Biochemical correlates of airway hyperreactivity in guinea pigs: role of lysophosphatidyl choline

Basis of rise in intracellular sodium in airway hyperresponsiveness and asthma

The aim of this study was to investigate the basis of disturbances in sodium transport in asthma and in airway hyperresponsiveness without symptoms of asthma (asymptomatic AHR). We measured the intracellular sodium (Na(i)); activity of Na(+)/K(+)-ATPase in unstimulated cells (resting activity) and in cell homogenate under optimal conditions (maximal activity); and sodium influx, in mixed leukocytes of 15 normal subjects, 12 subjects with asymptomatic AHR, and 26 asthmatics with or without active symptoms. Resting Na(+)/K(+)-ATPase activity was the same as sodium influx, consistent with homeostasis. Compared with normal subjects, those with asymptomatic AHR or asthma with controlled symptoms had a twofold increase in sodium influx and Na(i). Symptomatic asthmatics also had a twofold increase in sodium influx but a fourfold elevation of Na(i). Maximal Na(+)/K(+)-ATPase activity was reduced by half in symptomatic asthmatics compared with normal subjects. The reduction of maximal Na(+)/K(+)-ATPase activity was associated with a significant decrease in ATP turnover per Na(+)/K(+)-ATPase molecule but not number of Na(+)/K(+)-ATPase molecules per cell. In summary, airway hyperresponsiveness with or without asthma is associated with increased sodium influx and Na in leukocytes. Resting activity of Na(+)/K(+)-ATPase is also increased as a compensatory response to the increased sodium influx, but it is achieved at the expense of higher Na(i). Symptomatic asthma is additionally associated with reduction in maximal activity of Na(+)/K(+)-ATPase, resulting in reduced capacity to handle the increase in sodium influx and consequent severe elevations in Na(i).

Inhaled lysophosphatidylcholine provokes bronchoconstriction in guinea pigs in vivo

Lysophosphatidylcholine is increased in the airway of bronchial asthma, but its role is not clear. We investigated the role of lysophosphatidylcholine in asthma in anaesthetized, mechanically ventilated guinea pigs. Pressure at the airway opening was measured as an index of bronchial response. Increasing doses of lysophosphatidylcholine (1--10 mg/ml) were inhaled and then bronchoalveolar lavage was carried out. 100 and 200 microg/ml methacholine were inhaled 10 min after inhalation of 2.5 mg/ml lysophosphatidylcholine, 10 mg/ml dipalmitoyl phosphatidylcholine and 10 mg/ml glycerophosphocholine, all of which per se did not change the pressure at the airway opening. Effect of 1.0 microg/kg salbutamol, or 60 mg/kg diphenhydramine on the lysophosphatidylcholine-induced increase in the pressure at the airway opening was investigated. Inhalation of lysophosphatidylcholine dose-dependently increased the pressure at the airway opening and increased bronchial responsiveness to methacholine. On the other hand, inhalation of dipalmitoyl phosphatidylcholine decreased the pressure at the airway opening and decreased bronchial responsiveness to methacholine. Intravenously administered salbutamol, but not diphenhydramine, prevented the lysophosphatidylcholine-induced increase in the pressure at the airway opening. The percentage of leukocytes in bronchoalveolar lavage fluid did not change significantly at least within 20 min after the lysophosphatidylcholine inhalation. Lysophosphatidylcholine causes bronchoconstriction and enhances bronchial responsiveness without inducing leukocyte infiltration in the airway, suggesting that lysophosphatidylcholine may be a new bronchoconstrictor mediator in bronchial asthma.

Effects of Vitamin E on airway responses and biochemical parameters in guinea pigs sensitized to ovalbumin

The effect of dietary supplementation with Vitamin E was studied in sensitized guinea pigs. After measurement of baseline airway reactivity and sensitization with ovalbumin, the animals were randomized into two groups: Group A, on a commercial feed and Group B, on dietary supplementation with oral Vitamin E (0.7 IU/kg). These were challenged with inhaled ovalbumin after 4 weeks. The following outcomes were studied: airway responses to ovalbumin inhalation, airway reactivity, sodium and calcium ion influx in isolated tracheal cells, Na+ K+ ATPase and Ca2+ ATPase activity in tracheal homogenate and plasma malonaldehyde. Sensitization increased airway reactivity in Group A but not in Group B. The tracheal cells of animals in Group B showed significantly lower rates of 45Ca and 22Na influx and lower activities of tracheal Na+ K+ ATPase and Ca2+ ATPase as compared to Group A. Plasma malonaldehyde was similar between two groups. We concluded that Vitamin E suppresses the increase in airway reactivity following sensitization and has membrane stabilizing actions.

