Temperature- and structure-dependent interaction of pyrethroids with the sodium channels in frog node of Ranvier

Abstract

(1) The interaction of a series of pyrethroid insecticides with the Na + channels in myelinated nerve fibres of the clawed frog, Xenopus laevis, was investigated using the voltage clamp technique. (2) Out of 11 pyrethroids 9 insecticidally active compounds induce a slowly decaying Na + tail current on termination of a step depolarization, whereas the Na + current during depolarization was hardly affected. These tail currents are most readily explained by a selective reduction of the rate of closing of the activation gate in a fraction of the Na + channels that have opened during depolarization. (3) The rate of decay of the Na + tail current varies considerably with pyrethroid structure. After α-cyano pyrethroids the decay is at least one order of magnitude slower than after non-cyano pyrethroids. The decay always follows a single-exponential time course and is reversibly slowed when the temperature is lowered from 25 to 0°C. Arrhenius plots in this temperature range are linear. (4) These results indicate that the relaxation of the activation gate in pyrethroid-affected Na + channels is governed by an apparent first order, unimolecular process and that the rate of relaxation is limited by a single energy barrier. Application of transition state theory shows that after α-cyano pyrethroids this energy barrier is 9.6 kJ/mol higher than after non-cyano pyrethroids. (5) Differences in rate of decay of the Na + tail current account for the reported differences in repetitive nerve activity induced by various pyrethroids. In addition, the effect of temperature on the rate of decay explains the increase in repetitive activity with cooling.