Cholinesterases and postnatal development of the negative chronotropic effects of acetylcholine in albino rats.

The negative chronotropic effects of acetylcholine were studied in the isolated atria of the hearts of albino rats aged 2, 15, 29 and 47 days and adult. In untreated preparations, i.e. with full cholinesterase activity, the strongest effects were observed in newborn animals; with advancing age the reaction grew weaker. If cholinesterase activity was inhibited with physostigmine, the differences between the various age groups were obliterated. It is thus evident that the actual acetylcholine sensitivity of the sinoatrial node tissue does alter during postnatal life, but that growing cholinesterase activity reduces the amount of acetylcholine diffusing from the medium into the acetylcholine receptor zone. The change which takes place in cholinesterase activity in the myocardial tissue during postnatal life is so great that is must be taken into account when considering the development of cholinergic control of cardiac function.