Avian intrapulmonary chemoreceptor discharge rate is increased by anion exchange blocker 'DIDS'

Effects of aerobic and anaerobic metabolic inhibitors on avian intrapulmonary chemoreceptors

Birds have rapidly responding respiratory chemoreceptors [intrapulmonary chemoreceptors (IPC)] that provide vagal sensory feedback about breathing pattern. IPC are exquisitely sensitive to CO(2) but are unaffected by hypoxia. IPC continue to respond to CO(2) during hypoxic and even anoxic conditions, suggesting that they may generate ATP needed for signal transduction anaerobically. To assess IPC energy metabolism, single-cell action potential discharge and acid-base status were recorded from 26 pentobarbital-anesthetized Anas platyrhynchos before and after intravenous infusion of the glycolytic blocker iodoacetate (10-70 mg/kg), mitochondrial blocker rotenone (2 mg/kg), and/or mitochondrial uncoupler 2,4-dinitrophenol (5-15 mg/kg). After 5 min exposure at the highest dosages, iodoacetate inhibited IPC discharge 65% (15.9 +/- 0.3 s(-1) to 5.5 +/- 0.3 s(-1), P < 0.05), rotenone inhibited discharge 80% (12.9 +/- 0.5 s(-1) to 2.6 +/- 0.6 s(-1), P < 0.05), and 2,4-dinitrophenol inhibited discharge 19% (14.0 +/- 0.3 s(-1) to 11.3 +/- 0.3 s(-1), P < 0.05). These results suggest that IPC utilize glucose, require an intact glycolytic pathway, and metabolize the products of glycolysis to CO(2) and H(2)O by mitochondrial respiration. The small but significant effect of 2,4-dinitrophenol suggests that ATP production by glycolysis may be sufficient to meet IPC energy demands if NADH can be oxidized to NAD experimentally by uncoupling mitochondria, or physiologically by transient lactate production. A model for IPC spike frequency adaptation is proposed, whereby the rapid onset of phasic IPC discharge requires ATP from anaerobic glycolysis, using lactate as the electron acceptor, and the roll-off in IPC discharge reflects transient acidosis due to intracellular lactic acid accumulation.

Evidence for TREK-like tandem-pore domain channels in intrapulmonary chemoreceptor chemotransduction

Intrapulmonary chemoreceptors (IPC) are carbon dioxide sensing neurons that innervate the lungs of birds, control breathing pattern, and are inhibited by halothane and intracellular acidosis. TASK and TREK are subfamilies of tandem-pore domain potassium leak channels, important in setting resting membrane potential, that are affected by volatile anesthetics and acidosis. We hypothesized that such channels might underlie signal transduction in IPC. We treated mallard ducks with four volatile anesthetics in increasing concentrations to test their effects on IPC discharge through single cell, extracellular recording from vagal fibers. Isoflurane inhalation attenuated IPC discharge only at 8.25% inspired (alpha=0.05). Halothane attenuated IPC discharge significantly (alpha=0.05) at all treatment levels. Chloroform at 3.8%, 5.6%, and 8.25% significantly attenuated IPC discharge (alpha=0.05). Ether at 1.9%, 2.9%, and 3.8% significantly attenuated IPC discharge (alpha=0.05), abolishing IPC discharge at 3.8% inspired. The pharmacological signature of IPC discharge attenuation suggests that IPC express tandem-pore domain leak channels similar to TREK channels, which are inhibited by intracellular acidosis.

