Responses of slowly and rapidly adapting receptors in the airways of rabbits to changes in the Starling forces

Mechanisms Involved in the Stimulatory and Inhibitory Effects of 5-Hydroxytryptamine on Vagal Mechanosensitive Afferents in Rat Lung

Mechanosensitive vagal afferents in the lung, rapidly and slowly adapting receptors (RARs and SARs, respectively), play an important role in eliciting the reflexes that regulate the normal airway function. A profound bronchoconstrictive effect of 5-hydroxytryptamine (5-HT) has been extensively reported in various animal species, but its influence on the SAR and RAR activity is not known. This study investigated the effect of 5-HT on these receptors, and the possible mechanisms involved. Single-fiber activities of these afferents were measured in anesthetized, open-chest, and mechanically ventilated rats. Our results showed that intravenous injection of 5-HT evoked a consistent and pronounced stimulation of phasic RARs. In contrast, 5-HT generated an inconsistent and paradoxical action on SARs: no effect in 29% (5 of 17) of the SARs; stimulation in 35% (6 of 17); and inhibition in the remainder. These responses of both RARs and SARs to 5-HT were reproducible and dose-dependent. After the injection of a high dose of 5-HT (16 μg/kg), the receptor responses slowly reached a peak (after ∼8 s) and returned toward the baseline in ∼20 s, accompanied by a consistent increase in total pulmonary resistance and a decrease in dynamic lung compliance in a temporal pattern very similar to the increased receptor activity. When these changes in lung mechanics induced by 5-HT were prevented by pretreatment with salbutamol, a β₂ adrenergic receptor agonist, the delayed responses of both RARs and SARs to 5-HT were also abolished, except that the immediate stimulatory effect on a subset of RARs, the silent RARs, was not affected. In conclusion, 5-HT generated a delayed stimulatory effect on RARs and a paradoxical effect on SARs, which resulted primarily from the 5-HT-induced changes in mechanical properties of the lung.

Role of the paraventricular nucleus in the reflex diuresis to pulmonary lymphatic obstruction in rabbits

The changes in urine flow and renal sympathetic nerve activity (RSNA) due to pulmonary lymphatic obstruction (PLO) were examined in anesthetized, artificially ventilated New Zealand white rabbits. PLO was produced by pressurizing an isolated pouch created in the right external jugular vein at the points of entry of the right lymphatic ducts. During this maneuver, urine flow increased from 8.5 ± 0.3 mL/10 min to 12 ± 0.5 mL/10 min (P < 0.0001) and RSNA increased from 24.0 ± 4 to 40.0 ± 5 μV·s (P < 0.0001). Bilateral lesioning of the paraventricular nucleus (PVN) of the hypothalamus or cervical vagotomy abolished these responses. PLO increased c-fos gene expression in the PVN. The increase in urine flow due to PLO was attenuated by muscimol and abolished by kynurenic acid microinjections into the PVN. The results show that (i) neurons in the PVN are an important relay site in the reflex arc, which is activated by PLO; and (ii) this activation is regulated by glutamatergic and partly by GABAergic input to the PVN.

Sensory nerves in lung and airways

Sensory nerves innervating the lung and airways play an important role in regulating various cardiopulmonary functions and maintaining homeostasis under both healthy and disease conditions. Their activities conducted by both vagal and sympathetic afferents are also responsible for eliciting important defense reflexes that protect the lung and body from potential health-hazardous effects of airborne particulates and chemical irritants. This article reviews the morphology, transduction properties, reflex functions, and respiratory sensations of these receptors, focusing primarily on recent findings derived from using new technologies such as neural immunochemistry, isolated airway-nerve preparation, cultured airway neurons, patch-clamp electrophysiology, transgenic mice, and other cellular and molecular approaches. Studies of the signal transduction of mechanosensitive afferents have revealed a new concept of sensory unit and cellular mechanism of activation, and identified additional types of sensory receptors in the lung. Chemosensitive properties of these lung afferents are further characterized by the expression of specific ligand-gated ion channels on nerve terminals, ganglion origin, and responses to the action of various inflammatory cells, mediators, and cytokines during acute and chronic airway inflammation and injuries. Increasing interest and extensive investigations have been focused on uncovering the mechanisms underlying hypersensitivity of these airway afferents, and their role in the manifestation of various symptoms under pathophysiological conditions. Several important and challenging questions regarding these sensory nerves are discussed. Searching for these answers will be a critical step in developing the translational research and effective treatments of airway diseases.

