The impact of low-frequency, low-force cyclic stretching of human bronchi on airway responsiveness.

BACKGROUND: In vivo, the airways are constantly subjected to oscillatory strain (due to tidal breathing during spontaneous respiration) and (in the event of mechanical ventilation) positive pressure. This exposure is especially problematic for the cartilage-free bronchial tree. The effects of cyclic stretching (other than high-force stretching) have not been extensively characterized. Hence, the objective of the present study was to investigate the functional and transcriptional response of human bronchi to repetitive mechanical stress caused by low-frequency, low-force cyclic stretching. METHODS: After preparation and equilibration in an organ bath, human bronchial rings from 66 thoracic surgery patients were stretched in 1-min cycles of elongation and relaxation over a 60-min period. For each segment, the maximal tension corresponded to 80% of the reference contraction (the response to 3 mM acetylcholine). The impact of cyclic stretching (relative to non-stretched controls) was examined by performing functional assessments (epithelium removal and incubation with sodium channel agonists/antagonists or inhibitors of intracellular pathways), biochemical assays of the organ bath fluid (for detecting the release of pro-inflammatory cytokines), and RT-PCR assays of RNA isolated from tissue samples. RESULTS: The application of low-force cyclic stretching to human bronchial rings for 60 min resulted in an immediate, significant increase in bronchial basal tone, relative to non-cyclic stretching (4.24 ± 0.16 g vs. 3.28 ± 0.12 g, respectively; p < 0.001). This cyclic stimulus also increased the affinity for acetylcholine (-log EC50: 5.67 ± 0.07 vs. 5.32 ± 0.07, respectively; p p < 0.001). Removal of airway epithelium and pretreatment with the Rho-kinase inhibitor Y27632 and inward-rectifier K+ or L-type Ca2+ channel inhibitors significantly modified the basal tone response. Exposure to L-NAME had opposing effects in all cases. Pro-inflammatory pathways were not involved in the response; cyclic stretching up-regulated the early mRNA expression of MMP9 only, and was not associated with changes in organ bath levels of pro-inflammatory mediators. CONCLUSION: Low-frequency, low-force cyclic stretching of whole human bronchi induced a myogenic response rather than activation of the pro-inflammatory signaling pathways mediated by mechanotransduction.

Spectrum of myelinated pulmonary afferents (II).

Recently, it has been recognized that a single airway sensory unit may contain multiple receptive fields and that each field houses at least one encoder. Since some units respond to both lung inflation and deflation, we hypothesized that these units contain heterogeneous encoders for sensing inflation and deflation, respectively. Single unit activities were recorded from the cervical vagus nerve in anesthetized, open chest, and mechanically ventilated rabbits. Fifty-two airway sensory units with multiple receptive fields that responded to both lung inflation and deflation were identified. Among them, 13 units had separate receptive fields for inflation and deflation, where one of the fields could be blocked by local injection of 2% lidocaine (10 µl). In 8 of the 13 units, the deflation response was blocked without affecting the unit's response to inflation, whereas in the remaining five units, the inflation response was blocked without affecting the deflation response. Our results support the hypothesis that a single mechanosensory unit may contain heterogeneous encoders that can respond to either inflation or deflation.

A historical perspective of pulmonary rapidly adapting receptors.

Bronchopulmonary mechanosensors play an important role in the regulation of breathing and airway defense. Regarding the mechanosensory unit, investigators have conventionally adhered to 2 doctrines: one-sensor theory (one afferent fiber connects to a single sensor) and line-labeled theory. Accordingly, lung inflation activates 2 types of mechanosensors: slowly adapting receptors (SARs) and rapidly adapting receptors (RARs) that also respond to lung deflation to produce Hering-Breuer deflation reflex. RARs send signals to a particular brain region to stimulate breathing (labeled as excitatory line) and SARs to a different region to inhibit breathing (inhibitory line). Conventionally, RARs are believed to be mechanosensors, but are also stimulated by a variety of chemicals and mediators. They are activated during different disease conditions and evoke various respiratory responses. In the literature, RARs are the most debatable sensors in the airway. Recent physiological and morphological studies demonstrate that a mechanosensory unit consists of numerous sensors with 4 types, i.e., an afferent fiber connects to multiple homogeneous or heterogeneous sensors (multiple-sensor theory). In addition to SARs and RARs, there are deflation-activated receptors (DARs), which can adapt slowly or rapidly. Each type senses a specific force and generates a unique response. For example, RAR (or SAR) units may respond to deflation if they house DARs responsible for the Hering-Breuer deflation reflex. Multiple-sensor theory requires a conceptual shift because 4 different types of information from numerous sensors carried in an afferent pathway violates conventional theories. Data generated over last eight decades under one-sensor theory require re-interpretation. Mechanosensors and their reflex functions need re-definition. This detailed review of the RARs represents our understanding of RARs under the conventional doctrines, thus it provides a very useful background for interpretation of RAR properties and reflex function against the new proposed multiple-sensor theory.

