Intrapulmonary C02 receptors in the duck: II. Comparison with mechanoreceptors

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.

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.

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

Avian intrapulmonary chemoreceptors (IPC) are neurons that sense lung P(CO(2)) and provide phasic feedback for the control of breathing in birds. To try to understand mechanisms of CO(2) transduction and intracellular pH regulation in IPC, the anion exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) was used to block transmembrane Cl(-)/HCO(3)(-) transport. Single-unit IPC discharge rates were measured at steady intrapulmonary CO(2) levels and during step changes in CO(2) in 15 anesthetized, unidirectionally ventilated adult mallard ducks (Anas platyrhynchos). Measurements were repeated after giving 50, 100 and 200 micromol/kg cumulative i.v. dosages of DIDS. Mean IPC discharge rates at steady (tonic) P(CO(2)) levels were significantly increased by 100 and 200 micromol/kg DIDS, but not by 50 micromol/kg DIDS. Mean dynamic (phasic) IPC responses to CO(2) steps were not significantly affected by DIDS. Results indicate that the DIDS-sensitive Cl(-)/HCO(3)(-) membrane exchanger is involved with tonic CO(2) signal transduction in IPC. However, because some individual IPC were unaffected by DIDS, yet still altered their discharge rate with CO(2), additional mechanisms besides the Cl(-)/HCO(3)(-) exchange are probably required for CO(2) chemotransduction in IPC.

Central control of the cardiovascular and respiratory systems and their interactions in vertebrates

This review explores the fundamental neuranatomical and functional bases for integration of the respiratory and cardiovascular systems in vertebrates and traces their evolution through the vertebrate groups, from primarily water-breathing fish and larval amphibians to facultative air-breathers such as lungfish and some adult amphibians and finally obligate air-breathers among the reptiles, birds, and mammals. A comparative account of respiratory rhythm generation leads to consideration of the changing roles in cardiorespiratory integration for central and peripheral chemoreceptors and mechanoreceptors and their central projections. We review evidence of a developing role in the control of cardiorespiratory interactions for the partial relocation from the dorsal motor nucleus of the vagus into the nucleus ambiguus of vagal preganglionic neurons, and in particular those innervating the heart, and for the existence of a functional topography of specific groups of sympathetic preganglionic neurons in the spinal cord. Finally, we consider the mechanisms generating temporal modulation of heart rate, vasomotor tone, and control of the airways in mammals; cardiorespiratory synchrony in fish; and integration of the cardiorespiratory system during intermittent breathing in amphibians, reptiles, and diving birds. Concluding comments suggest areas for further productive research.

Response of roosters to resistive loads at constant chemical drive to breathe

This study investigated the role of mechanoreceptors in the respiratory responses to resistive loading in roosters. Adult roosters were unidirectionally ventilated (maintaining a constant chemical drive to breathe). Electrical circuits assessed the respiratory muscle pressure (Pmus) and controlled the relationship between Pmus and the respiratory volume changes. Respiratory volume changes similar to those achieved by flow-resistive unloading or loading were produced by the circuits, imposing a 'virtual' resistance (Rv). When Rv was doubled (decreased rate of volume change, n = 6), tidal volume (VT, measured by whole body plethysmography) decreased significantly (28%), while thoracic volume (VRIP, measured by respiratory inductance plethysmography) did not change. When RV was quadrupled (n = 4) VT and VRIP decreased significantly (53% and 24%, respectively). Changing RV to one half the normal value (n = 5) did not affect these parameters. Inspiratory time and Pmus were not significantly altered at any RV. It is concluded that, at constant chemical drive, mechanoreceptors play a minimal role in maintaining tidal volume during impeded breathing in roosters. Comparative differences which may explain these results are discussed.

Afferent vagal activity during hyperthermic polypnea in the pigeon

Respiration-modulated activity in afferent vagal fibers was recorded in 10 pigeons during euthermic breathing and thermal panting. Of these fibers, 13 were identified as intra-pulmonary chemoreceptors (IPCs), that increased discharge with diminishing lung gas PCO2, and 13 as mechanoreceptors, that increased firing with lung inflation. Two types of IPC were observed that were distinct by their firing pattern during panting. Phasic IPCs displayed phasic discharge within the respiratory cycle, even at respiratory frequencies (fresp) as high as 400 min-1. Tonic IPC fired tonically and increased their discharge as fresp increased. Several IPCs were silent during euthermic breathing, but discharged tonically as fresp increased with thermal polypnea. Discharge of neither type of IPC was consistently related to PaCO2. Discharge from mechanoreceptors was phasic with respiration, up to values of fresp as high as 350 min-1. However, the average number of impulses per breath decreased as fresp increased. We conclude that discharge from phasic intrapulmonary chemoreceptors and mechanoreceptors may contribute to setting the respiratory pattern during hyperthermic polypnea.

