Alterations in the hypoxic ventilatory response with advancing age in awake rats

This study seeks to determine the influence of aging on hypoxic ventilatory responsiveness. We addressed the issue by comparing the hypoxic ventilatory responses in three age-groups of conscious rats: 3, 12, and 24 months old animals. Ventilation was recorded in a whole body rodent plethysmograph. Minute ventilation (V(E)), respiratory rate, and tidal volume were considered for analysis. The rats were subjected to two levels of acute hypoxia: 14 and 11% O(2) in N(2). Hypoxic exposures were separated by a 15 min recovery interval in air. The part of the study between the 12 and 24 months age interval was longitudinal in that the same animals were studied twice, whereas the youngest animals belonged to a separate breed. All data were normalized to body mass. All hypoxic responses were biphasic with the stimulatory peak at 0.5 min after onset of hypoxia. The results demonstrate that there were no appreciable differences in magnitude of the peak hypoxic V(E) responses between 3 and 12 months old rats. The hypoxic V(E) responses and also the hypoxic ventilatory gain were, however, enhanced in the senescent rats. In these rats, the increment in peak V(E) from 14 to 11% hypoxia amounted to 364.1+/-95.8 ml.min(-1).kg(-1), which was more than double compared with 12 and 3 months old rats (P<0.02). We conclude that ventilatory responsiveness is not curtailed in senescent rats. The respiratory system is able to compensate for any age-related handicaps in the respiratory system to maintain a stimulatory response to ventilatory stress.