Effects of phalloidin on electrical and mechanical activity of frog muscle fibres

Antamanide antagonizes the phalloidin-induced negative inotropic effect and blocks voltage dependently the fast outward K+ current in voltage-clamped frog muscle fibres

The effects of antamanide (10(-14)-10(-5) M) and N-acetyl-secophalloidin (10(-7)-5 X 10(-3) M) a neutral non-toxic derivative of phalloidin, were tested on voltage-clamped single frog muscle fibres. Antamanide protected muscle fibres against the negative inotropic effect of phalloidin but blocked the fast potassium permeability in the same concentration range and the same voltage-dependent manner as did phalloidin. N-Acetyl-secophalloidin exhibited a strongly attenuated blocking effect on K+ permeability in a 1,000-fold higher concentration range than phalloidin. Neither antamanide nor N-acetyl-secophalloidin affected the contractile properties. These results suggest the existence in the frog muscle membrane of a receptor with two sites for phalloidin and antamanide which acts on potassium conductance.

The voltage-dependent blocking effect of phalloidin on the delayed potassium current of voltage-clamped frog skeletal muscle fibres

The effects of phalloidin (10(-14)-10(-6) M) were tested on voltage-clamped isolated frog muscle fibres. The toxin reversibly blocked the potassium current similarly in both detubulated and intact fibres. Neither the reversal potential nor the activation curve of the current were affected by the toxin (10(-8) M). The inactivation curve was shifted toward negative values at holding potentials more than +20 mV from the reference potential. This shift enhanced the potential-dependent facilitation of the current block observed between -40 and +40 mV from the reference holding potential (the higher the depolarization, the greater the blocking effect, which reached 100% at +40 mV). Contrary to what was seen with the current, hyperpolarization did not relieve the mechanical block. The effect of phalloidin did not seem to be frequency-dependent.