Binding properties of a novel calcium channel activating dihydropyridine in monolayer cultures of beating myocytes.

Binding characteristics of [3H]BAY K 8644, a new class of pharmacologically potent compounds, the calcium channel activating dihydropyridines (DHP), were demonstrated in cultured myocardial cells. [3H]BAY K 8644 exhibited reversible and saturable binding to myocytes, and specific binding was Ca2+-dependent. The equilibrium dissociation constant, Kd, was 35.2 nM, and maximal binding capacity, Bmax, was 1.07 pmol/mg protein. Binding of the 3H-ligand was highly specific for various potently displacing DHP derivatives (either the calcium channel activating BAY K 8644, or the Ca2+ entry blockers of the nifedipine type) with inhibition constants (Ki values) in the nanomolar range. BAY K 8644, on the other hand, showed very low affinity to other receptors tested in brain and heart membranes. Displacement potency of BAY K 8644 correlated well with data of the functional pharmacology; e.g., the enhanced myocardial contractility. Results from competition studies using [3H]BAY K 8644 and [3H]nimodipine support the conclusion that both the channel activating and inhibiting DHP structures interact with the same specific receptor site that might be associated with the putative Ca2+-channel.