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.

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 ( fimp) was related to transpulmonary pressure (Ptp) and tidal volume (Vt). Both the total number of impulses and maximal fimp 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 fimp (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.

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.

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.

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.

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.

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.

Effects of acetazolamide and 4-aminoprydine on the responses of deflationary slowly adapting pulmonary stretch receptors to CO2 inhalation in the rat

The inhibitory effect of CO(2) on deflationary slowly adapting pulmonary stretch receptors (deflationary SARs) was investigated before and after administration of acetazolamide, a carbonic anhydrase (CA) inhibitor, or 4-aminopyridine (4-AP), a K(+) channel blocker, in anesthetized, artificially ventilated rats after unilateral vagotomy. CO(2) inhalation (maximum tracheal CO(2) concentration ranging from 9 to 12%) for approximately 60 s decreased the impulse activity of deflationary SARs but had no significant effect on tracheal pressure (P(T)) as an index of bronchomotor tone. Acetazolamide treatment (20 mg/kg) diminished the inhibitory response of deflationary SARs to CO(2) inhalation. 4-AP (0.7 and 2.0 mg/kg) dose-dependently attenuated the decrease in deflationary SAR activity induced by CO(2) inhalation. When comparing the maximum attenuation due to 4-AP (2.0 mg/kg) and acetazolamide (20 mg/kg) in CO(2)-induced deflationary SAR inhibition, blockade of K(+) channels had a more pronounced effect. These results suggest that inhibition of deflationary SARs by CO(2) inhalation may be largely mediated by the stimulating action of 4-AP-sensitive K(+) currents in the nerve terminals of the receptors.

Functional morphology and physiology of slowly adapting pulmonary stretch receptors

Since the original work by Hering and Breuer (1868) on slowly adapting pulmonary stretch receptors (SARs), numerous studies have demonstrated that these receptors are the lung vagal afferents responsible for eliciting the reflexes evoked by moderate lung inflation. SARs play a role in controlling breathing pattern, airway smooth muscle tone, systemic vascular resistance, and heart rate. Both anatomical and physiological studies support the contention that SARs, by their close association with airway smooth muscle, continuously sense the tension within the myoelastic components of the airways caused by lung inflation, smooth muscle contraction, and/or tethering of small intrapulmonary airways to the lung parenchyma. As a result, the receptor field location within the tracheobronchial tree of a SAR plays an important role in its discharge pattern, with variations in airway transluminal pressure and airway smooth muscle orientation being important modulating factors. The disruption of airway myoelastic components in various pulmonary diseases would be expected to alter the discharge pattern of SARs, and contribute to changes in breathing pattern and airway smooth muscle tone.

Activation of lung vagal sensory receptors by circulatory endotoxin in rats

Although endotoxin is known to induce various pulmonary responses that are linked to the function of lung vagal sensory receptors, its effects on these pulmonary receptors are still not clear. This study investigated the effects of circulatory endotoxin on the afferent activity of lung vagal sensory receptors in rats. We recorded afferent activity arising from vagal pulmonary C fibers (CFs), rapidly adapting receptors (RARs), tonic pulmonary stretch receptors (T-PSRs), and phasic pulmonary stretch receptors (P-PSRs) in 64 anesthetized, paralyzed, and artificially ventilated rats. Intravenous injection of endotoxin (50 mg/kg; lipopolysaccharide) stimulated 7 of the 8 CFs, 8 of the 8 RARs, and 4 of the 8 T-PSRs studied, while having no effect on the 8 P-PSRs tested. The stimulation started 3-16 min after endotoxin injection and lasted until the end of the 90-min observation period. The evoked discharge of either CFs or RARs was not in phase with the ventilatory cycle, whereas that of T-PSRs showed a respiratory modulation. Injection of a saline vehicle caused no significant change in the discharge of these receptors. Additionally, endotoxin significantly produced an increase in total lung resistance, and decreases in dynamic lung compliance and arterial blood pressure. Our results demonstrate that a majority of lung vagal sensory receptors are activated following intravenous injection of endotoxin, and support the notion that these pulmonary receptors may function as an important afferent system during endotoxemia.

Excitatory mechanism of deflationary slowly adapting pulmonary stretch receptors in the rat lung

The excitatory responses of deflationary slowly adapting pulmonary stretch receptor (SAR) activity to lung deflation ranging from approximately -15 to -25 cm of H(2)O for approximately 5 s were examined before and after administration of flecainide, a Na(+) channel blocker, and K(+) channel blockers, such as 4-aminopyridine (4-AP) and tetraethylammonium (TEA). The experiments were performed in anesthetized, artificially ventilated rats after unilateral vagotomy. The deflationary SARs increased their activity during lung deflation and its effect became more pronounced by increasing the degree of negative pressure. During lung deflation the average values for the deflationary SAR adaptation index (AI) were below 40%. Intravenous administration of veratridine (50 microg/kg), an Na(+) channel opener, stimulated deflationary SAR activity: one maintained excitatory activity mainly during deflation and the other receptors showed a tonic discharge during both deflation and inflation. Despite the difference in deflationary SAR firing patterns after veratridine administration, flecainide treatment (6.0 mg/kg) blocked veratridine-induced deflationary SAR stimulation and also caused strong inhibition of the excitatory responses of deflationary SARs to lung deflation. Under these conditions, the average values for deflationary SAR AI were over 90%. The responses of deflationary SARs and deflationary SAR AI to lung deflation were not significantly altered by pretreatment with either 4-AP (0.7 and 2.0 mg/kg) or TEA (2.0 and 6.0 mg/kg). These results suggest that the excitatory effect of lung deflation on deflationary SAR activity is mediated by the activation of flecainide-sensitive Na(+) channels on the nerve terminals of deflationary SARs.

