Chronic hypercapnia resets CO2 sensitivity of avian intrapulmonary chemoreceptors

Avian intrapulmonary chemoreceptors (IPC) are vagal sensory neurons that pa rticipate in the control of breathing. IPC action potential frequency is in versely proportional to Pco(2), but it is unclear whether low Pco(2) Or hig h pH is the immediate stimulus for signal transduction in IPC, To address t his question, comparisons were made between single cell neural responses of 34 IPC recorded in 6 anesthetized ducks (Anas platyrhynchos) acclimatized 12 days to 7.5% inspired CO2 and 22 IPC recorded in 9 normal anesthetized d ucks. We hypothesized that if respiratory-linked pH changes determine IPC a ctivity, action potential frequency as a function of inspiratory Pco(2) (PI co2) should be greater after acclimatization due to metabolic acid-base com pensation and higher pH. Conversely, if Pco(2) alone determines IPC dischar ge, action potential frequency vs. Pco(2) should be unchanged by acclimatiz ation. Results indicate that after acclimatization ventilation was depresse d at 28 and 42 Torr PIco2 (P < 0.05) and mean plasma pH at 40 Torr Pco(2) i ncreased from 7.38 +/- 0.03 to 7.56 +/- 0.02 (P < 0.05), indicating signifi cant metabolic acid-base compensation and HCO, retention. Mean IPC discharg e rate was elevated by CO2 acclimatization at all Pco(2) studied. In acclim atized vs. normal animals, regression analysis of IPC discharge as a functi on of lnPco(2) showed increased mean intercepts of 81.1 +/- 4.0 vs. 48.4 +/ - 3.6 impulses/s (P < 0.05) and increased mean slopes of -19.0 +/- 1.0 vs. -12.0 +/- 1.1 impulses . s(-1) . lnPco(2)(-1) (P < 0.05). Results indicate that IPC response to CO2 is mediated by H+ from CO2 hydration and not by CO 2 directly.