CO₂ hydration in intrapulmonary chemoreceptors: Effects of acetazolamide and benzolamide

Intrapulmonary chemoreceptors (IPC) are CO₂-sensitive sensory neurons that innervate the lungs of birds. Current evidence suggests that IPC CO₂ response requires carbonic anhydrase (CA) and that pH is the likely cellular determinant of IPC sensory discharge. In this study we test whether CA enzyme is located intra- or extra-cellularly in IPC by quantifying the effect of relatively membrane permeant lacetazolamide) and relatively impermeant (benzolamide) CA inhibitors on IPC response to CO₂. Single fiber recordings were made from 9 anesthetized, unidirectionally ventilated Anas platyrhynchos. Inspired CO₂ was stepped from 0% to 7% to identify IPC, and then held steady at levels between 1% and 7% to measure steady IPC response to CO₂. Without CA inhibition, IPC discharge rate was inversely proportional to inspired PCO₂ (slope -8.9 ± 1.0 sec^-1.InTorr^-1; μ±sem, n = 9). Infusion of 25 mg/kg benzolamide (i.v.) had no significant effect on IPC CO₂ sensitivity (slope -8.5±1.4 sec^-1.InTorr^-1, n = 7), but 25 mg/kg acetazolamide nearly eliminated sensitivity (slope -0.3 ± 1.0 sec^-1.InTorr^-1, n = 2). High doses of benzolamide (100 mg/kg) decreased CO₂ sensitivity to -3.2 ± 0.6 sec^-1.InTorr^-1, n = 3, but sensitivity remained greater than with 25mg/kg acetazolamide. Our results suggest that an intracellular rather than extracellular carbonic anhydrase catalyzes hydration of CO₂ to carbonic acid in IPC, and that intracellular H⁺ controls IPC discharge rate.