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.