Chronic hypercapnia resets CO2 sensitivity of avian intrapulmonary chemoreceptors

Avian intrapulmonary chemoreceptors (IPC) are vagal sensory neurons that participate in the control of breathing. IPC action potential frequency is inversely proportional to PCO2, but it is unclear whether low PCO2 or high pH is the immediate stimulus for signal transduction in IPC. To address this 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 ducks. We hypothesized that if respiratory-linked pH changes determine IPC activity, action potential frequency as a function of inspiratory PCO2 (PICO2) should be greater after acclimatization due to metabolic acid-base compensation and higher pH. Conversely, if PCO2 alone determines IPC discharge, action potential frequency vs. PCO2 should be unchanged by acclimatization. Results indicate that after acclimatization ventilation was depressed at 28 and 42 Torr PICO2 (P < 0.05) and mean plasma pH at 40 Torr PCO2 increased from 7.38 +/- 0. 03 to 7.56 +/- 0.02 (P < 0.05), indicating significant metabolic acid-base compensation and HCO-3 retention. Mean IPC discharge rate was elevated by CO2 acclimatization at all PCO2 studied. In acclimatized vs. normal animals, regression analysis of IPC discharge as a function of lnPCO2 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. lnPCO2-1 (P < 0.05). Results indicate that IPC response to CO2 is mediated by H+ from CO2 hydration and not by CO2 directly.