Airway irritation and cough evoked by acid: from human to ion channel

Inhalation or aspiration of acid solution evokes airway defense responses such as cough and reflex bronchoconstriction, resulting from activation of vagal bronchopulmonary C-fibers and Aδ afferents. The stimulatory effect of hydrogen ion on these sensory nerves is generated by activation of two major types of ion channels expressed in these neurons: a rapidly activating and inactivating current mediated through ASICs, and a slow sustaining current via activation of TRPV1. Recent studies have shown that these acid-evoked responses are elevated during airway inflammatory reaction, revealing the potential convergence of a wide array of inflammatory signaling on these ion channels. Since pH in the airway fluid drops substantially in patients with inflammatory airway diseases, these heightened stimulatory effects of acid on airway sensory nerves may play a part in the manifestation of airway irritation and excessive cough under those pathophysiological conditions.

Control of breathing during acute change in cardiac preload in a patient with partial cardiopulmonary bypass

We recently had the opportunity to investigate the ventilatory effects of changing the rate of venous return to the heart (and thus pulmonary gas exchange) in a patient equipped with a venous-arterial oxygenated shunt (extracorporeal membrane oxygenation (ECMO) support). The presence of the ECMO support provided a condition wherein venous return to the right heart could be increased or decreased while maintaining total aortic blood flow and arterial blood pressure (ABP) constant. The patient, who had received a heart transplant 12 years ago, was admitted for acute cardiac failure related to graft rejection. The clinical symptomatology was that of right heart failure. We studied the patient on the 4th day of ECMO support, while she was breathing spontaneously. The blood flow diverted through the ECMO system represented 2/3 of the total aortic flow (4 l min(-1)). With these ECMO settings, the baseline level of ventilation was low (3.89+/-0.99 l min(-1)), but PET(CO2) was not elevated (37+/-2 mmHg). When Pa(CO2) in the blood coming from the ECMO was increased, no stimulatory effect on ventilation was observed. However, when the diversion of the venous return to the ECMO was stopped then restored, minute ventilation respectively increased then decreased by more than twofold with opposite changes in PET(CO2). These maneuvers were associated with large changes in the size of the right atrium and ventricle and of the left atrium. This observation suggests that the change in venous return affects breathing by encoding some of the consequences of the changes in cardiac preload. The possible sites of mediation are discussed.

EFFECTS OF MITRAL REGURGITATION ON THE REFLEX DIURESIS TO PULMONARY LYMPHATIC OBSTRUCTION IN RABBITS

Increasing the extravascular fluid of the airways acutely by obstructing pulmonary lymph drainage causes a reflex diuresis mediated by neuronal nitric oxide synthase in the renal medulla. The authors examined this reflex in rabbits with a chronic increase in extravascular fluid of the airways resulting from surgically induced mitral regurgitation. Intact rabbits served as controls. Renal neuronal (nNOS) and endothelial (eNOS) nitric oxide synthase expressions were also examined. The reflex was absent in rabbits with mitral regurgitation. There were significant increases in medullary and cortical nNOS mRNA compared to controls. The observed changes in mRNA levels correlated with nNOS protein levels. eNOS mRNA was unaffected.

