Dexamethasone inhibits ovine corticotrophin-releasing factor (oCRF), arginine vasopressin (AVP), and oCRF + AVP stimulated release of ACTH during the last third of pregnancy in the sheep fetus

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Abstract
We examined the hypothesis that in fetal sheep during late pregnancy exogenous glucocorticoids might affect differentially the pituitary response, measured as changes in plasma ACTH concentrations, to the systemic administration of ovine corticotrophin-releasing factor (oCRF), arginine vasopressin (AVP), or oCRF + AVP. At d 113–116 of pregnancy, equimolar injections of oCRF and AVP given separately provoked similar significant increases in plasma ACTH; the change in ACTH over basal values was significantly greater than the sum of the two separate responses when AVP + oCRF were given together. Exogenous dexamethasone did not affect basal ACTH concentrations, but suppressed significantly the responses to oCRF, AVP, and oCRF + AVP. At d 126–130, there was a significant ACTH response to CRF alone and to AVP + oCRF, but not to AVP alone. The response during the first 30 min postinjection to oCRF was significantly less than that to AVP + oCRF. Plasma Cortisol rose after each peptide injection. Exogenous dexamethasone suppressed both basal and stimulated responses to each peptide. At the amounts injected, there was no significant ACTH or Cortisol response to oCRF, AVP, or oCRF + AVP at d 136–140, but dexamethasone suppressed basal ACTH and Cortisol concentrations at this time. We conclude that stimulated, but not basal, release of ACTH is subject to the negative feedback effect of exogenous glucocorticoid by d 113–116 of gestation in fetal sheep. Both basal and stimulated release of ACTH and Cortisol are suppressed after d 125. At the amount of exogenous dexamethasone given, oCRF, AVP, and oCRF + AVP-stimulated responses are affected similarly. Our results suggest different controls of basal and stimulated ACTH release from the pituitary at d 113–116 of gestation. Our findings would be consistent with the pituitary as a level of action for the negative feedback effect of corticosteroids on stimulated ACTH release throughout the last third of pregnancy in fetal sheep.