The action of pyrethroids on sodium channels in myelinated nerve fibres and spinal ganglion cells of the frog.
The interaction of pyrethroids with the voltage-dependent sodium channel was studied in voltage-clamped nodes of Ranvier and isolated spinal ganglion neurons of the clawed frog, Xenopus laevis. In the node, pyrethroids prolonged the sodium tail current associated with a step repolarization of the membrane. It was found that the amplitude of the slow, pyrethroid-induced, sodium tail current (PIT) first increased and then decreased as a function of the duration of membrane depolarization (to -5 mV). This decrease of the PIT amplitude was absent when depolarizations to the sodium equilibrium potential (+40 mV) were used. Measurements of changes in sodium reversal potential indicated that sodium ion depletion in the perinodal space is largely responsible for the inactivation of the pyrethroid-modified sodium current. Inactivation is not completely abolished by pyrethroid treatment since the probability of channel opening, measured in membrane patches excised from spinal ganglion cells, decreased slowly during prolonged depolarization. Analysis of unitary currents indicated that both activation and inactivation are retarded by pyrethroids. The arrival of sodium channels in the pyrethroid-modified open state followed a time course that was slower than both activation and inactivation of unmodified sodium channels. Our findings indicate that sodium channels are modified when in the closed resting state and that both opening and closing kinetics are delayed by pyrethroids.
de Weille
J R
JR
Department of Veterinary Pharmacology, Pharmacy and Toxicology, University of Utrecht, The Netherlands.
Leinders
T
T
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D005727:Ganglia, Spinal / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D009413:Nerve Fibers, Myelinated / Q000187:drug effects / Q000502:physiology*; D011722:Pyrethrins / Q000494:pharmacology*; D011901:Ranvier's Nodes / Q000187:drug effects / Q000502:physiology*; D012584:Sciatic Nerve; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology*; D014982:Xenopus laevis / Q000502:physiology*
D011722:Pyrethrins; D015222:Sodium Channels