Binding of a 1,4-dihydropyridine calcium channel activator, (-) S bay K 8644, to cardiac preparations

The 1,4-dihydropyridine Ca2+ channel activator, (-) [³H]Bay K 8644, binds to cardiac membranes and polarized [5 mM K+] and depolarized [50 mM K+] cardiac cells. Binding to microsomal membranes at 25°C indicates a single set of binding sites, K_D = 2.9 × 10^-9M and a site density, 337 fmoles/mg protein, not different from that measured by antagonist 1,4-dihydropyridines. Binding to neonatal rat myocytes at 37°C was independent of membrane potential with a K_D value of 5 × 10^-8M and a site density, 63 fmoles/mg protein, not significantly different from that measured by PN 200 110. These results indicate that 1,4-dihydropyridine activators and antagonists label the same number of binding sites in cardiac tissue, but that activator binding to intact myocytes is voltage-independent.