A number of clonal cell lines derived from a rat pituitary tumour, collectively termed GH cells, have retained a range of differentiated cell functions, including their ability to secrete the hormones prolactin and growth hormone in response to stimuli such as thyrotropin-releasing hormone (TRH). The mechanisms underlying this release process involve, at least in part, an increase in cytosolic free calcium levels, and the cells have proved useful as a model system in studies of receptor-controlled calcium mobilization. The initial response of the cells to the addition of TRH now appears to be the interaction of the occupied TRH receptor with a GTP-binding protein. A sophisticated signalling system is then activated which initially involves the phosphodiesteratic hydrolysis of phosphatidylinositol 4,5-bisphosphate to 1,2-diacylglycerol and inositol 1,4,5-trisphosphate. Both of these products are important intracellular messengers, and their formation leads to a plethora of biochemical and electrical changes which culminate in the biphasic release of hormone from the cell. The changes in cytosolic free calcium that occur following TRH addition follow a complex temporal pattern. Within 1 s, the concentration starts to increase from a resting level, in the range 100–150 nmol l-1, to a peak value of around 1 mumol l-1 which is attained within 6–8 s. This ‘spike’ of calcium is almost exclusively derived from intracellular stores, probably the endoplasmic reticulum, in response to the formation of inositol 1,4,5-trisphosphate. With high concentrations of the peptide, the cytosolic free calcium concentration declines promptly, due to the activation of a protein kinase C-mediated extrusion and/or sequestration process. This inhibitory phase is less marked at low agonist concentrations but, in all cases, is superseded by a second increase in free calcium, which is due to the stimulated influx of the cation through dihydropyridine-sensitive calcium channels. These biphasic changes in calcium, in concert with the activation of protein kinase C, appear sufficient to regulate prolactin secretion.
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JOURNAL ARTICLE| 01 September 1986
Inositol lipid metabolism and signal transduction in clonal pituitary cells
A. H. Drummond
Online Issn: 1477-9145
Print Issn: 0022-0949
© 1986 by Company of Biologists
J Exp Biol (1986) 124 (1): 337–358.
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A. H. Drummond; Inositol lipid metabolism and signal transduction in clonal pituitary cells. J Exp Biol 1 September 1986; 124 (1): 337–358. doi: https://doi.org/10.1242/jeb.124.1.337
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