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D M Maxwell

Publications and source records attributed to D M Maxwell.

50 records · Page 3Linked to original sources
PubMed1988

Effect of carboxylesterase inhibition on carbamate protection against soman toxicity.

The ability of the carbamates pyridostigmine and physostigmine to protect against the lethal effects of soman, an extremely toxic anticholinesterase agent, was measured in rats, guinea pigs and rabbits. Pharmacologically equivalent doses of these carbamates that inhibited 70% of the blood acetylcholinesterase in each species were injected i.m. 25 min before s.c. injection with soman. Pretreatment with either carbamate, in combination with 17.4 mg/kg of atropine, produced protection against soman toxicity in all species. When protection was expressed as the ratio between the soman LD50 values in carbamate-protected animals and control animals, this protective ratio varied 3-fold between species (2.1-6.1 for pyridostigmine; 2.2-6.6 for physostigmine). When protection was expressed as the difference in the soman LD50 values between carbamate-protected animals and control animals, this protective difference was consistent among species (126 +/- 19 micrograms/kg). Species variation in protective ratios was observed largely because the control LD50 values defining soman toxicity in unprotected animals varied among species (20 micrograms/kg in rabbits, 28 micrograms/kg in guinea pigs and 126 micrograms/kg in rats). The species variation of the soman LD50 values in control animals was eliminated by pretreating animals with cresylbenzodioxaphosphorin oxide, which reduced the species variation in soman detoxification. The LD50 values for soman in cresylbenzodioxaphosphorin oxide-treated animals (9.8-15.6 micrograms/kg) did not differ significantly between species. Similarly, protective ratios for carbamates against soman in cresylbenzodioxaphosphorin oxide-treated animals were also clustered in a narrow range (8.5-11.4 for pyridostigmine; 9.0-13.4 for physostigmine) that did not differ significantly, regardless of species or carbamate.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholinesterase↗
PubMed1987

The effects of blood flow and detoxification on in vivo cholinesterase inhibition by soman in rats.

The in vivo time course of cholinesterase inhibition was measured in brain, lung, spleen, hind limb skeletal muscle, diaphragm, intestine, kidney, heart, liver, and plasma of rats receiving 90 micrograms/kg soman, im. This dose of soman produced severe respiratory depression and transient hypertension, but no significant changes in the cardiac output or heart rate of anesthetized rats. The rate and maximal extent of in vivo cholinesterase inhibition by soman varied widely among the tissues. Although cardiac output was unchanged by soman administration, the blood flow in heart, brain, and lung (bronchial arterial flow and arteriovenous shunts) was increased, whereas blood flow in spleen, kidney, and skeletal muscle was decreased. The relative importance of tissue blood flow, tissue levels of cholinesterase and acetylcholinesterase, and tissue levels of soman-detoxifying enzymes (diisopropyl-fluorophosphatase and carboxylesterase) in determining the in vivo rate and maximal extent of cholinesterase inhibition was examined by multiple regression analysis. The best multiple regression model for the maximal extent of cholinesterase inhibition could explain only 63% of the observed variation. The best multiple regression model for the in vivo rate of cholinesterase inhibition contained three independent variables (blood flow, carboxylesterase, and cholinesterase) and could account for 94% of the observed variation. Of these three variables blood flow was the most important, accounting for 79% of the variation in the in vivo rate of cholinesterase inhibition. This suggests that it may be possible to use a flow-limited physiological pharmacokinetic model to describe the kinetics of in vivo cholinesterase inhibition by soman.

Acetylcholinesterase↗
PubMed1987

The effect of carboxylesterase inhibition on interspecies differences in soman toxicity.

Subcutaneous administration of 2 mg/kg cresylbenzodioxaphosphorin oxide (CBDP) produced complete inhibition of carboxylesterase activity in plasma and lung of mice, rats, guinea pigs and rabbits, without inhibition of acetylcholinesterase activity in either brain or diaphragm. This CBDP treatment also reduced the subcutaneous soman LD50 in these species by 48-90% in comparison to the soman LD50 in control animals. The interspecies differences in the soman LD50 values that were seen in control animals were absent in CBDP-treated animals. The soman LD50 values in control animals were 125 micrograms/kg (mouse), 116 micrograms/kg (rat), 32.3 micrograms/kg (guinea pig) and 22.8 micrograms/kg (rabbit), whereas the soman LD50 values in CBDP-treated animals from these species were clustered in a narrow dose range (11.8-15.6 micrograms/kg) and were not significantly different. This suggests that the amount of CBDP-sensitive carboxylesterase available for detoxification of soman in each species may be an important determinant of interspecies differences in soman toxicity.

Animals↗
PubMed1986

A simple organ bath for electrical stimulation and superfusion of rat brain slices.

