Effect of chlorophos (dipterex, trichlorphon) on high-threshold potassium and calcium channels of the neuronal membrane

Abstract

The effect of chlorophos (dipterex, trichlorphon) on high-threshold potassium and calcium currents is studied on isolated snail neurons using the patch-clamp technique. Chlorophos (10-I000 ~mol/liter) is found to reversibly lower the peak amplitude of a high-threshold potassium current by 30% on average and exerts two independent effects on a high-threshold calcium current: reversible lowering of the peak amplitude by 35% on average and, in 30% of cases, reversible inhibition of its activation, inactivation, and deactivation. This effect is abolished by adding diltiazem (a calcium channel antagonist) in a concentration of 100 p.mol/liter to the medium. Key Words: insecticides; chlorophos; potassium channels; calcium channels; snarl neuronsThe mechanisms by which organic pesticides exert their neurotoxic effect are currently being studied by neuropharmacologists. It has been demonstrated that critical targets of organic pesticides in nervous tissue are ion channels of the surface cell membrane [1]. Close attention in this respect has been paid to the pyrethroids and organochlorines. Pesticides of these classes are able to lower current amplitude through voltage-dependent calciu m [12,14,15], potassium [12][13][14]16], and chlorine [10] channels and modulate sodium channel gating in a complex fashion by inhibiting activation and deactivation of these channels [1,8,14,17].At the same time, the effect of organophosphorus pesticides (dichlorvos, chlorophos, malathion, etc.) on ion channels remains unclear. These compounds are effective inhibitors of acetylcholinesterase, which is thought to be the chief mechanism of their neurotoxic action [4,6,7,11,18]. However, some neurotropic effects of organophosphorus compounds cannot be attributed to the above mechanism, for instance, chlorophos-induced depolarization of the presynaptic terminal [7] or lowered rate of impulse transmission [5,7]. These changes may be due to the action of the pesticide on the ionic conductivity of the neuronal membrane.The aim of the present study was to investigate the effect of chlorophos on the voltage-dependent potassium and calcium channels of the common snail. MATERIALS AND METHODSThe experiments were carried out on isolated nonidentified neurons of the snail Helix pomatia using the patch-clamp technique. Two microelectrodes filled with potassium citrate (2 mol/liter) were introduced into the cell to fix the potential and to record the transmembrane currents. The solution used for recording the potassium currents contained (in mmol/ liter): 100 NaCI, 4 KCI, 5 CaCI 2, 4 MgCI 2, and 5 tris-(oxymethyl)-aminomethane, and that for recording the calcium currents 4 KCI, 10 CaCI 2, 4 MgCI 2, 5 tris-(oxymethyl)-aminomethane, and 95 tetraethylammo-

Discovery and identification of O, O-diethyl O-(4-(5-phenyl-4, 5-dihydroisoxazol-3-yl) phenyl) phosphorothioate (XP-1408) as a novel mode of action of organophosphorus insecticides

“…Ca 2+ -dependent K + currents also had been reported to underlie the mechanism of hyperexcitability induced by paraoxon in snail neurons 8 . However, in toxicology, there were rare studies about the relationship between AChE inhibitors such as OP insecticides with I K(A) and I K(DR channels 35 – 37 .…”