The proposed research involves defining the detailed molecular mechanisms by which thyrotropin-releasing hormone (TRH, thyroliberin) stimulates the secretion (release of preformed hormone) of thyrotropin (thyroid-stimulting hormone, TSH) and prolactin (PRL) from the anterior pituitary gland. Two homogeneous populations of pituitary cells are used in these studies so that one may unambiguously assign changes in various aspects of cellular calcium homeostasis to the appropriate cell type. These are a mouse (LAF1/J) thyrotropic pituitary tumor from which cells are derived for short-term studies in culture (TtT cells) and a cloned cell line (GH3 cells) originally derived from a rat mammotropic pituitary tumor. Some comparative studies in which measurement only of TSH and PRL are made will be performed in vitro using cells derived from normal female Sprague-Dawley rats. It has recently been demonstrated that TRH causes a rapid elevation of cytoplasmic free Ca2+ concentration ([Ca2+])i and it has been proposed that this increase in [Ca2+]i serves to couple, at least in part, stimulation by TRH to secretion of TSH and PRL. In this proposal, further studies of the effects of TRH on [Ca2+]i will be made and additive, synergistic and/or antagonistic effects of other regulators of TSH and PRL secretion will be determined. The cellular pools of calcium mobilized by TRH will be determined in intact and detergent-permeabilized cells. A possible second messenger fuction if inositoltrisphosphate in TRH mobilization of cellular calcium and the mechanism of inositoltrisphosphate action will be studied in detergent-permeabilized cells and in isolated cellular organelles. Lastly, studies will be made of secretagogues, such as arachidonic acid and phorbol esters, that appear to act without causing an elevation of [Ca2+]i. The additivity or synergism of stimulation of secretion by arachidonic acid and phorbol esters with secretagogues tht act via elevating [Ca2+]i will be studied as will their effects on cellular calcium homeostasis. It is believed that a more complete understanding of how TRH acts will serve to expand our understanding of the mechanism of action of many cell-surface interacting regulators and allow us to understand better some pathologic conditions that involve abnormalities in cell regulation.
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