Role of lysophosphatidylcholine in eosinophil infiltration and resistance in airways

1. Lysophosphatidylcholine (Lyso-PC), which is synthesized by phospholipase A2, is generally considered to induce adhesion molecules. However, little is known about the involvement of Lyso-PC in the pathogenesis of bronchial asthma. The present study was designed to examine whether pre-exposure to Lyso-PC causes eosinophil recruitment and an increase in resistance in airways. 2. Eosinophils in bronchoalveolar lavage fluid (BALF) and the airway walls were enumerated after inhalation of 0.5 mg/mL Lyso-PC to guinea-pigs for 10 min. Respiratory resistance (Rrs) was recorded continuously over 6 h after inhalation of an equi-dose of Lyso-PC for an equivalent period. 3. The proportion of eosinophils was increased from 10.7 +/- 3.3 to 27.5 +/- 3.1% (P < 0.0001) in BALF 6 h after inhalation of Lyso-PC, whereas the proportion of neutrophils and lymphocytes was not increased. Histological examination also showed uniform distribution of eosinophils in the airway wall of bronchi and bronchioles 6 h after inhalation of Lyso-PC. The number of eosinophils (/10 h.p.f.) in the bronchi and bronchioles was increased from 43.5 +/- 16.8 to 154.8 +/- 21.7 (P < 0.0001) and from 34.8 +/- 0.7 to 106.0 +/- 26.6 (P < 0.01), respectively. This eosinophil infiltration was similarly observed 24 h later. 4. Next, we examined the effects of eosinophil infiltration induced by Lyso-PC on Rrs. Inhalation of Lyso-PC caused a slow increase in Rrs and the percentage increase in Rrs was 19.8 +/- 1.9% (P < 0.0001) 6 h later. Eosinophil infiltration and an increase in Rrs did not occur after inhalation of physiological saline. These phenomena induced by Lyso-PC were diminished by pretreatment with dexamethasone (6 micro g/kg per day for 3 days). 5. Lysophosphatidylcholine causes eosinophil infiltration and a subsequent increase in resistance in airways. Our results indicate that Lyso-PC may be involved in the pathophysiology of bronchial asthma.

Relationship between intracellular calcium and airway reactivity in guinea pigs

The present study was carried out to examine the relationship between intracellular free calcium ion concentrations and its regulatory enzymes, sodium potassium adenosine triphosphatase (Na(+),K(+)-ATPase) and calcium adenosine triphosphatase (Ca(2+)-ATPase), with airway reactivity to inhaled histamine in guinea pigs. Forty-nine guinea pigs were included in this study. Of these, 34 animals responded to histamine bronchoprovocation challenge in vivo with a greater than 35% fall in specific airways conductance and were labeled as "reactive," and the remaining 15 were "nonreactive." The dose of histamine producing a 35% fall in specific airways conductance was labeled as ED(35) SGaw. The animals were then sacrificed, and the following biochemical measurements were carried out: intracellular free calcium ion concentrations [Ca(2+)](i) in leukocytes and isolated tracheal smooth muscle cells, activities of Na(+),K(+)-ATPase and Ca(2+)-ATPase in tracheal homogenate, and plasma levels of lysophosphatidylcholine (LPC). Reactive guinea pigs showed significantly higher [Ca(2+)](i) and Na(+),K(+)-ATPase and Ca(2+)-ATPase activities. Airway reactivity (ED(35) SGaw) had significant negative correlation with [Ca(2+)](i), with activities of each of the ATPases and with plasma lysophosphatidylcholine. It is concluded that the level of [Ca(2+)](i) is an important determinant of airway reactivity. Intracellular calcium levels modulate airway response to histamine with higher levels being associated with greater reactivity.

