ELECTROPHYSIOLOGICAL AND RADIOLIGAND BINDING-STUDIES OF ELGODIPINE AND DERIVATIVES IN PORTAL-VEIN MYOCYTES

The effects of a novel dihydropyridine, elgodipine, and of three derivatives have been studied on the calcium channel currents of isolated cells from rat portal vein by the patch-clamp technique, and on specific (+)-[H-3]isradipine binding to vascular membranes. Elgodipine inhibited both T- and L-type calcium channels in a concentration-dependent manner. Half-inhibitions of T- and L-type calcium channel current were obtained at concentrations of 32 and 2.3 nM, respectively. Currents activated repetitively were similarly inhibited than those after a rest period, indicating absence of use-dependent inhibition by elgodipine. When cells were held at depolarized membrane potentials at which T- or L-type calcium channels were inactivated, the inhibitory effects of elgodipine were enhanced on both calcium channel currents, indicating that the elgodipine-induced inhibition was voltage-dependent, The elgodipine concentration which blocked the inactivated calcium channels were 5 to 7 times lower than those which blocked the resting calcium channels. The inhibition constant for elgodipine obtained from the displacement of (+)-[H-3]isradipine binding to the L-type calcium channels in vascular membranes was identical to the dissociation constant calculated from electrophysiological data on inactivated calcium channels. At concentrations that completely inhibited calcium channels, elgodipine had no effect on chloride and potassium channels, and did not interfere with the intracellular calcium stores. The elgodipine derivatives (in which modifications of ester substituents on position 3 and 5 of the 1,4-dihydropyridine ring were carried out) had no effect on the T-type calcium channels. Their order of potency on (+)-[H-3]isradipine binding and L-type calcium channel current was M3-875 > M9-875 > M10-875. At 100 nM, only M3-875 and M9-875 partially inhibited L-type calcium current. These data show that elgodipine inhibits both T- and L-type calcium channels and that the three derivatives are less effective than elgodipine.