Carotid body hypercapnia does not elicit ventilatory acclimatization in goats

Abstract

The carotid body (CB) perfusion model utilizes surgical vascular ligations to allow isolated blood supply to a single in situ CB in awake goats. The contralateral CB was excised. By use of an extracorporeal pump-oxygenator system the blood gas composition perfusing the CB can be controlled independently from that of the systemic arterial system including the brain. Using this model we compared the responses of systemically normoxic goats to CB hypercapnia and CB hypoxia. In 6 goats CB stimulation with hypercapnic-normoxic blood (mean PcbCO2 = 78 Torr, mean PcbO2 congruent to 100 Torr) produced acute hyperventilation (mean decrease in PaCO2 of 5.2 Torr, P less than 0.05) which remained constant over the 4-h perfusion period. Lack of a progressively increasing hyperventilation indicates that ventilatory acclimatization did not occur with hypercapnic CB perfusion. Hypoxic-normocapnic CB stimulation (mean PcbO2 = 40 Torr, mean PcbCO2 = 39 Torr) produced an acute mean decrease in PaCO2 of 5.5 Torr (P less than 0.05) in 6 additional goats. In contrast to CB hypercapnia, the acute hyperventilation induced by CB hypoxia was followed by a progressive time-dependent additional mean decrease in PaCO2 of 5.6 Torr (P less than 0.05) over a 4-h period (ventilatory acclimatization). These data are compatible with the concept of separate receptor mechanisms for hypercapnia and hypoxia in the CB and suggest that the early phase of ventilatory acclimatization to hypoxia in goats may result from a time-dependent increase in CB afferent output.