Transient in utero nicotine exposure stimulates mechanosensory-dependent lung development.

Nicotine receptors are present in the developing lung yet their function is unknown. The transient role of nicotine receptors in lung development has not been addressed. In this study, nicotine's direct effect on smooth muscle contraction, necessary for mechanosensory-dependent fetal lung development, is examined after transient nicotine stimulation to determine the relationship between nicotine exposure, smooth muscle contraction, and fetal lung development. Rat fetuses at 16 days' gestation were exposed in utero to 5 different concentrations of nicotine or control injected directly into the amniotic fluid. Specific concentrations of in utero nicotine increased the phosphorylated Western blot analysis and immunohistochemistry of muscle contraction proteins. Respiratory function tests on nicotine-exposed rat pups showed a statistically significant decrease in airway resistance earlier in life compared to control and an upward shift of the pressure-volume curve pointing towards a structural maturation of the in utero nicotine-exposed lung. These results are consistent with transient nicotine exposure during intrauterine life stimulating stretch-induced lung organogenesis by altering phosphorylation of muscle contraction proteins. The increase in smooth muscle phosphorylation may stimulate stretch-induced lung organogenesis, which affects lung development and accelerates lung maturation in rats.

Mechanical stretch promotes fetal type II epithelial cell differentiation via shedding of HB-EGF and TGF-alpha.

The mechanisms by which mechanical forces promote fetal lung development are not fully understood. Here, we investigated differentiation of fetal type II epithelial cells via the epidermal growth factor receptor (EGFR) in response to mechanical strain. First, we showed that incubation of embryonic day (E) 19 fetal type II cells with recombinant heparin-binding EGF-like growth factor (HB-EGF) or transforming growth factor (TGF)-alpha, but not with amphiregulin (AR), betacellulin (BTC) or epiregulin (EPR), increased fetal type II cell differentiation, as measured by surfactant protein B/C mRNA and protein levels. Next, we demonstrated that 5% cyclic stretch of E19 monolayers transfected with plasmid encoding alkaline phosphatase (AP)-tagged ligands shed mature HB-EGF and TGF-alpha into the supernatant and promoted type II cell differentiation. Release of these ligands was also observed in E19 cells subjected to higher degrees of cyclic strain, but not in cells exposed to continuous stretch. Interestingly, the addition of fibroblasts to type II cell cultures did not enhance release of HB-EGF. Whereas HB-EGF shedding was also detected in E18 cells exposed to 5% cyclic stretch, release of this ligand after 2.5% sustained stretch was restricted to cells isolated on E18 of gestation. In addition, mechanical stretch released EGF, AR and BTC. We conclude that mechanical stretch promotes fetal type II cell differentiation via ectodomain shedding of HB-EGF and TGF-alpha. The magnitude of shedding varied depending on gestational age, ligand, and strain protocol. These studies provide novel mechanistic information potentially relevant to fetal lung development and to mechanical ventilation-induced lung injury.

Cough reflex responses during pulmonary C-fibre receptor activation in anesthetized rabbits.

