Caffeine stores and dopamine differentially require Ca(2+) channels in goldfish somatotropes.

The regulation of growth hormone (GH) secretion by intracellular Ca(2+) stores was studied in dissociated goldfish somatotropes. We characterized a caffeine-activated intracellular store that had been shown to mediate GH release in response to gonadotropin-releasing hormone. The peak response of caffeine stimulation was reduced by approximately 28% by 100 microM ryanodine in a use-dependent manner suggesting that the first 10 min of GH release is partially mediated by a caffeine-activated ryanodine receptor. The temporal sensitivities of caffeine- and dopamine-evoked GH release to blockade of Cd(2+)-sensitive Ca(2+) channels were compared. We demonstrated that the initial phase of dopamine-evoked release was dependent on Ca(2+) channels, whereas the initial phase of caffeine-evoked release was sensitive only to pretreatment blockade. This would suggest that the maintenance of one class of caffeine-activated intracellular stores requires entry of Ca(2+) through Cd(2+)-sensitive Ca(2+) channels. This differential temporal requirement for Ca(2+) channels in Ca(2+) signaling may be a mechanism to segregate intracellular signaling pathways of multiple neuroendocrine regulators in the teleost pituitary.