Impact of Airline Secondhand Tobacco Smoke Exposure on Respiratory Health and Lung Function Decades After Exposure Cessation.

BACKGROUND: Twenty-five percent to 45% of COPD is caused by exposures other than active smoking. Secondhand tobacco smoke (SHS) has been suggested as an independent cause of COPD, based on its association with increased respiratory symptoms and a small decrease in lung function, but its impact on respiratory health and lung function after exposure cessation has not been explored. RESEARCH QUESTION: What are the consequences of airline SHS exposure on respiratory health and lung function decades after cessation? STUDY DESIGN AND METHODS: We performed a cohort study involving flight attendants because of their exposure to SHS that stopped > 20 years ago. We included subjects ≥ 50 years of age with > 1 year vs ≤ 1 year of airline SHS exposure (ie, exposed vs unexposed). Respiratory quality of life, as determined by the St. George's Respiratory Questionnaire (SGRQ), was the primary outcome for respiratory health. Key secondary outcomes included general quality of life (the Rand Corporation modification of the 36-item Short Form Health Survey Questionnaire; RAND-36), respiratory symptoms (COPD Assessment Test; CAT), and spirometry. RESULTS: The study enrolled 183 SHS-exposed and 59 unexposed subjects. Exposed subjects were 66.7 years of age, and 90.7% were female. They were hired at 23.8 years of age, were exposed to airline SHS for 16.1 years, and stopped exposure 27.5 years before enrollment. Prior SHS exposure was associated with worsened SGRQ (6.7 units; 95% CI, 2.7-10.7; P = .001), RAND-36 physical and social function, and CAT vs unexposed subjects. SHS exposure did not affect prebronchodilator spirometry or obstruction, but was associated with lower postbronchodilator FEV1 and FEV1/FVC, total lung capacity, and diffusing capacity of the lungs for carbon monoxide in a subset of subjects. Former smoking and SHS exposure synergistically worsened SGRQ (ß = 8.4; 95% CI, 0.4-16.4; P = .04). SHS exposure in people who never smoked replicated primary results and was associated with worsened SGRQ vs unexposed people (4.7 units; 95% CI, 0.7-7.0; P = .006). INTERPRETATION: Almost three decades after exposure ended, airline SHS exposure is strongly and dose-dependently associated with worsened respiratory health, but less robustly associated with airflow abnormalities used to diagnose COPD.

Disparity in the effect of morphine on eupnea and gasping in anesthetized spontaneously breathing adult rats.

The goal of this study was to examine the effects of systemic morphine on the pattern and morphology of gasping breathing during respiratory autoresuscitation from transient anoxia. We hypothesized that systemic morphine levels sufficient to cause significant depression of eupnea would also cause depression of gasping breathing. Respiratory and cardiovascular variables were studied in 20 spontaneously breathing pentobarbital-anaesthetized adult male rats. Sham (saline) injections caused no significant change in resting respiratory or cardiovascular variables (n = 10 rats). Morphine, on the other hand, caused significant depression of eupneic breathing, with ventilation and peak inspiratory flow decreased by ∼30-60%, depending on the background condition (n = 10 rats). In contrast, morphine did not depress gasping breathing. Duration of primary apnea, time to restore eupnea, the number and amplitude of gasping breaths, average and maximum peak flows, and volume of gasping breaths were not significantly different postinjection in either condition. Blood pressures were all significantly lower following morphine injection at key time points in the process of autoresuscitation. Last, rate of successful recovery from anoxia was 80% in the morphine group (8/10 rats) compared with 100% (10/10 rats) in the sham group, postinjection. We conclude that the mechanisms and/or anatomic correlates underlying generation of gasping rhythm are distinct from those underlying eupnea, allowing gasping to remain robust to systemic morphine levels causing significant depression of eupnea. Morphine nevertheless decreases likelihood of recovery from transient anoxia, possibly as a result of decreased tissue perfusion pressures at critical time points during the process of respiratory autoresuscitation.

