ANTAGONISTS OF BRADYKININ THAT STABILIZE A G-PROTEIN-UNCOUPLED STATE OF THE B2 RECEPTOR ACT AS INVERSE AGONISTS IN RAT MYOMETRIAL CELLS

Several B2 bradykinin (BK) receptor-specific antagonists including HOE 140, NPC17731, and NPC567 exhibited negative intrinsic activity, which was observed as a decrease in basal phosphoinositide hydrolysis in pr imary cultures of rat myometrial cells, and this response was opposite to that elicited by the agonist BK. The order of potency of the antag onists in attenuating basal activity was essentially the same as that in competing both [H-3]BK and [H-3]NPC17731 for binding to B2 receptor s on both intact rat myometrial cells and bovine myometrial membranes. We previously proposed a three-state model for the binding of agonist s to G-protein-coupled B2 receptors in bovine myometrial membranes (Le eb-Lundberg, L. M. F and Mathis, S. A. (1990) J. Biol. Chem. 265, 9621 -9627). This model was based on the ability of BK to promote the seque ntial formation of three receptor binding states where formation of th e third, equilibrium state was blocked by Gpp(NH)p (guanyl-5'-yl imido diphosphate) identifying it as the G-protein-coupled state of the rece ptor. Here, we show that, in contrast to BK, these antagonists bound p referentially to a G-protein uncoupled state of the receptor. These re sults indicate that B2 receptor antagonists that stabilize a G-protein -uncoupled state of the receptor act as inverse agonists. Furthermore, these results provide strong evidence that endogenous G-protein coupl ed receptors exhibit spontaneous activity in their natural environment in the absence of agonist occupancy.