Calcium dependence of somatostatin (SRIF) release and cyclic AMP levels in cultured diencephalic neurons.

Calmodulin has been reported to be involved in the Ca2+-dependent hypothalamus in vitro. The present experiments were undertaken to determine whether at an early stage of development (in diencephalic primary cultures secreting SRIF, on the 11th day) the activation of a Ca2+-calmodulin kinase system is also involved in the release of the peptide. Since a calmodulin-dependent adenylate cyclase activity has been detected in the brain, we measured intracellular cyclic AMP accumulation as an additional parameter of calmodulin activity. SRIF release and cyclic AMP accumulation were stimulated by K+ (56 mM) and by the Ca2+ ionophores ionomycin (0.5 microM) and A 23187 (in a dose-dependent manner). Incubation of cells in Ca2+-free Locke medium or in the presence of Co2+ (1 mM) completely blocked ionophore-induced SRIF release and cyclic AMP accumulation. Three calmodulin antagonists (calmidazolium, W-7, and chlorpromazine) and two blockers of calmodulin-dependent kinase (phenytoin and diazepam) were tested on evoked-SRIF release and cyclic AMP formation. Neither W-7 nor calmidazolium modified A 23187-induced SRIF release at any dose tested, although they inhibited, in a dose-dependent manner, the stimulatory effect of the Ca2+ ionophore on cyclic AMP accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)