Circadian rhythms: variation in sensitivity of isolated rat artria to acetylcholine.

Experiments were performed at two time points in the 24hour cycle to determine whether isolated right atria of rats would vary in sensitivity, with a circadian rhythm, to two concentrations of acetylcholine. The beating rate of the atria decreased more in response to the drug if the atria were isolated at 1100 than if isolated at 2300 hours.