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PubMed1991

The effects of neonatal capsaicin administration on trigeminal nerve chemoreceptors in the rat nasal cavity.

Trigeminal nerve fibers in the nasal cavity respond to a variety of volatile chemical stimuli. Some of these trigeminal nerve fibers have been suggested to be capsaicin-sensitive and thus belong to a class of pain receptor rather than constituting a separate class of chemoreceptor. Our current results confirm this suggestion. Trigeminal nerve responses to volatile chemical stimuli were eliminated in rats which were injected with capsaicin on the second day of life. Animals whose nerves were unresponsive to chemical stimuli also exhibited a loss of intraepithelial peptide-immunoreactive fibers in their nasal cavities. The results of this study suggest that trigeminal nerve fibers in the nasal cavity which respond to chemical stimuli may be polymodal nociceptors which contain substance P, calcitonin gene-related peptide, or perhaps other neuropeptides.

Animals↗
PubMed1995

Synaptic interactions of substance P immunoreactive nerve terminals in the baro- and chemoreceptor reflexes of the cat.

The neurochemical anatomy and synaptic interactions of morphologically identified chemoreceptor or baroreceptor afferents in the nucleus of the solitary tract (NTS) are poorly understood. A substantial body of physiological and light microscopic evidence suggests that substance P (SP) may be a neurotransmitter contained in first order sensory chemo- or baroreceptor afferents, however ultrastructural support of this hypothesis is lacking. In the present report we have traced the central projections of the carotid sinus nerve (CSN) in the cat by utilizing the transganglionic transport of horseradish peroxidase. Medullary tissues including the commissural NTS (cNTS) were processed for the histochemical visualization of transganglionically labeled CSN afferents and for the immunocytochemical detection of SP by dual labeling light and electron microscopic methods. At the light microscopic level, dense bilateral labeling with TMB was found in the tractus solitarius (TS) and cNTS, caudal to the obex. Rostral to the obex, significant ipsilateral TMB labeling was detected in the dorsal, dorso-lateral, and medial subnuclei of the NTS, as well as in the TS. Significant staining of SP immunoreactive processes was detected in most subnuclei of the NTS. The cNTS was examined by electron microscopy. Either HRP or SP were readily identified in single labeled unmyelinated axons, myelinated axons, and nerve terminals in the cNTS. SP immunoreactivity was also identified in unmyelinated axons, myelinated axons, and nerve terminals in the cNTS which were simultaneously identified as CSN primary afferents. These ultrastructural data support the hypothesis that SP immunoreactive first order neurons are involved in the origination of the chemo- and baroreceptor reflexes. Axo-axonic synapses were observed between CSN primary afferent terminals and: (a) unlabeled nerve terminals; (b) other CSN primary afferent terminals; and (c) terminals containing SP. Axo-axonic synapses were also observed between CSN primary afferents which contained SP, and other SP terminals. These observations may mediate the morphological bases for multiple forms of presynaptic inhibition in the cNTS, including those involved in cardiorespiratory integration. In conclusion, our results indicate that SP immunoreactive nerve terminals may be important in both the origination and the modulation of the chemo- and/or baroreceptor reflexes.

Animals↗
PubMedDate not supplied

A model of regional ventilation-perfusion inhomogeneity in the avian lung. Implications for gas exchange and intrapulmonary chemoreceptor microenvironment.

We recorded discharge frequencies of 32 intrapulmonary chemoreceptors (IPC) during caudocranial and craniocaudal ventilation in the perfused duck lung. Blood gases, ventilatory gas flow, inspired PCO2 and PO2, and expired PCO2 measured simultaneously were used to predict regional CO2 and O2 gradients within the lung. Gas exchange was modelled in 7 log normal ventilation-perfusion compartments using mass balance differentials with an adjustable step size. CO2 and O2 interactions during exchange were modelled using the Bohr effect, P50, blood acid-base status, and the CO2 dissociation relationship. Close agreement (+/- 1.0 Torr) between simulated arterial and expired PCO2 and observed values was achieved after forcing simulated PaO2 to converge on observed PaO2 by an iterative adjustment of the perfusive shunt or the log standard deviation of the ventilation-perfusion distribution. Using the IPC static CO2 sensitivity measured in the non-perfused lung and the CO2 gradients generated by the model, we have found evidence for a distributed multi-ending receptor system in the duck lung.