Behaviours of pulmonary sensory receptors during development of acute lung injury in the rabbit

We tested the hypothesis that oleic acid-induced acute lung injury activates pulmonary nociceptors, that is, C fibre receptors (CFRs) and high-threshold Adelta fibre receptors (HTARs). Single-unit activity was recorded in the cervical vagus nerve and assessed before and after injecting oleic acid (75 microl kg(-1) i.v.) into anaesthetized, open-chest, mechanically ventilated rabbits. Unit activities increased within seconds and peaked within a few minutes (from 0.3 +/- 0.1 to 1.4 +/- 0.9 impulses s(-1) for CFRs and from 0.5 +/- 0.1 to 1.7 +/- 0.3 impulses s(-1) for HTARs, both n = 8 and P < 0.05). These activities were sustained while pulmonary oedema developed and dynamic lung compliance decreased over the 90 min observation period. Activities in slowly adapting receptors and rapidly adapting receptors were also increased; however, their responsiveness to airway pressure stimulation decreased progressively. We conclude that pulmonary nociceptors are stimulated during acute lung injury. The dual nociceptor system, consisting of both non-myelinated CFRs and myelinated HTARs, may play an important role in the pathophysiological process of acute lung injury-induced respiratory responses.

Altitude-related cough

Cough is a troublesome condition which affects many visitors to high altitude. Traditionally it has been attributed to the inspiration of the cold, dry air which characterizes the high altitude environment. This aetiology was brought into question by observations and experiments in long duration hypobaric chamber studies in which cough still occurred despite controlled temperature and humidity. Anecdotally however, exercise, possibly via the associated increase in ventilation, does appear to precipitate cough at altitude. It is likely that the term, altitude-related cough, covers a number of conditions and aetiologies. These aetiologies are discussed and include water loss from the respiratory tract; high altitude pulmonary oedema; acute mountain sickness; bronchoconstriction; respiratory tract infections; vasomotor rhinitis and post-nasal drip; and alterations in the central control of respiration. We hypothesize that there are two forms of altitude-related cough: a cough which may occur at relatively low altitudes and which is related to exercise and persists despite descent and a cough which does not occur at altitudes below 5000-6000 m and which improves rapidly with descent to lower altitude. The treatment of altitude-related cough is symptomatic and frequently ineffective. Further work is required to understand the nature and aetiology of the cough which occurs at high altitude before effective therapies can be developed.

The rapidly adapting receptors in mammalian airways and their responses to changes in extravascular fluid volume

In this short review, we shall focus on some recent findings on the physiological stimulus for the rapidly adapting receptors (RAR) of the airways. They are readily activated by a sustained inflation of the lungs and they are usually identified by their rapid adaptation to this stimulus. They are also activated by both tactile stimuli and irritant gases applied to the epithelium of the airways. The investigations reviewed here suggest that these receptors are activated by changes in extravascular fluid volume. The principal factors governing fluid flux from the microcirculation are identified in the Starling equation. These are the hydrostatic pressure, plasma oncotic pressure and capillary permeability. Findings from recent studies suggest that all these factors increase the activity of RAR. In addition, these receptors are also activated by obstruction of lymph drainage from the lung. Evidence is presented to show that manipulation of Starling forces also increases the extravascular fluid volume of the airways in areas where the RAR are located. On the basis of these findings, it is suggested that, along with mechanosensitivity to stimuli such as stretch, inflation and deflation, another physiological stimulus to the RAR is a change in extravascular fluid volume in the regions of the airways where these receptors are located.

Role of adrenergic receptors in the reflex diuresis in rabbits during pulmonary lymphatic obstruction

The role of adrenergic receptors in the reflex diuresis in response to pulmonary lymphatic drainage was examined in anaesthetized, artificially ventilated New Zealand White rabbits. Pulmonary lymphatic drainage was obstructed by raising the pressure in a pouch created from the right external jugular vein. This pulmonary lymphatic obstruction results in a reflex increase in urine flow and sodium excretion. This reflex is abolished by renal denervation and by administration of L-NAME, a non-selective inhibitor of nitric oxide synthase. Also, infusion of the relatively selective neuronal nitric oxide synthase blocker, 7-nitroindazole sodium salt, into the renal medulla abolished the reflex diuresis. In this study the effects of adrenergic receptor antagonists on the reflex increase in urine were observed. Both ureters were cannulated in order to determine urine flow from both kidneys separately. Prazosin, an alpha1 adrenergic receptor antagonist, was infused into the renal medulla of the right kidney, while the left kidney acted as control. Administration of prazosin in this manner did not block the reflex diuresis in response to pulmonary lymphatic obstruction in either kidney. However, rauwolscine, an alpha2 adrenergic receptor antagonist, abolished the reflex increase in urine and sodium excretion in the ipsilateral kidney while preserving it in the contralateral kidney. These findings suggest that the increase in urine flow in rabbits caused by pulmonary lymphatic obstruction is dependent upon activation of alpha2 adrenergic receptors within the renal medulla.

