Effect of "calcium antagonist" D-600 on the postsynaptic membrane

Using the technique of dialysis of the cell interior (Kostyuk et al., 1978), the effect of "calcium antagonist" D-600 on voltage clamped isolated neurones of mollusc Limnaea stagnalis was studied. Identified giant neurones responding to acetylcholine (ACh) application with changes in conductance for Cl- ions only were used. 1. D-600, 10-100 microM, blocks electroexcitable inward Ca2+-Na+ current in the neurone soma membrane. In the same concentrations, D-600 reduces the amplitude of ACh-induced current and also the time constant of desensitization onset. 2. Voltage jump relaxation experiments show D-600 to decrease the average lifetime, tau, of the Cl- channels opened by ACh, although the effect of d-tubocurarine (d-TC), 10-20 microM, on tau is insignificant. 3. The lifetime of open Cl- channels increases with membrane hyperpolarization; but while the voltage dependence of tau is not altered by d-TC, it is reduced by D-600. 4. The results obtained allow us to suggest that D-600 blocks open Cl- channels. The calculated rate constant of association of D-600 with open channels is of the order of 10(5) M-1 s-1.