Age dependent changes in choline uptake of the chick iris.

The kinetics of the uptake of choline, the rate-limiting substrate in the in vivo synthesis of acetylcholine, were studied during the period 1--7 years in the iris of the chick. These changes were correlated to the endogenous levels of acetylcholine and choline in the same organ. Vmax values per iris decrease significantly at 5 years and continue to decrease at 7 years, to 64% and 37% of the one year value, respectively. If the variation in Vmax is calculaed per protein the decline is 34% between 1 and 5 years. Km does not change significantly during the period 1--7 years. Total acetylcholine and choline levels follow a similar trend, decreasing progressively from 1 to 7 years. Sensitivity to hemicholinium (5.5 X 10(-5) M) decreases significantly between 5 days of incubation (d.i.) and 5 years. Inhibition by ouabain (10(-4) M) shows an opposite trend, increasing significantly from 20% at 5 d.i. to 52% at 3 months. At 5 years sensitivity to ouabain is the same as at 3 months. Nardependence decreases significantly between 5 d.i. and 5 years but no significant changes are seen between 5 d.i. and 3 months. Uptake at 27 degrees C decreases from 59% of control at 5 d.i. to 45% at 3 months, and is not changed at 5 years. Our observations suggest that the effect of aging on peripheral cholinergic neurons is not generalized, but is specifically directed toward the neuronal periphery (terminals) as opposed to cell bodies.