Non-cholinergic lethality following intravenous injection of carbamate insecticide in rabbits

Propoxur: a novel mechanism for insecticidal action and toxicity

Propoxur is a carbamate insecticide that has recently attracted considerable attention as a possible treatment option for addressing the bedbug epidemic. The generally accepted mechanism of toxicity for propoxur involves the inhibition of ChE, as is the case for many agents in the category. Considerable research supports the concept that most physiologically active substances induce their effects through multi-faceted action. In this review, we provide evidence that ET--ROS--OS participate mechanistically in both the action and in human toxicity of pesticides, including propoxur. Propoxur is a catechol derivative that contains carbamate and isopropyl groups on the oxygens in its moiety. Metabolic studies with propoxur reveal hydrolysis of the carbamate and dealkylation of the isopropyl group to yield the parent catechol. In addition, nuclear hydroxylation produces a hydroquinone derivative. Both the catechol and this hydroquinone derivative are potentially able to undergo redox cycling with the corresponding quinone to produce ROS. It is primarily for these reasons that we believe propoxur may be similar to other classes of physiologically active compounds in producing effects through ET-ROS-OS. Generally, reactive ROS are generated by metabolic processes that yield ET entities, and this occurs with propoxur as well. Although ROS are commonly associated with toxicity, there is little recognition in the literature that they can also play a role in therapeutic action.

Functional assays for neurotoxicity testing

Neurobehavioral and pathological evaluations of the nervous system are complementary components of basic research and toxicity testing of pharmaceutical and environmental chemicals. While neuropathological assessments provide insight as to cellular changes in neurons, behavioral and physiological methods evaluate the functional consequences of disruption of neuronal communications. The underlying causes of certain behavioral alterations may be understood, but many do not have known direct associations with specific brain pathologies. In some cases, however, rapidly expanding mouse models (transgenic, knock-out) are providing considerable information on behavioral phenotypes of altered pathology. Behavior represents the integrated sum of activities mediated by the nervous system, and functional tests used for neurotoxicity testing tap different behavioral repertoires. These tests have an advantage over pathologic measures in that they permit repeated evaluation of a single animal over time to determine the onset, progression, duration, and reversibility of a neurotoxic injury. Functional assays range from a screening-level battery of tests to refined procedures to tap specific forms of learning and/or memory. This article reviews common procedures for behavioral toxicity testing and provides examples of chemical-specific neurobehavioral-pathological correlations in order to inform interpretation and integration of neuropathological and behavioral outcomes.

A carbamate-type cholinesterase inhibitor 2-sec-butylphenyl N-methylcarbamate insecticide blocks L-type Ca2+ channel in guinea pig ventricular myocytes

2-sec-Butylphenyl N-methylcarbamate (BPMC) is a carbamate-type cholinesterase (ChE) inhibitor with unique toxicological properties such as noncholinergic cardiovascular collapse. Effects of BPMC on L-type Ca2+ channel currents (ICa(L)) were studied in isolated guinea pig ventricular myocytes using the whole-cell patch-clamp technique, since the examination of cardiovascular responses indicated its Ca2+ antagonistic action. BPMC induced bradycardic and hypotensive responses in vivo and inhibited contraction of isolated papillary muscles (IC50 = 1.3 x 10(-4) M) in guinea pigs. BPMC produced reversible block of ICa(L) in the concentration range of 10(-4) - 10(-3) M. At test potentials between -30 mV and +20 mV, BPMC at 3 x 10(-4) M caused marked acceleration of decay rate of ICa(L) with moderate reduction of peak ICa(L) amplitude. BPMC (3 x 10(-4) M) shifted the steady-state inactivation curve to the hyperpolarizing direction by 12.7 mV. Decay rate of Ba2+ currents (IBa(L)) was also accelerated by BPMC. Fitting analysis of inactivation kinetics of IBa(L) with a two-exponential equation revealed that BPMC accelerates the slow inactivation component. At concentrations for blocking peak IBa(L) by ca. 30%, the inactivation kinetics of IBa(L) were significantly accelerated by BPMC, but merely slightly accelerated by Ca2+ channel antagonists such as diltiazem, nifedipine, or verapamil. These results indicate that BPMC, in addition to the inhibition of ChE, blocks L-type Ca2+ channels by accelerating voltage-dependent inactivation.