Effects of gadolinium chloride on slowly adapting and rapidly adapting receptors of the rabbit lung

Effects of gadolinium chloride, an inhibitor of stretch-activated channels, on the responses of slowly adapting receptors (SARs) and rapidly adapting receptors (RARs) to hyperinflation were investigated. The increase in activity of RARs resulting from sustained elevations of left atrial pressure (LAP) was also assessed with gadolinium chloride application. Action potentials (AP) of SARs and RARs during hyperinflation were recorded from the vagus nerve of anesthetized New Zealand White rabbits before and after application of gadolinium chloride (20mM) directly on the receptor area of the nerve endings. There was a significant reduction of activity in SARs (n = 9) and RARs (n = 7) after application of gadolinium chloride. Activity of RARs (n = 6) increased when the LAP was elevated by 5 and 10 mmHg. This effect was abolished after gadolinium chloride was applied to receptor endings and the activity was restored when gadolinium chloride was removed. This suggests that stretch-activated channels play a role in SARs and RARs activity.

Role of nitric oxide in the reflex diuresis in rabbits during pulmonary lymphatic obstruction

The role of nitric oxide in the reflex diuresis in response to pulmonary lymphatic drainage was examined in anaesthetized, artificially ventilated New Zealand White rabbits. Pulmonary lymphatic drainage was obstructed by raising the pressure in a pouch created from the right external jugular vein. Pulmonary lymphatic obstruction resulted in a significant increase in urine flow from an initial control value of 8.9 +/- 0.5 ml (10 min)(-1) to 12.1 +/- 0.6 ml (10 min)(-1) during lymphatic obstruction (mean +/-s.e.m.; n= 17, P < 0.001). This increase in urine flow was accompanied by a significant increase in the excretion of sodium. Additionally, renal blood flow remained unchanged during the increase in urine flow caused by lymphatic obstruction. Intravenous infusion of L-NAME, a non-selective inhibitor of nitric oxide synthase (NOS), abolished the reflex diuresis. Furthermore, intraperitoneal administration of the relatively selective neuronal NOS blocker, 7-nitroindazole also abolished the response. It was observed that infusion of a more soluble neuronal NOS blocker, 7-nitroindazole sodium salt (7-NINA), into the renal medulla also abolished the reflex diuresis. These findings suggest that the increase in urine flow in rabbits caused by pulmonary lymphatic obstruction is dependent upon the integrity of neuronal NOS activity within the renal medulla.

Responses of bronchial C-fiber afferents of the rabbit to changes in lung compliance

Rapidly adapting receptors (RAR) in the lung are stimulated when the lung compliance is reduced. The present investigation was undertaken to determine whether bronchial C-fibers are also activated when lung compliance is decreased since both RAR and bronchial C-fibers are influenced by extra-vascular fluid in the airways. Action potentials (AP) were recorded from bronchial C, pulmonary C, RAR and slowly adapting receptor (SAR) afferents in the cervical vagus in open chest New Zealand White rabbits ventilated at a constant rate and tidal volume. AP were recorded during (a) positive end-expiratory pressure (PEEP) of 2-3 cmH2O (control), (b) zero end-expiratory pressure (ZEEP), (c) negative end-expiratory pressure (NEEP) of -4 cmH2O, (d) restoration of PEEP and (e) final control after hyper-inflation. Both RAR and bronchial C-fiber activity increased significantly compared with control when lung compliance was decreased (bronchial C-fibers: 35 +/- 5 vs. 66 +/- 13 impulses per 30 sec and RAR: 3 +/- 1 vs. 94 +/- 14 impulses per 30 sec).

