intracellular Calcium Transients Mediated by Human ET_A Endothelin Receptor

The family of endothelins, endothelin-1 (ET-1), ET-2 and ET-3, act on two subtypes of receptors called ET_A and ET_B receptors. BQ123 is an ET_A-selective competitive antagonist. In this study, however, BQ123 inhibited ET-1-induced transients of cytosolic Ca²⁺ concentrations ([Ca²⁺]i) in an apparently noncompetitive manner in mouse L cell stably expressing cloned human ET_A receptor. BQ123 (3 - 30 nM) did not change the EC₅₀ of ET-1 (6.6 nM), but reduced the maximum responses down to 15.0 %. In an non-equilibrium binding assay which mimicks the conditions of [Ca²⁺]i transient assay (cells were incubated with [¹²⁵I]ET-1 only for 30 s), BQ123 (30 nM) constantly decreased the specific [¹²⁵I]ET-1 binding to (equivalent to)13% of that without the antagonist. Thus, the apparent noncompetitive antagonism by BQ123 of ET_A receptor-mediated [Ca²⁺]_i transient is due to the assay condition where the interactions of the agonist, antagonist and receptor remained non-equilibrium state.