A simple organ bath was designed for the rapid screening of drugs using electrically stimulated brain slices. Control parameters for the release of 3H-5-HT from rat caudate slices using a simple pulse sequence were developed. Field-stimulated release of serotonin was shown to be calcium dependent and was not inhibited by tetrodotoxin. Preliminary experiments into the inhibition of 3H-5-HT release by lysergic acid diethylamide showed moderate inhibition of electrically stimulated overflow from rat caudate at low concentrations. The ratio of postdrug tritium overflow (S2) to predrug tritium overflow (S1) was determined and used as the measure of inhibition in these experiments. An investigation into drug-stimulated 3H-5-HT release from rat hippocampal slices by methylenedioxyamphetamine revealed significant stimulation of tritium overflow. Overall the new organ bath has fulfilled our need for a rapid central nervous system screening procedure by demonstrating experimental versatility and reproducible results.

3,4-Methylenedioxyamphetamine↗
PubMed1984

Kinetic properties of soluble and membrane-bound acetylcholinesterase from electric eel.

Using electric eel acetylcholinesterase (AChE) which was either membrane-bound (AChEm) or solubilized (AChEs), similar kinetics were seen in the absence of inhibitor or in the presence of edrophonium, trimethylammonium ion or paraoxon. Thus, both forms of the enzyme appear to behave similarly toward various inhibitors. However, in the presence of a probe sensitive to allosteric effects or changes in membrane fluidity, the two forms exhibit altered behavior. In the presence of F-, the relative rate of substrate hydrolysis by AChEm was reduced more rapidly than with AChEs, whether or not paraoxon was present. When inhibition by paraoxon (10(-7)-10(-4) M) was studied in the presence of F-, AChEs had a Hill coefficient of 1.0, whereas with AChEm the Hill coefficient changed from 0.8 to 1.5.

Acetylcholinesterase↗
PubMedDate not supplied

Temperature effects in cyanolysis using elemental sulfur.

As part of our studies directed at new treatments for cyanide poisoning we examined the effect of temperature on both the non-catalyzed and the albumin-catalyzed reactions of cyanide with a colloidal suspension of elemental sulfur (CSES). Using saturated sulfur solutions prepared in two solvents, pyridine (PY) and methyl cellosolve (MC), the reactions were studied at 15.0, 25.0, 30.0 and 37.5 degrees C. For all the cyanolysis reactions (non-catalyzed and albumin-catalyzed) there is an enhancement of reaction rate when the organic solvent for the sulfur is MC. Irrespective of the solvent for the CSES, the non-catalyzed reactions gave linear Arrhenius plots (PY, correlation coefficient = 0.998; MC, correlation coefficient = 0.997). In each case the entropy of activation was positive (14.1 cal K-1 mol-1 for PY and 56.4 cal K-1 mol-1 for MC). In contrast with these results the albumin-catalyzed reactions generated non-linear Arrhenius plots and negative entropies of activation. Non-linear plots were observed with the three albumins studied: human serum albumin, heat-shock bovine serum albumin and fatty acid-free bovine serum albumin. The non-linear plots are the result of a more complex reaction sequence than a simple cyanolysis reaction.

Colloids↗
PubMedDate not supplied

Cardiorespiratory effects of O-isobutyl S-[2-(diethylamino)-ethyl] methylphosphonothioate -- a structural isomer of VX.

O-Isobutyl S-[2-(diethylamino)ethyl]methylphosphonothioate (VR) is a structural isomer of a more well-known chemical warefare agent, O-ethyl S-[2(diisopropylamino)ethyl]methylphosphonothioate (code designation VX). In this study, cardiorespiratory and central nervous system (CNS) effects of VR (2LD50 or 22.6 microg kg(-1); s.c.) were evaluated in urethane-anesthetized (Group 1) and unanesthetized (Group 2) guinea pigs instrumented for concurrent recordings of electrocorticogram (ECoG) and a variety of cardiorespiratory activities. The first sign of intoxication was a state of progressive bradycardia, vascular hypotension and arrhythmia (Group 1, approximately 13 min post-VR; Group 2, approximately 6 min post-VR). Bradypnea, excessive salivation and compensatory changes in blood pressure typically did not emerge until 3-5 min prior to apnea (Group 1, approximately 28 min post-VR; Group 2, approximately 15 min post-VR). An idioventricular rhythm, which signalled a failing myocardium, appeared at the same time or shortly after the development of a bradypneic profile. Another notable toxicity component of VR, based on arterial pH, pO2/pCO2 and bicarbonate (HCO3-) level data, was a state of combined hypercapnia, acidemia and hypoxemia during the development of bradypnea. Taken together, findings from this study indicated that changes in medullary respiratory unit activity and ECoG data displayed little, if any, notable signs of CNS perturbation prior to the terminal stage (approximately 1 min prior to respiratory failure). Thus, in addition to displaying a greater sensitivity to perturbation by VR, the peripheral cardiorespiratory system components also appeared to play a more important role in precipitating a progressively dysfunctional cardiorespiratory status that ultimately led to collapse of central respiratory mechanisms and death.