Fgf10-Hippo Epithelial-Mesenchymal Crosstalk Maintains and Recruits Lung Basal Stem Cells.

The lung harbors its basal stem/progenitor cells (BSCs) in the protected environment of the cartilaginous airways. After major lung injuries, BSCs are activated and recruited to sites of injury. Here, we show that during homeostasis, BSCs in cartilaginous airways maintain their stem cell state by downregulating the Hippo pathway (resulting in increased nuclear Yap), which generates a localized Fgf10-expressing stromal niche; in contrast, differentiated epithelial cells in non-cartilaginous airways maintain quiescence by activating the Hippo pathway and inhibiting Fgf10 expression in airway smooth muscle cells (ASMCs). However, upon injury, surviving differentiated epithelial cells spread to maintain barrier function and recruit integrin-linked kinase to adhesion sites, which leads to Merlin degradation, downregulation of the Hippo pathway, nuclear Yap translocation, and expression and secretion of Wnt7b. Epithelial-derived Wnt7b, then in turn, induces Fgf10 expression in ASMCs, which extends the BSC niche to promote regeneration.

Receptor-type Guanylyl Cyclases Confer Thermosensory Responses in C. elegans.

Thermosensation is critical for optimal regulation of physiology and behavior. C. elegans acclimates to its cultivation temperature (Tc) and exhibits thermosensitive behaviors at temperatures relative to Tc. These behaviors are mediated primarily by the AFD sensory neurons, which are extraordinarily thermosensitive and respond to thermal fluctuations at temperatures above a Tc-determined threshold. Although cGMP signaling is necessary for thermotransduction, the thermosensors in AFD are unknown. We show that AFD-specific receptor guanylyl cyclases (rGCs) are instructive for thermosensation. In addition to being necessary for thermotransduction, ectopic expression of these rGCs confers highly temperature-dependent responses onto diverse cell types. We find that the temperature response threshold is determined by the rGC and cellular context, and that multiple domains contribute to their thermosensory properties. Identification of thermosensory rGCs in C. elegans provides insight into mechanisms of thermosensation and thermal acclimation and suggests that rGCs may represent a new family of molecular thermosensors.

Regulator of Calcineurin (RCAN-1) Regulates Thermotaxis Behavior in Caenorhabditis elegans.

Regulator of calcineurin (RCAN) is a calcineurin-interacting protein that inhibits calcineurin phosphatase when overexpressed, often upregulated under neuropathological conditions with impaired learning and memory processes, such as Down syndrome or Alzheimer's disease. Thermotactic behavior in the nematode Caenorhabditis elegans is a form of memory in which calcineurin signaling plays a pivotal role in the thermosensation of AFD neurons. In this study, we found that rcan-1 deletion mutants exhibited cryophilic behavior dependent on tax-6, which was rescued by expressing rcan-1 in AFD neurons. Interaction between RCAN-1 and TAX-6 requires the conserved PxIxIT motif of RCAN-1, without which thermotactic behavior could not be fully rescued. In addition, the loss of crh-1/CREB suppressed the thermotaxis phenotypes of rcan-1 and tax-6 mutants, indicating that crh-1 is crucial in thermotaxis memory in these mutants. Taken together, our results suggest that rcan-1 is an inhibitory regulator of tax-6 and that it acts in the formation of thermosensory behavioral memory in C. elegans.

CaMKI-dependent regulation of sensory gene expression mediates experience-dependent plasticity in the operating range of a thermosensory neuron.

