Heterogeneity of calcium channel agonist binding sites in the coronary artery

The binding properties (3H) BAY k 8644 a 1,4-dihydropyridine calcium channel agonist were studied in the subcellular membrane fraction isolated from the coronary artery by differential centrifugation. The specific binding of (3H) BAY k 8644 to microsomal membranes of the coronary smooth muscle was rapid, saturable, reversible and of both high and low affinity. The dissociation constants obtained from Scatchard analysis with (3H) BAY k 8644 and nitrendipine were 0.60 +/- 0.02 nmol.l-1 and 9.1 +/- 0.1 nmol.l-1 for the high and low affinity binding site respectively and the estimated maximal numbers of binding sites in the plasma membrane fraction were 0.76 +/- 0.02 and 3.15 +/- 0.18 pmol.mg-1 of protein respectively. The substituted dihydropyridine calcium channel antagonists nitrendipine and nifedipine competitively inhibited specific (3H)BAY k 8644 binding suggesting a common high affinity 1,4-dihydropyridine binding site in the coronary microsomal fraction for calcium channel activator and antagonists. The low affinity agonist binding sites were significantly inhibited by adding nucleoside carrier inhibitors, 2-deoxyadenosine and dipyridamole, and by -SH alkylating agent N-ethylmaleimide. The results suggests that the coronary artery contains both high and low affinity calcium channel binding sites (in a 1:5 ratio) with the low affinity calcium channel agonist binding sites being associated with nucleoside carrier and/or with-SH groups.