The circadian pattern of breathing in conscious adult rats.

Recently, a circadian oscillation in pulmonary ventilation (VE) was reported in conscious, undisturbed rats [Respir. Physiol. 120 (2000) 179], with a pattern similar to that of body temperature (Tb), oxygen consumption (V(O(2))) and activity. The present study explored the relationship between the daily VE pattern and these rhythms. Adult rats (n=23) were instrumented for measurements of Tb and activity by telemetry, and placed in a chamber for measurement of VE by the barometric method and V(O(2)) by an open-flow method. Simultaneous recordings were made continuously for 3 consecutive days in a 12-h light:12-h dark (L:D) cycle. All variables showed substantial daily oscillations, with significantly higher values in the D phase, and approximately proportionate changes in VE and V(O(2)). Daily changes in tidal volume (VT) relative to inspiratory time (TI), rather than in TI relative to total breath duration, accounted for the oscillation in VE. The VT rhythm was phase-advanced relative to those of V(O(2)) and Tb. L-D differences in VE persisted when comparison between the phases was made for the same level of either very low or very high activity. We conclude that the oscillation in VE does not depend on the daily changes in activity. Rather, the daily pattern of VE is likely shaped by the oscillations of multiple physiological variables, two of which may be Tb and V(O(2)).