We investigated the changes induced by pulmonary C-fibre receptor activation in the cough reflex evoked by mechanical stimulation of the tracheobronchial tree in pentobarbitone anesthetized, spontaneously breathing rabbits. Phrenic nerve and abdominal muscle activities were monitored along with tracheal and arterial blood pressures. The activation of pulmonary C-fibre receptors by means of right atrial injection of phenylbiguanide (PBG) caused the pulmonary chemoreflex characterized by tachypnea, bradycardia and hypotension. During the pulmonary chemoreflex, the time components (total cycle duration, inspiratory and expiratory times) of the cough motor pattern significantly decreased, whereas no consistent changes in peak phrenic and abdominal activity, peak tracheal pressure and number of coughs evoked by each stimulation trial were observed. At variance with previous findings in cats and dogs, the results show that tracheobronchial cough is not significantly reduced in the rabbit during PBG-induced chemoreflex. This study is the first to provide evidence supporting the hypothesis that the time components of the cough motor pattern are, to some extent, dependent upon the timing characteristics of the ongoing respiratory activity and suggests a novel mechanism leading to cough depression.

Maintained inspiratory activity during proportional assist ventilation in surfactant-depleted cats early after surfactant instillation: phrenic nerve and pulmonary stretch receptor activity.

BACKGROUND: Inspiratory activity is a prerequisite for successful application of patient triggered ventilation such as proportional assist ventilation (PAV). It has recently been reported that surfactant instillation increases the activity of slowly adapting pulmonary stretch receptors (PSRs) followed by a shorter inspiratory time (Sindelar et al, J Appl Physiol, 2005 [Epub ahead of print]). Changes in lung mechanics, as observed in preterm infants with respiratory distress syndrome and after surfactant treatment, might therefore influence the inspiratory activity when applying PAV early after surfactant treatment. OBJECTIVE: To investigate the regulation of breathing and ventilatory response in surfactant-depleted young cats during PAV and during continuous positive airway pressure (CPAP) early after surfactant instillation in relation to phrenic nerve activity (PNA) and the activity of PSRs. METHODS: Seven anesthetized, endotracheally intubated young cats were exposed to periods of CPAP and PAV with the same end-expiratory pressure (0.2-0.5 kPa) before and after lung lavage and after surfactant instillation. PAV was set to compensate for 75% of the lung elastic recoil. RESULTS: Tidal volume and respiratory rate were higher with lower PaCO2 and higher PaO2 during PAV than during CPAP both before and after surfactant instillation (p < 0.05; both conditions). As an indicator of breathing effort, esophageal deflection pressure and PNA were lower during PAV than during CPAP in both conditions (p < 0.02). Peak PSR activity was higher and occurred earlier during PAV than during CPAP (p < 0.01), and correlated linearly with PNA duration in all conditions studied (p < 0.001). The inspiratory time decreased as tidal volume increased when CPAP was changed to PAV, with the highest correlation observed after surfactant instillation (r = -0.769). No apneic periods could be observed. CONCLUSION: PSR activity and the control of breathing are maintained during PAV in surfactant-depleted cats early after surfactant instillation, with a higher ventilatory response and a lower breathing effort than during CPAP.

Surfactant replacement partially restores the activity of pulmonary stretch receptors in surfactant-depleted cats.

Single units of slowly adapting pulmonary stretch receptors (PSRs) were investigated in anesthetized cats during spontaneous breathing on continuous positive airway pressure (2-5 cmH2O), before and after lung lavage and then after instillation of surfactant to determine the PSR response to surfactant replacement. PSRs were classified as high threshold (HT) and low threshold (LT), and their instantaneous impulse frequency (f imp) was related to transpulmonary pressure (Ptp) and tidal volume (Vt). Both the total number of impulses and maximal f imp of HT and LT PSRs decreased after lung lavage (55 and 45%, respectively) in the presence of increased Ptp and decreased Vt. While Ptp decreased markedly and Vt remained unchanged after surfactant instillation, all except one PSR responded with increased total number of impulses and maximal f imp (42 and 26%, respectively). Some HT PSRs ceased to discharge after lung lavage but recovered after surfactant instillation. The end-expiratory activity of LT PSRs increased or was regained after surfactant instillation. After instillation of surfactant, respiratory rate increased further with a shorter inspiratory time, resulting in a lower inspiratory-to-expiratory time ratio. Arterial pH decreased (7.31 +/- 0.04 vs. 7.22 +/- 0.06) and Pco2 increased (5.5 +/- 0.7 vs. 7.2 +/- 1.3 kPa) after lung lavage, but they were the same after as before instillation of surfactant (pH = 7.21 +/- 0.08 and Pco2 = 7.6 +/- 1.4 kPa) during spontaneous breathing. In conclusion, surfactant instillation increased lung compliance, which, in turn, increased the activity of both HT and LT PSRs. A further increase in respiratory rate due to a shorter inspiratory time after surfactant instillation suggests that the partially restored PSR activity after surfactant instillation affected the breathing pattern.

