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

Rapidly adapting receptor activity during oxidative stress induced airway hyperresponsiveness

The responses of airway rapidly adapting receptors (RARs) to ovalbumin challenge and histamine were investigated in guinea pigs which were sensitized with ovalbumin. Sensitization alone increased the basal RAR activity. Antigen challenge stimulated them. Histamine doses which caused a 50% increase in airway resistance (ED50) were reduced immediately and 24h after antigen challenge indicating respectively early and late onset airway hyperresponsiveness. At these doses, there was a greater stimulation of the RARs compared to controls. An increase in lipid peroxidation and a decrease in glutathione peroxidase were observed also. With oral intake of vitamins C and E, attenuations in the basal RAR activity, the responses of RARs to antigen challenge and the oxidative stress were observed. With an increase in ED50, the RAR response to histamine became similar as in control. It is concluded that by decreasing the RAR responses to allergen and histamine, antioxidants may reduce reflex bronchoconstriction occurring in asthmatics.

Biomass smoke exposures: toxicology and animal study design

The International Biomass Smoke Health Effects (IBSHE) conference was convened in Missoula, MT, to define our current knowledge of smoke exposure and the potential health effects. In an effort to ascertain the relative health effects of an exposure to biomass smoke, numerous studies have utilized either animal or in vitro systems. A wide variety of systems that have been employed ranged from more mainstream animal models (i.e., rodents) and transformed cell lines to less common animal (piglets and dogs) and explant models. The Toxicology and Animal Study Design Workgroup at IBSHE was tasked with an analysis of the use of animal models in the assessment of the health effects of biomass smoke exposure. The present article contains a mini-review of models utilized historically, in addition to the adverse health effects assessed, and an overview of the discussion within the breakout session. The most common question that arose in discussions at the IBSHE conference was from local and federal health departments: What level of smoke is unhealthy? The present workgroup determined categories of exposure, common health concerns, and the availability of animal models to answer key health questions.

Development of avian intrapulmonary chemoreceptors

Although avian intrapulmonary chemoreceptors (IPC) have been studied extensively in adults, the maturation of IPC CO(2) sensitivity during development is completely unknown. To begin investigating IPC development we asked two fundamental questions: (1) Are IPC capable of sensing CO(2) during early development, and, if so, how early? And, (2) does IPC CO(2) sensitivity during early development exhibit postnatal maturation compared to IPC discharge characteristics in adult ducks? We addressed these questions by recording from single IPC Anas platyrhynchos ducklings beginning approximately 6 h prior to internal pipping through 4 days of postnatal development. We then compared mean IPC discharge characteristics during early development with mean IPC activity from adult ducks greater than 12 weeks old. In total, we recorded 28 individual IPC from 5 ducklings and 12 adult ducks. Results show that IPC were capable of responding to rapid step changes in CO(2) before hatching occurred, during the paranatal developmental period. We also found that mean IPC activity during early development had increased peak discharge frequencies, greater spike frequency adaptation, and less tonic CO(2) sensitivity when compared to adults (P< or =0.05). These results suggest that during early development phasic IPC CO(2) sensitivity is fully developed, yet tonic IPC CO(2) sensitivity exhibits postnatal maturation possibly associated with hatching. These results also suggest that the mechanisms that underlie phasic and tonic IPC action potential discharge, and therefore the degree of partial spike frequency adaptation, may be independent processes with different developmental trajectories.

Woodsmoke health effects: a review

The sentiment that woodsmoke, being a natural substance, must be benign to humans is still sometimes heard. It is now well established, however, that wood-burning stoves and fireplaces as well as wildland and agricultural fires emit significant quantities of known health-damaging pollutants, including several carcinogenic compounds. Two of the principal gaseous pollutants in woodsmoke, CO and NOx, add to the atmospheric levels of these regulated gases emitted by other combustion sources. Health impacts of exposures to these gases and some of the other woodsmoke constituents (e.g., benzene) are well characterized in thousands of publications. As these gases are indistinguishable no matter where they come from, there is no urgent need to examine their particular health implications in woodsmoke. With this as the backdrop, this review approaches the issue of why woodsmoke may be a special case requiring separate health evaluation through two questions. The first question we address is whether woodsmoke should be regulated and/or managed separately, even though some of its separate constituents are already regulated in many jurisdictions. The second question we address is whether woodsmoke particles pose different levels of risk than other ambient particles of similar size. To address these two key questions, we examine several topics: the chemical and physical nature of woodsmoke; the exposures and epidemiology of smoke from wildland fires and agricultural burning, and related controlled human laboratory exposures to biomass smoke; the epidemiology of outdoor and indoor woodsmoke exposures from residential woodburning in developed countries; and the toxicology of woodsmoke, based on animal exposures and laboratory tests. In addition, a short summary of the exposures and health effects of biomass smoke in developing countries is provided as an additional line of evidence. In the concluding section, we return to the two key issues above to summarize (1) what is currently known about the health effects of inhaled woodsmoke at exposure levels experienced in developed countries, and (2) whether there exists sufficient reason to believe that woodsmoke particles are sufficiently different to warrant separate treatment from other regulated particles. In addition, we provide recommendations for additional woodsmoke research.

