Vol. 298, Issue 3, 1067-1082, September 2001

Actions of Pyrethroid Insecticides on Sodium Currents, Action Potentials, and Contractile Rhythm in Isolated Mammalian Ventricular Myocytes and Perfused Hearts

C. Ian Spencer¹ , Kathryn H. Yuill, John J. Borg, Jules C. Hancox and Roland Z. Kozlowski

Departments of Pharmacology (C.I.S., J.J.B., R.Z.K.), Physiology, and Cardiovascular Research Laboratories (K.H.Y., J.C.H.), School of Medical Sciences, University of Bristol, University Walk, Bristol, United Kingdom

Pyrethroid insecticides are known to modify neuronal sodium channels, inducing persistent, steady-state sodium current at depolarized membrane potentials. Cardiac myocytes are also rich in sodium channels but comparatively little is known about the effect of pyrethroids on the heart, or on the cardiac sodium channel isoform. In the present study therefore, we determined the actions of type I and type II pyrethroids against rat and guinea pig ventricular myocytes under current and voltage clamp, and on isolated perfused rat hearts. In myocytes, tefluthrin (type I) and fenpropathrin and -cypermethrin (type II) prolonged action potentials and evoked afterdepolarizations. The time course of sodium current (I_Na) was also prolonged by these compounds. Pyrethroids delayed I_Na inactivation, when measured under selective conditions as current sensitive to 30 µM tetrodotoxin, by increasing the proportion of slowly inactivating current at the expense of fast inactivating current. Further experiments, focusing on fenpropathrin, revealed that its effects on I_Na inactivation time course were dose-dependent, and the Na⁺ "window-current" was increased in its presence. In unstimulated, isolated hearts perfused with the same pyrethroids, the variability in contraction amplitude increased due to variations in the intervals between heartbeats. These potentially arrhythmogenic changes are consistent with the effects observed at the cellular level. The type I pyrethroid tetramethrin had little effect in any of the preparations. These findings suggest that some pyrethroids possess considerable mammalian cardiac arrhythmogenic potential, the manifestation of which in vivo may depend on the route of exposure.