Umbilical cord occlusion but not increased plasma T3 or norepinephrine stimulate brown adipose tissue thermogenesis in the fetal sheep.
Nonshivering thermogenesis is normally inactive in utero but increases with supplemental oxygenation and again after occlusion of the umbilical cord. To test the hypothesis that brown fat responses are triggered by the surge in triiodothyronine (T3) which occurs at birth, we studied 7 fetal sheep at 132-143 days gestation. Fetuses were first cooled 2-3 degrees C by circulating cold water through an external coil in the amniotic fluid and then ventilated with oxygen in utero to raise arterial PO2 to 109 +/- 10 (SEM) mmHg. An hour later T3 was infused intravenously to elevate and maintain plasma levels at 39.8 +/- 6.1 nmol/l, some 40-50 times basal levels. Indices of brown heat production did not rise during the next 30 min. Following snaring of the umbilical cord, however, plasma free fatty acid levels increased 400% to 423 +/- 91 mEq/l, plasma glycerol rose 350% to 766 +/- 168 mmol/1, and the temperature difference between brown fat and body core widened to 0.59 +/- 0.13 degrees C during the next 30 min. Whole body oxygen consumption peaked at 23.1 +/- 2.8 ml.min-1.kg-1 body weight. These responses to cord occlusion were similar with and without T3 administration. Changes in plasma catecholamines during these experiments did not correlate with the onset of nonshivering thermogenesis. We conclude that the rise in T3 or the changes in plasma catecholamines which occurs at birth are not causally related to the onset of nonshivering thermogenesis.
Power
G G
GG
Department of Pediatrics, University of Auckland, Auckland & Princess Margaret Hospital, Christchurch, New Zealand.
Gunn
T R
TR
Johnston
B M
BM
Nichols
G
G
Gluckman
P D
PD
D002001:Adipose Tissue, Brown / Q000502:physiology*; D000818:Animals; D001833:Body Temperature Regulation; D003080:Cold Temperature; D004837:Epinephrine / Q000097:blood; D005230:Fatty Acids, Nonesterified / Q000097:blood; D005260:Female; D005333:Fetus / Q000502:physiology*; D005990:Glycerol / Q000097:blood; D007700:Kinetics; D009638:Norepinephrine / Q000097:blood; D010101:Oxygen Consumption; D011247:Pregnancy; D012756:Sheep; D014284:Triiodothyronine / Q000097:blood* / Q000494:pharmacology; D014470:Umbilical Cord / Q000502:physiology*
D005230:Fatty Acids, Nonesterified; D014284:Triiodothyronine; D005990:Glycerol; D009638:Norepinephrine; D004837:Epinephrine