Effect of ouabain on the afterhyperpolarization of slowly adapting pulmonary stretch receptors in the rat lung.

In anesthetized, artificially ventilated rats with one vagus nerve section, the purposes of the present study were to investigate whether release from phasic consecutive hyperinflations (inflation volume=3 tidal volumes) results in the afterhyperpolarization (AHP) of the slowly adapting pulmonary stretch receptor (SAR) activity and whether the effect of ouabain, a Na+-K+ ATPase inhibitor, alters AHP of the SAR activity seen after release from maintained inflations. Release from 10 consecutive phasic hyperinflations did not cause any significant inhibition of SAR activity. Release from maintained inflations (for approximately 10 and 15 cmH2O) for 5 s produced the induction of disappearance of SAR activity, corresponding with the AHP. Intravenous administration of ouabain (20 and 40 microg/kg) had no significant effects on the responses of SAR activity and SAR adaptation index (AI) to maintained inflations, but ouabain treatment with at 40 microg/kg resulted in a significant increase in the SAR activity after stopping the respirator and significantly attenuated the AHP of the SAR activity. In the immunohistochemical study, we found Na+-K+ ATPase alpha3-subunit-isoforms-like immunoreactivity in SAR terminals, forming leaflike extensions in the intrapulmonary bronchioles at different diameters, and those terminals were buried in the smooth muscle. In the same sections, the alpha1 subunit immunoreactivity of SAR terminals was not found. These results suggest that the mechanism of generating the AHP of SARs is mainly mediated by the activation of Na+-K+ ATPase alpha3 subunit isoform.

The inhibitory effect of ouabain on the response of slowly adapting pulmonary stretch receptors to hyperinflation in the rabbit.

The combined effects of ouabain (Na(+)-K(+) ATPase inhibitor) and hyperinflation (inflation volume=three tidal volumes) on slowly adapting pulmonary stretch receptors (SARs) were studied before and after administration of nifedipine (an L-type Ca(2+) channel blocker) and KB-R7943 (a reverse-mode Na(+)-Ca(2+) exchanger blocker) in anesthetized, artificially ventilated rabbits after bilateral vagotomy. Before ouabain administration, hyperinflation stimulated SAR activity. After 20 min of ouabain administration (30 microg/kg) the SARs increased discharge rates in normal inflation. Under these conditions, hyperinflation initially stimulated SAR activity but subsequently inhibited the activity at peak inflation. Additional administration of 60 microg/kg ouabain (total dose=90 microg/kg) caused a further stimulation of SAR activity, but 20 min later both normal inflation and hyperinflation resulted in a greater inhibition of the receptor activity. The hyperinflation-induced SAR inhibition in the presence of ouabain (30 microg/kg) was not significantly altered by administration of either nifedipine (2 and 4 mg/kg) or KB-R7943 (1 and 3 mg/kg). In another series of experiments, we further examined the combined effects of ouabain and hyperinflation in veratridine (a Na(+) channel opener, 40 microg/kg)-treated animals. After recovery from the veratridine effect on SAR activity, which vigorously stimulated the receptor activity, ouabain treatment (30 microg/kg) that silenced the receptor activity at peak inflation greatly inhibited hyperinflation-induced SAR stimulation. These results suggest that hyperinflation-induced SAR inhibition in the presence of ouabain may be related to a Na(+) overload, but not to a Ca(2+) influx via activation of L-type Ca(2+) channels, in the SAR endings.

Airway sensory innervation as a target for novel therapies: an outdated concept?

Sensory nerves in the airways regulate central and local reflex events such as bronchoconstriction, airway plasma leakage, mucus secretion and cough. Sensory nerve activity can be enhanced during inflammation and, as a result, these protective reflexes become exacerbated and deleterious. The development of drugs that directly inhibit sensory nerve function has again become an attractive target for the pharmaceutical industry. In particular, the focus is on inhibition of the symptoms associated with airway inflammatory diseases such as asthma, chronic obstructive pulmonary disease and cough of any aetiology.

