Threshold of hormonal and biophysical responses to acute hypoxemia in fetal sheep at different gestational ages

Abstract

We examined whether there was a threshold for change in fetal arterial [Formula: see text] to elicit alterations in plasma adrenocorticotropic hormone, arginine vasopressin, or cortisol, or to affect the incidence of fetal breathing movements or eye movements and we determined whether such a threshold changed with gestational age. Fetal sheep were exposed to two levels of hypoxemia, mild (4.6–5.3 mmHg [Formula: see text] drop) and moderate (8.3–8.8 mmHg [Formula: see text] drop) (1 mmHg = 133.322 Pa) for 1 h without pH change at 125–129 or 134–147 days of gestation within 7 days of spontaneous labor. Hypoxemia was induced by altering the inspired percent oxygen of the mother. No significant hormonal and biophysical changes were observed in mild hypoxemia at either age. In moderate hypoxemia at 125–129 days of gestation, there were significant increases of fetal adrenocorticotropic hormone, arginine vasopressin, and cortisol concentrations, and a decreased incidence of fetal breathing movements and eye movements. At 134–147 days of pregnancy, moderate hypoxemia induced a significant increase in adrenocorticotropic hormone, but the response was less than at 125–129 days of gestation. The arginine vasopressin response was similar to that at 125–129 days and there was no significant change in cortisol. There was a significant decrease in fetal breathing movements but not in eye movements. We conclude that a threshold of fetal arterial [Formula: see text] drop exists between 5 and 8 mmHg to elicit endocrine or biophysical responses to hypoxemia in fetal sheep at 125–129 days of gestation. There was a similar threshold for hypoxemia effects at 134–147 days of gestation within 7 days of spontaneous labor on adrenocorticotropic hormone, arginine vasopressin, and fetal breathing movements, but not on cortisol and fetal eye movements.Key words: fetal sheep, hypoxemia, adrenocorticotropic hormone, arginine vasopressin, cortisol, fetal breathing movements, fetal eye movements.