ABSTRACT
In experiments on adult albino rats the authors used the substances BW 284 C51 (1.5-bis(allyldimethylammoniumphenyl)-pentane-3-one-dibromide) as a specific inhibitor of acetylcholinesterase (AChE) and ethopropazine (10-(2-diethylaminopropyl) phenothiazine hydrochloride) as a specific inhibitor of butyrylcholinesterase (BuChE) to determine the two enzyme activities in atrial homogenates and to investigate changes after AChE or BuChE inhibition of the negative chronotropic effect of acetylcholine (ACh) on atria incubated in vitro. AChE accounted for only 12% and BuChE for 88% of the total ability of atrial homogenates to hydrolyse acetylcholine. The concentration of exogenous ACh needed to reduce the spontaneous frequency of contractions of the isolated right atrium by 30, 60, or 90/min fell by 78%, 79% and 84% respectively after BW 284 C51 inhibition of AChE and by 95%, 94% and 94% after simultaneous inhibition of AChE and BuChE. The significance of AChE in control of the negative chronotropic effect of ACh is thus evidently significantly greater than would correspond to the percentual proportion of AChE in cholinesterase activities in the atria of the rat heart. In can be assumed that AChE is functionally associated with parasympathetic innervation of the heart and that it is probably present in a high concentration in the primary pacemaker region.
