Relationship of calcium and adenylate cyclase messenger systems in rat brain synaptosomes

The effects of cyclic AMP on the rise in cytosolic free calcium concentration, [Ca2+]i, after stimulation with 15 mM K+ in rat brain synaptosomes were investigated. The fluorescent chelating agent Quin-2 was employed to monitor alterations of K+-evoked [Ca2+]i. Under normoxic conditions, clonidine (1, 10 microM), an alpha 2-adrenoceptor agonist, decreased the 15 mM K+-evoked [Ca2+]i. Although yohimbine (1, 10 microM), an alpha 2-adrenoceptor antagonist, had little or no effect on K+-evoked [Ca2+]i, the inhibitory effects of clonidine were blocked by yohimbine. 8-Bromo cyclic AMP, a cyclic AMP analogue, (50-500 microM), increased K+-evoked [Ca2+]i in a dose-dependent manner. The addition of cyclic AMP analogues subsequent to clonidine treatment reversed the clonidine-induced suppression of K+-evoked [Ca2+]i. On the other hand, under hypoxic conditions, K+-evoked [Ca2+]i was reduced by about 50-60%. 8-Bromo cyclic AMP and the adenylate cyclase activators, yohimbine (1-10 microM) and isoproterenol, a beta-adrenoceptor agonist, (0.1-10 microM), transiently reversed the reduction of the K+-evoked [Ca2+]i caused by hypoxia. These results indicate that the activation of alpha 2-adrenoceptor produces a rapid, sustained decrease in [Ca2+]i which may be due to a decrease in the levels of intracellular cyclic AMP. In addition, the increase in cellular levels of cyclic AMP reversed the reduction of the Ca2+ response to high K+ stimulation caused by hypoxia. If this is so, there is the possibility that increased cyclic AMP might improve the hypoxic damage.