Serial changes in nasal potential difference and lung electrical impedance tomography at high altitude

Recent work suggests that treatment with inhaled beta(2)-agonists reduces the incidence of high-altitude pulmonary edema in susceptible subjects by increasing respiratory epithelial sodium transport. We estimated respiratory epithelial ion transport by transepithelial nasal potential difference (NPD) measurements in 20 normal male subjects before, during, and after a stay at 3,800 m. NPD hyperpolarized on ascent to 3,800 m (P < 0.05), but the change in potential difference with superperfusion of amiloride or isoprenaline was unaffected. Vital capacity (VC) fell on ascent to 3,800 m (P < 0.05), as did the normalized change in electrical impedance (NCI) measured over the right lung parenchyma (P < 0.05) suggestive of an increase in extravascular lung water. Echo-Doppler-estimated pulmonary artery pressure increases were insufficient to cause clinical pulmonary edema. There was a positive correlation between VC and NCI (R(2) = 0.633) and between NPD and both VC and NCI (R(2) = 0.267 and 0.418). These changes suggest that altered respiratory epithelial ion transport might play a role in the development of subclinical pulmonary edema at high altitude in normal subjects.

Responses of C fiber afferents of the rabbit airways and lungs to changes in extra-vascular fluid volume

Effects of changes in extra-vascular fluid volume produced by pulmonary lymphatic obstruction and plasmapheresis on the activities of bronchial and pulmonary C fiber receptors and rapidly adapting receptors (RARs) were investigated in New Zealand White rabbits. In intact rabbits, pulmonary lymphatic obstruction either alone or in combination with plasmapheresis did not stimulate pulmonary C fiber receptors. Only the combined stimulus activated the bronchial C fiber receptors. Bronchial C fiber receptors were also stimulated by graded increases in left atrial pressure (+5 and +10 mmHg). In contrast, RARs were activated by lymphatic obstruction either alone or in combination with plasmapheresis. These procedures increase the extra-vascular fluid volume in the carina and bronchi but not in the lungs (alveoli). In rabbits with chronic pulmonary venous congestion secondary to mitral valve damage, bronchial C fiber receptors were not stimulated by these increments in left atrial pressure which were insufficient to increase the extra vascular fluid content of the airways. However, both pulmonary and bronchial C fiber receptors were stimulated when the left atrial pressure was raised to 25 mmHg in these animals to cause pulmonary edema.

Rapidly adapting receptors in a rabbit model of mitral regurgitation

1. Unlike in normal rabbits, pulmonary rapidly adapting receptors (RARs) in rabbits with chronic mitral regurgitation (MR) do not respond to small changes in extravascular fluid (EVF) volume in major airways. The present study examined the effect of shrinking the EVF volume in rabbits with chronic MR by infusing hypertonic albumin, to see whether this response of RARs is restored. The effect of raising the left atrial pressure (LAP) acutely above 25 mmHg (to cause pulmonary oedema) on RARs was also investigated. 2. Mean RAR activities in rabbits with MR (n = 6) at initial control, LAP +5 mmHg, LAP +10 mmHg and final control periods were 20.9 +/- 9. 5, 18.8 +/- 11.3, 27.0 +/- 11.2 and 17.2 +/- 9.8 action potentials min-1, respectively (P > 0.05, ANOVA). After infusion of 35 % bovine serum albumin i.v. these values were 9.4 +/- 3.2, 30.6 +/- 14.6, 48. 9 +/- 10.1 and 18.4 +/- 7.3 action potentials min-1, respectively (P < 0.01, ANOVA). In rabbits with chronic MR (n = 7) raising the LAP above 25 mmHg stimulated RARs. 3. EVF content of the airways and lungs was measured in rabbits with MR and in control rabbits, at baseline and after elevation of the LAP by 10 or 25 mmHg for 20 min. In control rabbits the EVF contents in the lower trachea, carina and bronchi at baseline and at LAP +10 mmHg were 52.1 +/- 1.2 and 57.8 +/- 1.7 %, respectively (P < 0.05, Student's t test). In rabbits with MR these values were 58.3 +/- 1.5 and 56.9 +/- 1.9 %, respectively. When the LAP was elevated by 25 mmHg the EVF content increased to 62.4 +/- 1.1 % (P < 0.05, t test compared with baseline and LAP +10 mmHg). 4. We concluded that in rabbits with chronic MR, RARs are unable to respond to acute, small elevations of LAP because there is no concomitant increase in EVF content in the vicinity of these receptors. Furthermore, these receptors can be activated in these animals by elevating the LAP above 25 mmHg or can be made sensitive to acute small elevations of LAP by shrinking the chronically expanded EVF compartment.

