Circadian pattern of ventilation during prolonged hypoxia in conscious rats

In mammals, metabolic rate is well known to present a circadian oscillation. Because metabolism is a major determinant of the magnitude of the hypoxic ventilatory response, we hypothesized that the level of this response would follow a circadian pattern. To this end, we measured pulmonary ventilation (VE), oxygen consumption (V(O(2))), body temperature (Tb) and activity simultaneously and continuously in conscious adult rats by non-invasive methods. Measurements were made in a 12:12-h light (L):dark (D) cycle for 3 days in air, and then for 4 days in 10.5% normobaric hypoxia (HX). In normoxia, all variables oscillated, with the highest values in D. In HX, the circadian Tb and V(O(2)) oscillations were blunted, due to a decrease in their D values. The hypoxic VE response (% increase in VE from the corresponding air value) was greater in L than in D. This L-D difference was proportionate to that of the V(O(2)) response such that the hyperventilatory response (% increase in VE/V(O(2))) was similar throughout the day. The VE/V(O(2)) response was also similar between L and D when it was compared for the same level of activity; in this case, however, there was no L-D difference in the VE or V(O(2)) response. We conclude that the circadian oscillation in the hypoxic VE response was related to the time-of-day changes in the effect of HX on V(O(2)), and that, in the rat, the presence of a circadian clock does not compromise the hyperventilatory response to HX, as this response presents no time-of-day variation.