Abstract

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.