Subcellular localization of [3H]-nitrendipine binding sites in guinea-pig ileal smooth muscle

The binding of [3H]-nitrendipine was studied in microsomal fractions isolated from guinea-pig ileal smooth muscle. Only one class of specific binding sites was detected, with a KD of 0.4 nM. For various dihydropyridine derivatives, including the stereoisomers of nimodipine and the 'Ca agonist' Bay K 8644, the potency for inhibition of [3H]-nitrendipine binding correlated well with the reported pharmacological potency in smooth muscle preparations. To establish the subcellular localization of [3H]-nitrendipine binding sites, untreated and digitonin-treated microsomal fractions were subfractionated by isopycnic density gradient centrifugation. The density distribution of [3H]-nitrendipine binding was markedly shifted by digitonin towards higher densities, as were the distributions of 5'-nucleotidase and [3H]-ouabain binding, whereas the distributions of NADPH:cytochrome c reductase and NADH:cytochrome c reductase were hardly modified by digitonin. It is concluded that most, if not all, [3H]-nitrendipine binding sites in guinea-pig ileal smooth muscle are present in the plasma membrane, in agreement with the postulated mode of action of dihydropyridines as inhibitors of plasmalemmal Ca channels.