The catalytic domain of human neuropathy target esterase mediates an organophosphate-sensitive ionic conductance across liposome membranes.

In humans and other vertebrates, reaction of organophosphates with a neuronal membrane protein, neuropathy target esterase (NTE), initiates events which culminate in axonal degeneration. The initiation process appears to involve modification of a property of the protein distinct from its esterase activity, subsequent to formation of a negatively charged adduct with the active site serine residue. Here, we show that membrane patches from liposomes containing NEST, a recombinant hydrophobic polypeptide comprising the esterase domain of human NTE, display a transmembrane ionic conductance with both stable and high-frequency flickering components. An asymmetric current-voltage relationship suggested that ion flow was favoured in one direction relative to the membrane and its associated NEST molecules. Flow of anions was slightly favoured compared with cations. The flickering current formed a much larger proportion of the overall conductance in patches containing wild-type NEST compared with the catalytically inactive S966A mutant form of the protein. The conductance across patches containing NEST, but not those with the S966A mutant, was significantly reduced after adding neuropathic organophosphates to the bathing medium. By contrast, non-neuropathic covalent inhibitors of the catalytic activity of NEST did not reduce NEST-mediated conductance. Future work may establish whether NTE itself mediates an organophosphate-sensitive ion flux across intracellular membranes within intact cells.

Pyrethroid insecticides: poisoning syndromes, synergies, and therapy.

BACKGROUND: Pyrethroid insecticides are widely used, but there have been relatively few reports of systemic poisoning. These reports have, however, shown that pharmacotherapy is difficult and that the duration of poisoning can be unexpectedly long. Pyrethroids are ion channel toxins prolonging neuronal excitation, but are not directly cytotoxic. Two basic poisoning syndromes are seen. Type I pyrethroids produce reflex hyperexcitability and fine tremor. Type II pyrethroids produce salivation, hyperexcitability, choreoathetosis, and seizures. Both produce potent sympathetic activation. Local effects are also seen: skin contamination producing paresthesia and ingestion producing gastrointestinal irritation. The slow absorption of pyrethroids across the skin usually prevents systemic poisoning, although a significant reservoir of pyrethroid may remain bound to the epidermis. Carboxyesterase inhibitors can enhance pyrethroid toxicity in high-dose experimental studies. Hence, the unauthorized pyrethroid/organophosphate mixtures marketed in some developing countries may precipitate human poisoning. Pyrethroid paresthesia can be treated by decontamination of the skin, but systemic poisoning is difficult to control with anticonvulsants. Pentobarbitone, however, is surprisingly effective as therapy against systemic type II pyrethroid poisoning in rats, probably due to its dual action as a chloride channel agonist and a membrane stabilizer.

The role of voltage-gated chloride channels in type II pyrethroid insecticide poisoning.

Pyrethroids act on mammalian sodium channels, but we have previously shown that low concentrations of the type II pyrethroid deltamethrin also decrease the open channel probability (P(o)) of voltage-gated chloride channels. This effect would be expected to amplify the sodium channel-mediated signs of poisoning produced by pyrethroids. In the present study we evaluated potential chloride channel agonists in vitro, and then tested the most effective of these on pyrethroid-poisoned rats to determine the practical significance of chloride channel effects in vivo. Patch clamp experiments showed that, for voltage-gated maxi chloride channels in excised, inside-out patches from mouse N1E 115 neuroblastoma cells, ivermectin (10(-7) M) and pentobarbitone (10(-6) M) significantly increased open channel probability (p

Actions of pyrethroid insecticides on voltage-gated chloride channels in neuroblastoma cells.

