Benzolamide, acetazolamide, and signal transduction in avian intrapulmonary chemoreceptors

Abstract

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 PCO(2) (-9.0 +/- 0.8 s(-1). lnTorr(-1); means +/- SE, n = 16) and exhibited phasic responses to rapid PCO(2) changes. Benzolamide (25 mg/kg iv) raised the discharge rate but did not alter tonic IPC PCO(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 PCO(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.

Spike firing allometry in avian intrapulmonary chemoreceptors: matching neural code to body size

“…Colonic temperature (T b ) was regulated to 41±1°C in all birds except lovebirds, where T b was regulated to 40±1°C. Unidirectional ventilation followed previously described procedures (Hempleman and Bebout, 1994;Hempleman et al, 2000). In birds where each lung was independently ventilated, airflow through the two lungs exceeded reported ventilatory flow rates by an average factor of 1.1-1.9.…”

“…In birds where each lung was independently ventilated, airflow through the two lungs exceeded reported ventilatory flow rates by an average factor of 1.1-1.9. All lovebirds, most of the quail and two ducks were unidirectionally ventilated with a continuous stream of warmed, humidified gas that was delivered via an endotracheal tube, and exited the opened interclavicular air sac (Hempleman et al, 2000;Shoemaker and Hempleman, 2001). Insufflation rates exceeded published ventilatory flow rates in these birds by a factor of 4-5.…”

“…Insufflation rates exceeded published ventilatory flow rates in these birds by a factor of 4-5. Carbon dioxide was added to the ventilatory gas stream going to the left lung or through the trachea with an electronic valve to make step changes in lung CO 2 concentration between low Allometry of neural coding (0-1%) and high (5%-7%; Hempleman and Bebout, 1994;Hempleman et al, 2000). Arterial blood gases were periodically assessed to ensure the adequacy of ventilation.…”

“…IPC were identified by their prompt response to step reductions in ventilatory CO 2 , and single IPC action potentials were identified by the constant shape and amplitude of their spike waveform (Hempleman and Bebout, 1994;Hempleman et al, 2000;Shoemaker and Hempleman, 2001). Occurrence times of action potentials and CO 2 stimulus steps were recorded on-line using an Intel 8085-based interrupt driven assembly language program sampling at 14·500·s -1 (Hempleman and Bebout, 1994;Hempleman et al, 2000;Shoemaker and Hempleman, 2001). Data were analyzed off-line to produce cycle triggered stimulus histograms (averaged spike frequency vs time) and raster plots (spike occurrence vs time) of IPC spike activity (e.g.…”

CO2transduction in avian intrapulmonary chemoreceptors is critically dependent on transmembrane Na+/H+exchange

“…Many presumptive chemoreceptor neurons located in the mammalian medullary raphe, however, have also been shown to be inhibited by high PCO 2 (41, 54), as are reptilian IPC, mammalian pulmonary stretch receptors, and mammalian laryngeal CO 2 chemoreceptors (10,32,39,44). Thus the inverse CO 2 response (discharge rate inhibited by high CO 2 ) may be a common variant of respiratory chemosensitivity, just somewhat overlooked.IPC, like some other chemoreceptors that are excited by decreasing PCO 2 levels, are critically dependent on carbonic anhydrase (CA) activity for CO 2 transduction (11,27,38,43,44). Pharmacological inhibition of intracellular CA with acetazolamide (but not extracellular CA with benzolamide) abolishes avian IPC response to CO 2 and causes sustained high-frequency action potential discharge (27,43).…”