Sensory adaptation represents a form of experience-dependent plasticity that allows neurons to retain high sensitivity over a broad dynamic range. The mechanisms by which sensory neuron responses are altered on different timescales during adaptation are unclear. The threshold for temperature-evoked activity in the AFD thermosensory neurons (T*(AFD)) in C. elegans is set by the cultivation temperature (T(c)) and regulated by intracellular cGMP levels. We find that T*(AFD) adapts on both short and long timescales upon exposure to temperatures warmer than T(c), and that prolonged exposure to warmer temperatures alters expression of AFD-specific receptor guanylyl cyclase genes. These temperature-regulated changes in gene expression are mediated by the CMK-1 CaMKI enzyme, which exhibits T(c)-dependent nucleocytoplasmic shuttling in AFD. Our results indicate that CaMKI-mediated changes in sensory gene expression contribute to long-term adaptation of T*(AFD), and suggest that similar temporally and mechanistically distinct phases may regulate the operating ranges of other sensory neurons.

The balance between cytoplasmic and nuclear CaM kinase-1 signaling controls the operating range of noxious heat avoidance.

Through encounters with predators, competitors, and noxious stimuli, animals have evolved defensive responses that minimize injury and are essential for survival. Physiological adaptation modulates the stimulus intensities that trigger such nocifensive behaviors, but the molecular networks that define their operating range are largely unknown. Here, we identify a gain-of-function allele of the cmk-1 CaMKI gene in C. elegans and show that loss of the regulatory domain of the CaMKI enzyme produces thermal analgesia and shifts the operating range for nocifensive heat avoidance to higher temperatures. Such analgesia depends on nuclear CMK-1 signaling, while cytoplasmic CMK-1 signaling lowers the threshold for thermal avoidance. CMK-1 acts downstream of heat detection in thermal receptor neurons and controls neuropeptide release. Our results establish CaMKI as a key regulator of the operating range for nocifensive behaviors and suggest strategies for producing thermal analgesia through the regulation of CaMKI-dependent signaling.

Augmented breaths ('sighs') are suppressed by morphine in a dose-dependent fashion via naloxone-sensitive pathways in adult rats.

Morphine treatment can eliminate augmented breaths (ABs; 'sighs') during spontaneous breathing. In the present study, unanesthetized rats were studied to: (1) determine the involvement of naloxone-sensitive receptor pathways, and (2) establish the dose-response relationship of this side effect. At a dosage of 5mg/kg (2-10mg/kg is recommended range for analgesia) morphine eliminated ABs from the breathing rhythm across nearly 100 min post-administration (vs. 6.2 ± 1.6 ABs in 15 min, control condition, p<0.001). This occurred despite no apparent effect on indices of ventilation. By contrast, when naloxone was co-administered with morphine, the occurrence of ABs was not different compared to control. The suppression of ABs by morphine followed a sigmoidal pattern across the low-mid dosage range (R(2)=0.83), whereas tidal volume and breathing frequency were unaffected. We conclude that the opioid-induced suppression of ABs is mediated by naloxone-sensitive opioid receptor pathways, and that this side effect is potent across the low-mid dosage range, and cannot be simply avoided by restricting dosage.

Control of breathing in invertebrate model systems.

The invertebrates have adopted a myriad of breathing strategies to facilitate the extraction of adequate quantities of oxygen from their surrounding environments. Their respiratory structures can take a wide variety of forms, including integumentary surfaces, lungs, gills, tracheal systems, and even parallel combinations of these same gas exchange structures. Like their vertebrate counterparts, the invertebrates have evolved elaborate control strategies to regulate their breathing activity. Our goal in this article is to present the reader with a description of what is known regarding the control of breathing in some of the specific invertebrate species that have been used as model systems to study different mechanistic aspects of the control of breathing. We will examine how several species have been used to study fundamental principles of respiratory rhythm generation, central and peripheral chemosensory modulation of breathing, and plasticity in the control of breathing. We will also present the reader with an overview of some of the behavioral and neuronal adaptability that has been extensively documented in these animals. By presenting explicit invertebrate species as model organisms, we will illustrate mechanistic principles that form the neuronal foundation of respiratory control, and moreover appear likely to be conserved across not only invertebrates, but vertebrate species as well.