Effect of chronic mitral valve damage on activity of pulmonary rapidly adapting receptors in the rabbit

1. The effects of acute pulmonary venous congestion on the activity of rapidly adapting receptors (RARs) were determined in intact (control and sham-operated) rabbits and in rabbits 6 and 12 weeks after surgical destruction of the mitral valve. 2. Destruction of the mitral valve increased the mean left atrial pressure (LAP) by approximately 2.6 and 3.8 mmHg, 6 and 12 weeks after surgery, respectively. These changes were accompanied by significant increases in left ventricular weight. The effect of acute increments in LAP on RAR activity was examined against this background of chronic pulmonary venous congestion. 3. In intact control and sham-operated animals RAR activity increased from 48.8 +/- 0.9 to 83. 5 +/- 3.6 and 121.1 +/- 4.7 action potentials min-1 when the LAP was raised by 5 and 10 mmHg, respectively, above control values. Six weeks after surgery only 40 % of RARs were activated in this way. 4. In animals maintained for 12 weeks after surgery, RAR activity at LAPs of 6.6 +/- 1.2 (control), 11.6 +/- 1.2 and 16.6 +/- 1.2 (mmHg) were 35.5 +/- 2.3, 33.8 +/- 14.4 and 34.0 +/- 3.4 action potentials min-1, respectively. These changes were statistically not significant. 5. Slowly adapting receptors (SARs) in the lung showed a small but statistically significant increase in activity when the left atrial pressure was acutely elevated in both intact and mitral valve damaged animals . 6. It is concluded that chronic pulmonary venous congestion resulting from destruction of the mitral valve attenuates the ability of RARs to respond to acute moderate elevations of LAP.

Effect of pulmonary lymphatic obstruction on rabbit urine flow

1. The effects of pulmonary lymphatic obstruction on urine flow, sodium and potassium excretion were examined on anaesthetized, artificially ventilated New Zealand White rabbits. Pulmonary lymphatic obstruction was produced by raising the pressure in a pouch created from the right external jugular vein. The experiments were performed on two groups of rabbits (non-hydrated and hydrated). 2. Pulmonary lymphatic obstruction caused a significant increase in urine flow in both groups of rabbits. After release of the obstruction, the urine flow returned to basal values. Urine flow (ml (10 min)-1) for both groups was initial control, 5.3 +/- 0.9; lymphatic obstruction, 8.9 +/- 1.0; final control, 6.2 +/- 0.7 (means +/- S.E.M.; n = 21, P < 0.025). 3. The increase in urine flow was not accompanied by significant changes in concentration of sodium and potassium in urine. Sodium excretion increased significantly only in the hydrated rabbits. 4. The increase in urine flow was abolished by bilateral cervical vagotomy and by renal nerve sectioning. Cooling the cervical vagi to 8 degrees C also abolished the response. 5. Pulmonary lymphatic obstruction did not produce any significant change in heart rate, mean arterial blood pressure, mean right atrial pressure and peak airway pressure. 6. These findings suggest that obstructing the lymph drainage from the lung results in a reflex increase in urine flow. The afferent pathway for this reflex resides in the myelinated fibres of the vagi and the efferent pathway in the renal nerves. The rapidly adapting receptors of the airways are likely to be the receptors involved.

