Influence of Mg++ on the effect of diltiazem to increase dihydropyridine binding to receptors on Ca++-channels in chick cardiac and skeletal muscle membranes.

The influence of Mg++ on the effect of diltiazem to increase ligand binding to a high-affinity state of dihydropyridine receptors on voltage-dependent Ca-channels has been studied in chick cardiac and skeletal muscle membranes at 25 degrees C. The high-affinity binding of the Ca-channel inhibitors (+)-[3H]PN 200-110 and [3H]nitrendipine to cardiac membranes was depressed markedly by EDTA and restored fully by the addition of free Mg++ (Ptasienski et al., Biochem. Biophys. Res. Commun. 129: 910-917, 1985). Similar results have now been obtained with skeletal muscle membranes. In the presence of EDTA alone, diltiazem, which binds to another receptor on the Ca-channel, increased the high-affinity binding of both ligands to cardiac and skeletal muscle membranes. However, in the presence of added Mg++, diltiazem had smaller or no effects on the binding of these dihydropyridines. Analyses of the data indicated that both Mg++ and diltiazem could increase the maximum binding (Bmax) for these ligands, but the effect of diltiazem was smaller than, and not additive to, that of Mg++. Specific binding of the Ca-channel activator [3H]Bay k 8644 was only observed in assays containing Mg++ in excess of EDTA. The Bmax for [3H]Bay k 8644 in skeletal muscle membranes was less than that for [3H]PN 200-110 and [3H]nitrendipine, whereas with cardiac membranes equal Bmax values were obtained for all ligands. Diltiazem increased the Bmax for [3H]Bay k 8644 in skeletal muscle, but not in cardiac membranes.(ABSTRACT TRUNCATED AT 250 WORDS)