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Reduction of K+‐Stimulated 45Ca2+ Influx in Synaptosomes with Age Involves Inactivating and Noninactivating Calcium Channels and Is Correlated with Temporal Modifications in Protein Dephosphorylation

Abstract: The voltage‐dependent calcium uptake in rat brain synaptosomes was measured under conditions in which [Ca2+]o/[Na+]i exchange was minimized to characterize the voltage‐sensitive calcium channels from rats of different ages. In solutions of CaCl2 concentrations of <500 μM, the initial (5‐s) calcium uptake declined by ∼ 20–50% in 12‐ and 24‐month‐old rats relative to 3‐month‐old adults. Depolarization of synaptosomes from 3‐month‐old rats in a calcium‐free medium or in the presence of 0.5 mM CaCl2 led to an exponential decline of the calcium uptake rate after 20 s (voltage‐ or voltage‐and‐calcium‐dependent inactivation) to ∼ 66 and 34% of the initial value with a t1/2 of 1.6 or 0.7 s, respectively. The presence of 1 μM nifedipine resulted in a 15–25% reduction of 45Ca2+ uptake rates, which appeared to affect noninactivating calcium channels, but addition of the calcium channel agonist Bay K 8644 was without effect. In 24‐month‐old rats, inactivation of 45Ca2+ uptake in calcium‐free media was nondetectable, and in the presence of 0.5 mM CaCl2, the rate and extent of inactivation were also much lower than in 3‐month‐old animals (the t1/2 was 0.9 s, and the calcium uptake rate at 20 s was 55% of its initial value). Moreover, the presence of 1 μM nifedipine was without effect on initial calcium uptake or inactivation in synaptosomes from 24‐month‐old rats. These results indicate that the decrease in calcium channel‐mediated 45Ca2+ uptake involves an inhibition or block of both dihydropyridine‐resistant and ‐sensitive calcium channels. It is interesting that the large decrease in calcium uptake via calcium channels that occurs in 24‐month‐old rats is correlated with a slower dephosphorylation of the synaptosomal phosphoprotein P96, a process that is intimately related to the opening and ensuing calcium flow through voltage‐gated calcium channels.