Hypoxia abolishes the morning–night differences of metabolism and ventilation in 6-day-old rats

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Abstract
Rat pups have a higher metabolic rate at night than in the morning; we questioned to what extent this was accompanied by changes in ventilation [Formula: see text] and the effect of hypoxia on the morning–night differences. From birth, the pups were mother reared under a 12 h light: 12 h dark cycle (light on from 07:00 to 19:00). At day 6, oxygen consumption ([Formula: see text], by a flow-through method) and [Formula: see text] (by airflow plethysmography) were measured at 07:30 and 19:30, and again at 07:30 of the next day. In normoxia, all values were higher at 19:30 than at 07:30. At all hours, [Formula: see text] remained constant (36.5 at 33 °C, 32 at 29 °C), because changes in [Formula: see text] matched those of [Formula: see text]. In acute hypoxia (inspired O2 = 10%), [Formula: see text] and [Formula: see text] dropped significantly from normoxia, and more so at 19:30 than at 07:30, resulting in no difference in [Formula: see text], [Formula: see text], or [Formula: see text] among hours. These results suggest that in the rat pup [Formula: see text] control is sufficiently developed to finely track changes in metabolic rate, and that hypoxia interferes with the metabolic and ventilatory morning–night differences.Key words: circadian rhythmicity, neonatal respiration, oxygen consumption, thermoregulation.