Effect of bilateral vagotomy on arterial acid-base stability during panting in the pigeon

Bilateral, cervical vagotomy denervates, among other receptors, the known peripheral chemoreceptors in birds. To test the importance of afferent vagal input to cardiorespiratory control during thermal panting, we measured responses to an increase in body temperature (Tb) induced by ambient heating in 6 bilaterally, cervically vagotomized pigeons. These responses were compared with those we previously reported in intact pigeons (control animals). At thermoneutral conditions, respiratory frequency (fR) was lower after vagotomy compared to controls, and PaCO2 was higher. During increases in Tb in both control and vagotomized pigeons, fR, PaO2, pHa, heart rate and oxygen consumption increased, and PaCO2 decreased. However, fR rose less, PaCO2 decreased more, and pHa increased more in the vagotomized pigeons than in controls. All three were more variable than in controls. Heart rate and blood pressure were higher than controls; blood pressure was more variable. We conclude that bilateral, cervical vagotomy compromises the ability of pigeons to maintain stable respiratory and cardiovascular responses to hyperthermia.

Breathing pattern in anesthetized chickens: CO2 inhalation and vagotomy

The breathing patterns of 20 anesthetized chickens were studied during progressive suppression of intrapulmonary chemoreceptors (IPC) by various concentrations of CO2 and following bilateral vagotomy. The vagotomy breathing pattern, characterized by a marked accentuation of expiratory times with prolonged expiratory pauses, was markedly different from that induced by CO2 inhalation. Removal of neural input to the central respiratory centers from IPC does not appear to be solely responsible for the altered breathing pattern following vagotomy in birds.

Ventilatory pressure loading at constant pulmonary FCO2 in Gallus domesticus

Seven White Leghorn roosters were unidirectionally ventilated at constant flows and CO2 concentrations. The birds were awake and stood or crouched in a plethysmograph. A servo system clamped the pressure in the air sacs at constant values from -10 to +10 cm H2O in 2 cm H2O increments. Therefore, the animals could inflate or deflate the air sacs with breathing movements without affecting intrapulmonary pressures. Decreasing air sac pressure less than atmospheric caused inspiratory duration (TI), expiratory duration (TE), total period (TTOT) and tidal volume (VR) to decrease, and the ratio, TI/TE to increase. Increasing air sac pressures to 6 cm H2O above atmospheric caused, TE to increase, TI and TI/TE to decrease and VT and TTOT to change very little. After bilateral vagotomy air sac pressure changes caused little or no changes in TI, TE, TTOT or TI/TE, but produced percentage changes in VT similar to before vagotomy. Comparison of end expiratory volumes with apneic volumes (produced by lowering CO2 in the insulfating gas) over the range of air sac pressures clamped shows: (1) chickens actively exhale at pressures as low as -10 cm H2O, and (2) the change of mean air sac volume due to imposed pressure is less during breathing than during apnea. These findings, we believe, are due to a reflex initiated by mechanoreceptors with projections in the vagus nerves.

Factors affecting the respiratory and cardiovascular responses to hypercapnic hypoxia, in mallard ducks

Experiments were performed to determine the factors responsible for the differences in heart rate and blood flow to the leg between ducks after 60 sec head submersion and those spontaneously breathing a hypercapnic hypoxic gas mixture; blood gases were similar in both cases. It is concluded that, in forcibly submerged ducks, full development of the reduction in heart rate and of the accompanying cardiovascular adjustments is dependent upon the cessation of central respiratory activity and of respiratory movements. The CO2-sensitive receptors in the lungs account for approximately one third of the antagonism to these changes in ducks spontaneously breathing a hypoxic hypercapnic gas mixture. Other contributions are from central inspiratory neurons (a quarter of total), musculo-skeletal and cardiovascular mechanoreceptors (a quarter of total) and pulmonary mechanoreceptors (one sixth of total).

