COMPETITIVE-BINDING EXPERIMENTS REVEAL DIFFERENTIAL INTERACTIONS FOR DIHYDROPYRIDINE CALCIUM-CHANNEL ACTIVATORS AND ANTAGONISTS AT DIHYDROPYRIDINE RECEPTORS ON MOUSE-BRAIN MEMBRANES

The binding of the dihydropyridine (+/-)-202-791 and its corresponding calcium channel activating and calcium channel antagonist enantiomers ((+)-S-202-791 and (-)-R-202-791, respectively) to dihydropyridine re ceptors on mouse brain membranes was studied through competition for [ H-3]nitrendipine binding and H-3-labelled (+/-)-BAY K8644 ((+/-[K-3]BA Y K8644). Direct binding studies with (+/-)-[H-3]BAY K8644 and [H-3]ni trendipine revealed high affinity binding to a homogeneous set of dihy dropyridine calcium channel activator and antagonist receptors on mous e brain membranes, (+/-)-[H-3]BAY K8644 binding to approximately one h alf as many receptors as did [H-3]nitrendipine. Competition binding st udies revealed a significant discrimination of both high and low affin ity receptors for (-)-R-202-791 and a homogeneous set of receptors for (+)-S-202-791 regardless of whether (+/-)-[H-3]BAY K8644 or [H-3]nitr endipine was the competing radioligand. Molar ratios (1:1, 5:1, 10:1) of (+)-S-202-792 to (-)-R-202-791 inhibited [H-3]nitrendipine binding with displacement binding isotherms substantially different from those predicted on the basis of the binding properties of the individual en antiomers. These data suggest that dihydropyridine calcium channel ant agonists and activators bind to different allosterically linked recept ors or domains of the dihydropyridine protein associated with the volt age-dependent calcium channels. Furthermore, these results support the concept of multiple binding sites for dihydropyridine ligands.