Effects of potassium channel blockers on hyperinflation-induced rapidly adapting pulmonary stretch receptor stimulation in the rabbit

The effects of K+ channel blockers, such as 4-aminoprydine (4-AP) and tetraethylammonium (TEA), on the excitatory responses of rapidly adapting pulmonary stretch receptor (RAR) activity to hyperinflation (inflation volume=3 tidal volumes) were investigated in anesthetized, artificially ventilated rabbits after vagus nerve section. The changes in the RAR adaptation index (AI) produced by constant-pressure (approximately 30 cmH2O, 29.7+/-0.2 cmH2O) inflation of the lungs were also examined before and after pretreatment with 4-AP and TEA. The administration of 4-AP (0.7 and 2.0 mg/kg) potentiated hyperinflation-induced RAR stimulation in a dose-dependent manner. During hyperinflation after 2.0 mg/kg 4-AP administration the discharge of RARs showed a relatively regular firing pattern in both inflation and deflation phases. The RAR AI values during constant-pressure inflation of the lungs were significantly reduced by 4-AP treatment (2.0 mg/kg). TEA treatment (2.0 and 7.0 mg/kg) did not significantly alter either the excitatory response of RAR activity to hyperinflation or the RAR AI values seen during constant-pressure inflation of the lungs. These results suggest that during hyperinflation in in vivo experiments on rabbits, RARs may be maintained at a lower activity by opening the 4-AP-sensitive K+ channels on the receptor endings, which can determine accommodation of the receptor discharge.

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.

Activation of sympathoadrenomedullary system increases pulmonary nitric oxide production in the rabbit

Nitric oxide (NO) is continuously produced in the lung and can be measured in exhaled gas of different species. To investigate a possible neuro-humoral regulation of pulmonary NO production in vivo we injected veratrine, an activator of Na(+) channels known to activate the sympathoadrenal system, in anaesthetized, mechanically ventilated and laparotomized rabbits. Exhaled NO concentration increased by 38+/-3% when plasma adrenaline rose from 12.3+/-3.1 to 49.5+/-10.7 pmol ml(-1) in response to veratrine (500 microg kg(-1), i.v.). Pretreatment with atenolol, a beta(1)-adrenoceptor antagonist (1 mg kg(-1)), or bilateral ligation of adrenal blood vessels inhibited the increase in exhaled NO in response to veratrine. Atenolol also decreased basal NO, suggesting an endogenous regulation of pulmonary NO by adrenaline. Neither phentolamine (1 mg kg(-1)), atropine (1 mg kg(-1)) nor vagotomy inhibited the veratrine-induced pulmonary NO production. These results suggest a role of the sympathoadrenal system in the regulation of pulmonary NO production.

Interaction of vagal lung afferents with inhalation of histamine aerosol in anesthetized dogs

In seven alpha-chloralose anesthetized dogs we examined the contribution of lung afferents to the rapid, shallow breathing induced by inhalation of 10 breaths of histamine aerosol. In four spontaneously breathing dogs, the inhalation of histamine caused an increased respiratory frequency, decreased tidal volume, and decreased dynamic lung compliance. Selective blockade of pulmonary C-fibers abolished a reflex-induced increase in respiratory frequency but did not significantly affect the reductions in tidal volume or lung compliance. Terbutaline treatment in combination with C-fiber blockade abolished the reductions in tidal volume and lung compliance induced by histamine. In three separate alpha-chloralose anesthetized, open-chest, mechanically ventilated dogs, we recorded an increase in the inspiratory activity of rapidly adapting pulmonary stretch receptors (RARs) induced by the inhalation of histamine aerosol. Selective C-fiber blockade abolished histamine-induced increases in RAR activity while only partially attenuating reductions in lung compliance. We conclude that the increase in RAR activity induced by histamine depends on intact C-fibers and not on a direct effect of histamine on RARs or an indirect effect of histamine reducing lung compliance. In addition, our data illustrate the multiple interactions that occur between the various vagal afferents and their roles in the reflexes induced by histamine inhalation.

Effects of ouabain and flecainide on CO(2)-induced slowly adapting pulmonary stretch receptor inhibition in the rabbit

The inhibitory effect of CO2 on slowly adapting pulmonary stretch receptors (SARs) was examined before and after administration of ouabain, a Na+-K+ ATPase inhibitor, and flecainide, a Na+ channel blocker. The experiments were performed in anesthetized, artificially ventilated rabbits after vagus nerve section. CO2 inhalation (maximal tracheal CO2 concentration ranging from 9.2 % to 10.4%) for about 60 sec decreased the receptor activity during both inflation and deflation. The magnitude of decreased SAR activity during deflation was greater than that seen during inflation. Administration of ouabain (25 microg/kg) initially stimulated SAR activities during inflation and deflation, and after 20 min, the SAR response was still kept excitatory in both inflation and deflation phases. Under these conditions, CO2 inhalation inhibited SAR activities during inflation and deflation. Flecainide treatment (3 mg/kg) that abolished veratridine (30 microg/kg)-induced SAR excitation had no significant effect on the inhibitory responses of SAR activity to CO2. These results suggest that the inhibitory effect of CO2 occurs when ouabain results in intracellular Na+ concentration ([Na+]i) increases in the SAR endings, and that CO2-induced SAR inhibition may not be related to the reduction of influx of Na+ through voltage-gated Na+ channels.