Hypoxia-induced arterial chemoreceptor stimulation and hydrogen sulfide: too much or too little?

This brief review presents and discusses some of the important issues surrounding the theory which asserts that endogenous hydrogen sulfide (H(2)S) is the mediator of, or at least an important contributor to, hypoxia-induced arterial chemorereceptor stimulation. The view presented here is that before H(2)S can seriously be considered as a candidate for transducing the O(2)-signal in the carotid bodies (CB), fundamental contradictions need to be resolved. One of these major contradictions is certainly the discrepancy between the levels of H(2)S endogenously present in the CB during hypoxia compared to the levels required to stimulate the arterial chemoreceptors in vitro. Very small amounts of H(2)S are thought to be produced endogenously during a given level of hypoxia, yet the partial pressure of tissue H(2)S which is needed to produce an effect commensurate with that of hypoxia is thousands to millions of times higher. This review discusses this and other contradictions in light of what is known about H(2)S concentration and production in various tissues, the lessons we have learnt from the response to exogenous sulfide and the ability of the blood and the mitochondria to oxidize very large amounts of sulfide. These considerations suggest that the increased production of H(2)S in hypoxia and exogenous sulfide cannot produce the same effect on the carotid bodies and breathing. While the effects of the endogenous H(2)S on breathing remains to be established, the effects exogenous sulfide can be accounted for by its long established toxicity on cytochrome C oxidase.

The "other" respiratory effect of opioids: suppression of spontaneous augmented ("sigh") breaths.

The purpose of this study was to examine the effects of a clinically relevant opioid on the production of augmented breaths (ABs) in unanesthetized animals breathing normal room air, using a dosage which does not depress breathing. To do this we monitored breathing noninvasively, in unrestrained animals before and after subcutaneous injection of either morphine, or a saline control. The effect of ketamine/xylazine was also studied to determine the potential effect of an alternative sedative agent. Last, the effect of naloxone was studied to determine the potential influence of endogenous opioids in regulating the normal incidence of ABs. Morphine (5 mg/kg) had no depressive effect on breathing, but completely eliminated ABs in all animals in room air (P = 0.027). However, when animals breathed hypoxic air (10% O(2)), animals did express ABs, although their incidence was still reduced by morphine (P < 0.001). This was not a result of sedation per se, as ABs continued at their normal rate in room air during sedation with ketamine. Naloxone had no effect on breathing or AB production, and so endogenous opioids are not likely involved in regulating their rate of production under normal conditions. Our results show that in unanesthetized animals breathing normal room air, a clinically relevant opioid eliminates ABs, even at a dose that does not cause respiratory depression. Despite this, hypoxia-induced stimulation of breathing can facilitate the production of ABs even with the systemic opioid present, indicating that peripheral chemoreceptor stimulation provides a potential means of overcoming the opioid-induced suppression of these respiratory events.

Hydrogen sulfide oxidation and the arterial chemoreflex: effect of methemoglobin.

Endogenous H(2)S has been proposed to transduce the effects of hypoxia in the carotid bodies (CB). To test this hypothesis, we created a sink for endogenously produced H(2)S by inducing ∼10% methemoglobinemia via the injection of 250 mg of sodium nitrite in spontaneously breathing anaesthetized sheep. Methemoglobinemia has been shown to catalyze the oxidation of large quantities of sulfide in the blood and tissues. We found that the presence of metHb completely abolished the ventilatory stimulation induced by 10 mg NaHS (i.v.), which in control conditions mimicked the effects of breathing 6-7 tidal volumes of nitrogen, confirming the dramatic increase in the oxidative power of the blood for sulfide. The ventilatory responses to hypoxia (10% O(2)), nitrogen and hyperoxia were in no way depressed by the metHb. Our results demonstrate that the ventilatory chemoreflex is not depressed in the presence of a high oxidative capacity for sulfide and challenge the view that H(2)S transduces the effects of hypoxia in the CB.