Respiratory mechanics of Pekin ducks under four conditions: pressure breathing, anesthesia, paralysis or breathing CO2-enriched gas

Impedance magnitude (Z) of the lower respiratory system was studied in Pekin ducks, using forced oscillations of a small volume at the airways opening in the range 1.6-16 Hz. The experiments were performed on 5 awake ducks enclosed in a body plethysmograph and spontaneously breathing ambient air at a transrespiratory pressure (Prs, the pressure difference between the lung and the body surface) which was varied in steps from -10 cm H2O (compression) to +10 cm H2O (distension). In 3 anesthetized birds, the effects of CO2 breathing and muscular paralysis were also studied. Analysis of end-expiratory Z data yielded estimates of respiratory resistance (R), inertance (I) and compliance (C). During positive or negative pressure breathing in conscious ducks, minute volume (V) and end-tidal CO2 (PETCO2) remained unchanged from normal (Prs = zero) while tidal volume (VT) and ventilatory period (Ttot) decreased. The respiratory system in late expiration can be modelled well with a simple series R-I-C mechanical model at Prs values of zero, +10 and -10 cm H2O. The value of Z increased at all frequencies studied during compression of the respiratory system (Prs = -10 cm H2O) and did not change much from normal (Prs = zero) during distension (Prs = +10 cm H2O). Both resistance and inertance increased during compression. During distension contradictory changes in resistance and inertance suggest that complex changes in flow profile and/or in flow pathways occurred with positive pressure breathing. Anesthesia or paralysis did not noticeably change the oscillatory resistance or inertance, but increased oscillatory compliance. CO2-breathing did not affect the respiratory impedance in late expiration, but reduced its flow dependence along the ventilatory cycle.

Functional localization of avian intrapulmonary CO2 receptors within the parabronchial mantle

To determine the location of avian intrapulmonary CO2 receptors, we changed the CO2 stimulus at different regions within the parabronchial mantle and measured the resulting changes in breathing pattern. Three procedures were used to vary the CO2 stimulus: (1) reverse the direction of pulmonary perfusion; (2) stop pulmonary ventilation while maintaining perfusion; and (3) stop pulmonary perfusion while maintaining ventilation. Right and left lungs of adult, anesthetized White Leghorn type chickens were independently, unidirectionally ventilated. The right lung was used to maintain the bird while the left pulmonary artery and vein were cannulated and connected to an extracorporeal gas exchanger, thereby isolating this lung's perfusion. The innervation to both lungs remained intact. When left pulmonary perfusion was reversed, the bird's breathing pattern remained unchanged. The change in breathing pattern that resulted from stopping left pulmonary ventilation was the same during forward perfusion (pulmonary artery to pulmonary vein) as during backward perfusion (pulmonary vein to pulmonary artery). The change in breathing pattern that resulted from stopping forward perfusion was the same as that resulting from stopping backward perfusion. The results indicate that CO2 receptors are not concentrated on the peripheral side of the parabronchial mantle, where venous blood would influence tham, or on the luminal side of the mantle, where arterialized blood would influence them. The CO2 receptors are either distributed symmetrically between the peripheral and luminal sides of the mantle or located in the epithelial lining of the parabronchial lumen.

Control of respiration in the chicken: effects of venous CO2 loading

To determine if ventilation in unanesthetized chickens is adjusted sufficiently to prevent alterations in the partial pressure of carbon dioxide in arterial blood (PaCO2) when the CO2 content of mixed venous blood is changed, hypercapnic (PCO2 about 533 Torr) and hypocapnic (PCO2 less than 10 Torr) blood was infused into the left jugular vein of decerebrate chickens at 38 ml . min-1 for 30 sec. Ventilation and PaCO2 were assessed by determining respiratory frequency (f), tidal volume (VT), and the end-tidal CO2 fraction while serial samples of arterial blood were withdrawn from the sciatic artery. Infusion of hypercapnic blood resulted in an increase VT and minute ventilation (VE) as well as an increase in PaCO2. Infusion of hypocapnic blood resulted in a decrease in VT and VE and a small, transient decrease in PaCO2; the PaCO2 often returned to control levels before the end of the infusion period. The respiratory control system in the chicken appears to be better able to maintain a constant PaCO2 when perturbed by a reduced venous CO2 load reaching the lung than when perturbed by a reduced venous CO2 load reaching the lung than when perturbed by an increased CO2 load. These results are consistent with the hypothesis that intrapulmonary CO2 receptors, whose sensitivity to PCO2 is highest at low PCO2, are involved in the breath-to-breath control of breathing in birds.