Effects of potassium channel blockers on CO2-induced slowly adapting pulmonary stretch receptor inhibition

In anesthetized, artificially ventilated rabbits with vagus nerve section, inhalation of CO(2) gas mixtures (tracheal CO(2) concentration ranging from 8.0 to 10.2%) for 60 s decreased slowly adapting pulmonary stretch receptor (SAR) activity during both inflation and deflation. The magnitude of decreased receptor activity during deflation had a more pronounced effect than that seen during inflation. CO(2) inhalation did not cause any significant change in tracheal pressure (P(T)) as an index of bronchomotor tone. Intravenous administration of 4-aminopyridine (0. 7 and 2.0 mg/kg i.v.), a K(+) channel blocker, which dose-dependently increased SAR activity during deflation and had no effect on P(T), abolished or attenuated the decrease in SAR activities induced by CO(2) inhalation in a dose-dependent manner. The K(+) channel blocker tetraethylammonium (2.0 and 6.0 mg/kg i.v.) that did not significantly alter either basal SAR discharge or P(T) had no effect on the inhibitory responses of receptor activity to CO(2) inhalation. These results suggest that the inhibitory mechanism of CO(2) inhalation on SARs may be involved in the activation of 4-aminopyridine-sensitive K(+) channels in the nerve terminals of SARs.

Excitatory mechanism of veratridine on slowly adapting pulmonary stretch receptors in anesthetized rabbits

The excitatory effects of veratridine on slowly adapting pulmonary stretch receptors (SARs) were studied before and after administration of ouabain (a Na+-K+ ATPase inhibitor) in anesthetized, artificially ventilated rabbits after vagus nerve section. Administration of veratridine (40 microg/kg) stimulated SAR activity but did not significantly alter tracheal pressure. Administration of ouabain (50 microg/kg) initially stimulated SAR activity during both inflation and deflation, but after 20 min, two different types of SAR responses were observed; one became silent at the peak, of inflation only, and the other maintained excitatory activity during both inflation and deflation phases. Veratridine usually inhibited SAR activity in ouabain-treated animals, irrespective of the difference of ouabain effects. These results suggest that veratridine-induced stimulation of SARs is closely related to the change in the Na+ ion gradient, which is regulated by Na+ pump activity.

Effects of halothane, enflurane, isoflurane, and sevoflurane on slowly adapting pulmonary stretch receptor activity in anesthetized dogs

The aim of this study was to evaluate the effects of halothane, enflurane, isoflurane, and sevoflurane on slowly adapting pulmonary stretch receptor (SAR) activity in dogs. Eight beagles were anesthetized with an intravenous injection of a mixture of urethane and alpha-chloralose as a basal anesthesia, then vagotomized, artificially ventilated, and chest opened. Single afferent activities from SARs were recorded from the peripheral nerve cut end of the left vagus. Changes in SAR activities with inhalation of halothane, enflurane, isoflurane, and sevoflurane at 1, 1.5, and 2 times the minimal alveolar anesthetic concentration (MAC) were measured, and differences in the discharges within and among four anesthetics were evaluated. As a result, two different types of SARs, low threshold SARs and high threshold SARs, were detected in this study. In all anesthetics, expiratory discharges of low threshold SARs decreased significantly in a dose-dependent manner, whereas inspiratory discharges did not change significantly at any anesthetic level. Discharges of high threshold SARs tended to decrease with increasing anesthetic level; however, no statistical significance was observed at any anesthetic level. Only one exception to these changes was observed at 1 MAC of halothane where no significant decrease in the expiratory discharge of low threshold SARs or significant increase in the discharge of high threshold SARs was induced against a control value. In conclusion, recent inhalation anesthetics, except for halothane at the light anesthetic level, tended to decrease SAR activities depending on the anesthetic level, suggesting attenuation of the Hering-Breuer inflation reflex.

Influence of surfactant on the activity of slowly adapting stretch receptors in the lung

Since surfactant has a major role in lung mechanics, this study was designed to determine whether it influences afferent neural feedback to the brainstem. Rats were anaesthetised, paralysed, artificially ventilated and unilaterally vagotomised before recording neural action potentials responding to ventilation stroke (NAPvS) from single units of slowly adapting receptors. The mean (+/-SEM) NAPvs decreased by 7.4% (+/-2.3%) at post-instillation of Tween-20 (a commercial surfactant previously employed in pulmonary studies) from their pre-instillation levels, whereas, by contrast, NAPvs increased by 12.4% (+/-2.3%) at post-instillation of saline--a significant finding (P=0.029). These changes support the concept that surfactant influences afferent neural feedback to the brainstem from mechanoreceptors in the lung, with implications to surfactant deficiency in respiratory control disorders.

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.

