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1. The effects of palytoxin (PTX) on transmembrane potentials and currents of frog skeletal muscle were analyzed by intracellular microelectrode techniques and the double sucrose-gap voltage clamp method. 2. PTX irreversibly depolarized the membrane. The depolarization was Na-sensitive. 3. Under voltage clamp, PTX induced an inward resting current which did not inactivate, was inhibited by external Na+ removal and was a function of external Na concentration. 4. This resting current could be carried either by Na+, Li+, K+ or by guanidinium according to the permeability sequence K+ less than Li+ less than Na+ less than Gua+. 5. The PTX-induced current was only weakly sensitive to tetrodotoxin. It was reversibly and dose-dependently inhibited by amiloride with a one to one stoichiometry and a KD of 0.3 mM. 6. Acidic pH partially inhibited the current induced by PTX which was also highly sensitive to external Cd2+ and La3+. The inhibitory sequence for divalent cations was: Mg2+ less than Ca2+ = Ba2+ = Mn2+ less than Cd2+; with La3+ greater than Cd2+. 7. The amplitude of the PTX-induced I(rest) was markedly reduced in the absence of external Ca2+. 8. PTX induced a Na+ resting conductance in frog skeletal muscle. The size of the channel induced by PTX is larger than the guanidinium ion. External membrane Ca2+ might be a cofactor involved in the mode of action of PTX.

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 μM 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 applyed 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.