CO2-sensitivity of stretch receptors in the marsupial lung

Discharge activity in single fibres of the vagus of the brush tailed possum was examined for evidence of pulmonary CO2 receptors by artificially ventilating the lungs with gas mixtures which preserved, abolished or reversed the normal tidal oscillation in FCO2. No specific CO2 receptors were observed. A quantitative study of the CO2 sensitivity of thirteen pulmonary stretch receptors was carried out after stabilizing FACO2 at high (6.4-7.8%), low (1.4-2.5%) as well as intermediate values. In addition receptor responses to a series of sustained augmenting inflations were examined at different intrapulmonary CO2 concentrations. All thirteen receptors showed CO2 sensitivity, their frequency of discharge being reduced by hypercapnia and increased by hypocapnia. Five were low threshold receptors which discharged throughout the ventilatory cycle while the remaining eight were only phasically active. High threshold receptors were more sensitive to FACO2 changes than were low threshold units. The results from the series of augmenting inflations suggest that it is the receptor's threshold, but not its sensitivity, to tracheal pressure that is modulated by the co2 signal.

Ventilation response to CO2 in birds. II. Contribution by intrapulmonary CO2 receptors

The CO2 sensitivity of intrapulmonary CO2 receptors (IPC) in the duck was studied, before (Control) and after blockade of carbonic anhydrase by Diamox, by recording single unit afferent activity in the vagus nerve. During Control, IPC activity decreased with increasing airway CO2 concentration. After Diamox administration, the discharge from IPC was higher at all levels of airway PCO2, and the receptors' CO2 sensitivity was markedly attenuated. Comparing these results with measurements on ventilation and blood gases of the duck under similar experimental conditions (Powell et al., 1978b) suggests that IPC play a role in the adjustment of ventilation to altered concentrations of inspired CO2; IPC may thus be a significant component in the control of breathing under physiological conditions.

Are avian intrapulmonary CO2 receptors chemically modulated mechanoreceptors or chemoreceptors?

Openings of paleopulmonic parabronchi in paralyzed, unidirectionally ventilated geese were photographed through small holes in the birds' mediodorsal secondary bronchi during single-unit recording from intrapulmonary CO2 receptors. Changes in the discharge frequency of the receptors as fractional CO2 concentration of ventilating gas was alternated between 0 and 0.05 were compared with the changes in cross-sectional areas of randomly selected parabronchial lumina. Intrapulmonary CO2 receptors, similar to those found in other avian species, are also present in geese. Changes in intrapulmonary CO2 concentration greatly influenced the discharge of these receptors but did not induce movement of parabronchial smooth muscle in this region of the lung. If most of the receptors are located in the paleopulmonic parabronchi, as currently appears to be the case, we must conclude that changes in receptor discharge in response to changes in intrapulmonary CO2 concentration do not result from mechanical distortion of the receptors induced by smooth muscle contraction; intrapulmonary CO2 receptors appear to be true chemoreceptors.

Chicken intrapulmonary chemoreceptors: discharge at static levels of intrapulmonary carbon dioxide and their location

We studied 54 intrapulmonary chemoreceptors in the unidirectionally ventilated left lungs of 12 thoracotomized cockerels. We ligated the left pulmonary artery to eliminate CO2 contributed by mixed venous blood. At zero PCO2 many units discharge irregularly, and some cease discharging after several seconds. Discharge frequencies at 13.7 torr PCO2 and above are described by logarithmic regressions. The slopes and intercepts of the logarithmic regressions are correlated so that the average response can be written: frequency = 3.86 -B . 1n (24.5 PCO2-1). Afferent activity above 6.8 torr PCO2 is described by 0.073 + 78.6 exp (-0.11 PCO2) -63.3 exp (-0.15 PCO2). For each unit, receptive site PCO2 in a perfused lung was assumed to be the PCO2 in the unperfused lung which gave the same discharge frequency. Location of the receptor was determined as the fraction of ventilation-perfusion region which had the same PCO2 as receptive site PCO2. Two major concentrations of receptors accounted for 85% of the total, one near the entering gas and one near the middle of the gas-exchange region. Sensitivity of individual receptors did not vary systematically with location.

