Chronic hypercapnia resets CO2 sensitivity of avian intrapulmonary chemoreceptors

Abstract

Avian intrapulmonary chemoreceptors (IPC) are vagal sensory neurons that participate in the control of breathing. IPC action potential frequency is inversely proportional to [Formula: see text], but it is unclear whether low [Formula: see text] 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[Formula: see text]([Formula: see text]) should be greater after acclimatization due to metabolic acid-base compensation and higher pH. Conversely, if[Formula: see text] alone determines IPC discharge, action potential frequency vs.[Formula: see text] should be unchanged by acclimatization. Results indicate that after acclimatization ventilation was depressed at 28 and 42 Torr[Formula: see text]( P < 0.05) and mean plasma pH at 40 Torr [Formula: see text] increased from 7.38 ± 0.03 to 7.56 ± 0.02 ( P < 0.05), indicating significant metabolic acid-base compensation and[Formula: see text] retention. Mean IPC discharge rate was elevated by CO2acclimatization at all [Formula: see text] studied. In acclimatized vs. normal animals, regression analysis of IPC discharge as a function of[Formula: see text] 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 ⋅ [Formula: see text] −1( P < 0.05). Results indicate that IPC response to CO2 is mediated by H+ from CO2 hydration and not by CO2 directly.