Ouabain stimulates slowly adapting pulmonary stretch receptors.

Ouabain, a Na(+)/K(+)-ATPase inhibitor, induces slowly adapting pulmonary stretch receptors (SARs) to discharge paradoxically. Paradoxical discharge is characterized by increased SAR activity during lung deflation coupled with silence during lung inflation. We hypothesized that over-excitation silences the SARs. Accordingly, if cyclic inflation pressure was reduced so as to lower SAR stimulation, paradoxical discharge would be prevented. In the present study, single-unit activity of SARs was recorded in anesthetized, open-chest and mechanically ventilated rabbits with positive-end-expiratory pressure (PEEP). After microinjection of ouabain into the receptive field, SAR activity initially increased and then gradually became paradoxical. During paradoxical cycling, SAR activity started and stopped abruptly, oscillating between high frequency discharge during lung deflation and silence during peak inflation. Removing PEEP reduced basal cyclic stimulation and returned the discharge pattern to normal, that is, SAR activity was highest at peak inflation pressure but silent during deflation. It is speculated that stretching SARs causes Na(+) influx, producing generator potential (GP). Normally, GP recovers by Na(+) extrusion via Na(+)/K(+)-ATPase. Ouabain inhibits the ATPase, which limits Na(+) extrusion, and thus sustains the GP. Therefore, after ouabain microinjection, lung inflation will further increase GP, causing over-excitation to silence the SARs.

Effects of potassium channel and Na+-Ca2+ exchange blockers on the responses of slowly adapting pulmonary stretch receptors to hyperinflation in flecainide-treated rats.

1. The effects of K(+) channel blockers, such as 4-aminopyridine (4-AP) and tetraethylammonium (TEA), and a reverse-mode Na(+)-Ca(2+) exchange blocker, 2-[2-[4-(4-nitrobenzyloxyl) phenyl] ethyl] isothiourea methanesulphonate (KB-R7943), on the responses of slowly adapting pulmonary stretch receptor activity to hyperinflation (inflation volume=3 tidal volumes) were investigated in anaesthetized, artificially ventilated, unilaterally vagotomized rats after pretreatment with a Na(+) channel blocker flecainide. The administration of flecainide (9 mg kg(-1)) at a dose greater than that which abolished 50 microg kg(-1) veratridine-induced SAR stimulation also inhibited hyperinflation-induced stimulation of SARs. 2. In flecainide-treated animals, administration of 4-AP (0.7 and 2 mg kg(-1)) stimulated SAR activity during normal inflation and also caused a partial blockade of hyperinflation-induced SAR inhibition. 3. The discharges of SARs during normal inflation in flecainide-treated animals were not significantly altered by administration of either TEA (2 and 7 mg kg(-1)) or KB-R7943 (1 and 3 mg kg(-1)), but both K(+) channel and Na(+)-Ca(2+) exchange blockers partially attenuated hyperinflation-induced SAR inhibition. 4. These results suggest that hyperinflation-induced SAR inhibition in the presence of flecainide (9 mg kg(-1)) involves the activation of several K(+) conductance pathways.

Response of slowly adapting pulmonary stretch receptors to reduced lung compliance.

We examined the steady-state response of slowly adapting pulmonary stretch receptors (SAPSRs) to reduced lung compliance in open-chest cats with lungs ventilated at eupneic rate and tidal volume (VT) and with a positive end-expiratory pressure (PEEP) of 3-4 cmH2O. Transient removal of PEEP decreased compliance by approximately 30% and increased transpulmonary pressure (Ptp) by 1-2.5 cmH2O. Reduction of compliance significantly decreased SAPSR discharge in deflation and caused a small increase in discharge at the peak of inflation; it had little effect on discharge averaged over the ventilatory cycle. Increasing VT to produce a comparable increase in Ptp significantly increased peak discharge. Thus unlike rapidly adapting receptors, whose discharge is increased more effectively by reduced compliance than by increased VT, SAPSRs are stimulated by increased VT but not by reduced compliance. We speculate that the most consistent effect of reduced compliance on SAPSRs (the decrease in deflation discharge) was due to the decreased time constant for deflation in the stiffer lung. This alteration in firing may contribute to the tachypnea evoked as the lungs become stiffer.