Effects of sustained constant artificial ventilation on rapidly adapting pulmonary stretch receptors and lung mechanics in rabbits

After hyperinflation to produce maximal dynamic lung compliance (Cdyn), the effects of artificial ventilation at constant frequency (f) and tidal volume (VT) for over a 2-hour period on rapidly adapting pulmonary stretch receptor (RAR) activity and lung mechanics were investigated in anesthetized rabbits. At a 20 min from maximal Cdyn the RARs began to significantly increase their activity, and this increase coincided with the increase in tracheal pressure (PT). Afterward, the increases in RAR activity and PT developed more slowly. Throughout the sustained constant artificial ventilation, the values of VT and respiratory airflow were not significantly different from controls. At 2 hours after maximal Cdyn, atropine (2 mg/kg) did not significantly alter either the increases in RAR activity and total lung resistance (RL) or the decrease in Cdyn, but hyperinflation reverted those responses to the control levels. These results suggest that the sustained artificial ventilation increases lung stiffness to invariably stimulate RARs, but that the response is not involved in vagally mediated bronchoconstriction.

Action of moguisteine on the activity of tracheobronchial rapidly adapting receptors in the dog

We have studied the effects of moguisteine, a new non-narcotic, peripherally acting antitussive compound, on tracheobronchial rapidly adapting irritant receptors (RARs). Experiments were carried out on dogs anaesthetized with a mixture of urethane and alpha-chloralose, paralysed with gallamine, vagotomized and artificially ventilated. Single unit action potentials identified as originating from tracheobronchial RARs were recorded from the peripheral cut end of the right vagus nerve. The activity of these receptors was recorded together with oesophageal pressure and arterial blood pressure. Fourteen RARs were challenged with moguisteine (200 microg x kg[-1] i.v.) in 0.4% dimethylsulphoxide (DMSO) or 0.4% DMSO alone (vehicle). Receptor activity was recorded before (control) and at 2, 5, 10, 15, 20, 30 and 45 min after administration of the challenging compounds. When the results at intervals of 2, 5, 10, 15 and 20 min were averaged for each dog, it was found that moguisteine decreased the mean activity of the 14 receptors to 75% of the control value (p<0.05); the greatest inhibition occurred 10-20 min after moguisteine administration. DMSO did not significantly affect the activity of these endings. Oesophageal pressure, arterial blood pressure and cardiac frequency were not altered during the experimental procedures. The overall results indicate the presence of an inhibitory effect of moguisteine on rapidly adapting irritant receptors that could account for the antitussigenic effect of this compound.

Sevoflurane has less effect than halothane on pulmonary afferent activity in the rabbit

We have compared the effects of sevoflurane and halothane on the discharge frequencies of 19 slowly adapting and four rapidly adapting lung receptors in the rabbit by recording from single vagal fibres. Both agents reduced the discharge frequency of slowly adapting receptors during expiration (P < 0.0005), halothane having a greater effect than sevoflurane (P < 0.0005). Neither agent had any effect on discharge frequency at the end of inspiration when discharge frequency is at a maximum. Neither agent affected the discharge frequency of rapidly adapting receptors.

Protoveratrines A and B increase sleep apnea index in Sprague-Dawley rats

The action of protovertarines A and B, which stimulate carotid sinus baroreceptors and vagal sensory endings in the heart as well as pulmonary bed, were assessed on spontaneous and postsigh central sleep apneas in freely moving Sprague-Dawley rats. During the 6-h recording period, animals were simultaneously monitored for sleep by using electroencephalogram and electromyogram recordings, for respiration by single-chamber plethysmography, and for blood pressure and heart period by using radiotelemetry. After administration of 0.2, 0.5, or 1 mg/kg sc of protoveratrines, cardiopulmonary changes lasting at least 6 h were observed in all three behavioral states [heart period increased up to 23% in wakefulness, 21% in non-rapid-eye-movement (non-REM) sleep, and 20% in REM sleep; P < 0.005 for each]. At the same time, there was a substantial increase in the number of spontaneous (375% increase; P = 0.04) and postsigh (268% increase, P = 0.0002) apneas. Minute ventilation decreased by up to 24% in wakefulness, 25% in non-REM, and 35% in REM sleep (P < 0.05 for each). We conclude that pharmacological stimulation of baroreflexes promotes apnea expression in the sleeping rat.

Central and peripheral effects of the non-neural substances on respiration before and after vagotomy

The central effects of capsaicin, veratrine, histamine and bradykinin were studied by injecting them directly into the oerebrospinal fluid and their peripheral effects were examined by injecting into femoral vein. Our experiments were performed in Na-pentobarbital-anaesthetized dogs. Tidal volume (VT), respiratory frequency (f/min), systemic arterial pressure (BP) were recorded. A significant increase in f, and an initial apnea or hypoventilation followed by a significant increase in VT were observed with central and peripheral capsaicin. Vagotomy removed the peripheral VT response, but not the central one. While central capsaicin administration increased BP, peripheral administration decreased. After vagotomy, a significant increase was observed in BP for both administrations. Respiratory responses to central and peripheral administrations of veratrine were similar to those of capsaicin. Significant increases were observed in f and VT of the intact group in response to central and peripheral administration of histamine. Response to peripheral administration disappeared after vagotomy. While central and peripheral bradykinin increased VT significantly, there was no significant change in f. Vagotomy only removed the increase in VT in response to peripheral administration. In conclusion, respiratory responses to central administration of capsaicin and veratrine are due to direct effects of these substances on respiratory neurons. In peripheral administration, disappearance of the responses after vagotomy indicate that the responses are brought about by stimulation of the lung receptors.

