Intrapulmonary Chemoreceptors Research Articles

Lung neuroendocrine system: current state and prospects for further research (review of literature)

The review of literature focuses on histotopography, ultrastructural organization of the neuroendocrinocytes of the lungs and their role in the prenatal and postnatal periods of ontogenesis. The purpose of the work: to analyze the morphofunctional and histotopographical features of the neuroendocrine system of the lungs in order to ascertain their participation in the development of diseases of the respiratory system in the prenatal and postnatal periods of ontogenesis. Conclusion. The strategic location of the neuroepithelial bodies with the apical part of the cells protruding into the lumen in the space of the airway bifurcation and their afferent-efferent innervation prove these structures provide an intrapulmonary chemoreceptor system that precisely regulates the balance between ventilation and perfusion. In prenatal ontogenesis, NEBs (neuroepithelial bodies) contribute to the normal development of the fetal lungs. The involvement of neuroendocrinocytes of the lungs in the processes of regeneration and carcinogenesis in adulthood has not been sufficiently studied.

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is Crucial for CO2 Signal Transduction in Avian Intrapulmonary Chemoreceptors (IPC)

CFTR is an anion channel and ABC transmembrane protein which also modulates other anion channels and transporters. Since CFTR is widely expressed in cells that transport anions, and we hypothesize that anion transport in IPC is critical for CO2 transduction, we tested for the presence of CFTR in IPC. Glibenclamide, a non‐selective CFTR inhibitor, and CFTR‐172inh, a highly specific CFTR inhibitor were given separately to pentobarbital‐anesthetized mallards (Anas platyrhynchos, n =15) while recording single cell IPC spike train responses to inspired CO2 steps (0%–7%, 11sec period). Both drugs produced similar results, where IPC discharge with the lowest 2 μmol/kg doses was unchanged from control, while the 4 and 8 μmol/kg doses significantly decreased both mean IPC discharge and phasic adaptation (ANOVA, p<0.01). IPC CO2 sensitivity was completely abolished in 4 CFTR‐172inh experiments and 4 glibenclamide experiments. Results suggest that CFTR is a crucial anion regulating channel involved in IPC CO2 signal transduction.Support or Funding InformationSupport: NIH R15 HL087269‐02.

Blocking Volume Regulated Anion Channels (VRAC) with DCPIB Depresses Avian Intrapulmonary Chemoreceptor (IPC) Discharge more in Tonic than Phasic Units

Intrapulmonary chemoreceptors (IPC) are respiratory vagal afferents that sense PCO2 changes and are responsible for controlling breathing in birds. Volume regulated anion channels (VRAC) are responsible for volume regulation in a large variety of cell types. Small doses of DCPIB (1 to 8 μmol/kg), a specific blocker of VRAC, were used to test for VRAC presence in IPC of pentobarbital‐anesthetized ducks (Anas platyrhyncos, n=11). Cycle triggered stimulus histograms of IPC action potential discharge to 0%–7% inspired CO2 steps were recorded and separated into 2 groups according to their spike frequency adaptation: phasic (n=4), tonic (n=7). The IPC discharge for phasic units remained unchanged from the control for the 1 to 4 μmol/kg doses, while the 8 μmol/kg dose reduced the peak phasic adaptation (ANOVA, p<0.01). Tonic units progressively reduced discharge for all doses (ANOVA, p<0.01). Since DCPIB more strongly affects IPC discharge in tonic units compared to phasic units the results suggest that: 1. Tonic and phasic units may have different mechanisms for VRAC activation, or 2. Tonic units express more VRAC than phasic units.Support or Funding InformationSupport: NIH R15 HL087269‐02.

Effects of prolonged lung inflation or deflation on pulmonary stretch receptor discharge in the alligator (Alligator mississippiensis)

