Hypoxic inhibition of respiratory neural regulation in anesthetized rats.

To determine the neural mechanism of hypoxic respiratory inhibition, discharge patterns of efferent phrenic (Phr), vagal superior laryngeal (Xsl), and vagal pharyngeal (Xphar) nerves were analyzed during systemic hypoxia in the urethane-anesthetized, vagotomized and artificially ventilated rat. In the carotid sinus nerve (CSN) intact rat, moderate hypoxia (end-tidal Po2, 40-50 mmHg) caused an initial increase in respiratory activity which was followed by inhibition due to reduction in respiratory frequency (f). The decrease in f was associated with prolongation of decremental Xphar expiratory (E) activity and retardation of the onset of inspiratory (I) activity. Integrated peak Phr or Xs1 I and Xphar E activities remained augmented during respiratory inhibition. After bilateral CSN section, moderate hypoxia produced an extreme reduction in f due to delayed onset of I activity and a strong reduction in the Xphar E activity. Phr and Xs1 I activities were little affected, and changes in inspiratory time were small. These results suggest that hypoxia centrally inhibits the process of initiating the onset of rhythmic I activity and the activity of decremental Xphar E motoneurons. Carotid chemoreceptor stimulation was inadequate to offset the central inhibitory effect of hypoxia on the onset of I activity.