Integrated phrenic responses to carotid afferent stimulation in adult rats following perinatal hyperoxia.

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1. Hypoxic ventilatory responses are greatly attenuated in adult rats exposed to moderate hyperoxia (60% O2) during the first month of life (perinatal treated rats). The present study was designed to test the hypothesis that perinatal hyperoxia impairs central integration of carotid chemoreceptor afferent inputs, thereby diminishing the hypoxic ventilatory response. 2. Time-dependent phrenic nerve responses to electrical stimulation of the carotid sinus nerve (CSN) and steady-state relationships between CSN stimulation frequency and phrenic nerve output were compared in control and perinatal treated rats. The rats were urethane anaesthetized, vagotomized, paralysed and artificially ventilated. End-tidal CO2 was monitored and maintained at isocapnic levels; arterial blood gases were determined. 3. Two stimulation protocols were used: (1) three 2 min episodes of CSN stimulation (20 Hz, 0.2 ms duration, 3 x threshold), separated by 5 min intervals; and (2) nine 45 s episodes of CSN stimulation with stimulus frequencies ranging from 0.5 to 20 Hz (0.2 ms duration, 3 x threshold), separated by 4 min intervals. 4. The mean threshold currents to elicit phrenic responses were similar between groups. Burst frequency (f, burst min-1), peak amplitude of integrated phrenic activity (integral of Phr), and minute phrenic activity (integral of Phr x f) during and after CSN stimulation were not distinguishable between groups in either protocol at any time or at any stimulus intensity (P > 0.05). 5. Perinatal hyperoxia does not alter temporal or steady-state phrenic responses to CSN stimulation, suggesting that the central integration of carotid chemoreceptor afferent inputs is not impaired in perinatal treated rats. It is speculated that carotid chemoreceptors per se are impaired in perinatal treated rats.

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