Submitted on July 24, 2008
Revised on December 16, 2008
Accepted on December 30, 2008
Postnatal development of metabolic rate during normoxia and acute hypoxia in rats: implication for a sensitive period
Qiuli Liu1, Charles Fehring1, Timothy F. Lowry1, and Margaret T. T. Wong-Riley1*
1 Medical College of Wisconsin
* To whom correspondence should be addressed. E-mail: mwr{at}mcw.edu.
Previously, we reported that the hypoxic ventilatory response (HVR) in rats was weakest at postnatal day P13, concomitant with neurochemical changes in respiratory nuclei. A major determinant of ventilation (VE) is reportedly the metabolic rate (O2 consumption and CO2 production). The present study aimed at testing our hypothesis that daily metabolic rates changed in parallel with ventilation during development, and that a weak HVR at P13 was attributable mainly to an inadequate metabolic rate in hypoxia. Ventilation and metabolic rates were monitored daily in P0-21 rats. We found that a) ventilation and metabolic rates were not always correlated, and VE/VO2 and VE/VCO2 ratios were not constant during development; b) metabolic rate and VE/VO2 and VE/VCO2 ratios at P0-P1 were significantly different from the remaining first postnatal week in normoxia and hypoxia; c) at P13, metabolic rates and VE/VO2 and VE/VCO2 ratios abruptly increased in normoxia and were compromised in acute hypoxia, unlike more stable trends during the remaining second and third postnatal weeks; and d) the respiratory quotient (VCO2/VO2) was stable in normoxia and fluctuated slightly in hypoxia at P0-21. Thus, our data revealed heretofore unsuspected metabolic adjustments at P0-1 and P13. At P0-P1, ventilation and metabolic rates were uncorrelated, whereas at P13, they were closely correlated under normoxia and hypoxia. The findings further strengthened the existence of a critical period of respiratory development around P13, when multiple physiological and neurochemical adjustments occur simultaneously.
