Effect of chronic resistive loading on ventilatory control in a rat model.

Acute resistive loading of the airway has been shown to activate the endogenous opioid system, with subsequent depression of ventilation. The present investigation was designed to assess the effect of chronic airway loading on ventilation and CO2 sensitivity, and to determine whether the endogenous opioid system contributes to long-term modulation of ventilatory control in this setting. A flow-resistive ventilatory load was imposed in 2-mo-old rats by surgical implantation of a circumferential tracheal band that approximately tripled tracheal resistance. Respiration and CO2 sensitivity were serially and noninvasively assessed by barometric plethysmography over a period of 21 wk. Ventilatory output was assessed as minute inspiratory effort, which was defined as the product of plethysmograph signal amplitude, inspiratory time, and respiratory rate (RR). CO2 sensitivity was calculated as the percent change in minute inspiratory effort from room air to CO2 exposure. The effect of naloxone administration on these parameters was also determine. Arterial blood gases demonstrated hypercapnia with maintenance of normoxia in loaded rats; these findings persisted for the duration of the study. Two days after surgery, rats with tracheal obstruction demonstrated a lower RR than controls during room air breathing and during CO2 stimulation. CO2 sensitivity was significantly depressed in obstructed animals at this time. Escape from suppression of RR and CO2 sensitivity was evident by 14 to 21 d after obstruction; however, suppression of these parameters reappeared and was maintained from 56 to 147 d after obstruction. Naloxone augmented minute inspiratory effort during CO2 stimulation at 2 d after obstruction but not thereafter; naloxone had no effect in control rats. These data indicate that chronic airway loading suppresses RR and CO2 sensitivity in a triphasic manner. The early suppression is partially reversible by naloxone; late-appearing suppression is unaffected by naloxone and is presumably mediated by mechanisms that do not involve endogenous opioids.