The release of pituitary GH appears to be critically dependent on alterations in the free intracellular Ca2+concentration ([Ca2+];). However, little is known about the nature of Ca2+signalling within normal pituitary cells. We, therefore, examined [Ca2+]ipatterns in individual cultured pituicytes of adult male rats under basal conditions and in response to GH regulatory agents, using the calcium-sensitive dye fura-2 together with digital imaging microscopy. Perfusion of cultured anterior pituitary cells with GH-releasing factor (GHRF) resulted in a marked increase in [Ca2+]iin specific pituitary cells. These cells did not respond to other hypothalamic secretagogues (GnRH, TRH, or CRF), and there was no evidence of desensitization on repetitive administration of GHRF. Somatotrophs (n = 134) exhibited spontaneous oscillations of [Ca2+]iin the basal state, with considerable heterogeneity of oscillatory patterns among cells. After application of a near-maximal stimulatory dose of GHRF (1 pm), there was a striking 2.2-fold increase in the amplitude of [Ca2+]ioscillations and only a modest increase in their frequency. Forskolin (1 µM) augmented somatotroph [Ca2+]iin patterns similar to those of GHRF. Somatostatin (10 nM) abolished the [Ca2+]iresponse to GHRF (n = 26); this reflected a marked reduction in the amplitude of [Ca2+]ioscillations and a slight reduction in their frequency. Ca2+-free medium or the Ca2+channel antagonist nimodipine (0.1-1 µM) suppressed the Ca2+stimulatory effect of GHRF. Conversely, the Ca2+channel agonist BAY K8644 (1 µM) strikingly augmented the GHRF-induced rise in [Ca2+]i, with a major stimulatory effect on the amplitude of [Ca2+]i; oscillations and no observed effect on their frequency. In summary, GHRF and other hypothalamic secretagogues increase [Ca2+]iin pituitary cells in a highly specific manner, consistent with the known specificity of their effects on hormone release. Somatotrophs exhibit spontaneous rhythmic oscillation of [Ca2+]iin the basal state. Known regulators of GH release markedly alter the [Ca2+]ioscillatory pattern in characteristic manners, exerting predominant effects on the amplitude of [Ca2+]ipulses and lesser effects on their frequency. These striking effects of GH regulatory agents on pituitary Ca2+signalling are consistent with the concept that modulation of [Ca2+]i; is a critical mediator of somatotroph function.
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