EFFECT OF CHRONIC RESISTIVE LOADING ON HYPOXIC VENTILATORY RESPONSIVENESS

Depression of ventilation mediated by endogenous opioids has been obse rved acutely after resistive airway loading. We evaluated the effects of chronically increased airway resistance on hypoxic ventilatory resp onsiveness shortly after load imposition and 6 wk later. A circumferen tial tracheal band was placed in 200-g rats, tripling tracheal resista nce. Sham surgery was performed in controls. Ventilation and the venti latory response to hypoxia were measured by using barometric plethysmo graphy at 2 days and 6 wk postsurgery in unanesthetized rats during ex posure to room air and to 12% O-2-5% CO2-balance N-2. Trials were perf ormed with and without naloxone (1 mg/kg ip). Room air arterial blood gases demonstrated hypercapnia with normoxia in obstructed rats at 2 d ays and 6 wk postsurgery. During hypoxia, a 30-Torr fall in Po-2 occur red with no change in PCO2. Hypoxic ventilatory responsiveness was sup pressed in obstructed rats at 2 days postloading. Naloxone partially r eversed this suppression. However, hypoxic responsiveness at 6 wk was not different from control levels. Naloxone had a small effect on vent ilatory pattern at this time with no overall effect on hypoxic respons iveness. This was in contrast to previously demonstrated long-term sup pression of CO2 sensitivity in this model, which was partially reversi ble by naloxone only during the immediate period after load imposition . Endogenous opioids apparently modulate ventilatory control acutely a fter load imposition. Their effect wanes with time despite persistence of depressed CO2 sensitivity.