Effects of hypoxia and ambient temperature on gaseous metabolism of newborn rats.

Whole body metabolic rate was measured by open-flow respirometry in 2-day-old rats at ambient temperatures (Ta) between 40 and 15 degrees C, changed at a rate of 0.5 degrees C/min, during normoxia or hypoxia (10% inspired O2). In normoxia, the thermoneutral range was found to be very narrow, at around 33 degrees C, suggesting a limited effectiveness in the mechanisms controlling heat dissipation. At lower or higher Ta, metabolism was at first increased; this increase could be maintained for at least 30 min at 30 and 35 degrees C, i.e., slightly below or above thermoneutrality, but it was not maintained at lower Ta. The metabolic response to Ta was not sufficient to maintain body temperature (Tb). In hypoxia, at all Ta, oxygen consumption (VO2) was consistently less than in normoxia and was linearly related to Ta (Q10 approximately 1.4). The rat's specific heat was 4 J.g-1.degrees C-1, and the time constant of passive heat exchange was 2.2 +/- 0.5 min; from these values it was calculated that the normoxic VO2 of the 2-day-old rat could be per se sufficient to maintain Tb of 35-36 degrees C at Ta of 33 degrees C, while at lower Ta the metabolic response could not be adequate to maintain Tb. In hypoxia, Tb was directly dependent on Ta.(ABSTRACT TRUNCATED AT 250 WORDS)