Thyrotropin-releasing hormone stimulation of prolactin secretion is coordinately but not synergistically regulated by an elevation of cytoplasmic calcium and 1,2-diacylglycerol.
The precise roles of the calcium and lipid pathways in TRH-stimulated PRL secretion from rat pituitary (GH3) cells are controversial. In particular, it is debated whether elevation of cytoplasmic free Ca2+ concentration [( Ca2+]i) is sufficient to cause burst secretion (0-2 min) or whether an increase in 1,2-diacylglycerol must accompany the Ca2+ elevation. In this study, the effects of TRH, which elevates 1,2-diacylglycerol, on [Ca2+]i and stimulation of burst secretion were compared with those of depolarization by high extracellular K+, which does not increase 1,2-diacylglycerol. A maximal concentration of TRH (1 microM) and depolarization by 17.5 mM K+ caused elevation of [Ca2+]i from the resting level of 140 +/- 20 nM to 470 +/- 70 nM and 514 +/- 60 nM, respectively, and stimulated burst secretion from 0.6 +/- 0.2 ng/10(6) cells/min to 3.3 +/- 0.8 and 3.1 +/- 0.4 ng/10(6) cells/min, respectively, when a small component of TRH-stimulated secretion that is independent of elevation of [Ca2+]i was subtracted. A detailed comparison of multiple levels to which [Ca2+]i was elevated (up to 600 nM) and the degree of stimulation of burst phase secretion demonstrated the same positive linear correlation (correlation coefficient = 0.96) for TRH and K+ depolarization. Hence, elevation of [Ca2+]i is sufficient to cause burst secretion irrespective of elevation of 1,2-diacylglycerol. Optimal stimulation by TRH of sustained secretion of PRL did not depend on elevation of [Ca2+]i; sustained PRL secretion stimulated by 10 nM TRH was 2.6 +/- 0.4 and 2.7 +/- 0.2 ng/10(6) cells/min in control cells and arachidonic acid-pretreated cells in which [Ca2+]i was not elevated, respectively. The data from this and previous studies demonstrate that elevation of [Ca2+]i and 1,2-diacylglycerol may act coordinately, but not synergistically, to mediate TRH stimulation of PRL secretion from GH3 cells.
D000818:Animals; D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D002460:Cell Line; D003593:Cytoplasm / Q000502:physiology; D004075:Diglycerides / Q000502:physiology*; D005989:Glycerides / Q000502:physiology*; D007295:Inositol Phosphates / Q000378:metabolism; D008564:Membrane Potentials; D010902:Pituitary Gland / Q000378:metabolism*; D011388:Prolactin / Q000378:metabolism*; D011493:Protein Kinase C / Q000502:physiology; D051381:Rats; D012636:Secretory Rate / Q000187:drug effects; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*
D001095:Arachidonic Acids; D004075:Diglycerides; D005989:Glycerides; D007295:Inositol Phosphates; D016718:Arachidonic Acid; D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin; D011493:Protein Kinase C; D002118:Calcium