Diethylphosphorylation of rat cardiac M2 muscarinic receptor by chlorpyrifos oxon in vitro.

The acute toxicity of chlorpyrifos oxon (CPO), the metabolically-activated form of the major organophosphorus insecticide chlorpyrifos, is attributable to diethylphosphorylation of acetylcholinesterase at its esteratic site. As a secondary effect, CPO is known to compete with agonist binding to the M2 muscarinic acetylcholine receptor (mAChR). This study tests the hypothesis that [ethyl-1,2-(3)H]CPO labels the M2 mAChR in rat cardiac membrane proteins. Of four labeled protein regions observed, only one had an apparent molecular mass (70-75 kDa) consistent with that of glycosylated M2 mAChR. It was identified as M2 muscarinic receptor by Western blotting and immunoprecipitation using a cardiac-specific M2 mAChR monoclonal antibody, providing the first direct evidence for diethylphosphorylation of a muscarinic receptor. This may be a functionally important M2 mAChR site, but the toxicological relevance and species and organ specificity of diethylphosphorylation are unknown.