Interactions of the pyrethroid fenvalerate with nerve membrane sodium channels: temperature dependence and mechanism of depolarization

Depolarization of nerve membranes is an important component of the mode of action of pyrethroids, and its negative temperature dependence parallels that of insecticidal activity. We studied the mechanism and temperature dependence of depolarization of crayfish giant axons by pyrethroids, using intracellular microelectrode and voltage clamp techniques. Membrane depolarization caused by tetramethrin and fenvalerate was greater at 10 degrees C than at 21 degrees C, and was reversible upon changing the temperature. Short-duration depolarizing pulses in voltage-clamped fenvalerate-treated axons induced prolonged sodium currents that are typical of other pyrethroids, but the decay of the tail current following repolarization was extremely slow, lasting several minutes at the large negative holding potential of -120 mV. At the normal resting potential, the tail current did not decay completely, and even without stimulation, a steady-state sodium current developed, which could account for the depolarization. The steady-state current induced by fenvalerate at the resting potential was much larger at 8 degrees C than at 21 degrees C, accounting for the negative temperature dependence of the depolarization. The negative temperature dependence of the steady-state current seems to be due ultimately to the great stabilizing effect of low temperature on the open-modified channel. When the steady-state current was induced at the resting potential, hyperpolarization to more negative potentials caused it to decay with exactly the same time course as tail currents induced by short-duration depolarizing pulses, indicating that both types of currents are carried by identically-modified channels. The modified channels were shown to be inactivated very slowly at potentials more positive than - 100 mV, accounting for the limited depolarization observed in micro-electrode experiments. Even when applied directly to the internal face of the membrane, the effect of fenvalerate on the sodium channel developed slowly, taking more than 90 min to reach its final level. Fenvalerate did not significantly affect potassium currents.