Regulation of response properties and operating range of the AFD thermosensory neurons by cGMP signaling.

BACKGROUND: The neuronal mechanisms that encode specific stimulus features in order to elicit defined behavioral responses are poorly understood. C. elegans forms a memory of its cultivation temperature (T(c)) and exhibits distinct behaviors in different temperature ranges relative to T(c). In particular, C. elegans tracks isotherms only in a narrow temperature band near T(c). T(c) memory is in part encoded by the threshold of responsiveness (T∗(AFD)) of the AFD thermosensory neuron pair to temperature stimuli. However, because AFD thermosensory responses appear to be similar at all examined temperatures above T∗(AFD), the mechanisms that generate specific behaviors in defined temperature ranges remain to be determined. RESULTS: Here, we show that the AFD neurons respond to the sinusoidal variations in thermal stimuli followed by animals during isothermal tracking (IT) behavior only in a narrow temperature range near T(c). We find that mutations in the AFD-expressed gcy-8 receptor guanylyl cyclase (rGC) gene result in defects in the execution of IT behavior and are associated with defects in the responses of the AFD neurons to oscillating thermal stimuli. In contrast, mutations in the gcy-18 or gcy-23 rGCs alter the temperature range in which IT behavior is exhibited. Alteration of intracellular cGMP levels via rGC mutations or addition of cGMP analogs shift the lower and upper ranges of the temperature range of IT behavior in part via alteration in T∗(AFD). CONCLUSIONS: Our observations provide insights into the mechanisms by which a single sensory neuron type encodes features of a given stimulus to generate different behaviors in defined zones.

Breathing patterns during cardiac arrest.

The absence of respiratory movements is a major criterion recommended for use by bystanders for recognizing an out-of-hospital cardiac arrest (CA), as the persistence of eupneic breathing is considered to be incompatible with CA. The basis for CA-related apnea is, however, uncertain, since brain stem Po(2) is not expected to drop immediately to the critical level where anoxic apnea occurs. It is therefore essential on both clinical and physiological grounds to determine whether and when breathing stops after the onset of CA. In eight patients, we measured the ventilatory response at the onset of ventricular fibrillation (VF) for 12-15 s during the placement of an implantable cardioverter-defibrillator device. We found that regular eupneic breathing was maintained unchanged despite the cessation of systemic and pulmonary blood flow generated by the heart. We extended these findings in adult sheep and found that, as in humans, the normal ventilatory pattern persists unchanged for the first 15 s despite the drop in blood pressure, followed by a progressive increase in minute ventilation, which was sustained for up to 164 s. The ensuing apnea was disrupted by typical gasps occurring at a very slow frequency. These observations suggest a complete "decoupling" between the return of CO(2) to the pulmonary circulation and continued effective respiratory activity, contrary to what we predicted. This delayed cessation of eupneic breathing during the absence of cardiac pump function is likely related to the time needed for brain stem anoxia to develop. These findings challenge the notions that 1) ventilation stops as pulmonary blood flow/cardiac output ceases and 2) the presence of eupneic breathing is a reliable sign of effective cardiac pumping activity.

Respiratory effects of changing the volume load imposed on the peripheral venous system.

This study was designed to determine if acute distension of the hindlimb venous circulation stimulates breathing, thereby contributing to the respiratory responses to rapid changes in total blood volume. In 10 spontaneously breathing anesthetized sheep, we withdrew 15 ml kg(-1) of blood from a femoral vein over approximately 1-2 min. We then compared the respiratory effects of infusing this venous blood back into the femoral veins across two conditions: the inferior vena cava (IVC) was either unobstructed or obstructed by a balloon-tipped catheter. We found that when blood was withdrawn and blood volume decreased, an absolute increase in breathing often occurred, but more importantly that a relative hyperventilation was always observed. When this blood was re-infused into the animal, effectively increasing blood volume, the respiratory response depended upon whether or not the IVC was open or obstructed. With the IVC unobstructed, a relative hypoventilation occurred, accompanied by an increase in alveolar PCO(2). In contrast, when the venous blood was re-infused and the IVC was obstructed, ventilation increased significantly, and the response was often hypocapnic. These results indicate that increasing the volume load in the venous circulation increases breathing, and that the transduction mechanism is contained within the peripheral venous system. Further, the respiratory drive from this sensory mechanism is subject to modulation via changes in the circulatory status, most likely within the arterial side.