Activation of pulmonary rapidly adapting receptors does not induce bronchoconstriction in dogs

Activation of pulmonary rapidly adapting receptors (RARs) is believed to constrict airways by a vagally mediated reflex. We tested this hypothesis in dogs anesthetized with sufentanil citrate. We ventilated both lungs separately at a positive end-expiratory pressure of 4 cmH2O. We stimulated RARs in one lung under three different conditions: 1) deflation of the lung; 2) decrease in lung compliance; and 3) aerosolization of methacholine. We monitored the airway pressure in the nonstimulated lung as an index for airway muscle tone and could not detect increases in the pressure swing under these conditions. On the other hand, electrical stimulation of the distal end of cervical vagus nerve increased the pressure swing bilaterally (ipsilateral dominant), suggesting that reflex response could be detected in our preparation. Moreover, deflation (or inflation) of either lung increased (or suppressed) diaphragmatic activity. The results indicate intact vagal afferents and central response in our preparation. We conclude that activation of RARs located below the carina does not induce bronchoconstriction by a centrally mediated reflex. If any effect is present, it appears to be small.

CO2-sensitive olfactory and pulmonary receptor modulation of episodic breathing in bullfrogs

Breathing was monitored during normocarbia, hypercarbia (6% CO2 in air), and the period immediately after the return to normocarbic conditions in intact, olfactory-denervated, and vagotomized bullfrogs. In intact frogs, ventilation increased during hypercarbia, but the breathing pattern remained episodic. Immediately upon return to air, there was a further paradoxical increase in breathing frequency, and breathing became continuous in most frogs. Results obtained from animals after olfactory receptor denervation indicate that tonic stimulation of olfactory receptors by airway CO2 inhibited breathing during hypercarbia. Measurements of the kinetics of changes in airway and arterial blood CO2 levels support the suggestion that the sudden release of this inhibition on the return to normocarbic conditions was responsible for the posthypercarbic hyperpnea. Vagotomy increased ventilation during normocarbia. Hypercarbia now caused a change in breathing pattern but had no net effect on total ventilation, suggesting that pulmonary vagal feedback inhibited ventilation during normocarbia but stimulated ventilation during hypercarbia. Although olfactory and pulmonary receptor feed-back shape the breathing pattern, they were not responsible for initiating or terminating the episodes of breathing.

Flecainide blocks the stimulatory effect of veratridine on slowly adapting pulmonary stretch receptors in anaesthetized rabbits without changing lung mechanics

We examined the responses of slowly adapting pulmonary stretch receptors (PSRs), total lung resistance (RL) and dynamic lung compliance (Cdyn) to administered veratridine before and after pretreatment with atropine or flecainide in anaesthetized, artificially ventilated rabbits with bilateral vagotomy. Administration of veratridine (10 and 30 micrograms kg-1) caused vigorous stimulation of PSRs, resulting in a tonic discharge of receptors during both inflation and deflation, but did not significantly alter either RL or Cdyn. The veratridine-induced PSR stimulation became more prominent, as the dose of this alkaloid was increased. Pretreatment with atropine (1 or 2 mg kg-1) had no significant effect on the excitatory response of PSRs to veratridine. The veratridine-induced PSR stimulation was inhibited by treatment with flecainide (1, 2 and 3 mg kg-1), a sodium channel blocker, and this inhibition was dose-dependent. These results suggest that activation of PSRs following veratridine administration probably related to the increased influx of sodium ions into the receptive terminals but does not depend upon bronchoconstriction.

The response of rapidly adapting pulmonary stretch receptors elicited by sodium cyanide is augmented by vagotomy in the rabbit

The responses of rapidly adapting pulmonary stretch receptor (RAR) activity, tidal volume (VT), inspiratory airflow (VI), transpulmonary pressure (Ptrans) and dynamic lung compliance (Cdyn) to administration of NaCN (20 and 40 micrograms/kg) were compared before and after vagal section in anesthetized, spontaneously breathing rabbits. The responses of RARs were recorded from a partially dissected branch of the left vagus nerve (LVN). Before vagal section, hyperpnea following NaCN administration led to increases in RAR activity, VT, VI and Ptrans, but caused a decrease in Cdyn. Those responses obtained became more prominent by increasing the does of NaCN. Under these conditions the increased receptor activity fired throughout the whole respiratory cycle. The responses of RAR activity, VT and Ptrans to NaCN administration were augmented by a vagal section that was produced by denervating the rest of the LVN still intact and right vagus nerve, and the discharge of receptors showed a predominantly inspiratory pattern that corresponded to augmentation of both Ptrans and VT. Vagal section, however, had no significant effect on the changes of VI and Cdyn induced by NaCN. In addition, mean percent changes of increased VT and Ptrans produced by NaCN administration after vagal section were similar to those before vagal section. Furthermore, augmentation of VT and elevation in baseline VT, when NaCN after bilateral vagotomy was administered i.v., were preceded by simultaneously increased Ptrans and RAR activity. These results suggest that augmentation of NaCN-induced RAR stimulation in animals without intact vagus nerves is related to concomitant changes in the respiratory mechanics such as Ptrans and VT, probably involving the increased sensitivity of receptors to these two factors.

Role of the parabrachial nuclei in the airway dilation evoked by the Hering-Breuer reflex

We tested the hypothesis that blockade of glutamatergic receptors in the parabrachial nucleus (PBN) of chloralose-anesthetized cats attenuated the reflex airway dilation evoked by activation of pulmonary stretch receptors. Unilateral microinjection of kynurenic acid (50 nl, 100 mM) into the PBN reversibly attenuated the reflex relaxation of the trachealis muscle in 7 cats. These findings suggest that the PBN is part of the central pathway mediating the airway dilation component of the Hering-Breuer reflex.

