HYPOXIA ABOLISHES THE MORNING NIGHT DIFFERENCES OF METABOLISM AND VENTILATION IN 6-DAY-OLD RATS

Rat pups have a higher metabolic rate at night than in the morning; we questioned to what extent this was accompanied by changes in ventilat ion (Ve) and the effect of hypoxia on the morning-night differences. F rom birth, the pups were mother reared under a 12 h light : 12 h dark cycle (light on from 07:00 to 19:00). Al day 6, oxygen consumption (Vo (2), by a flow-through method) and Ve (by airflow plethysmography) wer e 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, Ve/Vo(2) remained constant (36.5 at 33 degrees C, 32 at 29 degrees C) , because changes in Ve matched those of Vo(2). In acute hypoxia (insp ired O-2 = 10%), Vo(2) and Ve dropped significantly from normoxia, and more so at 19:30 than at 07:30, resulting in no difference in Vo(2), Ve, or Ve/Vo(2) among hours. These results suggest that in the rat pup Ve control is sufficiently developed to finely track changes in metab olic rate, and that hypoxia interferes with the metabolic and ventilat ory morning-night differences.