Calcium signalling in single growth hormone-releasing factor-responsive pituitary cells.
The release of pituitary GH appears to be critically dependent on alterations in the free intracellular Ca2+ concentration ([Ca2+]i). However, little is known about the nature of Ca2+ signalling within normal pituitary cells. We, therefore, examined [Ca2+]i patterns 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+]i in 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+]i in the basal state, with considerable heterogeneity of oscillatory patterns among cells. After application of a near-maximal stimulatory dose of GHRF (1 nM), there was a striking 2.2-fold increase in the amplitude of [Ca2+]i oscillations and only a modest increase in their frequency. Forskolin (1 microM) augmented somatotroph [Ca2+]i in patterns similar to those of GHRF. Somatostatin (10 nM) abolished the [Ca2+]i response to GHRF (n = 26); this reflected a marked reduction in the amplitude of [Ca2+]i oscillations and a slight reduction in their frequency. Ca(2+)-free medium or the Ca2+ channel antagonist nimodipine (0.1-1 microM) suppressed the Ca2+ stimulatory effect of GHRF. Conversely, the Ca2+ channel agonist BAY K8644 (1 microM) 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+]i in 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+]i in the basal state. Known regulators of GH release markedly alter the [Ca2+]i oscillatory pattern in characteristic manners, exerting predominant effects on the amplitude of [Ca2+]i pulses 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.
Cuttler
L
L
Department of Pediatrics, Case Western Reserve University, Cleveland, Ohio 44106.
Glaum
S R
SR
Collins
B A
BA
Miller
R J
RJ
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D003346:Corticotropin-Releasing Hormone / Q000494:pharmacology; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D007700:Kinetics; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D011919:Rats, Inbred Strains; D015398:Signal Transduction / Q000187:drug effects; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology
D005576:Colforsin; D007987:Gonadotropin-Releasing Hormone; D013973:Thyrotropin-Releasing Hormone; D003346:Corticotropin-Releasing Hormone; D013007:Growth Hormone-Releasing Hormone; D002118:Calcium