Animals↗
PubMed1978

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.

Action Potentials↗
PubMed1978

Response of intrapulmonary chemoreceptors in the duck to changes in PCO2 and pH.

We have estimated the relative importance of changes in blood PCO2 and pH in determining activity of intrapulmonary chemoreceptors (IPC) in the unidirectionally ventilated duck. The response of single unit vagal afferents from IPC to changing lung gas PCO2 was tested before and after changing blood pH by intravenous infusion of NaHCO3. Using multiple linear regression analysis, we calculated how much of the change in IPC activity for a given change in PCO2 was due to the changing PCO2 at constant pH (CO2 sensitivity) or to the change in pH concomitant with the change in PCO2 (H+ sensitivity). For 10 IPC, the CO2 sensitivity was on the average 2.3 times larger than the H+ sensitivity. Changes in pH as well as PCO2 of lung blood should be considered in assessing the role of IPC in control of breathing.

Action Potentials↗
PubMed1978

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.

Action Potentials↗
PubMed1979

Carbon dioxide sensitivity of aortic chemoreceptors in the cat.

Steady state CO2 sensitivity in the aortic chemoreceptors was variable. For 70.3% of the steady state response lines to increasing CO2 the slope was not significantly different from zero. For 24.3%, it was significantly positive and for 5.4% significantly negative. In contrast, in 86.7% of cases the oscillating response to alternate breaths of high and low CO2 suggested a transient response line which was significantly steeper than the steady state line. Removing CO2 from a high CO2 content inspirate for one breath caused a distinct drop in discharge on 83.3% of occasions. The significance of such transient CO2 sensitivity in the absence of clear steady state sensitivity is discussed.

Animals↗
PubMed1980

Discharge of intrapulmonary chemoreceptors and its modulation by rapid F1CO2 changes in decerebrate ducks.

Single-unit activity was recorded from intrapulmonary chemoreceptors (IPC) in decerebrate ducks inspiring room air (fresh air or control breath), or a short pulse of room air preceded and followed by 5% CO2 (fresh air pulse or experimental breath). Of 36 IPC studied, 28 fired a burst of impulses of similar duration to the fresh air pulse; delaying the fresh air pulse until later and later in inspiration progressively delayed the IPC burst. The remaining 8 IPC did not respond discretely to the fresh air pulse, rather their discharge was reduced diffusely in one or both of the ventilatory phases. The average discharge of the IPC population had a cyclic character during control breathing, with peak discharge mid-way through inspiration and essentially a constant discharge during expiration. An experimental breath had a similar IPC discharge pattern but peak inspiratory discharge was reduced; delaying the fresh air pulse delayed the population IPC burst. The results indicate that IPC could mediate the previously reported changes in T1 and TE that occur when the timing of a fresh air pulse is manipulated in conscious chickens [(Tallman et al. (1979). Am. J. Physiol. 237: R260-R265)].

Action Potentials↗
PubMed1981

Carotid chemoreceptor function in ventilatory and circulatory O2 convection of exercising dogs at low and high altitude.