Pyrethroid insecticides have the potential to act at several sites in excitable tissues in addition to their primary effect on sodium channels. We have assessed one such site, the voltage gated chloride channel. Following our previous demonstration of actions of deltamethrin on the voltage-gated chloride channel in rats and in neuroblastoma cells, we examined the dose-response relationship for deltamethrin by patch clamp analysis of voltage-gated channels in partially differentiated NIE115 cells. Open channel probability (P0) was assessed in channels stepped from 0 to +20 mV for 10 seconds. Deltamethrin significantly decreased P0 by 0.237+/-0.070 at 10(-10) M and, although construction of a dose-response relationship was rendered difficult by the very low water solubility of the pyrethroids, a decrease of 0.968+/-0.140 was reached at the highest concentration tested, 10(-4) M. A second Type II pyrethroid cypermethrin also decreased P0, by 0.430+/-0.09 at 5 x 10(-6) M, but the Type I pyrethroid cismethrin produced a decrease of only 0.188+/-0.08 at 2 x 10(-5) M. These effects were seen in 340 pS conductance, calcium-independent channels. A sample of 9 calcium-dependent channels with a conductance of 225 pS failed to show any response to deltamethrin. We conclude that actions on calcium-independent voltage-gated chloride channels are likely to contribute significantly to type II pyrethroid toxicity.

Inhibition of a neuronal voltage-dependent chloride channel by the type II pyrethroid, deltamethrin.

Following the previous finding that the Type II pyrethroid, deltamethrin, increased membrane resistance in peripheral nerve and muscle in a chloride-dependent manner, the action of deltamethrin on neuronal voltage-dependent chloride channels was assessed using inside-out patches from NIE-115 neuroblastoma cells. These were bathed in symmetrical solutions, containing 149 mM chloride and the membrane potential stepped from 0 mV to voltages ranging from +/- 10 to 80 mV for 2 or 5 sec. Active patches contained large conductance channels (343 +/- 11 pS, n = 8), which inactivated relatively slowly during the voltage step and could be resolved into a number of substates. The channels were confirmed as being chloride specific on the basis of substitution experiments with isethionate and pharmacological blockade by 9-anthracene carboxylic acid (9-ACA). Within 20 min of adding deltamethrin (2 microM) to the bath solution, open channel probability (Po) fell from 0.50 +/- 0.06 to 0.24 +/- 0.04 (n = 11) a highly significant result. Glycerinformal solvent alone (0.1% v/v) caused a non-significant rise to 0.65 +/- 0.09 (n = 4). The decreased open channel probability after deltamethrin was due to an increased incidence of both the closed channel state and low conductance substates. In addition, deltamethrin frequently caused flickering between substrates similar to that seen after 9-ACA. Deltamethrin did not change single channel conductance, current-voltage relationship or time-dependent channel inactivation, but decreased open channel probability over the complete range of membrane voltage tested.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel action of deltamethrin on membrane resistance in mammalian skeletal muscle and non-myelinated nerve fibres.

The pyrethroids, deltamethrin and cismethrin, were assessed for their ability to change membrane conductance directly in skeletal muscle and indirectly in non-myelinated nerve fibre preparations from the rat. In diaphragm muscle fibres of the rat, input resistance was significantly increased (35%) by deltamethrin but not by cismethrin, compared with solvent alone. In perfused vagus nerve from the rat, the amplitude of the post-tetanic hyperpolarization was significantly increased (100%) by deltamethrin both in vitro and ex vivo but not by cismethrin or solvent. In both test systems the actions of deltamethrin were abolished by changing the perfusate to a low chloride solution. The enhancement of amplitude of post-tetanic hyperpolarization by deltamethrin was reversed by ivermectin, a compound known to increase the resting chloride flux in neuronal preparations. Depolarizing afterpotentials, indicative of a prolongation in sodium conductance, did not develop until 0.5-1.0 hr after the enhancement of the amplitude of post-tetanic hyperpolarization by deltamethrin in the vagus preparations. The amplitude of post-tetanic hyperpolarization was not enhanced by exposure of the vagus to veratrine. These observations reinforce the conclusion that the enhancement of post-tetanic hyperpolarization by deltamethrin is not the result of intracellular accumulation of sodium. In addition, the chloride-dependent nature of the effects of deltamethrin, in both muscle and non-myelinated nerve, suggests that they are in both cases due to a reduction in resting membrane chloride conductance. This novel action of deltamethrin would be expected to amplify the effect of prolonged sodium current and thus influence the actions on excitable membranes both directly and indirectly.

The effects of two types of pyrethroid on rat skeletal muscle.

