Mouse hypothalamic somatostatin release: roles of calcium and calmodulin

We employed a short term system of dispersed adult mouse hypothalamic cells, as previously described in the rat, to examine the roles of calcium and calmodulin in SRIF release. Incubation studies were performed 24 h after hypothalamic cell dispersion. SRIF release, as determined by RIA was stimulated in a dose-dependent manner by the membrane-depolarizing agents KCl, ouabain, and veratridine as well as by the calcium ionophore A23187. The stimulation of SRIF release induced by depolarizing agents was abolished or diminished by 1) omission of extracellular calcium, 2) chelation of extracellular calcium by EGTA, and 3) the calcium channel blocker verapamil, indicating calcium dependence of this process. Three chemically distinct groups of calmodulin inhibitors were employed to study the role of calmodulin in stimulus-secretion coupling of hypothalamic SRIF. The neuroleptic calmodulin inhibitors trifluperazine (1 microM), chlorpromazine (10 microM), and promethazine (10 microM) as well as the naphthalene sulfonamide calmodulin inhibitor W7 (1 microM) and compound 48/80 (50 micrograms/ml) were all demonstrated to have an inhibitory effect on the stimulation of SRIF release induced by membrane depolarization. No inhibitory effect of the less potent naphthalene sulfonamide agent W5 was observed at 1 microM, although an inhibitory effect was seen at 10 microM. The stimulation of SRIF release induced by A23187 was not inhibited by omission of extracellular calcium or by verapamil, but was inhibited by EGTA and trifluperazine. These data demonstrate the role of calcium in membrane depolarization-induced stimulus-secretion coupling of mouse hypothalamic SRIF. Inhibition of the stimulatory response by low concentrations of three distinct groups of calmodulin inhibitors, i.e. neuroleptics, naphthalenesulfonamide agents and compound 48/80, suggests a role for calmodulin in this process.