Calcium and avian intrapulmonary chemoreceptor response to CO2

Intrapulmonary chemoreceptors (IPC) are highly responsive respiratory chemoreceptors that innervate the lungs of birds and diapsid reptiles. IPC are stimulated by low levels of lung Pco(2), inhibited by high levels of lung Pco(2), and their vagal afferents serve as a sensory limb for reflex adjustments of breathing depth and rate. Most IPC exhibit both phasic and tonic sensitivity to CO(2), and spike frequency adaptation (SFA) contributes to their phasic CO(2) responsiveness. To test whether CO(2) responsiveness and SFA in IPC is modulated by a Ca(2+)-linked mechanism, we quantified the role of transmembrane Ca(2+) fluxes and Ca(2+)-related channels on single-unit IPC function in response to phasic changes in inspired Pco(2). We found that 1) broad-spectrum blockade of Ca(2+) channels using cadmium or cobalt and blockade of L-type Ca(2+) channels using nifedipine increased IPC discharge; 2) activation of L-type Ca(2+) channels using BAY K 8644 reduced IPC discharge; 3) blockade of Ca(2+)-activated potassium channels using charybdotoxin (antagonist of large-conductance Ca(2+)-dependent K(+) channel) increased IPC discharge, but neither charybdotoxin nor apamin affected SFA; and 4) blockade of chloride channels, including Ca(2+)-activated chloride channels, with niflumic acid decreased IPC discharge at low Pco(2) and increased IPC discharge at high Pco(2), resulting in a net attenuation of the IPC CO(2) response. We conclude that Ca(2+) influx through L-type Ca(2+) channels has an inhibitory effect on IPC afferent discharge and CO(2) sensitivity, that spike frequency adaptation is not due to apamin- or charybdotoxin-sensitive Ca(2+)-activated K(+) channels in IPC, and that chloride channels blocked by niflumic acid help modulate IPC CO(2) responses.

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

Biological rates in small animals are usually higher than those in large animals, yet the maximal rate of action potential (spike) generation in sensory neurons encoding rate functions is similar in all animals, due to the conserved genetics of voltage-gated ion channels. Therefore, sensory signals that vary at rates approaching maximal spike generation rate, as might occur in animals of diminished body size, may require specialized spike coding to convey this information. To test whether spike coding scales allometrically in sensory neurons monitoring signals that change frequency with body size, we recorded action potentials from 70 avian intrapulmonary chemoreceptors (IPC), respiratory neurons that detect lung CO2 changes during breathing, in five different avian species ranging in size from body mass Mb=0.045 kg (lovebirds) to 5.23 kg (geese). Since breathing frequency scales approximately to Mb-1/4 (higher in small birds, lower in large birds), we reasoned that IPC discharge frequencies may also scale to maintain spike information transmission within each breath. We found that phasic action potential discharge pattern, as quantified by the peak discharge rate and the magnitude of spike frequency adaptation, scaled between Mb-0.22 and Mb-0.26, like breathing rate (P<0.05). Previously published values of peak discharge rate in IPC also fit this allometric relationship. We suggest that mass-dependent scaling of neural coding may be necessary for preserving information transmission with decreasing body size.

Imidazole binding reagent diethyl pyrocarbonate (DEPC) inhibits avian intrapulmonary chemoreceptor discharge in vivo

Data indicate that avian intrapulmonary chemoreceptors (IPC) transduce CO2 stimuli by sensing the products of CO2 hydration, [H+] and [HCO3-]. The alphastat regulation hypothesis of physiological pH sensitivity suggests that proteins sense [H+] through changes in the ionization state of imidazole groups (alphaIm). To test whether imidazole is involved with IPC CO2 sensitivity, we administered diethyl pyrocarbonate (DEPC) intravenously while recording from IPC exposed to varying levels of inspired CO2. At physiological pH, DEPC converts pH sensitive imidazole groups to pH-insensitive N-carbethoxyhistidyl residues. Single cell extracellular neural recordings were made from vagal filaments in anesthetized, unidirectionally ventilated Anas platyrhynchos. Without DEPC, IPC discharge rate was inversely proportional to inspired CO2 with characteristic dynamic responses to rapid CO2 alterations (n = 10). After DEPC treatment (> or = 15 mM), mean sensitivity of IPC discharge to static inspired CO2 levels was decreased 75% (P < 0.05), and mean peak dynamic IPC discharge rate was decreased 80% (P < 0.05). Additionally, we tested whether DEPC might alter IPC discharge by binding imidazole groups in the enzyme carbonic anhydrase (CA), but we found no effect on CA catalytic rate. We conclude that DEPC inhibits IPC CO2 signal transduction by modifying imidazole groups on acid-sensitive proteins other than CA, possibly membrane acid-base exchangers or ion channels. These data support the alphastat regulation hypothesis in IPC CO2 respiratory chemoreception and suggests a more direct link between H+ and membrane excitability.

