Postnatal changes of the tonic influence of the vagus nerves on the heart rate, and of the activity of choline acetyltransferase in the heart atria of rats

Postnatal changes in the resting heart rate and in its parasympathetic tonic inhibition have been measured in awake rats and compared with changes in the activity of choline acetyltransferase (ChAT) in the heart atria. The heart rate at rest increased from 372.min-1 on the 1st to 456 and 442.min-1 on the 15th and 24th day of life and then again decreased to 358 and 356.min-1 in 60-day-old and adult rats. Until the 15th day of postnatal life, the administration of atropine did not bring about an increase in the heart rate; the cardio-acceleratory effect of atropine (indicating the presence of tonic vagal inhibition of the heart) appeared only on the 18th day and increased steeply up to the 40th day of postnatal life. The activity of ChAT in the heart atria was measured as the difference between the synthesis of acetylcholine in atrial homogenates incubated in the absence and in the presence of bromoacetylcholine (BrACh), a specific inhibitor of ChAT; this procedure eliminated the contribution of carnitine acetyltransferase to the synthesis of acetylcholine. The activity of ChAT was found to increase steeply from the 1st to the 25th days of postnatal life; the steepest increase in the activity of the enzyme occurred between the 4th and the 15th days. Temporal correlation between the changes in the activity of ChAT, in the content of acetylcholine in the heart atria (Kuntscherová and Vlk 1979) and in the efficiency of transmural stimulation of sinoatrial region on the heart rate (Vlk 1979) indicate that the functional maturation of intracardiac cholinergic neurones, proceeding in rats during the first three weeks of their postnatal life, plays an important role in the onset and temporal development of the tonic parasympathetic inhibition of the heart rate.