The mechanical and electrophysiological effects of two synthetic pyrethroids, cismethrin (Type I) and deltamethrin (Type II) have been studied in whole rat gastrocnemius muscle and in isolated perfused rat diaphragm. In both directly or indirectly stimulated preparations deltamethrin but not cismethrin had a positive inotropic effect which in the case of the whole animal could be reversed by increasing the stimulation frequency. In the isolated diaphragm the deltamethrin effect could be prevented by TTX suggesting it is primarily the result of an increased sodium conductance. Deltamethrin also reversed a partial curare blockade in the diaphragm. Electrophysiological studies showed that the inotropic effect of deltamethrin is accompanied by repetitive muscle action potentials. In addition deltamethrin significantly increased muscle fibre input resistance, a novel finding which may underlie the curare reversal effect. It is concluded that the effectiveness of deltamethrin compared with cismethrin in causing repetitive firing in the muscle membrane is attributable to the greater duration of the after-depolarization produced by deltamethrin.

Pharmacological effects of pyrethroids on the cardiovascular system of the rat.

The cardiovascular actions of two pyrethroids, deltamethrin and cismethrin, were assessed using the pithed rat, the isolated working heart, and perfused mesentery preparations. Deltamethrin but not cismethrin, increased mean arterial pressure and differential pressure in the pithed rat, and the aortic output and mean systolic aortic pressure in the working heart. Reserpine pretreatment reduced, but did not abolish, the pressor response to deltamethrin in the pithed rat and working heart, but did not reduce the increase in aortic output following deltamethrin in the working heart. In the perfused mesentery, deltamethrin did not modify the action of exogenous noradrenaline, but increased the response to 10 Hz stimulation. It is concluded that the cardiovascular effects of deltamethrin are due to both increased catecholamine release in peripheral vascular beds, and to a direct positive inotropic effect on the heart. The results also provide a distinction between the action of the two pyrethroids in terms of their cardiovascular activity.

Cardiac sensitization induced by phenobarbitone and prolonged by CS2.

The arrhythmogenic effect of 8 microgram/kg noradrenaline given i.v. was increased in male rats pretreated 1-2 days earlier with phenobarbitone and starved from the time of the first phenobarbitone injection (80 mg/kg followed by 50 mg/kg 6 h later). Daily exposure to 4.0 mg/l CS2 (first exposure 24 h after the first phenobarbitone injection) for 4 h prevented the decline in susceptibility on the 3rd and 4th days after phenobarbitone, when the reaction of unexposed rats to noradrenaline returned to normal.

The inhibitory effect of cadmium on neuromuscular transmission in the rat.

Cadmium (0.125-1 mM) was found to inhibit the isometric response of the isolated rat hemidiaphragm during indirect stimulation, but not during direct stimulation. This effect of cadmium (1 mM) was completely reversed by ethyleneglycol bis-(aminoethyl)-N,N,N',N'-tetra-acetic acid (2 mM) or by L-cysteine (2 mM) but only partially by increased calcium. Cadmium (10 micronM) significantly reduced the quantal release of transmitter in the isolated phrenic diaphragm and a concentration of 0.1 mM frequently caused a complete failure of the endplate response after 30 min. The effect of cadmium on neuromuscular transmission could not be readily reversed by washing with cadmium-free solution. Miniature endplate potential frequency and amplitude were not significantly affected by cadmium (0.1 or 0.5 mM). The results suggest that the effect of cadmium on the isolated phrenic nerve-diaphragm is due largely to inhibition of calcium function at presynaptic nerve terminals.

A study of the cardiac effects of DDT in laboratory animals.

The claim that the hearts of animals chronically exposed to DDT are unduly sensitive to the cardiotoxic effects of vasopressin was reexamined. Rats and rabbits were fed a diet containing DDT for 8 months and were given weekly doses of vasopressin, which causes a temporary myocardial ischaemia. Electrocardiograms, recorded at 2-week intervals, showed no significant increase in the incidence of cardiac arrhythmias in the DDT-fed animals. Intravenous noradrenaline given at the end of the 8-month period did not produce a greater incidence of arrhythmias in the DDT-fed animals than in the controls. Isolated heart muscle preparations from the DDT-fed and control rats did not differ in their peak developed tensions and maximum rates of tension development. DDT, TDE, and DDE levels in heart muscle and fat were below detectable levels in the control rats, whereas in the DDT-fed rats they were at least 100 times those normally found in human tissues.