The American alligator (Alligator mississippiensis) is a semi-aquatic diving reptile that has a periodic breathing pattern. Previous work identified pulmonary stretch receptors, that are rapidly and slowly adapting, as well as intrapulmonary chemoreceptors (IPC), sensitive to CO2, that modulate breathing patterns in alligators. The purpose of the present study was to quantify the effects of prolonged lung inflation and deflation (simulated dives) on pulmonary stretch receptors (PSR) and/or IPC discharge characteristics. The effects of airway pressure (0–20cm H2O), hypercapnia (7% CO2), and hypoxia (5% O2) on dynamic and static responses of PSR were studied in juvenile alligators (mean mass=246g) at 24°C. Alligators were initially anesthetized with isoflurane, cranially pithed, tracheotomized and artificially ventilated. Vagal afferent tonic and phasic activity was recorded with platinum hook electrodes. Receptor activity was a mixture of slowly adapting PSR (SAR) and rapidly adapting PSR (RAR) with varying thresholds and degrees of adaptation, without CO2 sensitivity. Receptor activity before, during and after 1min periods of lung inflation and deflation was quantified to examine the effect of simulated breath-hold dives. Some PSR showed a change in dynamic response, exhibiting inhibition for several breaths after prolonged lung inflation. Following 1min deflation, RAR, but not SAR, exhibited a significant potentiation of burst frequency relative to control. For SAR, the post-inflation receptor inhibition was blocked by CO2 and hypoxia; for RAR, the post-inflation inhibition was potentiated by CO2 and blocked by hypoxia. These results suggest that changes in PSR firing following prolonged inflation and deflation may promote post-dive ventilation in alligators. We hypothesize that PSR in alligators may be involved in recovery of breathing patterns and lung volume during pre- and post-diving behavior and apneic periods in diving reptiles.

KCC and NKCC activity affect spike frequency adaptation in CO2‐sensitive avian intrapulmonary chemoreceptors (1092.3)

IPC are respiratory chemoreceptors that exhibit SFA, have phasic and tonic CO2 sensitivity, and help control breathing in birds. In IPC, large changes in PCO2, pHi, and [HCO3]i occur with each breath. Here we test the hypothesis that Cl‐ transmembrane flux may be involved in IPC CO2 transduction to balance intracellular [HCO3‐] changes. Furosemide (FU), an equipotent inhibitor of both K+ ‐Cl‐ cotransport (KCC), which allows for Cl‐ efflux driven by the K+ gradient, and Na+ ‐K+ ‐2Cl‐ cotransport (NKCC), which allows for Cl‐ influx driven by the Na+ gradient, was given to anesthetized Anas platyrhynchos (N=6) while recording single‐unit IPC. FU progressively reduced SFA at 10‐40 mg/kg (p蠄0.0001). In contrast, tonic IPC discharge was decreased at low FU doses (0.5‐5 mg/kg, p蠄0.0001) but increased at higher dosages (10‐40 mg/kg, p蠄0.0001). Results suggest that IPC SFA and tonic CO2 sensitivity are influenced by KCC and/or NKCC, presumably by reducing ability of the cell to produce Cl‐ counter movements required by phasic [HCO3‐] changes.Grant Funding Source: Supported by NIH R15 HL087269‐02

Spike frequency adaptation in avian intrapulmonary chemoreceptors does not rely on Na+/H+ exchange (1092.9)

IPC are essential for avian breath‐by‐breath respiratory control and are inversely sensitive to lung PCO2. Evidence suggests IPC detect intracellular pH rather than molecular CO2, and that pHi reflects a dynamic interaction of intracellular CO2 hydration/dehydration, buffering, and transmembrane acid/base exchange. We used the Na+/H+ exchange (NHE) blocker dimethyl amiloride (DMA) to test NHE’s role in single‐unit IPC response to combined phasic and tonic CO2 step stimuli in anesthetized mallards (n=9). DMA depressed mean (tonic) IPC discharge response to CO2 step stimuli by 15%, 31%, 51%, and 58% at 1,3,8, and 13 µmol/kg, (p<0.05), but did not affect phasic discharge (spike frequency adaptation index, SFA). Depression of tonic CO2 response confirms earlier reports, and the new result that NHE has no effect on the phasic part of IPC step response demonstrates that other mechanisms must underlie SFA.Grant Funding Source: Supported by: NIH R15 HL087269‐02 and R25 GM056931‐15.