Role of lysophosphatidylcholine in the desensitization of beta-adrenergic receptors by Ca(2+) sensitization in tracheal smooth muscle

Lysophosphatidylcholine (Lyso-PC) is generally considered to promote tissue inflammation. To determine the involvement of exogenous Lyso-PC in the beta-adrenergic desensitization by phospholipase A2, we examined the inhibitory effects of isoproterenol (ISO) on tension and intracellular Ca(2+) concentration by methacholine (MCh) after continuous exposure to Lyso-PC in guinea-pig tracheal smooth muscle, using isometric tension recordings and fura-2 signal (F340/F380 ratio). Pre- exposure to 10 microM Lyso-PC markedly reduced subsequent inhibition by 0.3 microM ISO against 1 microM MCh-induced contraction in a time-dependent manner. In contrast, values of percent F340/F380 ratio for MCh with ISO were not affected after exposure to Lyso-PC. In the presence of Y-27632, a selective rho-kinase inhibitor, a reduction in subsequent relaxation by ISO after exposure to Lyso-PC was inhibited in a concentration-dependent manner. Preincubation with cholera toxin also inhibited reduced responsiveness to ISO by Lyso-PC. Pre-exposure to Lyso-PC did not attenuate subsequent relaxation by agents that bypass beta-adrenergic receptors. These results indicate that continuous exposure to Lyso-PC may cause homologous desensitization of beta-adrenergic receptors via an augmentation in sensitivity to Ca(2+) by rho, a small G protein, in airway smooth muscle, and that activation of the stimulatory G protein of adenylyl cyclase, G(s), may prevent this phenomenon.

Prednisolone hastens recovery from histamine-induced bronchospasm in asthmatics

Short-term treatment with oral steroids is very effective in control ling symptoms and improving lung function in asthma but has not been shown unequivocally to reduce bronchial hyperresponsiveness. Recently it has been shown to increase the activities of sodium-potassium and calcium adenosine triphosphatases, enzymes that regulate intracellular calcium levels. This action may be expected to promote recovery of cells from an excitatory stimulus. The present study was carried out to determine how prednisolone modulates airway response to histamine, including recovery from induced bronchospasm in asthmatics. Spirometry and measurement of bronchial responsiveness (forced expiratory volume in 1 sec [FEV1] and concentration of histamine causing a 20% reduction FEV1 [PD20 FEV1]) to inhaled histamine were carried out in 10 clinically stable asthmatics. Subsequently, all of the patients were prescribed oral prednisolone, 0.6-0.75 mg/kg body weight for 1 week. At the end of 1 week, spirometry was repeated and bronchial reactivity was measured again. Comparison of PD20 FEV1 values before and after treatment (geometric means 0.66 and 0.81 mg/mL, respectively) for the whole group did not show any significant change. The mean +/- SD time for 95% recovery from histamine-induced bronchospasm was 33.00 +/- 19.47 min before treatment and decreased significantly to 18.00 +/- 7.88 min after treatment. It was concluded that short-term benefits from oral prednisolone do not result from changes in bronchial responsiveness. These benefits may be related to effects on mechanisms that lead to recovery from an excitatory stimulus.

Secretory non-pancreatic phospholipase A2 and cyclooxygenase-2 expression by tracheobronchial smooth muscle cells

Lipid mediators of inflammation, contribute to airway hyper-reactivity in asthma. Since production of lipid mediators is largely regulated by phospholipase A2 (PLA2), and since PLA2 expression in mesenchymal cells is induced by cytokines and other signals, we examined PLA2 expression by rat tracheobronchial smooth muscle cells (TBSMC). PLA2 expression in TBSMC cultures was markedly increased by tumour-necrosis factor (TNF) alpha (130-fold) and interleukin-1beta (IL-1beta) (7.4-fold). Lipopolysaccharide (LPS;100 ng/ml) resulted in a 51-fold increase in extracellular PLA2 activity. PLA2 expression by LPS-stimulated or cytokine-stimulated cells was downregulated by dexamethasone. Whereas forskolin or dibutyrl cAMP increased PLA2 activity, inhibition of protein kinase A but not tyrosine kinase reduced PLA2 expression. Northern blot analysis showed that TNF alpha and IL-1beta increased both PLA2 and inducible cyclooxygenase (Cox-2) mRNA transcription. Addition of dexamethasone substantially blunted the increase in PLA2 and Cox-2 mRNA. In contrast, the level of Cox-1 mRNA was very low and did not change with the various treatments. Since proinflammatory lipid mediators have been implicated in the pathogenesis of asthma and PLA2 activity regulates generation of these lipid mediators, cytokine-stimulated synthesis and release of PLA2 by airway smooth cells may contribute to the potentiation of airway inflammation in asthma.