CO2 transduction in avian intrapulmonary chemoreceptors is critically dependent on transmembrane Na+/H+ exchange

Avian intrapulmonary chemoreceptors (IPC) are vagal respiratory afferents that are inhibited by high lung Pco(2) and excited by low lung Pco(2). Previous work suggests that increased CO(2) inhibits IPC by acidifying intracellular pH (pH(i)) and that pH(i) is determined by a kinetic balance between the rate of intracellular carbonic anhydrase-catalyzed CO(2) hydration/dehydration and transmembrane extrusion of acids and/or bases by various exchangers. Here, the role of amiloride-sensitive Na(+)/H(+) exchange (NHE) in the IPC CO(2) response was tested by recording single-unit action potentials from IPC in anesthetized ducks, Anas platyrhynchos. For each of the IPC tested, blockade of the NHE using dimethyl amiloride (DMA) elicited a marked (>50%) dose-dependent decrease in mean IPC discharge (P < 0.05), suggesting that NHE is important for pH(i) regulation and CO(2) transduction in IPC. In addition, activation of the NHE using 12-O-tetradecanoylphorbol 13-acetate stimulated six of the seven IPC tested, although the overall effect was not statistically significantly (P = 0.07). Taken together, these findings suggest that CO(2) transduction in IPC is dependent on transmembrane NHE although it is likely to be much slower than carbonic anhydrase-catalyzed hydration-dehydration of CO(2).

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.

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.

Airway receptors

There are many types of afferent receptor in the airways; at least five in the larynx: pressure, drive, cold, irritant and C-fibre; and at least four in the trachea and bronchi: slowly and rapidly adapting stretch receptors (SARs and RARs), C-fibre receptors, and those in neuroepithelial bodies (NEBs). Histologically enough sensory structures have been identified to account for the various patterns of afferent activity, but most correlations are poor. For the larynx, four or more sensory structures have not definitively been identified with afferent discharges and reflex responses. For the trachea and bronchi, only SARs have been clearly identified morphologically and physiologically. The reflexes and afferent discharges from RARs and C-fibre receptors are fairly clear, some at least of the sensory terminals lie in the epithelium, but receptor complexes have not been mapped out. Nerves in NEBs have been identified, but not their local and central reflex actions.

Responses of tracheobronchial receptors to inhaled furosemide in anesthetized rats

Inhalation of furosemide, a loop diuretic, has shown favorable effects on experimentally induced cough, bronchoconstriction, and dyspnea. The effect of inhaled furosemide on tracheobronchial receptors was studied in anesthetized, spontaneously breathing rats. Single unit or pauci unit activity was recorded from the right vagus nerve. Tracheobronchial receptors were classified into slowly and rapidly adapting receptors (SARs and RARs, respectively), based on their adaptation index (AI), which was derived from the decrease in spike frequency (sf) over 2 s, expressed as a percentage of the peak firing rate. There were 43 SARs (AI </= 25%) and eight RARs (AI >/= 50%). Inhalation of furosemide (n = 29) increased the slope of airway pressure (Paw) versus sf of SARs from 8.6 to 14.8 Hz/cm H(2)O with an increase in sf at Paw = 0 cm H(2)O from 18.0 to 49.5 Hz, resulting in an upward shift of the line. Neither inhalation of vehicle (n = 9) nor intravenous injection of furosemide (n = 5) changed this relationship. In addition, inhalation of furosemide attenuated the activity of RARs. These findings indicate that SARs are sensitized and RARs desensitized by inhalation of furosemide. We discuss possible mechanisms for this, and its relevance to clinical problems of dyspnea, bronchoconstriction, and cough.

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.