Analysis of Vagal and Cholinesterase Effects on the Heart of Chelydra serpentina

¹ From the Department of Physiology, University of Michigan, Ann Arbor, Michigan

Cholinesterase, applied to the isolated turtle heart, caused a decreased inotropic and chronotropic inhibition when the vagus nerve was stimulated with multiple shocks, but caused an increased inotropic inhibition when the vagus was stimulated with a single shock. The beating of the atrium is an active force in modifying the effect of endogenous acetylcholine on inotropic inhibition. This is indicated by: a) cholinesterase will not decrease the amount of inhibition caused by a single shock; b) when different ambient solutions are used, the amount of inhibition from a single shock to the vagus varies, but the number of beats required to obtain the lowest amplitude of contraction remains the same; c) at the end of a multiple stimulation which causes complete chronotropic inhibition, the atrium must still pass through the phase of decreasing amplitude before recovering; d) during cholinesterase, the inotropic and chronotropic inhibition during multiple stimulation allowed the recovery to start at a higher rate than before cholinesterase. Velocity constants were obtained from the recovery curves before, during and after cholinesterase. Computed curves based on the velocity constants and a geometric progression closely approximate the experimental data showing temporal summation of inotropic inhibition from submaximal stimulation of the vagus nerve before and during cholinesterase application.