INITIATION OF AUTOMATICITY BY ACETYLCHOLINE AND NOREPINEPHRINE IN ISOLATED QUIESCENT ATRIA OF THE CAT

¹ The Department of Pharmacology, School of Medicine and Dentistry, The University of Rochester, Rochester, New York

Rates and durations of spontaneous beating of isolated cat atria are the same in Chenoweth-Koelle solution and in Tyrode solution at 29°C. In Chenoweth-Koelle solution the spontaneous rate at 37°C is two times that at 29°C and the duration of beating at 37°C is one half that at 29°C. Rates of beating induced by acetylcholine in isolated quiescent cat atria in Chenoweth-Koelle solution and in Tyrode solution at 29°C are the same; rates induced by norepinephrine in Chenoweth-Koelle solution and in Tyrode solution at 29°C are the same. However, the rates induced by norepinephrine are two times those induced by acetyichohine.

The suppression of the ability of acetylcholine to start quiescent cat atria in Chenoweth-Koelle solution was abolished in Tyrode solution and proved to be K⁺ ion dependent. Differences in the actions of acetyicholine and norepinephrine in starting isolated cat atria suggest that different mechanisms of action are involved.