Abnormal pulmonary slowly adapting receptors in canine acrylamide neuropathy.

Slowly adapting lung stretch receptors (SARs) and their vagal afferents are considered to play an important role in the mediation of numerous respiratory reflexes. The understanding of such reflexes has been facilitated by altering the discharge properties of SARs or by preventing the conduction of SAR-generated impulses to the brain stem. In a number of naturally occurring diseases of the peripheral nervous system, the vagus nerve and vagal reflexes are damaged. We have studied the function of SARs in anesthetized dogs with acrylamide neuropathy, a distal axonopathy that has been used as a model of naturally occurring neuropathies. There was a marked increase in threshold and decrease in firing rate of SARs in dogs with moderate neuropathy. Abnormal SAR discharge patterns were observed, and there was a depletion of those units innervated by the fastest conducting vagal afferent fibers in treated animals. Acrylamide induced degeneration of myelinated fibers in bronchial branches of the vagus nerve. These abnormalities were partially reversed upon withdrawal of the neurotoxin. Acrylamide may be a useful agent in the study of vagally mediated respiratory reflexes. SAR function is likely to be abnormal in diseases of the peripheral nervous system.

Responsiveness of the guinea pig trachea to stretch: role of the epithelium and cyclooxygenase products.

The role of the epithelium and cyclooxygenase products was investigated in the responses of isolated airways to sudden stretch. Strips of guinea pig trachea, in some of which the epithelium had been removed mechanically, were suspended in organ chambers; isometric tension was recorded. After rapid stretching to their optimal tension, the preparations (with and without epithelium) relaxed initially and then contracted to a level close to the imposed tension. Afterward, tissues with epithelium maintained this level of tension, but those without epithelium relaxed. After treatment with papaverine or isoproterenol (at concentrations causing maximal relaxation), stretch was followed only by a decrease in tension; a similar response to stretch was also obtained in tissues treated with indomethacin or acetylsalicylic acid (inhibitors of cyclooxygenase). Dazmegrel (an inhibitor of thromboxane synthase) and SQ-29548 (an antagonist of prostaglandin H2 or thromboxane A2 receptors) did not affect the response of tissues with epithelium but abolished the stretch-induced contraction in those without epithelium. Tranylcypromine, which inhibits prostacyclin synthase, and tetrodotoxin, which blocks local reflexes, did not significantly affect the responses of the tissues to stretch. These observations suggest that thromboxane may mediate the epithelium-independent contraction and that another product of cyclooxygenase contributes to the maintenance of tension on stretching observed in tissues with epithelium.

Role of N-methyl-D-aspartate receptors in the pontine pneumotaxic mechanism in the cat.

Systemic injection of MK-801, an N-methyl-D-aspartate (NMDA) receptor-associated channel blocker, induces an apneusis in vagotomized cats similar to that produced by pontine respiratory group (PRG) lesions, suggesting the possible involvement of NMDA receptors in the pontine pneumotaxic mechanism. Previous results from our laboratory indicate that the efferent limb of the pontine pneumotaxic mechanism is unlikely to require NMDA receptor-mediated neurotransmission. Therefore, the present study examined the potential involvement of PRG NMDA receptors in the pontine pneumotaxic mechanism. Experiments were conducted in decerebrate, paralyzed, and ventilated adult cats. The effects on inspiratory time (TI) of MK-801 microinjection into PRG were tested in 12 cats. Pressure microinjection of MK-801 (15 mM, 80-3,000 nl) significantly prolonged TI in all animals when lung inflation was withheld. TI progressively increased in most animals for > or = 30 min. After this period, partial recovery of the effect occurred in eight cats as TI shortened toward predrug levels. In three animals, microinjection of MK-801 induced a complete apneusis in the absence of lung inflation from which there was no detectable recovery. Microinjections into regions approximately 2 mm distant from PRG produced little or no effect. These results provide evidence that NMDA receptors located in the region of PRG play an important functional role in the control of the breathing cycle.