Acidosis↗
PubMedDate not supplied

Behavioral decrements persist in rhesus monkeys trained on a serial probe recognition task despite protection against soman lethality by butyrylcholinesterase.

Recently, it has been demonstrated that an exogenously administered enzyme such as butyrylcholinesterase (BuChE) can prevent death in rhesus monkeys exposed to multiple-lethal doses of the acetylcholinesterase inhibitor soman when the enzyme is given prior to soman exposure (3). We report that despite BuChE protecting against soman-induced lethality, behavioral effects are seen in these monkeys which last for at least 6 days as measured by performance on a serial probe recognition (SPR) task. Analyses of the serial position curves showed that performance was lower on the probe trials when the probe items were from the middle of the list than when the probe items were from the beginning or end of the list which were unaffected. BuChE given alone also produced behavioral effects, causing all animals not to respond on the probe trials until 8 h following BuChE administration. Taken together, these findings suggest that the BuChE is not completely binding all of the soman and that a concentration of soman which is capable of causing behavioral effects may be entering the CNS.

Animals↗
PubMedDate not supplied

Quantitative structure-activity analysis of acetylcholinesterase inhibition by oxono and thiono analogues of organophosphorus compounds.

A comparison of the bimolecular rate constants (ki) for inhibition of electric eel acetylcholinesterase (AChE) by the oxono (i.e., P=O) and thiono (i.e., P=S) analogues of parathion, methylparathion, leptophos, fonofos, sarin, and soman revealed that the oxono/thiono ratios of ki values varied from 14 for soman to 1240 for parathion. Analysis of the relative importance of the dissociation equilibrium constant and the phosphorylation rate constant in producing this variation in ki values indicated that the oxono analogues and the phosphorylation rate constant values that varied in a narrow range from 8- to 14-fold greater than their thiono counterparts, while the oxono/thiono ratios for dissociation constants varied widely from 1 for soman to 82 for fonofos. The lower affinities of thiono analogues for AChE probably resulted from differences in the hydrophobic binding of oxono and thiono analogues to the active site of AChE, inasmuch as the hydrophobicities (i.e., octanol/water partition coefficients) of thiono organophosphorus compounds were much greater than the hydrophobicities of their oxono analogues. Quantitative structure--activity analysis indicated that the hydrophobic effects of oxono and thiono moieties correlated with log ki for AChE inhibition to a greater extent (r2 = 0.79) than their electronic effects (r2 less than or equal to 0.48). These observations suggest that the differences in hydrophobicity of oxono and thiono analogues of organophosphorus compounds may be as important as their electronic differences in determining their effectiveness as AChE inhibitors.

Acetylcholinesterase↗
PubMedDate not supplied

Oxime-induced reactivation of carboxylesterase inhibited by organophosphorus compounds.

A structure-activity analysis of the ability of oximes to reactivate rat plasma carboxylesterase (CaE) that was inhibited by organophosphorus (OP) compounds revealed that uncharged oximes, such as 2,3-butanedione monoxime (diacetylmonoxime) or monoisonitrosoacetone, were better reactivators than cationic oximes. Cationic oximes that are excellent reactivators of OP-inhibited acetylcholinesterase, such as pyridine-2-aldoxime or the bis-pyridine aldoximes, HI-6 and TMB-4, produced poor reactivation of OP-inhibited CaE. The best uncharged reactivator was 2,3-butanedione monoxime, which produced complete reactivation at 0.3 mM in 2 h of CaE that was inhibited by phosphinates, alkoxy-containing phosphates, and alkoxy-containing phosphonates. Complete reactivation of CaE could be achieved even after inhibition by phosphonates with highly branched alkoxy groups, such as sarin and soman, that undergo rapid aging with acetylcholinesterase. CaE that was inhibited by phosphonates or phosphates that contained aryloxy groups were reactivated to a lesser extent. The cause of this decreased reactivation appears to be an oxime-induced aging reaction that competes with the reactivation reaction. This oxime-induced aging reaction is accelerated by electron-withdrawing substituents on the aryloxy groups of phosphonates and by the presence of multiple aryloxy groups on phosphates. Thus, reactivation and aging of OP-inhibited CaE differ from the same processes for OP-inhibited acetylcholinesterase in both their oxime specificity and inhibitor specificity and, presumably, in their underlying mechanisms.

Animals↗