Correlations between the circadian patterns of body temperature, metabolism and breathing in rats

It had been demonstrated previously that the circadian patterns of activity and state of arousal are not essential for the manifestation of the daily patterns of pulmonary ventilation (V˙E), tidal volume (VT) and breathing frequency (f). In this study we investigated the extent of the linkage between the circadian pattern of breathing and those of body temperature (Tb) and metabolic rate (oxygen consumption, V˙O2, and carbon dioxide production, V˙CO2). Rats were instrumented for measurements of Tb (by telemetry), and placed in a chamber for continuous 13-day period of measurement of breathing (by a modification of the barometric methodology), and of V˙O2 and V˙CO2 (by an open flow method). After the first 4 days in control conditions under a 12 h light:12 h dark (L:D) cycle, a perturbation was introduced on day 4, with an L-phase prolongation of 12 h, and on day 9, with an D-phase prolongation of 12 h. During the control days 1-4, all variables had daily oscillations (higher values in D), in phase with each other. During the perturbations (days 4-13), changes in Tb, V˙O2 and V˙CO2, averaged over the whole period, correlated significantly better with f than with VT. Day-by-day X-Y loops indicated that V˙E, VT and f could lead significantly the changes of Tb, V˙O2 and V˙CO2, and that these relations changed throughout the perturbation period. In addition, f and VT did not change necessarily in phase with each other. It is concluded that neither the oscillation in Tb nor that in metabolism can be considered the direct cause of the daily oscillation of breathing. Presumably, the circadian pattern of breathing reflects the interplay of the daily patterns of many variables, none acting as the primary guide of the breathing daily rhythm.