Dual response to bumetanide in avian intrapulmonary chemoreceptors (1092.1)

IPC in avian lungs display unique chemosensitive properties: they respond rapidly (ms) and inversely to changes in lung PCO2, have very consistent spike train responses, and help terminate inspiration. We recorded action potentials from single vagal IPC afferents exposed to inspired CO2 stimuli (7%-0% on-off CO2 step, T=11s) in anesthetized Anas platyrhynchos. Bumetanide, a predictable sulfamyl loop diuretic that blocks NKCC and KCC cation-chloride co-transporters with about 500-fold greater affinity for NKCC than for KCC, excited IPC at low doses (0.025 - 0.125mg•kg-1) and inhibited IPC at high doses (1.0 - 40 mg•kg-1), p<0.0001, n=6. Results suggest that both KCC and NKCC are expressed in IPC, and together modulate IPC pH and CO2 chemosensitivity, perhaps by controlling mean intracellular [Cl-]. Grant Funding Source: Supported by: NIH R15 HL087269-02, R25 GM056931-15

Two potassium‐channel blockers differently affect afferent signal of avian intrapulmonary chemoreceptors

We investigated the roles of IA and IDR potassium channels in action potential generation of avian intrapulmonary chemoreceptors (IPC). IPC are CO2‐sensing vagal afferents in avian lungs, with fast and inverse response to airway CO2 partial pressure (PCO2). Discharge frequency of a single cell IPC was recorded in 11 anesthesized, ventilated ducks. FICO2 was electronically controlled. Intravenous injection of voltage‐gated potassium‐channel blockers 4‐aminopyridine (4AP) or tetraethylammonium (TEA) disrupted the response to CO2. 4AP blocks the transient K current (IA), and TEA blocks the delayed rectifier current (IDR). A dose of 7 mg/kg of 4AP eliminated both phasic and tonic IPC sensitivities to FICO2, resulting in constant high‐frequency AP firing (p &lt; 0.05). Low dosages of TEA had no effect on that sensitivity, although at high TEA doses (&gt;; 20 mg/kg) the phasic (p &lt; 0.05) but not tonic response to a step change in FICO2 disappeared. Our results suggest that transient voltage gated K channels (affected at low dosages of 4AP and high dosages of TEA) are involved in setting resting membrane potential in IPC. If IA in IPC is increased by CO2/H+ as it is in slowly adapting mammalian pulmonary stretch receptors, IA channel modulation by CO2/H+ may also underlie CO2 signal transduction in avian IPC. Support: NIH 2R15HL087269

Carbonic anhydrase activity underlies spike frequency adaptation in avian intrapulmonary chemoreceptors (IPC)

IPC are vagal respiratory afferents that sense intrapulmonary CO2 and control breathing in birds. Most IPC show spike frequency adaptation (SFA) which helps encode phasic and tonic information about CO2 changes within each breath. Carbonic anhydrase (CA) is required for CO2 transduction, and we hypothesized that rapid CA‐catalyzed CO2 hydration and dehydration underlies SFA in IPC. Small doses of CA inhibitor acetazolamide (AZ) were used to test if phasic IPC CO2 responses (SFA) could be inhibited independently of tonic CO2 responses. Single cell IPC spike trains were studied in seven pentobarbital‐anesthetized mallards (Anas platyrhynchos). SFA was decreased 23.0% and 66.2% (p&lt;0.05) at 0.5 and 2.0 mg/kg AZ, respectively, while tonic IPC responses were not affected until AZ dosages ≥5 mg/kg. Results suggest that SFA in IPC is due to rapid intracellular CA catalysis of CO2 hydration resulting in a kinetic overshoot of intracellular pH with abrupt changes in intrapulmonary CO2. We present a model of linked differential equations that explores this hypothesis. Support: NIH R15 HL087269–02 and R25 GM056931–15.

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.

Development of avian intrapulmonary chemoreceptors

Although avian intrapulmonary chemoreceptors (IPC) have been studied extensively in adults, the maturation of IPC CO(2) sensitivity during development is completely unknown. To begin investigating IPC development we asked two fundamental questions: (1) Are IPC capable of sensing CO(2) during early development, and, if so, how early? And, (2) does IPC CO(2) sensitivity during early development exhibit postnatal maturation compared to IPC discharge characteristics in adult ducks? We addressed these questions by recording from single IPC Anas platyrhynchos ducklings beginning approximately 6 h prior to internal pipping through 4 days of postnatal development. We then compared mean IPC discharge characteristics during early development with mean IPC activity from adult ducks greater than 12 weeks old. In total, we recorded 28 individual IPC from 5 ducklings and 12 adult ducks. Results show that IPC were capable of responding to rapid step changes in CO(2) before hatching occurred, during the paranatal developmental period. We also found that mean IPC activity during early development had increased peak discharge frequencies, greater spike frequency adaptation, and less tonic CO(2) sensitivity when compared to adults (P< or =0.05). These results suggest that during early development phasic IPC CO(2) sensitivity is fully developed, yet tonic IPC CO(2) sensitivity exhibits postnatal maturation possibly associated with hatching. These results also suggest that the mechanisms that underlie phasic and tonic IPC action potential discharge, and therefore the degree of partial spike frequency adaptation, may be independent processes with different developmental trajectories.