Effects of thromboxane A2 agonist STA2 on rapidly adapting pulmonary stretch receptors in vagotomized rabbits

We investigated the responses of rapidly adapting pulmonary stretch receptors (RARs) and tracheal pressure (PT) to right atrial injections of the thromboxane A2 (TXA2) stable analogue STA2 (0.3, 1.0, and 3.0 microgram/kg) before and after administration of atropine sulfate (1 mg/kg), isoprenaline (200 micrograms/kg), indomethacin (1 mg/kg), or S-145 (0.5 mg/kg) in artificially ventilated, bilaterally vagotomized rabbits. The RARs increased their activity after STA2 administration, and the increase was dose-dependent. However, intraatrial injections of STA2 at all the doses examined had no significant effect on PT. The excitatory responses of RAR activity to STA2 (0.3-3.0 micrograms/kg) were not significantly altered by administration of atropine sulfate (anticholinergic agent), isoprenaline (bronchodilator), or indomethacin (cyclooxygenase inhibitor). However, S-145 treatment (TXA2 antagonist) blocked the STA2-induced RAR stimulation. To determine whether or not administration of STA2 causes release of acetylcholine (ACh), we also examined the effects of vagal efferent stimulation (10-15 V, 10 Hz, 1 ms), STA2 administration (3.0 micrograms/kg), and their combination on PT in rabbits associated with both artificial ventilation and bilateral vagotomy. The vagally mediated bronchoconstriction that led to an increase in PT was not enhanced by simultaneous administration of STA2 at 3.0 micrograms/kg in all of the tested animals. These results suggest that the stimulation of RARs by STA2 is not mediated by the release of ACh from the nerve endings but is probably due to a local inflammatory bronchoconstriction that does not significantly alter the value of PT.

Effects of veratridine and nifedipine on ammonia-induced rapidly adapting pulmonary stretch receptor stimulation in vagotomized rabbits

We studied the effects of aerosol administration of veratridine (a sodium channel opener) or nifedipine (a calcium channel blocker) on the responses of rapidly adapting pulmonary stretch receptors (RARs) and dynamic lung compliance (Cdyn) to aerosols of 2 and 4% ammonia solutions in anesthetized spontaneously breathing rabbits without intact vagi. The RARs increased their activity following ammonia aerosol, and the increase was concentration-dependent. However, ammonia aerosol did not significantly alter the value of Cdyn. The RARs following aerosol administration of veratridine (about 200 micrograms) showed their characteristic firing pattern with several phases; each phase was characterized by the long high-frequency continuous discharges. Under these conditions, the response was not associated with any significant change in Cdyn. Even though the change in receptor activity produced by veratridine was restored to control level, subsequent aerosol application of ammonia led to similar firing patterns, as veratridine was given by aerosol, but had no significant effect on Cdyn. Following aerosol administration of nifedipine (about 1 and 2 mg) the RAR activity and Cdyn were similar to those during control. Furthermore, the ammonia-induced RAR stimulation was not significantly affected by nifedipine aerosol. These results suggest that the stimulation of RARs by ammonia in vagotomized rabbits is independent of changes in Cdyn and speculate that their excitatory effect is at least in part related to the activation of Na+ influx to the receptive terminals but is not involved in the secondary entry of Ca2+ ions to the receptor membrane, through voltage-dependent calcium channels.

Effects of isoprenaline on the responses of slowly adapting pulmonary stretch receptors to reduced lung compliance and to administered histamine

To define the difference between the responses of slowly adapting pulmonary stretch receptors (SARs) to reduced dynamic lung compliance (Cdyn) and to administered histamine, experiments were performed in open-chest, artificially ventilated, bilaterally vagotomized rabbits with positive end-expiratory pressure (PEEP). Both stimuli caused an increase in tracheal pressure and produced augmentation of SAR activities during inflation and deflation. Isoprenaline treatment that blocked the responses of SARs and PT to histamine had no effect on those to reduced Cdyn. The results suggest that the response characteristics of SARs provoked by histamine administration do not involve the contribution of decreased Cdyn.

Responses of tracheobronchial receptors to halothane, enflurane, and isoflurane in anesthetized dogs

We investigated the effects of halothane, enflurane, and isoflurane on the activity of 43 tracheobronchial slowly adapting stretch receptors (SARs) and 16 rapidly adapting irritant receptors (RARs) in 5 anesthetized, vagotomized, paralyzed, and artificially ventilated dogs. The 43 SARs were classified into 2 subtypes: (i) 17 low-threshold SARs with an expiratory discharge at FRC that were active throughout the respiratory cycle and (ii) 26 high-threshold SARs active only in inspiration. Ventilating the lungs with 5% of each anesthetic caused a significant increase in the inspiratory discharge of low-threshold SARs, whereas the expiratory discharge was inhibited or altogether silenced. While the activity of the majority of high-threshold SARs increased during the administration of the three volatile anesthetics, it decreased in those with a particularly high recruitment threshold. There was, however, a consistent increase in the pressure threshold at which all SARs were recruited. Ventilating the lungs with 5% of each anesthetic caused a significant decrease in activity of RARs. Our results indicate that all three halogenated anesthetics inhibit RARs at concentrations ranging from 1% to 5%.

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.

