Abstract
The circadian change of ventilatory responses to hypoxic stimuli have rarely been studied in experimental animals. We examined the ventilatory and metabolic responses to hypoxia in the light and dark periods in wild-type (WT) and histamine H1 receptors knockout (H1RKO) mice with a whole-body plethysmograph and an open-circuit system by means of magnetic-type mass spectrometry. Animals were subjected to a 20-min hypoxic gas exposure (7% O2 and 3% CO2 in N2) after acclimatization to the chamber for 90 min. In both groups of mice, hypoxia initially increased minute ventilation (VE) and then decreased it. However, we found differences in the ventilatory response to hypoxia (HVR) between WT and H1RKO mice. In WT mice, the VE response to hypoxia was higher in the dark period than those in the light period. Hypoxia decreased metabolic rate in WT mice. O2 consumption and CO2 excretion (VCO2) in WT mice were higher in the dark period than light period. However, in H1RKO mice, changes in VE between the light and dark periods were minimal. There were no changes in VCO2 between the light and dark periods, between normoxia and hypoxia. Thus, there was a circadian light/dark cycle in the HVR in WT mice, whereas the cycle disappeared in H1RKO mice. These results suggest that H1 receptors play a part in the light/dark cycle variations in HVR via affecting metabolic control. [J Physiol Sci. 2007;57 Suppl:S214]
