EFFECTS OF ACETYLCHOLINE ON MECHANICAL AND ELECTRICAL PROPERTIES OF ISOLATED RABBIT AURICLE: THEIR RELATIONSHIP TO THE GENESIS OF ARRHYTHMIAS

¹ Department of Pharmacology, School of Medicine, University of Washington, Seattle, Washington
² Physiology Division, Chemical Corps Medical Laboratories, Army Chemical Center, Maryland

Acetylcholine decreases the rate and force of spontaneous contractions in isolated rabbit auricle, but has little or no effect on conduction velocity of spontaneous action potentials. It does not alter the intensity of the minimally effective electrical stimulus delivered as a single shock.

Progressively decreasing the interval between paired stimuli applied to control preparations results in: decreased excitability, decreased contractile force and decreased conduction velocity, all with respect to the response to the second of a stimulus pair. The absolutely refractory period was estimated by decreasing the interval between paired stimuli. The effect of acetylcholine is to shift to the left the curves for conduction time and excitability, thus indicating a marked decrease in the duration of the absolutely refractory period. Ectopic beats appear when the interval between stimuli nears the value of the absolutely refractory period.

Hypothetical considerations regarding the nature of the observed ectopic rhythms are presented. A type of re-entry excitation based on the experimental data is discussed.