Usual dietary salt intake and asthma in children: a case-control study

BACKGROUND

A decline in host resistance due to an alteration in diet--primarily of salt--was recently put forward as a possible explanation for rising rates of asthma.

METHODS

A case-control study was conducted in participants in a prevalence survey which included 187 children with asthma (defined by prior diagnosis and/or a decline in forced expiratory volume in one second (FEV1) of > or = 10% after exercise) and 145 age and sex matched controls. Subjects were selected from 989 children aged 5-13 years attending 18 elementary schools on the island of Montreal. Usual dietary salt intake was estimated from a food frequency questionnaire administered to the mother, and a salt intake score was used to group the children into quartiles from I (lowest) to IV (highest salt intake). Bronchial hyperresponsiveness to methacholine was assessed by Yan's method. Cases and controls were combined in one group to examine the relationship of salt intake to bronchial hyperresponsiveness to methacholine. Methacholine responsiveness was expressed as a dose-response slope and ranks of dose-response slopes were used in the analysis.

RESULTS

After accounting for important confounding variables, there was no association between asthma and salt intake, while methacholine dose-response slope ranks increased with increasing salt intake and methacholine responsiveness was greater in the highest quartile than in the lowest quartile of salt intake. The median dose-response slopes in % fall in FEV1 per mumol methacholine for quartiles I, II, III, and IV were 5.4, 5.9, 7.7, and 8.7.

CONCLUSIONS

No association was found between asthma or exercise-induced bronchospasm and dietary salt intake. Bronchial hyperresponsiveness to methacholine did, however, appear to increase with greater salt intake, but the relevance of this association to asthma is unclear.

Magnesium antagonizes the actions of lysophosphatidyl choline (LPC) in myocardial cells: a possible mechanism for its antiarrhythmic effects

Patients with cardiac arrhythmias, ischemia, and infarction may benefit from administration of supplemental magnesium. However, the exact mechanisms for magnesium's beneficial effects remain unknown. Lysophosphatidyl choline (LPC), an amphipathic phospholipid released from cardiac cell membranes during ischemia, increases free intracellular calcium concentrations ([Ca]i) and has been implicated as a cause of cardiac arrhythmias and coronary artery spasm during myocardial ischemia. We postulated that magnesium acts by inhibiting cellular calcium overload induced by mediators such as LPC. Myocardial cells from male Sprague-Dawley rats were isolated from ventricular tissue samples and [Ca]i determined using the fluorescent dye, fura-2/acetoxymethyl ester, measured in a spectrofluorometer. The increase in [Ca]i after exposure to 100 and 200 microM LPC was recorded in cells suspended in modified Dulbecco's phosphate buffered saline solution with 0.2, 2.0, and 20 mM magnesium chloride. Differences were determined by analysis of variance with P < 0.05 considered significant. LPC significantly increased [Ca]i in the 100 microM (506 +/- 76 nM) and 200 microM (675 +/- 81 nM) concentrations, compared to baseline (301 +/- 25 nM). MgCl2 at both the 2.0 and 20 mM concentrations significantly blunted the increase in [Ca]i in myocardial cells exposed to LPC, whereas 0.2 mM MgCl2 was ineffective. LPC is a potent lipid mediator which increases myocyte [Ca]i in a concentration-dependent manner. Magnesium concentrations > or = 2.0 mM effectively antagonize the increase in [Ca]i induced by LPC. Thus, magnesium may limit intracellular calcium overload stimulated by ischemic-induced LPC release.

