Chronic hypoxia abolishes expiratory prolongation following carotid sinus nerve stimulation in the anesthetized rat

In anesthetized rats, increases in phrenic nerve (PN) amplitude and frequency during brief periods of hypoxia or electrical stimulation of the carotid sinus nerve (CSN) are followed by an increase in expiratory duration. We investigated the effects of chronic exposure to hypoxia on PN responses to CSN stimulation. In Inactin anesthetized (100 mg/kg) Sprague-Dawley rats PN discharge and arterial pressure responses to 10-120 s of CSN stimulation (20 Hz, 0.2 ms duration pulses) were recorded after 7-10 days exposure to hypoxia (10 +/- .5% O2). In normoxic rats, the degree of CSN-evoked expiratory prolongation was dependent upon the duration of CSN stimulation. CSN-evoked increases in PN burst amplitude were not different comparing chronic hypoxic rats to rats maintained at normoxia while CSN-evoked increases in PN burst frequency were greater in chronic hypoxic rats (p<.05). CSN-evoked expiratory prolongation was abolished in chronic hypoxic rats. Following chronic hypoxia, changes occur within the central processing of arterial chemoreceptor inputs so that CSN stimulation evokes an enhanced PN frequency response and no expiratory prolongation.