Substance P antagonist does not block the stimulation of rapidly adapting pulmonary stretch receptors by ammonia

We investigated the effects of the substance P (SP) blocker [D-Pro2,D-Trp7,9]-SP on the response of rapidly adapting pulmonary stretch receptors (RARs) to SP administered into the right atrium, or ammonia vapor inhaled into the lungs in anesthetized, spontaneously breathing rabbits. Right atrial administration of SP (0.3, 1.0, and 3.0 micrograms/kg) caused an increase in the RAR activity, and this increase became more prominent as the dose of SP was increased. The RARs increased their activity following inhalation of vapor from 5 and 10% ammonia solutions, and the increase was concentration dependent. The excitatory responses of RAR activity to SP at different doses were greatly diminished or completely blocked by administration of the selective SP antagonist (300 and 500 micrograms/kg). However, the ammonia-induced RAR stimulation was not significantly altered by prior treatment with the SP blocker (300 and 500 micrograms/kg). These results suggest that the stimulation of RARs by ammonia does not occur as a result of the release of SP from sensory nerves in the airways and lungs.

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.

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.

H1- and H2-receptor influences of histamine and ammonia on rapidly adapting pulmonary stretch receptor activities

Studies were designed to establish the difference of the excitatory mechanisms between histamine and ammonia on rapidly adapting pulmonary stretch receptors (RARs). We therefore examined the responses of RARs to histamine administered as an aerosol and ammonia inhaled as a vapor before and after aerosol administration of mequitazine or cimetidine in spontaneously breathing rabbits. The excitatory responses of RARs to aerosols of histamine at different concentrations were completely blocked by administration of aerosol mequitazine but potentiated by aerosol cimetidine. However, the increases of RAR activity produced by inhalation of ammonia vapor at different concentrations were not significantly affected by aerosol administration of either a H1-receptor blocker or a H2-receptor antagonist. These results suggest that the stimulation of RARs by aerosol histamine occurs as a result of the interaction between H1 (excitatory)- and H2 (inhibitory)-receptor effects, whereas these two receptor effects do not contribute to the ammonia-induced RAR stimulation.

Effects of calcium channel and H1-receptor blockers on the responses of slowly adapting pulmonary stretch receptors to histamine in vagotomized rabbits

We studied the effects of calcium channel antagonists (verapamil and nifedipine) and H1-receptor blockers (mequitazine) on changes in the slowly adapting pulmonary stretch receptors (SARs) located below the carina in response to right atrial injections of histamine (60 and 80 micrograms/kg) in anesthetized artificially ventilated rabbits with bilateral vagotomy. After histamine was injected into the right atrium, the SARs became more active during expiration but decreased their activity during inspiration. These changes were more pronounced by increasing the dosage of histamine. However, administration of histamine had no significant effect on tracheal pressure (PT). Verapamil treatment (1 mg/kg) did not alter the SAR response to histamine, whereas the responses of SARs to histamine at different dosages were significantly diminished by treatment with nifedipine (1 mg/kg). Mequitazine (1 mg/kg), a potent H1-receptor blocker, blocked completely all the responses of SAR activity to histamine. These results suggest that the effect of histamine 60-80 micrograms/kg on SAR activity is mediated by the activation of H1-receptors of the peripheral airway smooth muscle and that this activation, at least in part, involves the opening of calcium channels of the airway smooth muscle.

Effects of the thromboxane A2 mimetic, U46,619, on pulmonary vagal afferents in the cat

Release of thromboxane A2 (TxA2) or infusion of the TxA2 mimetic U46,619 in the cat elicits pulmonary hypertension and rapid shallow breathing (Shams et al., Respir. Physiol. 71: 169-183, 1988). The vagus nerve mediates the observed respiratory, but not the circulatory, effects (Shams and Scheid, J. Appl. Physiol. 68: 2042-2046, 1990). To identify the type of lung vagal afferent fibers involved in this respiratory response to TxA2, we have recorded the functional single-unit activity and its response to infusion of U46,619 in fine strands of the vagus nerve in the artificially ventilated cat and rabbit. The fibers were classified as originating from slowly adapting (SAR) or rapidly adapting (RAR) stretch receptors by their response to sustained pulmonary inflation (intrapulmonary pressure of 20-25 cmH2O) or as C-fibers, by their response to a bolus injection of phenylbiguanide. C-fibers responded variably to lung inflation. U46,619 infusion caused only a small increase in SAR or RAR activity along with increases in end-inspiratory tracheal airway pressure (Paw), but evoked a marked increase in the firing rate of C-fibers, independent of their response to lung inflation. This increase in C-fiber activity was unrelated to the increase in Paw, which accompanied the infusion of U46,619. Since these responses remained the same after indomethacin they appear to be due to a direct action of U46,619, and not to be mediated by prostanoids that might be released by U46,619. These data suggest that C-fibers are indeed involved in the respiratory effects of TxA2. Since the effects exerted on C-fibers by U46,619 were unrelated to increased Paw, TxA2 is likely to stimulate the nerve endings directly, rather than via smooth muscle contraction. On the other hand, the small stimulating effect of U46,619 on SAR and RAR may be mediated by bronchoconstriction.

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.

MDL 72222 (a selective 5-HT3 receptor antagonist) prevents stimulation of intrapulmonary C fibres by pulmonary embolization in anaesthetized rabbits

We have investigated the effects of the intravenous administration of 5-hydroxytryptamine upon the discharge frequency of pulmonary stretch receptors, irritant receptors and C fibres. Only C fibres were stimulated by this autacoid. MDL 72222 a selective 5-HT3 receptor antagonist blocks the C fibre stimulation by 5-hydroxytryptamine. It also blocks the C fibre response to miliary pulmonary embolism. These data confirm the hypothesis that 5-hydroxytryptamine is the humoral link between pulmonary embolism and tachypnoea.