Awake dogs were studied before (control) and after chronic bilateral carotid denervation (denervated) at rest and running for 3 min on a treadmill at 8 km . h-1 and at various grades, in an altitude chamber operated either at 140 m or at 4000 m for 3 h. Steady-state pulmonary ventilation (Vg) and breathing pattern (VT, fR), oxygen consumption (MO2), O2 concentrations (C) and pressures (P) in the arterial (a) and mixed venous blood (v), hematocrit (Ht) and acid-base status in arterial blood, and heart frequency (fH) were measured. From these data cardiac output (Vb) and stroke volume (Vs), ventilatory and circulatory requirements (Vg/Mo2, Vb/MO2), extraction of O2 from inspired gas (EairO2) and blood (EbO2), and capacitance coefficient of blood for oxygen (beta bO2) were calculated. Ventilatory responses to transient O2-inhalation were also studied and the aortic (AP) and pulmonary (PP) blood pressures measured in resting conditions. 1. After chronic carotid denervation the hypoxic chemoreflex drive of ventilation was reduced by about half, maximal MO2 remained unaffected at 140 m, but at 4000 m decreased 50% compared to 30% in controls. 2. In all experimental conditions, Vg/MO2, PaO2 and CaO2 were less in denervated animals than in controls, and EairO2, PaCO2 and H+ ion concentration were higher. 3. At 140 m, circulatory O2 convective transport was identical in the two groups of dogs. At 4000 m, beta bO2 increased similarly in both groups, but Vb and Vb/MO2 were higher in denervated dogs than in controls, in relation with reduced CaO2-CVO2 difference which contributed to restore PVO2 towards higher values. 4. At 140 m, mean resting AP and PP were similar in both groups of dogs. At 4000 m, AP increased not significantly in controls, and decreased in denervated animals; PP increased in controls, but not in denervated dogs. It is concluded that integrity of the arterial chemoreceptor drive is essential in determining the eupneic level of ventilation and normal acid-base status of the blood in both resting and exercising dogs, at low and at high altitude, and in reducing the O2 circulatory requirement at high altitude. At 4000 m, the lack of carotid chemosensitivity is accompanied by severe hypoxemia, in association with hypercapnia and acidosis, and by increased cardiac blood flow, most presumably due to decreased peripheral resistance and increased venous return; despite these compensatory changes in circulatory O2 convective transport, denervated animals reach a maximum O2 uptake at lower work load than controls.

Adaptation, Physiological↗
PubMed1982

Ventilation and carotid chemoreceptor discharge during venous CO2 loading via the gut.

A simple method of loading CO2 into the venous blood of anaesthetized or decerebrate cats is described. Heated humidified gas of high CO2 content is passed retrogradely through the gut. This increases CO2 elimination from the lungs by 1.8 to 3.6 fold, minute ventilation by 1.5 to 2.7 fold and cardiac output by 1.1 to 1.6 fold. This method of venous CO2 loading allows the recording of afferent nerve impulses. We show that it has little effect on the mean discharge frequency of carotid body chemoreceptors when end-tidal gases are held constant, but that it enhances greatly the amplitude of the respiratory oscillation of their discharge.

Animals↗
PubMed1983

Effect of temperature on the CO2 sensitivity of avian intrapulmonary chemoreceptors.

We determined linear regressions of discharge frequency on ln PCO2 of 23 intrapulmonary chemoreceptors (IPC) from eight hyperthermic, adult cockerels. At low PCO2, IPC in hyperthermic cockerels discharged slower than IPC measured in euthermic cockerels; above 25 torr PCO2, however, they discharged faster than euthermic IPC. Thus, IPC were less sensitive to PCO2 during hyperthermia. We calculate that a 1 degree C increase in the temperature of the lung (TL) causes the slope of the linear regression of discharge frequency on ln PCO2 to be less negative by 1.5 +/- 0.5 imp (sec X ln PCO2)-1 and that, for any increase in TL above normal (41.5 degrees C), the average IPC discharge frequency equals (-10.7 + 1.5 (TL -41.5] X (ln (PCO2/25.0] + 3.7. This relationship may be partly responsible for the increased tidal volume and decreased respiratory frequency observed when body temperature increases during constant PaCO2.

Alkalosis, Respiratory↗
PubMed1983

Exercise hyperpnea in the duck without intrapulmonary chemoreceptor involvement.