Pharmacological analysis of noncholinergic action of 2-sec-butylphenyl N-methylcarbamate insecticide on the isolated rabbit aorta

We investigated noncholinergic actions of 2-sec-butylphenyl N-methylcarbamate (BPMC) on the isolated rabbit thoracic aorta to help determine the mechanisms responsible for its unique toxicological properties, which are characterized by cardiovascular collapse and low lethality compared to its anticholinesterase (anti-ChE) activity. BPMC inhibited K(+)-induced contraction more effectively than it did norepinephrine (NE)-induced contraction. The inhibitory effect on K(+)-induced contraction was not altered by changing the external K(+) concentration, but it was decreased by adding an L-type Ca(2+) channel agonist, BAY K 8644. Simultaneous measurement of tension and cytosolic Ca(2+) levels elevated by K(+) stimulation revealed that BPMC decreased the Ca(2+) levels prior to and parallel to the tension. The magnitude of the inhibitory effect on Ca(2+) levels was increased by treating BPMC before Ca(2+) application in the depolarized preparation. However, BPMC did not inhibit caffeine- or NE-induced transient contraction in Ca(2+)-free medium. On the other hand, BPMC produced tonic contraction in the resting aorta. The contraction to BPMC did not develop after removing the adventitia. The contraction was inhibited by phentolamine or guanethidine but not by atropine or tetrodotoxin. These results suggest that BPMC inhibits depolarization- or agonist-induced contraction by inhibiting Ca(2+) entry through L-type Ca(2+) channels, whereas it produces vascular contraction in the resting state by releasing NE from sympathetic nerve terminals. These apparently opposing mechanisms may contribute to the unique noncholinergic toxicological properties of BPMC.

The role of cholinergic and noncholinergic mechanisms in the cardiorespiratory failure produced by N-methylcarbamate cholinesterase inhibitors in rabbits

We investigated the relative contribution of several cardiorespiratory components to acute lethality produced by N-methylcarbamate cholinesterase (ChE) inhibitors physostigmine, 2-sec-butylphenyl methylcarbamate (BPMC), and 2-isopropoxyphenyl methylcarbamate (PHC) in halothane-anesthetized rabbits. Intravenous injection of these compounds produced dose-dependent pressor and/or depressor responses related to each compound. A lethal dose of physostigmine resulted in cardiovascular collapse after a pressor response. That of PHC produced cardiovascular collapse after biphasic effects on blood pressure, a transient decrease followed by an increase. Unlike these compounds, BPMC elicited a rapidly developing depressor response followed by cardiovascular collapse. Artificial ventilation prevented cardiovascular collapse and lethal actions to physostigmine and PHC, but not BPMC. A degree of acute lethality to physostigmine and PHC depended on their anti-ChE activity, whereas BPMC exhibited a low degree of lethality relative to its anti-ChE activity. While the pressor response to physostigmine and PHC was ascribed to an atropine-sensitive increase in cardiac contractility, the depressor response to PHC and BPMC was attributed to an atropine-insensitive decrease in cardiac contractility and/or vascular resistance. Similar to the order for eliciting the depressor response in vivo, all three compounds inhibited contraction of the isolated cardiac and aortic smooth muscles with the order of their inhibition in terms of anti-ChE activity, i.e., BPMC > PHC > physostigmine. Thus, the primary cause of death with physostigmine and PHC is respiratory arrest subsequent to ChE inhibition, whereas BPMC exhibiting the low degree of lethality causes cardiovascular collapse mediated through direct inhibitory effects on cardiac and vascular smooth muscle contraction.

Absence of a protective effect of the oxime 2-PAM toward paraoxon-poisoned honey bees: acetylcholinesterase reactivation not at fault

We investigated the failure of 2-PAM to protect honey bees against poisoning with paraoxon. The protective effect of the oxime 2-PAM against inhibition of acetylcholinesterase (AChE) by paraoxon was estimated in vitro and in vivo and was correlated with the mortality of paraoxon-treated bees. In vitro, 2-PAM protected 90% of AChE activity in the presence of paraoxon and reactivated more than 90% of inhibited AChE. Minor soluble and major membrane-bound forms of bee AChE presented about similar extents of reactivation, but the first order rate constant of reactivation (kobs) of the soluble form is threefold higher than that of the membrane-bound form. However, this difference did not significantly influence the reactivation kinetics of total AChE; the constant kobs of the membrane-bound form reflected that of total AChE. The linear kinetic profile of total AChE reactivation supported the conclusion that there was a single population of reactivatable species. The bimolecular rate constant of reactivation (kr), the dephosphorylation rate constant (k2), and the dissociation constant (Kd) were 646 M-1.min-1, 0.84 min-1 and 1. 30 mM, respectively. In vivo, administration of 2-PAM, after paraoxon exposure, induced a complete protection of AChE activity, but did not elicit any significant effect on mortality in paraoxon-treated bees. The inefficiency of 2-PAM to antagonize paraoxon-induced mortality was not changed by the administration of 2-PAM in pretreatment-therapy and in therapy treatments. These results indicated that the mortality of paraoxon-poisoned honey bees was not due to a lack of AChE reactivation.