Ventilatory effects of specific carotid body hypocapnia and hypoxia in awake dogs.

Specific carotid body (CB) hypocapnia in the-10-Torr (less than eupneic) range reduced ventilation in the awake and sleeping dog to the same degree as did CB hyperoxia [CB PO2 (PCBO2); > 500 Torr; C.A. Smith, K.W. Saupe, K. S. Henderson, and J. A. Dempsey. J. Appl. Physiol. 79:689-699, 1995], suggesting a powerful inhibitory effect of hypocapnia at the carotid chemosensor over a range of PCO2 encountered commonly in physiological hyperpneas. The primary purpose of this study was to assess the ventilatory effect of CB hypocapnia on the ventilatory response to concomitant CB hypoxia. The secondary purpose was to assess the relative gains of the CB and central chemoreceptors to hypocapnia. In eight awake female dogs the vascularly isolated CB was perfused with hypoxic blood (mild, PCBO2 approximately equal to 50 Torr or severe, PCBO2 approximately equal to 36 Torr) in a background of normocapnia or hypocapnia (10 Torr less than eupneic arterial PCO2) in the perfusate. The systemic (and brain) circulation was normoxic throughout, and arterial PCO2 was not controlled (poikilocapnia). With CB hypocapnia, the peak ventilation (range 19-27 s) in response to hypoxic CB perfusion increased 48% (mild) and 77% (severe) due to increased tidal volume. When CB hypocapnia was present, these increases in ventilation were reduced to 21 and 27%, respectively. With systemic hypocapnia, with the isolated CB maintained normocapnic and hypoxic for > 70 s, the steady-state poikilocapnic ventilatory response (i.e., to systemic hypocapnia alone) decreased 15% (mild CB hypoxia) and 27% (severe CB hypoxia) from the peak response, respectively. We conclude that carotid body hypocapnia can be a major source of inhibitory feedback to respiratory motor output during the hyperventilatory response to hypoxic carotid body stimulation.