The relationship between cholinesterase inhibition in the chick biventer cervicis muscle and its sensitivity to exogenous acetylcholine.

The effect of physostigmine has been studied on cholinesterase in homogenates of chick biventer cervicis muscles and on the contractile responses of the intact muscles to acetylcholine and carbachol. The concentration of physostigmine required to produce the maximum increase in sensitivity to acetylcholine almost completely inhibited the cholinesterase in muscle homogenates. This concentration of physostigmine had no effect on muscle contractures elicited by carbachol. By taking account of the combined effects of acetylcholine diffusion and enzymic hydrolysis, a quantitative theoretical relationship has been derived between the level of cholinesterase activity in cylindrical muscles and the fractional occupancy of the acetylcholine receptors in these muscles in the presence of different concentrations of exogenous acetylcholine. This theory attributes the thousand-fold increase in sensitivity to exogenous acetylcholine produced by anticholinesterases in chick biventer cervicis muscles largely to an alteration in acetylcholine concentration gradient within the muscle and accounts satisfactorily for the shift in the dose-response curve for acetylcholine which occurs after treatment of the muscles with various concentrations of physostigmine.