The essential nature of the pituitary-adrenal system for normal fetal development is well recognized. Glucorticoids produced by the system subserve a diverse series of roles in fetal life from stimulating pulmonary surfactant production to inducing enzymes in the liver important for metabolism. However, maturation of pituitary-adrenal function remains poorly understood. We wish to test, in the present application, three specific, but interrelated, hypotheses focusing on maturation of fetal pituitary-adrenal function. The first hypothesis is that the decrease in CRH R1 receptor expression and decline in ACTH responses to CRH observed in late gestation are the result of the increase in fetal plasma cortisol occurring then. The second hypothesis is that the increase in ACTH responses to vasopressin stimulation found in late gestation are related to increased generation of the second messenger, inositol triphosphate, which mediates ACTH release from corticotrophs. The third hypothesis is that hypothalamic-pituitary disconnection which disrupts corticotroph maturation reduces the ratio of ACTH1-39 to ACTH precursors and thereby arrests the development of the fetal adrenal to secrete the primary glucocorticoid in sheep, cortisol. These three hypotheses are directed toward defining mechanisms involved in the normal phenotypic change in corticotrophs which take place in fetal life and the functional impact of these corticotroph changes on the adrenal. The fetal sheep is used for these experiments because we have established the nature of the phenotypic changes in corticotrophs which occur and we can surgically manipulate the fetus to interfere with corticotroph maturation. Our studies will employ specific assays for the measurement of ACTH1-39 and its precursors, sensitive RNase protection assays for quantifying message levels for the CRH R1, vasopressin V1b and ACTH receptors and silencer RNA techniques to ablate gene products in the fetal adrenal. Interpretation of mRNA levels will be complimented by determination of protein expression/receptor binding. Our approach will be to use cellular, molecular and whole animal techniques to clearly define the functional significance of the changes in corticotrophs and the mechanisms involved in these changes and in their impact at the fetal adrenal.
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