In vivo inhibition of rabbit whole blood cholinesterase with organophosphate inhibitors and reactivation with oximes.

Abstract

Whole blood cholinesterase was determined during and following intravenous infusion of a series of dimethyl organophosphate inhibitors followed by injections of oxime reactivators. Inhibitors were infused into the marginal ear vein of adult male New Zealand white rabbits; reactivators were injected immediately following infusion or at periodic intervals thereafter. Heparinized whole blood samples were obtained from a canula implanted in the external jugular vein; enzyme activity was determined in an electrometric constant pH titration assay using acetyl choline bromide substrate.The dimethyl phosphates, BidrinR, C-2307, DDVP, Gutoxon (the oxygen analog of GuthionR), and the oxygen analog of C-9491 produced dose- and time-related inhibition of cholinesterase during infusion; inhibited enzyme recovered spontaneously to 607–80% of normal activity within 60–90 minutes after infusion. AzodrinR-induced inhibition was dose- and time-related, but immediate spontaneous recovery was negligible. Injection of 2-PAM immediately following infusion produced rapid almost complete reactivation of cholinesterase (progressively less reactivation was seen at 1-3 hours following infusion of all dimethyl phosphates). Enzyme inhibited with SD-1652, a diethyl phosphate, was reactivated by 2-PAM up to at least 48 hours following inhibitor infusion. TMB-4 and P-2-S produced marked enzyme reactivation when injected immediately following inhibitor infusion; MINA and DAM were not effective at doses ≥100 mg/kg.Results suggest that spontaneous reactivation of inhibited cholinesterase may be due partially to a reversal of the enzyme inhibitor complex as well as to hydrolysis of the dialkyl phosphorylated enzyme.