Factors influencing the initiation of nonshivering thermogenesis

The aim of this study was to observe the sequential effects of oxygenation, umbilical cord occlusion, and finally cooling on circulating catecholamines and nonshivering thermogenesis in fetal sheep. We studied five fetal sheep at 132 ± 3 days' gestation. The fetuses were first ventilated with oxygen; Pao2 levels were maintained above 150 mm Hg, and by 60 minutes there was a significant rise in both plasma glycerol and free fatty acid levels. After umbilical cord occlusion there was a peak in plasma catecholamine and triiodothyronine levels but no significant increase in lipolysis. Cooling, by circulating cold water through a coil around the fetus, induced maximum lipolysis and temperature difference between brown fat and body core, when plasma catecholamine levels were falling. During this study the changes in plasma catecholamine levels did not correlate with the onset of nonshivering thermogenesis. Cutaneous cooling, which causes neurally mediated sympathetic stimulation of brown adipose tissue, is the major signal for the initiation of nonshivering thermogenesis and thus neonatal adaptation.