The pharmacology of airway hyperresponsiveness and inflammation

Hyperresponsiveness is a key abnormality in asthma and deserves recognition both in the clinical presentation of asthma and its pathogenesis. Available evidence strongly suggests that inflammatory processes are important in producing this aspect of disordered airway physiology. Pharmacologic studies have greatly increased our knowledge of mechanisms involved in hyperresponsiveness but fall short of a complete explanation of the phenomenon. It is most likely that hyperresponsiveness, as measured in the laboratory, has many different components and that any single mechanism will be an insufficient explanation.

Oxygen radicals in lung pathology

Pulmonary tissue can be damaged in different ways, for instance by xenobiotics (paraquat, butylated hydroxytoluene, bleomycin), during inflammation, ischemia reperfusion, or exposure to mineral dust or to normobaric pure oxygen levels. Reactive oxygen species are partly responsible for the observed pulmonary tissue damage. Several mechanisms leading to toxicity are described in this review. The reactive oxygen species induce bronchoconstriction, elevate mucus secretion, and cause microvascular leakage, which leads to edema formation. Reactive oxygen species even induce an autonomic imbalance between muscarinic receptor-mediated contraction and the beta-adrenergic-mediated relaxation of the pulmonary smooth muscle. Vitamin E and selenium have a regulatory role in this balance between these two receptor responses. The autonomic imbalance might be involved in the development of bronchial hyperresponsiveness, occurring in lung inflammation. Finally, several antioxidants are discussed which may be beneficial as therapeutics in several lung diseases.

Non-specific hyperreactivity to pharmacological stimuli in tracheal and lung parenchymal strips of actively sensitized guinea-pigs

The responsiveness of tracheal and lung parenchymal strips isolated from actively sensitized guinea-pigs to CaCl2 (in K+-depolarized tissue), KCl, acetylcholine and histamine was compared with that of strips from unsensitized animals. The concentration-response curves to the mentioned agonists exhibited, in the sensitized group, a left upward displacement (greater maximal effect, lesser effective concentration 50% and a steeper slope) compared with those obtained in the unsensitized group. These results indicate the existence of a non-specific increase in responsiveness in the airway smooth muscle from sensitized animals.

Modification by indomethacin of airway contractile responses in normal and sensitized guinea-pigs

Active sensitization of guinea-pigs resulted in an increase in responsiveness and sensitivity of tracheal and lung parenchymal strips to CaCl2 (in K+-depolarised tissue), KCl, acetylcholine and histamine. Indomethacin (5 microM) preferentially enhanced the response of tracheal strips from normal animals to histamine and to a lesser extent acetylcholine but not to CaCl2 or KCl. A similar trend was observed in sensitized tissues. Indomethacin pretreatment did not cause changes in responsiveness or sensitivity of lung parenchymal strips from normal or sensitized guinea-pigs to the agonists tested. It is concluded that immunological sensitization produced a non-specific hyperresponsiveness in trachea and lung parenchymal strips. Conversely, cyclooxygenase inhibition by indomethacin elicited a selective increase in the responsiveness to certain agonists in central but not in the peripheral airways.

The spasmogenic effect of caffeine in trachealis isolated from control and actively sensitized guinea-pigs

The spasmogenic activity of caffeine (10 mM) was evaluated in tracheal strips obtained from control and sensitized guinea-pigs then pretreated with indomethacin (2.8 microM) and cooled to 20 degrees C. The contraction elicited by caffeine was inhibited by verapamil (100 microM), trifluoperazine (100 and 500 microM) and dantrolene (50 and 500 microM) in the control and the sensitized tissues but was unaffected by disodium cromoglycate (39 microM). However, the same concentration of verapamil produced significantly less inhibition of the caffeine-induced contraction in sensitized compared to control tissues while the reverse was found for trifluoperazine and dantrolene. Exposure to a Ca2+-free, EGTA-containing medium resulted in 33% inhibition of the response to caffeine in control tissues but no inhibition in sensitized tissues. These results suggest the existence of differences in calcium movements in response to caffeine between control and sensitized tissues that may reflect abnormalities in calcium handling by the sensitized tissue.

Metabolic basis of asthma. A united hypothesis

Asthma is a heterogenous disease triggered by a large number of different stimuli. This article presents a theory of the metabolic mechanisms of asthma. The theory is based on the growing understanding of the activity of lysophosphatidylcholine (LPC). Since the effect of LPC on cell membranes, membrane bound enzymes and the various types of cells involved in the pathogenesis of asthma, this may represent a unifying link between the various types of asthma.

