SUMMARYThe 'engine' of fetal metabolism generates heat (3-4 W kg-' in fetal sheep) which has to be dissipated to the maternal organism. Fetal heat may move through the amniotic/allantoic fluids to the uterine wall (conductive pathway; total conductance, 1-1 W 0C-' kg-') and with the umbilical arterial blood flow (convective pathway) to the placenta. Because resistance to heat flow is larger than zero fetal temperature exceeds maternal temperature by about 0 5°C (0.3-1°C). Probably 85 % of fetal heat is lost to the maternal organism through the placenta, which thus serves as the main 'radiator'. Placental heat conductivity appears to be extremely high and this may lead to impaired heat exchange (guinea-pig placenta). A computer simulation demonstrates that fetal temperature is essentially clamped to maternal temperature, and that fetal thermoregulatory efforts to gain thermal independence would be futile. Indeed, when the late gestational fetus in uter^o is challenged by cold stress, direct and indirect indicators of (non-shivering) thermogenesis (oxygen consumption, increase of plasma glycerol and free fatty acid levels) change only moderately. In prematurely delivered lambs, however, cold stress provokes summit metabolism and maximum heat production. Only when birth is imitated in utero (by cord clamping, external artificial lung ventilation and cooling) do thermogenic efforts approach levels typical of extra-uterine life. This suggests the presence of inhibitors of thermogenesis of placental origin, e.g. prostaglandins and adenosine. When the synthesis of prostaglandins is blocked by pretreatment with indomethacin, sheep fetuses react to intra-uterine cooling with vigorous thermogenic responses, which can be subdued by infusion of prostaglandin E2 (PGE2). Since the sheep placenta is known to produce sufficient amounts of PGE2, it seems that the placenta controls fetal thermogenic responses to some extent. This transforms the fetus into an ectothermic organism, and yet allows the newborn the full exploitation of thermoregulatory responses typical of endothermic animals.

Because maximal nonshivering thermogenesis can commence only after occlusion of the umbilical cord, circulating stimulators and inhibitors were hypothesized to alter brown fat activity in the perinatal period. The roles of prostaglandin I2 (PGI2) and PGE2 in the initiation of nonshivering thermogenesis at birth were investigated. Indomethacin (45 mg bolus, 3 mg h-1 thereafter) was infused into 10 near-term fetal sheep to decrease prostanoid synthesis; 6 age-matched fetuses were infused with saline as controls. Sixteen hours later, birth was simulated in utero by sequentially cooling the fetus, ventilating its lungs with oxygen and occluding the umbilical cord. In the control fetuses, the plasma concentrations of PGI2 and PGE2 and free fatty acids, an index of nonshivering thermogenesis, were unaffected by cooling. Ventilation caused the concentration of PGI2 to increase 108% (P < 0.001) and that of PGE2 to decrease 26% (P < 0.05), while fatty acid concentrations increased 100% (P < 0.05). After cord occlusion, PGI2 concentrations remained elevated whereas PGE2 concentrations decreased a further 46% (P < 0.01), and fatty acid concentrations increased a further 100% (P < 0.05). In the indomethacin-treated fetuses, PGI2 and PGE2 concentrations decreased to 20% of the preinfusion values (P < 0.001) and did not change during the experiment. Cooling initiated a 300% increase in fatty acid concentrations (P < 0.05) and ventilation and cord occlusion induced no further significant changes. Thus, prostanoid concentrations follow changes in nonshivering thermogenic activity and support a regulatory role for PGI2 and PGE2 in the initiation of thermogenesis. Before birth, high concentrations of PGE2 favour suppression of thermogenesis, and after birth this inhibition is removed and there is stimulation by PGI2.

No abstract