The circadian pattern of breathing in conscious adult rats

Recently, a circadian oscillation in pulmonary ventilation ($\text{V}\text{̇}$e) was reported in conscious, undisturbed rats [Respir. Physiol. 120 (2000) 179], with a pattern similar to that of body temperature (Tb), oxygen consumption () and activity. The present study explored the relationship between the daily $\text{V}\text{̇}$e 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 $\text{V}\text{̇}$e by the barometric method and 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 $\text{V}\text{̇}$e and . 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 $\text{V}\text{̇}$e. The Vt rhythm was phase-advanced relative to those of and Tb. L–D differences in $\text{V}\text{̇}$e 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 $\text{V}\text{̇}$e does not depend on the daily changes in activity. Rather, the daily pattern of $\text{V}\text{̇}$e is likely shaped by the oscillations of multiple physiological variables, two of which may be Tb and .