To determine the involvement of intrapulmonary chemoreceptors (IPC) in the control of breathing during exercise, it is necessary to hold the PCO2 in the microenvironment of these receptors constant at the resting value. We accomplished this in unanesthetized Pekin ducks by ligating the left pulmonary artery and diverting all of the cardiac output to the right lung, which was denervated. The ducks were then unidirectionally ventilated with a constant gas stream (5% CO2, 19% O2, balance N2) at a flow rate of 12 L X min-1. This procedure provided a constant microenvironment for the receptors in the left lung despite any changes in the chemical composition or flow rate of the blood going to the right lung during exercise. Ventilatory effort increased during running exercise (1.44 km X h-1 at a 3 degrees incline) by an average of 145% over resting values because of an increase in both respiratory frequency and tidal volume. Because no altered stimuli were presented to the IPC using this procedure, the increased ventilation with exercise could not have resulted from changes in their discharge frequency. We conclude that ventilation can increase during exercise in the duck in the absence of IPC involvement and that other neural input, possibly from muscle, is responsible for the hyperpnea.

Animals↗
PubMed1984

Receptive fields of intrapulmonary chemoreceptors in the Pekin duck.

Reflex experiments indicate a uniform distribution of CO2 chemosensitivity in avian lungs, but neural recording experiments suggest a non-uniform distribution of intrapulmonary chemoreceptor (IPC) endings. To reconcile these observations, blood gases and PECO2 were measured while recording discharge frequencies of 32 IPC innervating the unidirectionally ventilated lungs of 14 Pekin ducks. IPC discharge frequencies, recorded from the left vagus, were determined while ventilating the perfused left lung with caudocranial and craniocaudal flows of 1% CO2 in air, and then while ventilating the unperfused left lung with known levels of CO2 in air. Lung PCO2 profiles were predicted using an eight-compartment computer model of cross-current gas exchange with log-normal ventilation-perfusion inequality and shunt. The PCO2 profiles and IPC discharge frequencies were used to calculate receptor location. At the 99% confidence limit, estimates of IPC location changed significantly in all but 7 IPC when the direction of ventilation was reversed, indicating many IPC have multiple endings. Eighteen of 32 IPC had receptive fields extending at least 50% of the parabronchial length, which may explain the uniform reflex chemosensitivity to intrapulmonary CO2 noted by others.

Animals↗
PubMed1985

Effects of changes in tidal volume on avian intrapulmonary chemoreceptor discharge.

It has been suggested that avian intrapulmonary CO2-sensitive receptors (IPC) may be capable of monitoring the rate and extent of CO2 washout from the lung during spontaneous breathing. The purpose of this study was to analyse IPC discharge activity (using computerised bin-averaging and counting techniques) in spontaneously breathing domestic fowl when VT was elevated from resting levels. This was accomplished by administration of almitrine (2 mg X kg-1 i.v.), a respiratory stimulant drug that has been shown to have a specific long-lasting stimulatory action on carotid body chemoreceptors. Unanaesthetized decerebrate chickens were tracheotomized and single unit activity was recorded from 14 IPC. When VT progressively increased following administration of almitrine (with little or no change in TI or TE), IPC activity increased in a linear relationship with the increased VT. IPC activity in expiration was also increased, and the delay period before the onset of IPC discharge in inspiration was shortened. It is concluded that IPC discharge is increased when VT is elevated in the spontaneously breathing chicken and hence the IPC are capable of monitoring the extent of each ventilatory effort. It is well known that IPC have strong inhibitory effects on ventilatory motor output and conceivably they could originate reflexogenic information to the respiratory centres in response to intrapulmonary PCO2 changes. The latter could arise from changes in CO2 delivery by the mixed venous blood or from changes in the extent of CO2 washout with each breath.

Almitrine↗
PubMed1985

Ventilatory and intrapulmonary chemoreceptor sensitivity to CO2 in the burrowing owl.