Effect of Diazinon PLUS on rapidly adapting receptors in the rabbit

The effects of Diazinon PLUS aerosol on the activities of rapidly adapting receptors (RARs) and slowly adapting receptors (SAR) of the airways were investigated in anesthetized rabbits. The effects on both the baseline activity and the responses to stimulation by increasing mean left atrial pressure were examined. Action potentials were recorded from the left cervical vagus nerve. Aerosols (particle size 3 microns) were generated by a Mini-HEART nebulizer. We observed that an aerosol of Diazinon PLUS (1:10 vol/vol dilution in normal saline) decreased the baseline RAR activity (n = 10) significantly (P < 0.05) from 209 +/- 77 to 120 +/- 40 impulses/min. In the post-Diazinon PLUS control period, the RAR activity recovered partially to 185 +/- 75 impulses/min and decreased significantly to 131 +/- 52 impulses/min (P < 0.05) after a second exposure of Diazinon PLUS (undiluted) aerosol. Aerosols of normal saline in the control state did not produce a significant change in the RAR activity. A group of SAR (n = 8) were examined under similar conditions, and it was found that only the exposure to Diazinon PLUS (undiluted) aerosol decreased the activity significantly (P < 0.05) from 1,536 +/- 206 to 1,367 +/- 182 impulses/min. The effect of Diazinon PLUS on the response to increasing mean left atrial pressure was examined in seven RARs. In the control state, RAR activity increased significantly (P < 0.05) during elevation of mean left atrial pressure. This response was abolished after exposure to Diazinon PLUS. These findings suggest that diazinon may interfere with airway defense mechanisms by reducing the activity of RARs.

Substance P contributes to rapidly adapting receptor responses to pulmonary venous congestion in rabbits

1. This study tested the hypothesis that substance P stimulates rapidly adapting receptors (RARs), contributes to the increase in RAR activity produced by mild pulmonary congestion, and evokes an augmented response from RARs when combined with near-threshold levels of pulmonary congestion. 2. RAR activity, peak tracheal pressure, arterial blood pressure and left atrial pressure were measured in paralysed, anaesthetized and ventilated rabbits. Substance P was given i.v. in one-half log incremental doses to a maximum of 3 micrograms kg-1. Mild pulmonary congestion was produced by inflating a balloon in the left atrium to increase left atrial pressure by 5 mmHg. Near-threshold levels of pulmonary congestion were produced by increasing left atrial pressure by 2 mmHg. 3. Substance P produced dose-dependent increases in RAR activity. The highest dose given increased the activity from 1.3 +/- 0.5 to 11.0 +/- 3.1 impulses bin-1. Increases in left atrial pressure of 5 mmHg increased RAR activity from 3.8 +/- 1.4 to 14.7 +/- 3.9 impulses bin-1. Blockade of NK1 receptors with CP 96345 significantly attenuated RAR responses to substance P and to mild pulmonary congestion. 4. Doses of substance P, which alone had no effect, stimulated the RARs when delivered during near-threshold levels of pulmonary congestion. 5. The findings suggest that substance P augments the stimulatory effect of mild pulmonary congestion on RAR activity, most probably by enhancing hydraulically induced microvascular leak.

Properties of rapidly adapting receptors of the airways in monkeys (Macaca mulatta)

The properties of rapidly adapting receptors (RARs) of the airways were examined in anaesthetised, artificially ventilated, paralysed and thoracotomised monkeys. The RARs were identified (i) by their rapid adaptation to a maintained inflation and forced deflation of the lungs and (ii) by their conduction velocity measurements. Right atrial (n = 17) and left atrial (n = 13) injections of histamine (10 micrograms/kg) stimulated the RARs. The stimulation was associated with an increase in peak intratracheal pressure. Right atrial injections of phenyl diguanide (n = 6, 10 micrograms/kg) and 5-hydroxytryptamine (n = 6, 10 micrograms/kg) did not produce a significant stimulation of the RARs. Administration of irritant vapours such as ammonia, (n = 12), cigarette smoke (n = 8), alcohol (n = 10), acetone (n = 10) and ether (n = 7), caused a significant stimulation of the RARs. This stimulation occurred in spite of a significant decrease or no change in peak intratracheal pressure. During mild degrees of pulmonary venous congestion produced by graded increments in mean left atrial pressure (+5 and +10 mmHg), there was a graded increase in RAR (n = 6) activity. The present study shows the existence of the RARs in the airways of the rhesus monkey. These receptors are stimulated (i) by administration of agents which cause bronchoconstriction (ii) by vapours which cause airway irritation and (iii) in conditions which cause an expansion of the extravascular space in airways.