The hypoxia-induced facilitation of augmented breaths is suppressed by the common effect of carbonic anhydrase inhibition.

The typical respiratory response to hypoxia includes a dramatic facilitation of augmented breaths (ABs) or 'sighs' in the breathing rhythm. We recently found that when acetazolamide treatment is used to promote CO(2) retention and counteract alkalosis during exposure to hypoxia, then the hypoxia-induced facilitation of ABs is effectively prevented. These results indicate that hyperventilation-induced hypocapnia/alkalosis is an essential factor involved in the hypoxia-induced facilitation of augmented breaths. However, acetazolamide is also known to decrease the sensitivity of the arterial chemoreceptors. Therefore, the question remains as to whether acetazolamide prevents the facilitation of ABs during hypoxia by offsetting the effects of respiratory alkalosis, or alternatively by suppressing carotid body afferent activity. In the present study, we addressed this question by studying the effects of treatment with an alternative carbonic anhydrase inhibitor, methazolamide, which has been reported to leave carotid body responsiveness to hypoxia intact. Respiratory variables were monitored before, during and after 2 days of methazolamide treatment (10 mg kg(-1) IP, bid) in unsedated and unrestrained adult male rats. Pre-treatment, the number of ABs observed in a 5 min observation window was 1.2 + or - 0.8 and 17.4 + or - 3.8 in room air and hypoxia, respectively. During methazolamide treatment, the facilitation of ABs in hypoxia was rapidly and reversibly suppressed such that ABs we no longer significantly more frequent than they were in room air. The present results demonstrate that the hypoxia-induced facilitation of ABs can be suppressed via the general effects of carbonic anhydrase inhibition, which are common to both acetazolamide and methazolamide. We discuss these results as they pertain to the mechanisms regulating augmented breath production, and the possible association between hypocapnia/alkalosis and sleep disordered breathing.

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

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

Acetazolamide suppresses the prevalence of augmented breaths during exposure to hypoxia.

Augmented breaths, or "sighs," commonly destabilize respiratory rhythm, precipitating apneas and variability in the depth and rate of breathing, which may then exacerbate sleep-disordered breathing in vulnerable individuals. We previously demonstrated that hypocapnia is a unique condition associated with a high prevalence of augmented breaths during exposure to hypoxia; the prevalence of augmented breaths during hypoxia can be returned to normal simply by the addition of CO(2) to the inspired air. We hypothesized that counteracting the effect of respiratory alkalosis during hypocapnic hypoxia by blocking carbonic anhydrase would yield a similar effect. We, therefore, investigated the effect of acetazolamide on the prevalence of augmented breaths in the resting breathing cycle in five awake, adult male rats. We found a 475% increase in the prevalence of augmented breaths in animals exposed to hypocapnic hypoxia compared with room air. Acetazolamide treatment (100 mg/kg i.p. bid) for 3 days resulted in a rapid and potent suppression of the generation of augmented breaths during hypoxia. Within 90 min of the first dose of acetazolamide, the prevalence of augmented breaths in hypoxia fell to levels that were no greater than those observed in room air. On cessation of treatment, exposure to hypocapnic hypoxia once again caused a large increase in the prevalence of augmented breaths. These results reveal a novel means by which acetazolamide acts to stabilize breathing and may help explain the beneficial effects of the drug on breathing stability at altitude and in patients with central forms of sleep-disordered breathing.