Postnatal development of the high-affinity uptake of choline and of the synthesis of acetylcholine in rat heart atria.

Isolated heart atria from rats of different ages were incubated in a medium containing (14C)choline and the rates of the uptake of (14C)choline into the tissue and of its conversion to (14C)acetylcholine (ACh) were measured. The synthesis of (14C)ACh (expressed per 1 g of fresh weight) increased from birth until 30 days of age and diminished after 40 days of postnatal life. The rate of (14C)ACh synthesis was considerably diminished when Na+ was omitted from the incubation medium or when hemicholinium-3 was added to it; these effects of the absence of Na+ and of hemicholinium-3 were already manifest on the 1st day after birth, indicating that the sodium-dependent high-affinity uptake of choline is operative and takes part in the synthesis of ACh in the heart from the start of postnatal life (if not earlier). In newborn rats, 4% of the (14C)choline that had been taken up by the atria was converted to (14C)Ach; this proportion rose to 7-9% at the age of 20 and 30 days and in adulthood. The total uptake of (14C)choline expressed per whole atria kept increasing from birth till adulthood when related to the whole atria, but it diminished when related to 1 g of atrial weight.