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 temp erature on the control of thermoregulation. Heat production was measur ed 20-30 h after birth during non-rapid-eye-movement sleep at 29 and 1 4 degrees C. At 29 degrees C there was no difference in heat productio n between groups, but at 14 degrees C cool-delivered (CD) lambs exhibi ted a 62% greater metabolic response. Irrespective of delivery tempera ture, 15 of the 18 lambs used shivering thermogenesis during cold expo sure, indicating a reduction in the ability to use nonshivering thermo genesis in brown adipose tissue (BAT). Mean plasma concentrations of t hyroxine and triiodothyronine were 35 and 45% greater, respectively, i n CD lambs than in warm-delivered lambs. The level of guanosine 5'-dip hosphate binding in BAT was lower than in normally delivered lambs and was not different between CD and warm-delivered lambs. Cesarean secti on delivery prevents the rise in BAT thermogenic activity, which resul ts in an increased reliance on shivering thermogenesis to maintain col onic temperature. Under these conditions, delivery into a cool environ ment increases the plasma concentration of thyroid hormones, which ben efits the neonate by enabling a greater thermogenic response via shive ring.