Dihydropyridine receptor in rat brain labeled with [3H]nimodipine.

Receptor binding sites for 1,4-dihydropyridine (DHP) calcium antagonists have been characterized by using [3H]-nimodipine, a potent analogue of nifedipine with cerebrovascular and neuro- and psychopharmacological properties. [3H]Nimodipine exhibited reversible and saturable binding to partially purified brain membranes. The equilibrium dissociation constant, Kd, was 1.11 nM and the maximal binding capacity, Bmax, was 0.50 pmol/mg of protein. The DHP receptor proved to be highly specific for various potently displacing DHP derivatives and discriminated between their optical isomers (stereoselectivity) with inhibition constants (Ki) in the nanomolar or even subnanomolar range. Structurally different calcium antagonists such as gallopamil (D-600), on the other hand, displayed much lower affinities, further substantiating the specificity of the receptor for DHP structures. Furthermore, the displacement potency of a series of DHP derivatives correlated well with that determined for inhibition of mechanical response in the intact smooth muscle over 5 orders of magnitude. [3H]Nimodipine binding thus may provide a molecular probe to elucidate the nature of the interaction of calcium entry blockers with specific membrane-located receptor sites that may be associated with the putative calcium channel. These receptor sites might well represent the loci of signaling events where the potent DHPs exert their pharmacological action.