Expiratory motor responses in the suckling opossum.

To evaluate developmental aspects of expiratory muscle utilization, ventilation and electromyographic (EMG) activity from abdominal muscles were measured in unanesthetized suckling opossums. From about 20-35 days of age, breathing against a continuous positive airway pressure (CPAP) or inhalation of hypercapnic and asphyxiant test gases (ventilatory chemostimulation) increased or initiated a sustained expiratory-phased abdominal EMG discharge. Using younger animals, such responses could not be demonstrated during test manipulations, or pattern of activation was not typical of that observed in the older group. In another series of experiments, vagal mechanisms underlying expiratory abdominal muscle responses were evaluated by using lightly anesthetized animals. Unilateral vagotomy reduced abdominal EMG levels during ventilatory chemostimulation while minimally affecting breathing pattern; in addition, abdominal muscle activity during CPAP was of slower onset than before vagotomy. Slow onset of abdominal muscle activity was also seen in relatively young intact unanesthetized animals during responses to CPAP. Thus vagal afferents mediate expiratory motor responses during chemostimulation of breathing and may account, in part, for maturation of expiratory responses to CPAP.