(+)-[3H]-PN 200-110 binding to cell membranes and intact strips of portal vein smooth muscle: characterization and modulation by membrane potential and divalent cations.

1. Specific binding of the calcium-antagonist dihydropyridine derivative, (+)-[3H]-PN 200-110 (isradipine), to cell membranes of equine portal vein smooth muscle was compared with binding to intact strips isolated from rat portal veins. 2. Specific binding to vascular smooth muscle membranes was of high affinity, saturable and reversible. The dissociation constant obtained from association and dissociation kinetics of (+)-[3H]-PN 200-110 was similar to that obtained from equilibrium binding and competition experiments. 3. Specific binding of (+)-[3H]-PN 200-110 was completely displaced by unlabelled dihydropyridines. Among other calcium antagonists, D888 and (+)-cis-diltiazem partially inhibited the binding at 25 degrees C. At 37 degrees C, only (+)-cis-diltiazem stimulated the binding. LaCl3, CdCl2, NiCl2, CoCl2 had inhibitory effects, whereas KCl and NaCl had no effect. 4. When intact strips of portal vein were incubated in high external potassium concentrations for 30 min, the Kd was lowered to 0.04 +/- 0.01 nM from the control value of 0.14 +/- 0.02 nM (n = 5), thereby indicating that (+)-[3H]-PN 200-110 bound to voltage-dependent calcium channels, with a higher affinity, in the depolarized state. 5. When external Ca2+ was removed or substituted with Ba2+ or Sr2+, Kd values increased suggesting that the dihydropyridine binding to intact strips was modulated by binding of Ca2+ ions to voltage-dependent calcium channels.