CO2 transduction mechanisms in avian intrapulmonary chemoreceptors: experiments and models

Intrapulmonary chemoreceptors (IPC) are neurons that sense tonic and phasic CO2 stimuli in the lungs of birds and diapsid reptiles. IPC are different from most other vertebrate respiratory CO2 receptors because: (1) they are stimulated by low PCO2 and inhibited by high PCO2, (2) they have extremely rapid response characteristics, (3) their CO2 sensitivity is nearly abolished by intracellular inhibitors of carbonic anhydrase, and (4) their CO2 sensitivity is strongly depressed by inhibiting Na+/H+ antiport exchange. Experimental evidence suggests that IPC respond to intracellular pH, not CO2 directly, and that intracellular pH and IPC discharge are determined by a kinetic balance between CO2 hydration/dehydration rates, transmembrane acid/base exchange rates, and intracellular buffering. We review experimental evidence for and against various mechanisms of IPC CO2 chemotransduction, present a conceptual and mathematical model of the proposed mechanisms, and compare this model to CO2 transduction in other respiratory chemoreceptors.

Cellular mechanisms involved in CO(2) and acid signaling in chemosensitive neurons

An increase in CO(2)/H(+) is a major stimulus for increased ventilation and is sensed by specialized brain stem neurons called central chemosensitive neurons. These neurons appear to be spread among numerous brain stem regions, and neurons from different regions have different levels of chemosensitivity. Early studies implicated changes of pH as playing a role in chemosensitive signaling, most likely by inhibiting a K(+) channel, depolarizing chemosensitive neurons, and thereby increasing their firing rate. Considerable progress has been made over the past decade in understanding the cellular mechanisms of chemosensitive signaling using reduced preparations. Recent evidence has pointed to an important role of changes of intracellular pH in the response of central chemosensitive neurons to increased CO(2)/H(+) levels. The signaling mechanisms for chemosensitivity may also involve changes of extracellular pH, intracellular Ca(2+), gap junctions, oxidative stress, glial cells, bicarbonate, CO(2), and neurotransmitters. The normal target for these signals is generally believed to be a K(+) channel, although it is likely that many K(+) channels as well as Ca(2+) channels are involved as targets of chemosensitive signals. The results of studies of cellular signaling in central chemosensitive neurons are compared with results in other CO(2)- and/or H(+)-sensitive cells, including peripheral chemoreceptors (carotid body glomus cells), invertebrate central chemoreceptors, avian intrapulmonary chemoreceptors, acid-sensitive taste receptor cells on the tongue, and pain-sensitive nociceptors. A multiple factors model is proposed for central chemosensitive neurons in which multiple signals that affect multiple ion channel targets result in the final neuronal response to changes in CO(2)/H(+).

CO2 transduction in avian intrapulmonary chemoreceptors is critically dependent on transmembrane Na+/H+ exchange

Avian intrapulmonary chemoreceptors (IPC) are vagal respiratory afferents that are inhibited by high lung Pco(2) and excited by low lung Pco(2). Previous work suggests that increased CO(2) inhibits IPC by acidifying intracellular pH (pH(i)) and that pH(i) is determined by a kinetic balance between the rate of intracellular carbonic anhydrase-catalyzed CO(2) hydration/dehydration and transmembrane extrusion of acids and/or bases by various exchangers. Here, the role of amiloride-sensitive Na(+)/H(+) exchange (NHE) in the IPC CO(2) response was tested by recording single-unit action potentials from IPC in anesthetized ducks, Anas platyrhynchos. For each of the IPC tested, blockade of the NHE using dimethyl amiloride (DMA) elicited a marked (>50%) dose-dependent decrease in mean IPC discharge (P < 0.05), suggesting that NHE is important for pH(i) regulation and CO(2) transduction in IPC. In addition, activation of the NHE using 12-O-tetradecanoylphorbol 13-acetate stimulated six of the seven IPC tested, although the overall effect was not statistically significantly (P = 0.07). Taken together, these findings suggest that CO(2) transduction in IPC is dependent on transmembrane NHE although it is likely to be much slower than carbonic anhydrase-catalyzed hydration-dehydration of CO(2).