Analysis of rapid calcium signals in synaptosomes

A combination of the stopped-flow technology with dual channel spectrofluorometry of Ca 2+-indicators was utilized for the measurement of rapid Ca 2+-signals in rat cerebral cortical synaptosomes evoked by K +-depolarization. There was no observable contribution of Ca 2+-ions from intracellular stores to the rise in [Ca 2+] i. The kinetics of the fast increase in intracellular Ca 2+ concentration was analysed in relation to the depolarization strength. The maximal increase in [Ca 2+] i and the time course of Ca 2+-channel inactivation were determined for depolarizations obtained by different extracellular K +-concentrations ([K +] o). An apparent threshold was observed at about 18 mM [K +] o; a maximal Ca 2+-signal amplitude was estimated at about 40 mM [K +] o. Pharmacological properties of the involved Ca 2+-channels were determined using selective Ca 2+-channel blockers (Dihydropyridines, ω-Conotoxin, ω-Agatoxins); the results suggest that a P-type voltage-dependent Ca 2+-channel is the relevant channel type, generating the evoked Ca 2+-signals in rat cerebral cortical synaptosomes.