We measured the ventilatory response of anesthetized, unidirectionally ventilated pigeons and burrowing owls to changes in intrapulmonary CO2 concentration and the static CO2 sensitivity of intrapulmonary chemoreceptors (IPC) in these species and the domestic goose. Compared with pigeons, burrowing owls showed a significantly reduced respiratory frequency and amplitude response to increases in FICO2 from 0.05 to 0.10, which corroborates similar findings in intact, awake individuals of the same species. The average static CO2 sensitivity of IPC in geese, pigeons, and burrowing owls, as reflected by the average slope of the linear regressions of receptor discharge frequency on ln(PICO2), was -7.96, -11.1 and -6.87 imp/sec . ln(PICO2), respectively. The sensitivities of individual receptors were normally distributed in geese and pigeons, but skewed in burrowing owls. Therefore, the median CO2 sensitivity [-5.50 imp/sec . ln(PICO2)] is a more appropriate measure of the typical CO2 sensitivity of IPC in burrowing owls. The static CO2 sensitivity of IPC in burrowing owls is the lowest reported for euthermic birds with a normal acid-base balance and may materially contribute to the blunted ventilatory response of these birds to the elevated CO2 levels they encounter in nature.

Animals↗
PubMed1985

Comparison of intrapulmonary chemoreceptor response to PCO2 in the duck and chicken.

Intrapulmonary chemoreceptors (IPC) in the burrowing owl are reported to be much less sensitive to PCO2 than IPC in the chicken. This blunted IPC sensitivity has been suggested to be a physiological adaptation to hypercapnic subterranean environments. To investigate the natural variation IPC responses in non-burrowing species, stimulus-response characteristics of 87 IPC in 22 anesthetized Pekin ducks were recorded and compared to those from 54 previously reported chicken IPC. Average logarithmic stimulus response curves were described by slopes of - 11.2 and - 10.7 imp X (sec X InPCO2)-1 for duck and chicken, respectively. Each had slopes steeper than the - 6.87 imp X (sec X InPCO2)-1 slope reported for the burrowing owl. As with chicken IPC, slopes and intercepts of the individual curves were highly correlated in the duck. It appears that a general mechanism of receptor transduction exists in birds, with some quantitative interspecies variation.

Animals↗
PubMed1986

Sensitivity of avian intrapulmonary chemoreceptors to venous CO2 load.

To investigate the response of individual intrapulmonary chemoreceptors (IPC) to venous CO2 loads approximating moderate muscular exercise, we recorded vagal discharge from 33 IPC arising from the left lungs of 9 anesthetized, unidirectionally ventilated Pekin ducks. Each IPC was studied during control conditions (PECO2 = 29.0 +/- 0.8 Torr, PVCO2 = 30.2 +/- 0.6 Torr) and during venous CO2 load (PECO2 = 29.5 +/- 0.7 Torr, PVCO2 = 51.5 +/- 1.4 Torr). Venous loading was produced by increasing the percentage of CO2 in the gas ventilating the right lung from 0 to 9-25% CO2. The flow of 1% CO2 through the left lung was adjusted to keep the left lung PECO2 constant. During venous loading, discharge frequencies indicated that the PCO2 at the receptive sites fell, on the average, 1.6 +/- 0.8 Torr.

Action Potentials↗
PubMed1986

Sodium bicarbonate infusion increases discharge frequency of intrapulmonary chemoreceptors only at high CO2.

We felt that earlier determinations of independent effects of extracellular pH and PCO2 on intrapulmonary chemoreceptors (IPC) discharge frequency were difficult to analyze because they used perfused lungs, and ventilation-perfusion changes among parabronchi could not be controlled. We decided to repeat these studies in non-perfused lungs. We cannulated both extrapulmonary bronchi of 10 thoracotomized Pekin ducks anesthetized with sodium pentobarbital (25-35 mg/kg) and unidirectionally ventilated each lung. The perfused right lung maintained gas exchange while the non-perfused left lung received 0.6 L/min of CO2 mixed in air. We recorded the discharge frequency of one IPC per duck at various PCO2, re-established circulation, and infused 3.0 mmol/kg of sodium bicarbonate intravenously. After 15 min, discharge frequencies were again measured from the same IPC in the nonperfused lung. The slopes and intercepts of discharge frequencies vs ln PCO2 relationship were depressed in six IPC, increased in two IPC and not significantly affected in two IPC. Arterial pH was increased significantly (0.11 unit) at 38 Torr arterial PCO2. We conclude that acutely increased extracellular sodium bicarbonate affects IPC discharge only by depressing sensitivity of most IPC to PCO2 and does not have an independent effect through pH.

Animals↗