BLOCKADE OF SODIUM-CHANNELS BY BISTRAMIDE-A IN VOLTAGE-CLAMPED FROG SKELETAL-MUSCLE FIBERS

The effect of Bistramide A, a toxin isolated from Bistratum lissoclinum Sluiter (Urochordata), on the peak sodium current (I(Na)) of frog skeletal muscle fibres was studied with the double sucrose gap voltage clamp technique. External or internal application of Bistramide A inhibited I(Na) without alteration of the kinetic parameters of the current nor of the apparent reversal potential for Na. The steady-state activation curve of I(Na) was unchanged while the steady-state inactivation curve of I(Na) was shifted towards more negative membrane potentials. Dose-response curves indicated an apparent dissociation constant for Bistramide A of 3.3-mu-M and a Hill coefficient of 1.2 which suggested a one to one relation between the toxin and Na channel. The inhibition of I(Na) occurred at rest, and was more important at more positive holding potentials. Bistramide A exhibited only a weak frequency-dependent effect. The toxin did not interact with the use-dependent effect of lidocaine. It mainly blocked Na channels at more depolarized holding potentials. The toxin blocked Na channels when it was internally applied and when the inactivation gating system has been previously destroyed by internal diffusion of iodate. The data suggest that Bistramide A inhibited the Na channel both at rest and in the inactivated state and occupied a site which was not located on the inactivation gate.