R56865 AS CA2+-CHANNEL BLOCKER IN PURKINJE NEURONS OF RAT - COMPARISON WITH FLUNARIZINE AND NIMODIPINE

The blocking action of recently synthesized benzothiazolamine derivative R56865 was compared with that of dihydropyridine (nimodipine) and diphenylalkylamine (flunarizine) on low-voltage-activated and non-inactivating high-voltage-activated Ca+ currents. The experiments were carried out on freshly isolated Purkinje neurons of rat cerebellum using patch-clamp technique in the whole-cell configuration. Among the substances tested R56865 was found to be the most effective blocker of the Ca2+ current. In the sequence R56865, flunarizine and nimodipine, apparent K(d) values for low-voltage-activated current are 0.1, 0.9 and 3.5 muM, and for high-voltage-activated current 3.1, 9.5 and 38 muM, respectively. The current-voltage relationships for both types of currents displayed little or no shift under either flunarizine or R56865 but showed a 10-mV shift in the positive direction under the action of nimodipine. The steady-state inactivation curves for low-voltage-activated calcium currents were shifted under the action of R56865, flunarizine and nimodipine (in concentrations which blocked 50-60% of the current) to more negative membrane potentials for 20, 10 and 6 mV, respectively. In contrast to R56865, flunarizine blocked both types of Ca2+ channel in a use-dependent manner. It is concluded that the order of potency of Ca2+ antagonist for both types of channels studied is R56865 > flunarizine > nimodipine. Strong shift of steady-state inactivation relationship by R56865 can further facilitate its blocking action in in vivo conditions.