The goals of this proposal are to study the regulation of growth hormone (GH) secretion, the hypothalamic releasing and inhibiting hormones (GH-releasing hormone (GRH) and somatostatin (SRIF)) that control the secretion of GH, and to search for alterations in neuroendocrine function that may be responsible for disorders of GH secretion in man. The identification of each of the component hormones in this regulatory system (GRH, SRIF, GH, and insulin-like growth factor-l (IGF-l), the mediator of many of the growth promoting action of GH) and their molecular cloning allows for an integrated approach to the study of this hypothalamic-pituitary system that may serve as a model for other systems. The proposal will utilize techniques of molecular biology, cell culture, protein chemistry, in vivo measurements of hormone levels in hypophyseal portal blood, and clinical studies to study different aspects of this system. The regulation of hypothalamic GRH gene expression will be studied in vivo in response to hormonal and metabolic (starvation, diabetes, obesity) perturbations, and in vitro using GRH gene transfection. The conversion of the GRH precursor to the mature hormone will be studied in transgenic animals expressing the human GRH gene and the role of processing enzymes as potential regulators will be determined. The metabolism of GRH will be studied in circulation and at the sites of GRH action and the enzymes involved will be characterized. The neuroendocrine regulation of GRH and SRIF will be studied by direct measurements of the two neurohormones in portal plasma and in hypothalamic perfusate fluid of unanesthetized sheep and the molecular forms of each hormone will be determined by high performance liquid chromatography. GRH secretion from GRH-containing neuronal cells in culture will be studied and the effects of secretogogues determined. GRH and SRIF action on the pituitary will be studied in the GH deficient lit/lit mouse and membrane fusion will be performed in an attempt to correct the previously reported abnormality presumed to reside in the GRH receptor. Our newly developed somatomammotroph cell line, the first that exhibits GRH sensitivity, will be used to study the effects of GRH and SRIF at cell membrane and nuclear levels. A comparison of exogenous and endogenous (by transfection of the GRH gene) GRH effects will also be compared. The molecular structure of circulating GRH in human plasma will be determined and the mechanism of GH feedback effects in normals and in subjects with disorders of GH secretion will be assessed. The results of these studies will provide a better understanding of the role of the central nervous system in clinical disorders of GH secretion and may ultimately lead to new therapeutic approaches.
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