DILTIAZEM DERIVATIVES MODULATE THE DIHYDROPYRIDINE-BINDING TO INTACT RAT VENTRICULAR MYOCYTES

To examine whether the modulation of the 1,4-dihydropyridine-binding b y diltiazem derivatives, which has been shown in cardiac and skeletal muscle membranes, takes place in intact cardiac myocytes, effects of d iltiazem derivatives on the specific binding of [H-3](+)-PN200-110 to freshly isolated adult rat ventricular myocytes were investigated in n ormal Tyrode solution at 37 degrees C. Diltiazem consistently potentia ted the [H-3](+)-PN200-110-binding in a concentration-dependent manner , while DTZ323 )-5-[2-[[2(3,4-dimethoxyphenyl)ethyl]-methylamino] -(4- methoxyphenyl)-1,5-benzothiazepin-4-(5H)-one), a potent diltiazem deri vative, inhibited it in a non-competitive manner. In saturation studie s, 100 mu M diltiazem decreased the K-d value of the [H-3](+)-PN200-11 0-binding (control, 0.102 +/- 0.008 vs. diltiazem, 0.074 +/- 0.004 (nM , n = 6), P < 0.05) without significant effect on B-max (control, 65.7 +/- 6.4 vs. diltiazem, 76.7 +/- 4.4 (fmol/mg protein, n = 6)). Moreov er, membrane-impermeant quaternary diltiazem also potentiated the [H-3 ](+)-PN200-110-binding in intact myocytes. These results suggest that diltiazem modulates the 1,4-dihydropyridine-binding even in intact car diac myocytes, and that the binding site of diltiazem is accessible fr om the extracellular side of the L-type Ca2+ channels.