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. V_max 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 V_max is calculated per protein the decline is 34% between 1 and 5 years.K_m 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 × 10⁻⁵M) decrease 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. Na⁺ dependence 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 °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 perophery (terminals) as opposed to cell bodies.