Effect of pulmonary lymphatic obstruction on respiratory rate and airway rapidly adapting receptor activity in rabbits

1. The effects on respiratory rate of obstruction of pulmonary lymph flow, reduction of plasma protein concentration and a combination of the two procedures were examined in anaesthetized rabbits. The former was achieved by raising the pressure in a pouch created from the right external jugular vein and the latter by batch plasmapheresis. 2. In spontaneously breathing rabbits, neither pulmonary lymphatic obstruction (n = 6) nor plasmapheresis (n = 5) produced a significant change in respiratory rate. However, their combination (n = 8) produced a significant increase in respiratory rate (P < 0.05). 3. Cooling of the cervical vagi to 8-9 degrees C (n = 4) and vagotomy (n = 7) abolished this response. 4. There was a significant increase in the activity of the airway rapidly adapting receptors (RARs; n = 9) during pulmonary lymphatic obstruction, plasmapheresis and their combination (P < 0.05). 5. It is concluded that in the rabbit, obstruction of lymphatic drainage from the lung after plasmapheresis causes a reflex increase in respiratory rate. The afferent pathway for this reflex response lies in the vagus nerve and the RARs are likely to be the receptors involved in this response.

Effect of bradykinin on respiratory rate in anaesthetized rabbits; role of rapidly adapting receptors

1. This study was performed in anaesthetized, spontaneously breathing rabbits: (a) to determine the effect of bradykinin administered into the right atrium on the respiratory rate, and (b) to elucidate the potential role of rapidly adapting receptors (RARs) in mediating this effect. The role of RARs was established by graded cooling of the cervical vagi. The respiratory rate was measured from an intrapleural pressure tracing. 2. Dose-response curves relating right atrial injections of bradykinin (0.25, 0.5, 1.0 and 1.5 micrograms/kg) to the respiratory rate were established in the control state (i.e. vagi at 37 degrees C). The respiratory rate increased significantly (P < 0.01, ANOVA) from a control value of 51.3 +/- 6.8 breaths/min by 12 +/- 3, 25 +/- 5, 43 +/- 7 and 58 +/- 11% respectively. At doses of 1.0 and 1.5 micrograms/kg I.V., the increase in rate was preceded by apnoea. 3. The dose-response curves were repeated with bolus injections of bradykinin (0.25, 0.5, 1.0 and 1.5 micrograms/kg) after cooling the cervical vagi to 8-9 degrees C. The increase in respiratory rate was attenuated significantly (P < 0.01 ANOVA). The rate increased from a control value of 27.2 +/- 2.1 breaths/min by 5 +/- 2, 6 +/- 2, 16 +/- 5 and 21 +/- 8% respectively. With vagi cooled, apnoea was increased in duration and occurred at lower doses. On rewarming vagi, the original responses were reestablished. 4. When the study was repeated after bilateral vagotomy, apnoea was abolished but there was a small residual increase in rate. This increase was similar to that seen after cooling the vagi (P > 0.05). 5. RAR (n = 5) activity was recorded from the cervical vagus. Right atrial injections of bradykinin (0.25-1.0 micrograms/kg) stimulated RARs. On cooling the vagi to 8-9 degrees C caudal to the recording site, the increase in activity was blocked. 6. These data support the proposition that bradykinin increases the respiratory rate in rabbits and that this response is, in part, a reflex mediated by RARs. In addition, bradykinin has other secondary effects on respiration: an aponea which is mediated by non-myelinated vagal afferents and a small stimulatory effect on respiration which persists after bilateral vagotomy.