Benzolamide, acetazolamide, and signal transduction in avian intrapulmonary chemoreceptors

Department of Biological Sciences, Northern Arizona University, Flagstaff, Arizona 86011-5640 Intrapulmonary chemoreceptors (IPC) are CO 2 -sensitive sensory neurons that innervate the lungs of birds, help control the rate and depth of breathing, and require carbonic anhydrase (CA) for normal function. We tested whether the CA enzyme is located intracellularly or extracellularly in IPC by comparing the effect of a CA inhibitor that is membrane permeable (iv acetazolamide) with one that is relatively membrane impermeable (iv benzolamide). Single cell extracellular recordings were made from vagal filaments in 16 anesthetized, unidirectionally ventilated mallards ( Anas platyrhynchos ). Without CA inhibition, action potential discharge rate was inversely proportional to inspired P CO 2 ( 9.0 ± 0.8 s 1 · lnTorr 1 ; means ± SE, n = 16) and exhibited phasic responses to rapid P CO 2 changes. Benzolamide (25 mg/kg iv) raised the discharge rate but did not alter tonic IPC P CO 2 response ( 9.8 ± 1.6 s 1 · lnTorr 1 , n = 8), and it modestly attenuated phasic responses. Acetazolamide (10 mg/kg iv) raised IPC discharge, significantly reduced tonic IPC P CO 2 response to 3.5 ± 3.6 s 1 · lnTorr 1 ( n = 6), and severely attenuated phasic responses. Results were consistent with an intracellular site for CA that is less accessible to benzolamide. A model of IPC CO 2 transduction is proposed. carbonic anhydrase; action potentials; sensory neurons; carbon dioxide