VAGAL AND CHEMORECEPTOR INFLUENCES ON ABDOMINAL MUSCLE-ACTIVITY IN AWAKE LAMBS DURING HYPOXIA

The ventilatory response to hypoxia is a complex phenomenon involving several control mechanisms. We designed this study to examine the dyna mic control of abdominal muscle expiratory electromyogram (EMG) activi ty during room-air breathing and hypoxia and then to analyze the relat ive contribution of the chemoreceptors and vagal afferents. We studied 12 11- to 22-day-old awake nonsedated lambs, six intact and six vagot omized. To assess the dynamic influence of peripheral chemoreceptors o n abdominal muscle expiratory activity, we performed transient testing of peripheral chemoreceptor function (pure O2 and N2 inhalation, KCN injection). To assess the influence of central chemoreceptor afferents , we compared results obtained during hypocapnic and isocapnic 15-min hypoxic runs (fractional concentration of inspired O2 0.08) in each la mb. We also compared results obtained in intact and vagotomized lambs so that the importance of vagal afferents could be assessed. We consis tently observed abdominal muscle expiratory EMG activity in each lamb, whether intact or vagotomized, during baseline room air breathing; fu rther recruitment was observed during hypoxia. We also consistently ob served abdominal musele expiratory recruitment during hypocapnic hypox ia in each lamb, although it was significantly less marked than during isocapnic hypoxia. Our transient testing of peripheral chemoreceptor function showed, furthermore, that peripheral chemoreceptor afferents dynamically modulate abdominal muscle expiratory activity. Thus, durin g hypoxia in 11- to 22-day-old awake nonsedated lambs, increased affer ent information from peripheral chemoreceptors forcefully enhances abd ominal muscle expiratory activity. This hypoxia-related recruitment pe rsists, although attenuated, in the presence of hypocapnia with or wit hout vagal information.