Action of pyrethroids on a nerve-muscle preparation of the clawed frog, Xenopus laevis

Abstract

The effects of a range of pyrethroids on end-plate potentials and muscle action potentials were studied in the pectoralis nerve-muscle preparation of the clawed frog, Xenopus laevis. The noncyano pyrethroids allethrin, cismethrin, bioresmethrin, and IR- cisphenothrin caused moderate presynaptic repetitive activity only, resulting in the occurrence of multiple end-plate potentials (epps). Trains of repetitive muscle action potentials without presynaptic repetitive activity were observed after the α-ethynyl pyrethroid S-5655 and after the α-cyano pyrethroids cypermethrin, deltamethrin, FCR 1272, and FCR 2769. An intermediate group of pyrethroids consisting of the non-cyano compounds 1R-permethrin, des-cyano-deltamethrin, NAK 1901 and NAK 1963, and the α-cyano pyrethroids cyphenothrin and fenvalerate caused both types of effect. The insecticidally inactive S-enantiomers of permethrin had no effect on the nerve-muscle preparation. Trains of repetitive action potentials in pyrethroid-treated muscle fibers were followed by a depolarizing afterpotential which in general decayed more rapidly for the non-cyano pyrethroids than for the α-cyano pyrethroids. The rate of decay of the depolarizing afterpotential decreased gradually as the temperature was lowered, whereas the pre- and postsynaptic repetitive activity remained largely unaffected over a large temperature range. It is concluded that in muscle membrane like in nerve membrane the pyrethroid-induced repetitive activity is due to a prolongation of the sodium current and that a clear distinction between non-cyano pyrethroids on the one hand and α-cyano compounds on the other cannot be made on the basis of the present results.