Peripheral chemoreceptor CO2 response during hyperoxia in the 14-day-old awake lamb.

We studied the interaction of O2 and CO2 at the peripheral chemoreceptors in 6 two-week-old awake lambs. The method used, which selectively tested the peripheral chemoreceptors, measured the immediate ventilatory (VE) response to pure O2 and then to O2 + CO2. From room air, the animal was switched abruptly to either pure O2 or O2 + 5% CO2, O2 + 7.5% CO2, or O2 + 10% CO2. VE was measured before and 8-10 sec after a step change in the inspired gas. In response to pure O2, VE/kg dropped 131 +/- 30 ml/min.kg (38%). Repeat O2 tests performed with the addition of CO2 showed that CO2 interacted to blunt the response to pure O2; the VE change from pre-test baseline to 8-10 sec being -77 +/- 42, -18 +/- 25, and +8.5 +/- 60 for 5%, 7.5%, and 10% added CO2 respectively. Carotid body denervation eliminated the immediate VE responses to O2 and CO2. We conclude that in the 2-week-old awake lamb, (1) hyperoxia suppresses the O2 drive output of the peripheral chemoreceptors, (2) during hyperoxia O2 and CO2 still interact at the chemoreceptor level, and (3) despite hyperoxia, the peripheral chemoreceptors retain a substantial graded response to incremental transient CO2 challenge.