Effect of ambient temperature during 1st day of life on thermoregulation in lambs delivered by cesarean section.

We examined the effect of delivering near-term twin lambs by cesarean section into a warm (30 degrees C) or cool (15 degrees C) ambient temperature on the control of thermoregulation. Heat production was measured 20-30 h after birth during non-rapid-eye-movement sleep at 29 and 14 degrees C. At 29 degrees C there was no difference in heat production between groups, but at 14 degrees C cool-delivered (CD) lambs exhibited a 62% greater metabolic response. Irrespective of delivery temperature, 15 of the 18 lambs used shivering thermogenesis during cold exposure, indicating a reduction in the ability to use nonshivering thermogenesis in brown adipose tissue (BAT). Mean plasma concentrations of thyroxine and triiodothyronine were 35 and 45% greater, respectively, in CD lambs than in warm-delivered lambs. The level of guanosine 5'-diphosphate binding in BAT was lower than in normally delivered lambs and was not different between CD and warm-delivered lambs. Cesarean section delivery prevents the rise in BAT thermogenic activity, which results in an increased reliance on shivering thermogenesis to maintain colonic temperature. Under these conditions, delivery into a cool environment increases the plasma concentration of thyroid hormones, which benefits the neonate by enabling a greater thermogenic response via shivering.