INHIBITION OF L-TYPE CA2+ CHANNELS IN PORTAL-VEIN MYOCYTES BY THE ENANTIOMERS OF OXODIPINE

We studied the effects of the enantiomers of the dihydropyridine derivative, 4-(2,3 methylenedioxyphenyl)-1,4-dihydro-2,6-dimethyl-3 carboxyethyl-5-carboxymethyl-pyridine (oxodipine), on voltage-dependent Ca2+ channels of rat portal vein myocytes by combining electrophysiological techniques and binding studies. (+)- and(-)-oxodipine depressed the L-type Ca2+ current in a concentration-dependent manner, with similar IC50 values (around 10 nM) but had no appreciable effect on the intracellular Ca2+ stores. The steady-state inactivation curve for the Ca2+ current was shifted along the voltage axis to negative membrane potentials indicating that the block of the Ca2+ current by oxodipine enantiomers increased with depolarization. The voltage-dependent inhibitory property of oxodipine was related to an increase in [H-3](+)-4-(benzo-2-oxa-1,3-diazol-4-yl)-1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylic acid 3-isopropyl, 5-methyl ester (isradipine) binding affinity without change in binding capacity. In normally polarized intact strips, interactions of (+)- and (-)-oxodipine with [H-3](+)-isradipine binding indicated a stimulation of the radioligand binding at low concentrations of(-)-oxodipine while the (+) enantiomer seemed to act as a competitive ligand. Depolarization of intact strips with 135 mM K+-solutions increased the apparent affinity of the enantiomers of oxodipine, and abolished the stimulating effect of(-)-oxodipine on the binding of [H-3](+)-isradipine. Inhibition of Ca2+ current was increased in the simultaneous presence of 1 nM of (+)- and (-)-oxodipine when compared to the inhibitions induced by 2 nM of each enantiomer. In addition, the Hill coefficients of the Ca2+ current inhibition curves for (+)- and (-)-oxodipine were found to differ from unity. Taken together these results suggest the existence of two cooperatively interacting high affinity binding sites for 1,4-dihydropyridines in L-type Ca2+ channels of vascular smooth muscle cells.