Intracellular energy metabolism in avian intrapulmonary chemoreceptors

Intrapulmonary chemoreceptors (IPC) are CO2-sensitive sensory neurons that innervate the lungs of birds. Although IPC neurons are present near the interface between air and blood, several lines of evidence suggests that they rely primarily on glycolysis and not on oxidative phosphorylation for ATP generation (Burger et al., 1974).The goal of the present study was to test this topic further by examining IPC discharge using pharmacological inhibitors of both glycolysis and oxidative phophorylation. We recorded the response of 12 single unit IPC from 12 adult Anas platyrhynchos to step changes in CO2 before and after intravenous administration of up to 70 mg/kg (0.38 mmole/kg) of the glycolytic inhibitor Na-iodoacetate (IAA; n=6) and 2 mg/kg of the oxidative phosphorylation inhibitor rotenone (n=6). In addition, we tested possible the acid-base affects of IAA on IPC discharge with up to 0.38 mmole/kg of Na-acetate (n=3). Results indicate that IAA cause > 70% reduction of IPC discharge at 70 mg/kg (P < 0.05) independent of acid-base disturbances. Similarly, 2 mg/kg rotenone also caused > 80% reduction in IPC discharge and CO2 sensitivity as well as > 50% reduction in whole body O2 consumption (P < 0.05). We conclude that IPC are dependent on glucose for ATP production, and that oxidative phosphorylation is also required for proper IPC function. These results suggest that glucose is completely oxidized to CO2 and H2O by aerobic metabolism within IPC. Support: NSF IBN-0217815

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.

Ventilatory roll off during sustained hypercapnia is gender specific in pekin ducks

The objective of the present study was to examine the relative roles of peripheral versus central mechanisms in producing ventilatory adjustments in pekin ducks during prolonged (5 h) hypercapnia (5% inspired CO2), and to determine whether these adjustments differed between male and female ducks. After 20 min of CO2 exposure, intact ducks increased total ventilation (VE) 2.5-3-fold above control values, due to large increases (approximately 200%) in tidal volume (VT) and slightly smaller increases (approximately 140%) in breathing frequency (fR). This response was accompanied by respiratory acidosis (pHa fell from approximately 7.46 to approximately 7.41) and hypercapnia (PaCO2 increased from approximately 35 to approximately 40 Torr). In males, VE fell progressively thereafter due exclusively to a fall in fR, in parallel with a rapid partial recovery of pH (to 7.44) while PaCO2 continued to climb (to approximately 42 Torr). In females, VE remained elevated during hypercapnia, and no pH recovery occurred. This suggests that a respiratory decline resulting from acid-base compensation (probably due to HCO3- mobilization) occurred in males but not in females. Bicarbonate mobilization, and thus pH compensation, may have been reduced in females due to the CaCO3 requirements of eggshell formation. In males, the acute ventilatory response was reduced slightly by denervation of the carotid bodies or intrapulmonary chemoreceptors, but there was no effect of denervation of either receptor group on the responses to prolonged CO2. We conclude that pH compensation triggered by constant or increasing PaCO2, acting at central chemoreceptors, likely mediates the respiratory adjustments seen in male pekin ducks during hypercapnia. Furthermore, we suggest that this ventilatory response be considered a gender-specific hypercapnic ventilatory roll off, in the context of the various time domains of the hypercapnic ventilatory response.

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.

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 (α = 0.05). Halothane attenuated IPC discharge significantly (α = 0.05) at all treatment levels. Chloroform at 3.8%, 5.6%, and 8.25% significantly attenuated IPC discharge (α = 0.05). Ether at 1.9%, 2.9%, and 3.8% significantly attenuated IPC discharge (α = 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.

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.

Cellular mechanisms involved in CO2 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 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.