Endocrine responses of fetal sheep to prolonged hypoxemia with and without acidemia: Relation to urine production

Our aim was to examine the endocrine changes associated with alterations in fetal urine production during 24 h of hypoxemia induced by either reduced uterine blood flow (RUBF) or maternal N₂ inhalation (N₂). In contrast to RUBF, which caused a diuresis, N₂ caused a transient antidiuresis; during both posthypoxemia periods (RUBF and N₂), fetal urine production was increased. RUBF, but not N₂, was associated with a transient acidemia. Fetal plasma arginine vasopressin (AVP) and atrial natriuretic factor (ANF) concentrations increased during RUBF and were inversely correlated to pH; there were no detectable AVP or ANF responses to N₂. Fetal prostaglandin E₂ (PGE₂) increased during the hypoxemia and posthypoxemia periods induced by both methods, but RUBF caused the greater increase. AVP and PGE₂ concentrations were positively correlated with urine production. Fetal arterial blood pressure increased during RUBF but not N₂. During RUBF, the increases in AVP and PGE₂ concentrations and/or fetal arterial blood pressure may have contributed to the diuresis. During N₂, we suggest that low, but increased, levels of AVP may have caused the transient antidiuresis, whereas the diuresis observed during both posthypoxemia periods may have been mediated by elevated PGE₂ concentrations and/or increased fetal arterial blood pressure.