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Department of Physiology, McGill University, Montreal, Quebec, Canada H3G 1Y6
Because the circadian rhythms of oxygen consumption
(
O2) and body temperature
(Tb) could be contributed to by differences in
thermogenesis and because hypoxia depresses thermogenesis in its
various forms, we tested the hypothesis that hypoxia blunts the normal
daily oscillations in O2
and Tb. Adult rats were instrumented for
measurements of Tb and activity by telemetry; O2 was measured by an
open-flow method. Animals were exposed to normoxia (21%
O2), hypoxia (10.5% O2), and normoxia again, each 1 wk in duration, in either a 12:12-h light-dark cycle
("synchronized") or constant light ("free running"). In
this latter case, the period of the cycle was ~25 h. In synchronized
conditions, hypoxia almost eliminated the Tb circadian
oscillation, because of the blunting of the Tb rise during
the dark phase. On return to normoxia, Tb rapidly increased
toward the maximum normoxic values, and the normal cycle was then
reestablished. In hypoxia, the amplitude of the activity and
O2 oscillations averaged,
respectively, 37 and 56% of normoxia. In free-running conditions, on
return to normoxia the rhythm was reestablished at the expected phase of the cycle. Hence, the action of hypoxia was not on the clock itself
but probably at the hypothalamic centers of thermoregulation. Hyperoxia
(40% O2) or hypercapnia (3% CO2) had no
significant effects on circadian oscillations, indicating that the
effects of hypoxia did not reflect an undifferentiated response to
changes in environmental gases. Modifications of the metabolism and
Tb rhythms during hypoxia could be at the origin of sleep
disturbances in cardiorespiratory patients and at high altitude.
biological rhythms; body temperature; chronic hypoxia; oxygen consumption
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