Indirect effects of histamine on pulmonary rapidly adapting receptors in cats

We did experiments to determine the relative importance of lung mechanical changes during histamine induced activation of pulmonary rapidly adapting receptors (RARs). In anesthetized, open-chest, artificially ventilated cats, we recorded RAR activity and injected histamine (25-50 micrograms/kg) into the right atrium. Histamine initially increased RAR activity from 1.1 +/- 0.2 to 3.6 +/- 0.6 imp/sec (n = 30) at 15.6 +/- 0.8 sec when dynamic lung compliance (CDYN) was decreased by 29.1 +/- 1.5%. The firing pattern of RARs changed from a relatively irregular pattern to a pronounced respiratory modulation. RAR activity reached its peak (5.6 +/- 0.8 imp/sec) at 36.3 +/- 3.3 sec. The firing pattern further changed to a cardiac modulation, and the activity closely correlated with cardiac output. Comparing the initial response of RARs to histamine with the response to mechanically decreasing CDYN, we found that the activities were similar when CDYN was decreased by the same amount. Our experiments suggest that in cats the initial increase of RAR activity in response to histamine is related to lung mechanical changes, but the later increase is related to cardiovascular functions.

Effects of histamine on slowly adapting pulmonary stretch receptor activities in vagotomized rabbits

Thirty-two SAR fibers located below the carina were used in 32 anesthetized, bilaterally vagotomized, and artificially ventilated rabbits. After intravenous administration of histamine (20, 40, and 80 micrograms/kg, n = 13) or ACh (10, 20, and 40 micrograms/kg, n = 13), SARs became active during expiration but decreased their inspiratory activity. The effects of both drugs were dose-dependent. The injection of histamine or ACh at all doses examined had no effect on tracheal pressure (PT). Atropine (3 mg/kg) and isoprenaline (100 micrograms/kg) blocked the responses of SAR activity to low doses of histamine (20 and 40 micrograms/kg) and to all doses of ACh. The response of SARs to 80 micrograms/kg of histamine was not altered by atropine (n = 10) nor by isoprenaline (n = 10). These results suggest that low-dose effects of histamine on SARs occur as the result of ACh release whereas with high doses the effect of histamine on the receptors is mainly independent of ACh released from the nerve endings. In another series of experiments (n = 6) where animals were treated with isoprenaline (100 micrograms/kg) and, subsequently, physostigmine (200 micrograms/kg), histamine (20 and 40 micrograms/kg) or ACh (10 and 20 micrograms/kg) increased the inspiratory discharge in SARs and the level of PT. From these results, it is assumed that the changes of SAR activity following histamine injection in vagotomized animals reflect a local bronchomotor effect which takes place in a peripheral bronchial tree that does not affect the level of PT. Histamine seems to release ACh and to elicit bronchoconstriction that can be manifested by physostigmine treatment.

Excitation of both slowly and rapidly adapting pulmonary stretch receptors attenuates tachypnea induced by ammonia

To elucidate the mechanism of attenuating the ammonia-induced tachypnea, the present study examined the discharge patterns and rates of slowly adapting pulmonary stretch receptors (SARs) and rapidly adapting pulmonary stretch receptors (RARs) in relation to the change in respiration produced by ammonia inhalation in anesthetized, spontaneously breathing rabbits. Extracellular action potentials of these two receptors were recorded at the peripheral cut-end of the left vagus nerve. A prolongation of expiration following ammonia inhalation occurred during the discharge of receptors increased continuously, particularly when the level of the discharge rate during expiration reached to approximately 20-fold, and under such circumstances the respiratory response was regularly associated with gasps. On the other hand, RARs increased their activity during only inspiration; this increased activity correlated with augmentation of inspiration. Furthermore, the prolongating effect of expiration due to ammonia inhalation was not observed after surgical denervation of the remaining right vagus nerve. These results suggest that vigorous stimulation of the SAR activity induced by ammonia inhalation can elicit a prolongation of expiration possibly resulting from augmentation of the Hering-Breuer inflation reflex and that augmentation of the increased RAR activity after ammonia inhalation counteracts the Hering-Breuer inflation reflex to shorten inspiration. Therefore, it is conceivable that strong stimulation of the SAR activity after ammonia inhalation counteracts the ammonia-induced tachypnea.

Location of pulmonary stretch receptors in the guinea-pig

The site of pulmonary slowly adapting stretch receptors (SRs) was investigated in anaesthetized, thoracotomized and artificially ventilated guinea-pigs. The location of SRs within the lungs and airways was determined by analyzing the changes of SR single fibres discharge patterns in response to (a) occlusion of the airways, (b) local probing, and (c) microinjection of the non-diffusible local anaesthetic cinchocaine into the presumed receptor site. The great majority (92%) of the 79 SRs examined was localized in small airways or in lung parenchyma ('peripheral SRs'), whereas only 8% were located in large airways, i.e., in the trachea, main bronchi and lobar bronchi ('central SRs'). The discharge responses to lung inflation and to ammonia inhalation slightly differed between these two SR groups. With the pronounced prevalence of peripheral SRs, the guinea-pig seems to take a unique position among the species examined hitherto.