Effects of acetylcholinesterase and butyrylcholinesterase inhibition on breathing in mice adapted or not to reduced acetylcholinesterase.

We investigated the contributions of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibition to the respiratory dysfunction produced by organophosphates in mice which were adapted or not to low AChE activity. Effects of acute selective inhibition of AChE and BChE on ventilation measured by whole-body plethysmography were compared in mice with either normal AChE activity (wild-type), or mice adapted to a null AChE activity (homozygotes for AChE gene deletion) or adapted to an intermediate level of activity (heterozygotes). In wild-type mice acute reduction of AChE by Huperzine A (1 mg/kg) to the level found in asymptomatic heterozygotes, induced tremors but no respiratory depression, whereas the same dose of Huperzine in heterozygote animals further reduced AChE activity, increased tidal volume (V(T)) and decreased breathing frequency (f(R)). A lethal dose of Huperzine in wild-type mice augmented these respiratory effects, but was ineffective in homozygotes. BChE inhibition by bambuterol was ineffective in wild-type mice and heterozygotes, decreased V(T) in homozygotes adapted to null AChE activity but increased V(T) in wild-type mice acutely treated with Huperzine, also aggravating the cholinergic syndrome. We conclude that: (1) Huperzine does not perturb respiration at a dose inhibiting 40% of AChE, and at a lethal dose does not affect any other enzyme important for respiration; (2) Respiratory function is more sensitive to anticholinesterases in heterozygotes than in wild-type mice; (3) BChE may play distinct roles in respiratory function, because its inhibition has opposite effects on tidal volume depending on whether the mouse has adapted to null AChE or whether AChE has been lowered acutely; (4) BChE inhibition may contribute to the respiratory toxicity of organophosphates.