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 (T(a)) 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 T(a), 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 T(a). The metabolic response to T(a) was not sufficient to maintain body temperature (T(b)). In hypoxia, at all T(a), oxygen consumption (VO2) was consistently less than in normoxia and was linearly related to T(a) (Q10 is similar to 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 T(b) of 35-36-degrees-C at T(a) of 33-degrees-C, while at lower T(a) the metabolic response could not be adequate to maintain T(b). In hypoxia, T(b) was directly dependent on T(a). The effects of hypoxia on the metabolic response to T(a) was also investigated in 2-day-old kittens, with results qualitatively similar to those of the rat pups, although the kitten's T(b) was maintained over a larger range of T(a). It is concluded that in the newborn rat during hypoxia 1) the metabolic response to T(a) is abolished, 2) VO2 becomes linearly dependent on T(a), and 3) VO2 is reduced mostly because of the decrease in thermoregulatory mechanisms.