Metabolic and hormonal responses to cooling the fetal sheep in utero.

The metabolic and hormonal effects of cooling 10 fetal sheep in utero (115-142 days of gestation) for 2h were studied. The fetal core temperature fell by 2.81 +/- 0.14 degrees C while the maternal temperature fell 0.86 +/- 0.15 degrees C. This hypothermia caused a significant rise in the fetal and maternal plasma glucose concentrations (P less than 0.001) and a fall in the fetal insulin concentrations (P less than 0.01). The fetal plasma lactate and cortisol concentrations rose rapidly (P less than 0.01) while the growth hormone fell (P less than 0.01) and remained low until cooling ceased when a rapid rebound occurred. There was no significant change in any of the fetal iodothyronines and no elevation of nonesterified free fatty acid concentrations, in contrast to the rapid rise (P less than 0.01) which occurred when newborn lambs were cooled. These observations demonstrate that appropriate glucose, insulin, lactate and cortisol responses to hypothermia have differentiated by 120 days of gestation. However, neither a thyroid hormone response nor an elevation in free fatty acid levels was observed. Thus not all components of the thermogenic response to hypothermia can be demonstrated in the late gestation fetail sheep in utero.