Reversible umbilical cord occlusion: effects on thermogenesis in utero.

ABSTRACT: The initiation of thermogenesis at birth is an important adaptation for survival. We examined the sequential effects of cooling, increased oxygenation, and repeated episodes of umbilical cord occlusion on nonshivering thermogenesis in six fetal sheep at 139 to 145 d of gestation. The fetal sheep were cooled by circulating cold water through a coil placed around the trunk for 4 h. The fetal core temperature fell 2.47 ± 0.24°C in the first 60 min of cooling with minimal changes in plasma FFA and glycerol levels. After fetal arterial O2 tension was increased above 6.65 kPa by ventilation, fetal temperature and thermogenic indices rose significantly in 60 min. After occlusion of the umbilical cord by a reversible occluder cuff, plasma FFA levels rapidly increased to 635 ± 69 μEq/L (p < 0.005) by 30 min, fetal temperature increased a further 0.96 ± 0.20°C (p < 0.001) and fetal O2 consumption peaked at 25.3 ± 4.9 mL·min-1·kg-1. Release of cord occlusion caused a rapid fall in FFA to 149 ± 23 μEq/L (p < 0.005) and a fall in fetal core temperature of 0.90 ± 0.13°C (p < 0.001) in 30 min. After irreversibly snaring the umbilical cord, the plasma FFA rose to 611 ± 83 μEq/L (p < 0.005) and the fetal temperature rose 0.78 ± 0.09°C (p < 0.02). The effects on thermogenesis of interrupting and reestablishing placental flow are rapid and reversible and suggest the presence of placental inhibitors of brown adipose tissue thermogenesis.