Impaired neuromuscular function by conjoint actions of organophosphorus insecticide metabolites omethoate and cyclohexanol with implications for treatment of respiratory failure

Abstract

Background

Ingestion of agricultural organophosphorus insecticides is a significant cause of death in rural Asia. Patients often show acute respiratory failure and/or delayed, unexplained signs of neuromuscular paralysis, sometimes diagnosed as “Intermediate Syndrome”. We tested the hypothesis that omethoate and cyclohexanol, circulating metabolites of one agricultural formulation, cause muscle weakness and paralysis.

Methods

Acetylcholinesterase activity of insecticide components and metabolites was measured using purified enzyme from eel electroplaque or muscle homogenates. Mechanomyographic recording of pelvic limb responses to nerve stimulation was made in anaesthetized pigs and isometric force was recorded from isolated nerve-muscle preparations from mice. Omethoate and cyclohexanol were administered intravenously or added to physiological saline bathing isolated muscle. We also assessed the effect of MgSO₄ and cooling on neuromuscular function.

Results

Omethoate caused tetanic fade in pig muscles and long-lasting contractions of the motor innervation zone in mouse muscle. Both effects were mitigated, either by i.v. administration of MgSO₄ in vivo or by adding 5 mM Mg²⁺ to the medium bathing isolated preparations. Combination of omethoate and cyclohexanol initially potentiated muscle contractions but then rapidly blocked them. Cyclohexanol alone caused fade and block of muscle contractions in pigs and in isolated preparations. Similar effects were observed ex vivo with cyclohexanone and xylene. Cyclohexanol-induced neuromuscular block was temperature-sensitive and rapidly reversible.

Conclusions

The data indicate a crucial role for organophosphorus and solvent metabolites in muscle weakness following ingestion of agricultural OP insecticide formulations. The metabolites omethoate and cyclohexanol acted conjointly to impair neuromuscular function but their effects were mitigated by elevating extracellular Mg²⁺ and decreasing core temperature, respectively. Clinical studies of MgSO₄ therapy and targeted temperature management in insecticide-poisoned patients are required to determine whether they may be effective adjuncts to treatment.

Keywords:

• Insecticide toxicity
• organophosphorus
• omethoate
• dimethoate
• organic solvents
• cyclohexanone
• cyclohexanol
• xylene
• neuromuscular transmission
• myography

Acknowledgments

We thank Professors David Beeson, John Harris, Clarke Slater, Angela Vincent, David Wyllie, and Drs John Tattersall, Richard Webster and Charlotte Whitmore for valuable discussions and comments on earlier drafts of the manuscript; Dr Stephen Greenhalgh, Ms Rachael Gregson and support staff at the Wellcome Trust Critical Laboratory in the Roslin Institute for veterinary assistance with in vivo recordings; and Drs Natalie Homer and Scott Denham for plasma measurements of insecticide metabolites.

Disclosure statement

None of the authors have any competing financial and/or non-financial interests in relation to the work described.

Additional information

Funding

This research was supported by the Medical Research Council under grant [MR/M024075]; the Wellcome Trust under grant [GR104972]; a Lister Research Prize Fellowship to ME, and a Charles Darwin Scholarship of the University of Edinburgh. FM is an MRC student on the Precision Medicine PhD Programme at Edinburgh University.