Diltiazem derivatives modulate the dihydropyridine-binding to intact rat ventricular myocytes.
To examine whether the modulation of the 1,4-dihydropyridine-binding by diltiazem derivatives, which has been shown in cardiac and skeletal muscle membranes, takes place in intact cardiac myocytes, effects of diltiazem on the specific binding of [3H](+)-PN200-110 to freshly isolated adult rat ventricular myocytes were investigated in normal Tyrode solution at 37 degrees C. Diltiazem consistently potentiated the [3H](+)-PN200-110-binding in a concentration-dependent manner, while DTZ323 (3-(acetyloxy)-5-[2-[[2- (3,4-dimethoxyphenyl)ethyl]-methylamino]ethyl]-2,3-dihydro-2(-4 methoxyphenyl)-1,5-benzothiazepin-4-(5H)-one), a potent diltiazem derivative, inhibited it in a non-competitive manner. In saturation studies, 100 microM decreased the Kd value of the 3[H](+)-PN200-110-binding (control, 0.102 +/- 0.008 vs. diltiazem, 0.074 +/- 0.004 (nM, n = 6), P < 0.05) without significant effect on Bmax (control, 65.7 +/- 6.4 vs. diltiazem, 76.7 +/- 4.4 (fmol/mg protein, n = 6). Moreover, membrane-impermeant quaternary diltiazem also potentiated the [3H](+)-PN200-110-binding in intact myocytes. These results suggest that diltiazem modulates the 1,4-dihydro-pyridine-binding even in intact cardiac myocytes, and the binding site of diltiazem is accessible from the extracellular side of the L-type Ca2+ channels.
Kanda
S
S
Department of Toxicology and Pharmacology, Faculty of Pharmaceuticals Sciences, University of Tokyo, Japan.
Kurokawa
J
J
Adachi-Akahane
S
S
Nagao
T
T
D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D020746:Calcium Channels, L-Type; D004110:Diltiazem / Q000494:pharmacology*; D017275:Isradipine / Q000378:metabolism; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D004110:Diltiazem; D017275:Isradipine