Inhibitory effect of inhaled wood smoke on the discharge of pulmonary stretch receptors in rats.

We investigated the inhibition of slowly adapting pulmonary stretch receptors (PSRs) by inhaled wood smoke. Impulses were recorded from PSRs in 68 anesthetized, open-chest, and artificially ventilated rats. Eighty-one of one hundred five PSRs were inhibited within one or two breaths when 6 ml of wood smoke were delivered into the lungs. As a group (n = 105), PSR activity significantly decreased from a baseline of 19.0 +/- 1.3 (SE) to a lowest level of 12.9 +/- 1.2 impulses/breath at the fourth or fifth breath after smoke delivery. This afferent inhibition usually persisted for 5-18 breaths. In contrast, smoke delivery did not affect transpulmonary pressure. Delivery of gas-phase smoke or a hypercapnic gas mixture containing CO2 at a concentration (15%) matching that in the smoke produced a nearly identical inhibition in the same PSRs (n = 10). This afferent inhibition was largely prevented by pretreatment with acetazolamide (an inhibitor of carbonic anhydrase; n = 10) but was not affected by pretreatment with the vehicle for acetazolamide (n = 8) or isoproterenol (a bronchodilator; n = 10). These results suggest that 1) an increase in H+ concentration resulting from hydration of CO2 in the smoke may be responsible for the inhibitory effect of wood smoke on the discharge of PSRs and 2) changes in lung mechanics are not the cause of this afferent inhibition.

Acute inhalation of ozone stimulates bronchial C-fibers and rapidly adapting receptors in dogs.

To identify the afferents responsible for initiating the vagally mediated respiratory changes evoked by acute exposure to ozone, we recorded vagal impulses in anesthetized, open-chest, artificially ventilated dogs and examined the pulmonary afferent response to ozone (2-3 ppm in air) delivered to the lower trachea for 20-60 min. Bronchial C-fibers (BrCs) were the lung afferents most susceptible to ozone, the activity of 10 of 11 BrCs increasing from 0.2 +/- 0.2 to 4.6 +/- 1.3 impulses/s within 1-7 min of ozone exposure. Ten of 15 rapidly adapting receptors (RARs) were stimulated by ozone, their activity increasing from 1.5 +/- 0.4 to 4.7 +/- 0.7 impulses/s. Stimulation of RARs (but not of BrCs) appeared secondary to the ozone-induced reduction of lung compliance because it was abolished by hyperinflation of the lungs. Ozone had little effect on pulmonary C-fibers or slowly adapting pulmonary stretch receptors. Our results suggest that both BrCs and RARs contribute to the tachypnea and bronchoconstriction evoked by acute exposure to ozone when vagal conduction is intact and that BrCs alone are responsible for the vagally mediated tachypnea that survives vagal cooling to 7 degrees C.

Bradykinin causes hypotension by activating pulmonary sympathetic afferents in the rabbit.

Bradykinin (BK) activates sympathetic afferents in the heart, intestine, and kidney, and it alters hemodynamics. However, we know little about the influence of pulmonary sympathetic afferents on circulation. Activation of pulmonary afferents by directly injecting stimulants into the lung parenchyma permits examination of reflexes that originate in the lung without confounding effects from the systemic circulation. In the present study, we tested the hypothesis that pulmonary sympathetic afferents exert a significant influence on hemodynamics. We examined reflex effects of injecting BK (1 microg/kg in 0.1 ml) into the lung parenchyma on circulation in anesthetized, open-chest, artificially ventilated rabbits. BK significantly decreased mean arterial blood pressure (BP) (27 +/- 3 mmHg) and heart rate (19 +/- 4 beats/min). Both effects remained after bilateral vagotomy. To rule out possible direct systemic vasodilation by BK, we examined renal sympathetic nerve activity (RSNA) in response to BK injection and examined BP responses to injection of ACh (0.1 ml of 10-4 M). BK suppressed the RSNA before and after vagotomy. ACh did not change BP when injected into the lung parenchyma, but it decreased BP (31 +/- 3 mmHg) when injected into the right atrium. Our data indicate that activating pulmonary sympathetic afferents reflexly suppresses hemodynamics.