Biogenic amine-containing cells in the chicken lung

Formaldehyde induced fluorescence was used to identify biogenic amine-containing cells in the adult chicken lung. Such cells, found in the parabronchial region, are sparsely distributed. There are at least two types of biogenic amine-containing cells in the lung: one type probably contains serotonin and the other a catecholamine. These cells might function as either a humoral or hormonal regulator of pulmonary ventilation and perfusion or as a receptor involved in control of breathing.

Carbon dioxide sensitivity of pulmonary receptors in the frog

Pulmonary mechano-receptors have been found in the frog lung that are sensitive to CO2 concentrations in the lungs and airways comparable to the physiological levels recorded in frogs. These results support the suggestion that a pulmonary receptor with distinct mechano- and chemosensitive properties may represent the functional precursor of the more specialized pulmonary receptor types which have evolved in birds and mammals.

Ventilatory responses to CO2 in the chicken: intrapulmonary and systemic chemoreceptors

The independent effects of pulmonary and arterial Pco2 on respiratory amplitude (RA) and respiratory frequency (f) were studied in unidirectionally ventilated chickens anesthetized with phenobarbital (160 mg-kg-1). Pulmonary Pco2 was set by the level of PIco2 ventilating the vascularly isolated right lung (VIL), whereas the systemic arterial Pco2 was set by the level of PIco2 ventilating the denervated left or gas exchange lung (GEL). The following results were obtained: 1) Increasing the PIco2 to the VIL from 0 to 35 torr and maintaining Paco2 constant at 2. torr increased RA from apnea to 76% of the animals' maximal hypercapnic response and decreased f: further increases in PIco2 to VIL had only minimal effects on RA and f. 2) increasing Paco2 from 19 to 61 torr and maintaining pulmonary Pco2 constant increased RA and decreased further increases in Paco2 had only slight effects on RA ulmonary chemoreflex and can dominate the control of RA during hypocapnic conditions, and (2) systemic CO2-sensitive chemoreceptors dominate the control of RA during hypercapnic conditions. It is suggested that the intrapulmonary chemoreceptors may act as a sensory system which plays a pertinent role in the regulation of parabronchial ventilation.

Intrapulmonary receptors in the Tegu lizard: I. Sensitivity to CO2

Single unit vagal recordings from intrapulmonary receptors were obtained in decerebrate, paralyzed lizards both during pump ventilation and during unidirectional ventilation on the cannulated, sack-shaped lung. Two types of receptors were identified: (1) CO2-receptors, which increased their discharge frequency as intrapulmonary CO2 concentration decreased but were not sensitive to stretch of the lung. (2) Mechanoreceptors, which rapidly increased discharge frequency when the lung was stretched. These receptors' CO2 sensitivity varied. Lungs of lizards thus appeared to possess both CO2 receptors, which have functional characteristics similar to those in birds, and mechanoreceptors with properties similar to stretch receptors in mammals.

Respiratory and cardiovascular interactions in ducks: the effect of lung denervation on the initation of and recovery from some cardiovascular responses to submergence

Lung denervation in ducks, by sectioning all vagal branches to one lung following mid-cervical vagotomy on the other side, resulted in immediate bradycardia and fall in breathing frequency. Some 3-5 weeks after lung denervation breathing frequency was within the normal range but the lung inflation reflex, present in unilaterally vagotomized sham-operated ducks, was abolished. During 2 min dives there were no significant differences between sham-operated and denervated ducks in heart rate, arterial blood pressure, blood gas tensions and pH(a). However, during recovery from diving heart rate increased more slowly in denervates and breathing rate was significantly below that attained by shams, although tidal volume rose to a maximum increase of 139% to a maximum of 225% of the pre-dive value in denervates in contrast to a maximum increase of 139% of pre-dive in sham-operated ducks. Both sham-operated and denervated ducks exhibited a significant fall in diastolic blood pressure 60 sec after emergence...

Intrapulmonary CO2 receptors in the duck: IV. Discharge pattern of the population during a respiratory cycle

We used single-unit vagal recordings to study the average discharge pattern during a respiratory cycle from 57 intrapulmonary CO2 receptors in 6 ducks artificially ventilated with a Starling pump. Peak discharge frequency occurred during inspiration in 51%, expiration in 19%, and both inspiration and expiration in 30% of the receptors. Average discharge frequency from all receptors was maximum at mid-inspiration, but frequency also increased slightly during early expiration. The results suggest that the brain receives a discharge pattern from the total population of intrapulmonary CO2 receptors which is phasic with respiration. Such a signal may control the rate and amplitude of breathing.