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      Differential Management of Ca 2+ Oscillations by Anterior Pituitary Cells: A Comparative Overview

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          Most electrical and ionic properties of anterior pituitary cells are common to all pituitary cell types; only gonadotropes exhibit a few cell specific features. Under basal conditions, the majority of pituitary cells in vitro, irrespective of their cell type, display spontaneous action potentials and [Ca<sup>2+</sup>]<sub>i</sub> transients that result from rhythmic Ca<sup>2+</sup> entry through L-type Ca<sup>2+</sup> channels. The main function of these action potentials is to maintain cells in a readily activable responsive state. We propose to call this state a ‘pacemaker mode’, since it persists in the absence of extrinsic stimulation. When challenged by hypothalamic releasing hormones, cells exhibit two distinct response patterns: amplification of pacemaker activity or shift to internal Ca<sup>2+</sup> release mode. In the internal Ca<sup>2+</sup> release mode, [Ca<sup>2+</sup>]<sub>i</sub> oscillations are not initiated by entry of external Ca<sup>2+</sup>, but by release of Ca<sup>2+</sup> from intracellular stores. In somatotropes and corticotropes, GHRH or CRH triggers the pacemaker mode in silent cells and amplifies it in spontaneously active cells. In contrast, in gonadotropes GnRH activates the internal Ca<sup>2+</sup> release mode in silent cells and switches already active cells from the pacemaker to the internal Ca<sup>2+</sup> release mode. Interestingly, homologous normal and tumoral cells display the same type of activity in vitro, in the absence or presence of hypothalamic hormones. Pacemaker and internal Ca<sup>2+</sup> release modes are likely to serve different purposes. Pacemaker activity allows long-lasting sequences of [Ca<sup>2+</sup>]<sub>i</sub> oscillations (and thus sustained periods of secretion) that stop under the influence of hypothalamic inhibitory peptides. In contrast, the time during which cells can maintain internal Ca<sup>2+</sup> release mode depends upon the importance of intracellular Ca<sup>2+</sup> stores. This mode is thus more adapted to trigger secretory peaks of large amplitude and short duration. On the basis of these observations, theoretical models of pituitary cell activity can be proposed.

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          Most cited references 12

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          Pit-1-dependent expression of the receptor for growth hormone releasing factor mediates pituitary cell growth.

           C-C Lin,  S. Lin,  C-P Chang (2016)
          In Snell (dw) and Jackson (dwJ) dwarf mice, mutations in the gene encoding Pit-1, a tissue-specific POU-domain transcription factor, lead to the absence of somatotroph, lactotroph and thyrotroph cells. Pre-somatotroph proliferation is stimulated by increased intracellular levels of cyclic AMP, normally induced by growth hormone releasing factor (GRF; refs 7-17). Here we report the cloning of mouse and rat complementary DNAs encoding a new member of the seven-transmembrane-helix, G-protein-coupled receptor family restricted to the pituitary gland, which mediates increases in intracellular cAMP and cAMP-dependent gene transcription in response to GRF. The receptor is expressed in a spatial and temporal pattern corresponding precisely to growth hormone gene expression, and neither is expressed in dw/dw mice. The pituitary hypoplasia in these mice thus appears to be due, at least in part, to the absence of GRF receptor, which is in turn due to the absence of functional Pit-1.
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            Oscillations of cytosolic Ca2+ in pituitary cells due to action potentials.

            Electrical activity in non-neuronal cells can be induced by altering the membrane potential and eliciting action potentials. For example, hormones, nutrients and neurotransmitters act on excitable endocrine cells. In an attempt to correlate such electrical activity with regulation of cell activation, we report here direct measurements of cytosolic free Ca2+ changes coincident with action potentials. This was achieved by the powerful and novel combination of two complex techniques, the patch clamp and microfluorimetry using fura 2 methodology. Changes in intracellular calcium concentration were monitored in single cells of the pituitary line GH3B6. We show that a single action potential leads to a marked transient increase in cytosolic free calcium. The size of these short-lived maxima is sufficient to evoke secretory activity. The striking kinetic features of these transients enabled us to identify oscillations in intracellular calcium concentration in unperturbed cells resulting from spontaneous action potentials, and hence provide an explanation for basal secretory activity. Somatostatin, an inhibitor of pituitary function, abolishes the spontaneous spiking of free cytosolic Ca2+ which may explain its inhibitory effect on basal prolactin secretion. Our data therefore demonstrate that electrical activity can stimulate Ca2+-dependent functions in excitable non-neuronal cells.
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              Calcium signalling in single growth hormone-releasing factor-responsive pituitary cells

               L Cuttler (1992)

                Author and article information

                S. Karger AG
                September 1998
                18 September 1998
                : 68
                : 3
                : 135-151
                Dynamique des Systèmes Neuroendocriniens, INSERM U159, Paris, France
                54360 Neuroendocrinology 1998;68:135–151
                © 1998 S. Karger AG, Basel

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                Page count
                Figures: 7, Tables: 2, References: 106, Pages: 17


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