Autocrine-paracrine inhibition of growth hormone and prolactin production by GH3 cell-conditioned medium

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

Feedback regulation of growth hormone synthesis and secretion in fish and the emerging concept of intrapituitary feedback loop

Growth hormone (GH) is known to play a key role in the regulation of body growth and metabolism. Similar to mammals, GH secretion in fish is under the control of hypothalamic factors. Besides, signals generated within the pituitary and/or from peripheral tissues/organs can also exert a feedback control on GH release by effects acting on both the hypothalamus and/or anterior pituitary. Among these feedback signals, the functional role of IGF is well conserved from fish to mammals. In contrast, the effects of steroids and thyroid hormones are more variable and appear to be species-specific. Recently, a novel intrapituitary feedback loop regulating GH release and GH gene expression has been identified in fish. This feedback loop has three functional components: (i) LH induction of GH release from somatotrophs, (ii) amplification of GH secretion by GH autoregulation in somatotrophs, and (iii) GH feedback inhibition of LH release from neighboring gonadotrophs. In this article, the mechanisms for feedback control of GH synthesis and secretion are reviewed and functional implications of this local feedback loop are discussed. This intrapituitary feedback loop may represent a new facet of pituitary research with potential applications in aquaculture and clinical studies.

Growth hormone. A paracrine growth factor?

A number of tissues, including the brain, pituitary, immune system, placenta, mammary gland, and testis, may be self-contained units of GH regulation, production, and action. The production of GH and GH-releasing factors outside the hypothalamo-pituitary axis complements, rather than replaces, the traditional endocrine interactions between GH-releasing factors, GH, and its target tissues.

Growth hormone receptor synthesis and release in tumorous somatolactotrophs

The growth hormone (GH) receptor (GHR) gene is expressed in pituitary somatotrophs and lactotrophs, in which GHR/GH-binding protein (GHBP) immunoreactivity is primarily located in secretory granules. The possibility that the GHR gene may be similarly expressed in GH(3) cells was therefore investigated, since these tumorous pituitary cells are used as models of GH- and prolactin-secreting cells but lack secretory granules. GHR/GHBP gene transcripts were detected in GH(3) cells and were homologous to hepatic GHR/GHBP transcripts. Immunoreactive GHRs were also detected by Western Blotting and GHBP/GHR immunoreactivity was localized by immunogold electron microscopy within each intracellular compartment. Although the immunoreactive GHR/GHBP content of these cells was less than that in rat tissues, GHBP release from GH(3) cells into incubation media (relative to tissue content) was much greater than GHBP release from rat tissue explants. These results demonstrate GHR expression in GH(3) cells, comparable with that in normal pituitary cells. These tumorous somatolactotrophs could thus provide a model for studies on GHR/GHBP synthesis and release and for studying GH effects on pituitary function. The release of GHBP from pituitary cells also indicates, for the first time, an extra-hepatic source of the plasma GHBP.

Medium flow rate modulates autocrine-paracrine feedback of GH and PRL release by perifused GH3 cells

We previously documented both the spontaneous acceleration of growth hormone (GH) and prolactin (PRL) production by GH3 cells during perifusion and the suppression of their production during plate culture. We here present the role played by medium flow itself in this differential behavior. Increasing rates of perifusion flow (pump rates of 1 to 5 ml/h, equivalent to chamber flow rates of 0.19 to 1.3 microliters.min-1.mm-2 of cross-sectional area) were associated with enhanced GH and PRL secretion. Flow rate-dependent basal hormone secretion rates were established quickly and were stable for the first 10 to 14 h of perifusion. The previously documented independent, spontaneous, and continuously accelerating production of both hormones that followed during the subsequent 40 (PRL) to 60 (GH) h of perifusion was also shown to be flow-rate related. Any time the rate of medium flow was changed within an experiment, the rate of hormone secretion was modulated. However, that modulation did not interrupt ongoing flow-associated acceleration of hormone production once the latter had begun. In addition, GH3 cell product(s) from one cell column reversibly inhibited secretion from cells in a downstream column. The inhibition did not occur when cells in the downstream column had been exposed to trypsin. Other work had suggested that neither GH, PRL, insulinlike growth factor-I, leucine, nor nutrient exhaustion were responsible for the effect. These data are consistent with autocrine-paracrine feedback regulation of GH3 cells by a secretory product(s). Feedback would thus provide a mechanism to effect flow-rate-dependent modulation of GH and PRL release, and to explain accelerating hormone production during perifusion.