Thermogenesis in newborn rats after prenatal or postnatal hypoxia

Oxygenconsumption (O₂)was measured in normoxia as ambient temperature(T_a) was lowered from 40 to15°C, at the rate of 0.5°C/min (thermoneutrality ~33°C).In 2-day-old rats born in hypoxia after hypoxic gestation, theT_a-O₂relationship was as in controls; their interscapular brown adiposetissue (IBAT) was hypoplastic (less proteins and DNA), with lowerconcentration of the mitochondrial uncoupling proteinthermogenin. In 8-day-old rats exposed to hypoxiapostnatally (day 2 today 8), at anyT_a below thermoneutralityO₂ was higher than incontrols; also, in this group IBAT was hypoplastic with decreasedthermogenin. Additional measurements under variousexperimental conditions indicated that the increased thermogeniccapacity was not explained by the smaller body mass and increased bloodoxygen content or by the eventuality of intermittent cold stimuliduring the chronic hypoxia. On the other hand, chronic hypercapnia (3%CO₂ in normoxia, fromday 2 to day8) also resulted in increased normoxic thermogenesis. We conclude that chronic hypoxia in the perinatal period1) reduces IBAT mass andthermogenin concentration and2) can increase the newborn's thermogenic capacity because of stress-related mechanisms not specific to hypoxia.