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 receptors on mouse brain membranes was studied through competition for [3H]nitrendipine binding and 3H-labelled (+/-)-BAY K8644 ((+/-)-[3H]BAY K8644). Direct binding studies with (+/-)-[3H]BAY K8644 and [3H]nitrendipine revealed high affinity binding to a homogeneous set of dihydropyridine calcium channel activator and antagonist receptors on mouse brain membranes, (+/-)-[3H]BAY K8644 binding to approximately one half as many receptors as did [3H]nitrendipine. Competition binding studies revealed a significant discrimination of both high and low affinity receptors for (-)-R-202-791 and a homogeneous set of receptors for (+)-S-202-791 regardless of whether (+/-)-[3H]BAY K8644 or [3H]nitrendipine was the competing radioligand. Molar ratios (1:1, 5:1, 10:1) of (+)-S-202-792 to (-)-R-202-791 inhibited [3H]nitrendipine binding with displacement binding isotherms substantially different from those predicted on the basis of the binding properties of the individual enantiomers. These data suggest that dihydropyridine calcium channel antagonists and activators bind to different allosterically linked receptors or domains of the dihydropyridine protein associated with the voltage-dependent calcium channels. Furthermore, these results support the concept of multiple binding sites for dihydropyridine ligands.