[Comparison of antagonistic effects of several drugs on calcium current in a giant neuron of an African snail (Achatina fulica Férussac)].

Antagonistic effects of several drugs on the calcium current (ICa) of a giant neurone, PON (periodically oscillating neurone), of an African snail (Achatina fulica Ferussac) were compared under the voltage clamp. According to their IC50 values and the confidence limit at 95%, the order of effectiveness of the drugs was: brovincamine, verapamil greater than or equal to isoperisone, eperisone, d-diltiazem greater than or equal to vincamine, l-diltiazem, tolperisone. Their IC50 values were assumed to be the dissociation constants of Ca2+ channels in the resting state (Kr). Voltage dependence of these drugs was examined by measuring the steady-state inactivation curves. From the results obtained, the dissociation constants of the channels in the inactivated state (Ki) and the Kr/Ki ratio were calculated for each drug. The order of Ki values of these drugs was: isoperisone, brovincamine, verapamil less than or equal to eperisone, l-diltiazem, d-diltiazem, tolperisone. The use dependence of the drugs was also examined by measuring the further decrease of ICa amplitude caused by high frequent stimulations. We observed that verapamil, brovincamine, d-diltiazem and l-diltiazem were use dependent, whereas eperisone, tolperisone and isoperisone were not.