Maturational change of KCl-induced Ca2+ increase in the rat brain synaptosomes

To investigate maturational change in the susceptibility of voltage-dependent calcium (Ca2+) channels (VDCC) in the brain to excessive depolarization, which is likely to occur during hypoxia or ischemia, we studied depolarization-induced increases in Ca2+ concentration in cortical synaptosomes ([Ca2+]i) obtained from young (8, 15, 22, 36, and 43-day-old) and adult rats using fura 2-AM as a Ca2+ indicator. The effects of Ca2+ antagonists on the increase were also studied. The maximal increase in [Ca2+]i caused by 50 mM KCl-induced depolarization was significantly lower in 8-day-old rats (73.3 nM) compared with that in adult rats (133.6 nM). On the other hand, the time necessary for [Ca2+]i to decrease to 50% of its maximal level (tau) was significantly shorter in immature rats compared with that in adult rats and was particularly short in 8- and 15-day-old rats (0.28 and 0.40 min vs. 3.85 for adult rats). The maximal increase in [Ca2+]i in 22-day-old rats and tau in adult rats were markedly reduced by verapamil, omega-agatoxin IVA, and omega-conotoxin GVIA (antagonists of L-, P-, and N-type Ca2+ channels, respectively) to similar extents, while a mixture of the three antagonists markedly decreased both maximal increase and tau in 8- and 22-day-old and adult rats. These results indicate that depolarization-induced Ca2+ influx through VDCCs in immature rat brain is less pronounced than that in adult rats, and suggest that the susceptibility of all of L-, N-, and P-type Ca2+ channels is increased during maturation in the first few weeks after birth. This lower susceptibility to depolarization might be involved in the resistance to hypoxia in immature animals.