Time course of the aging of the high affinity L-glutamate transporter in rat cortical synaptosomes.

Initial velocities of cortical synaptosomal glutamate uptake are measured as a function of both glutamate and sodium concentration in 10- and 18-month old animals and compared to previous results from 2- and 30-month old animals. As a percentage of 2-month values, mean velocity of uptake falls to 87% in the 10-month group and to 79% in the 18-month group, with no further change between 18 and 30 months. The data give minimal best fit to the same model for interaction of carrier with glutamate and sodium as found previously for 2- and 30-month old animals. Thus the basic mechanisms involved in transport do not change with age. However, the kinetic parameters which describe the transporter do change with age, the primary changes being in those parameters reflective of the transport capacity of the carrier. Va, apparent maximal velocity of uptake, declines most rapidly between 2 and 30 months, continues to decline rapidly between 10 and 18 months, and then declines slowly between 18 and 30 months. Jm, rate of uptake at infinite sodium concentration, declines most rapidly between 2 and 10 months, with a much slower decline between 10 and 30 months. Kt, apparent Michaelis-Menten constant, changes but little with age; at 30 months, Kt is only 6-11% less than at 2 months. Although the total change is only 9-10%, there appears to be a slow, steady decline in KNa, the sodium concentration giving an uptake equal to Jm/2, between 2 and 30 months of age.