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.
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 less than 500 microM, the initial (5-s) calcium uptake declined by approximately 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 approximately 66 and 34% of the initial value with a t1/2 of 1.6 or 0.7 s, respectively. The presence of 1 microM 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 microM 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.(ABSTRACT TRUNCATED AT 250 WORDS)
Martínez-Serrano
A
A
Departamento de Biología Molecular, CSIC-Universidad Autónoma de Madrid, Spain.
Bogónez
E
E
Vitórica
J
J
Satrústegui
J
J
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000375:Aging / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D002132:Calcium Radioisotopes; D004095:Dihydropyridines / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D008970:Molecular Weight; D009543:Nifedipine / Q000494:pharmacology; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D011188:Potassium / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*
D015220:Calcium Channels; D002132:Calcium Radioisotopes; D004095:Dihydropyridines; D010750:Phosphoproteins; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; C038806:1,4-dihydropyridine; D009543:Nifedipine; D011188:Potassium; D002118:Calcium