A diet rich in sodium may potentiate asthma. Epidemiologic evidence for a new hypothesis

The geographic variation in asthma prevalence and mortality has not been explained adequately. The marked increase in prevalence in communities adopting a more Western lifestyle suggested that some of this variation could be due to changes in sodium intake. The theory was tested using regional data from England and Wales and a strong correlation was found between table salt purchases and asthma mortality in both men and children. Asthma mortality in women was more closely related to prevalence of cigarette smoking. The association was not explained by differences in age, sex or prosperity.

Airway responses to inhaled ouabain in subjects with and without asthma

Challenges with ouabain and histamine were performed a week apart in 10 patients with asthma and 5 normal subjects. Concentrations were increased cumulatively until specific airway conductance decreased by 30% or the maximal concentration of 1.0% was reached. At low concentrations, ouabain induced bronchodilatation in six patients who had asthma. Bronchodilatation gradually decreased with increasing concentrations and was followed by bronchoconstriction in two patients with asthma who had high airway sensitivity to histamine. Ouabain caused only bronchoconstriction in three patients with severe asthma. The normal subjects showed mild bronchodilatation or no response to ouabain. Several possible biochemical mechanisms may be responsible for the bronchodilatory response to low doses of ouabain, such as stimulation of adenylate cyclase or (Na+,K+)-adenosine triphosphatase. The absence of a bronchodilatory response to ouabain in patients with severe asthma suggests an impairment in the activity of these enzymes.

Lysophosphatidylcholine increases airway and capillary permeability in the isolated perfused rat lung

The effects of lysophosphatidylcholine (lysoPC) on airway and capillary permeability in the isolated perfused rat lung were investigated. We determined the influence of lysoPC on the passage of different-sized (326-722 dalton) polyethylene glycols (PEGs), both from the airways to the pulmonary circulation and from the pulmonary circulation into the lung. We found that 1 mM lysoPC increased the overall passage of PEGs from the airways to the pulmonary circulation, and that 80 microM lysoPC increased the overall passage from the circulation into the lung. In both cases, the passage of the larger (502-722 dalton) PEGs increased more than the passage of the smaller (326-458 dalton) PEGs. We also found that the presence of lysoPC in the circulation increased the pulmonary arterial pressure, whereas deposition of lysoPC in the trachea did not. The pressure increase was blocked by indomethacine, BW755C, and quinacrine, inhibitors of arachidonic acid metabolism. These findings suggest that higher concentrations of lysoPC increase the airway permeability to larger molecules, and that lower concentrations of lysoPC increase the capillary permeability. The increase in capillary permeability may be due to a rise in capillary pressure mediated by arachidonic acid metabolites. The possibility that formation and accumulation of lysoPC is of importance for mediating inflammatory reactions in the lung is inferred.

Reactive airways dysfunction syndrome (RADS). Persistent asthma syndrome after high level irritant exposures

Ten individuals developed an asthma-like illness after a single exposure to high levels of an irritating vapor, fume, or smoke. In most instances, the high level exposure was the result of an accident occurring in the workplace or a situation where there was poor ventilation and limited air exchange in the area. In all cases, symptoms developed within a few hours and often minutes after exposure. We have designated the illness as reactive airway dysfunction syndrome (RADS) because a consistent physiologic accompaniment was airways hyperreactivity. When tested, all subjects showed positive methacholine challenge tests. No documented preexisting respiratory illness was identified nor did subjects relate past respiratory complaints. In two subjects, atopy was documented, but in all others, no evidence of allergy was identified. In the majority of the cases, there was persistence of respiratory symptoms and continuation of airways hyperreactivity for more than one year and often several years after the incident. The incriminated etiologic agent varied, but all shared a common characteristic of being irritant in nature. In two cases, bronchial biopsy specimens were available, and an airways inflammatory response was noted. This investigation suggests acute high level, uncontrolled irritant exposures may cause an asthma-like syndrome in some individuals which is different from typical occupational asthma. It can lead to long-term sequelae and chronic airways disease. Nonimmunologic mechanisms seem operative in the pathogenesis of this syndrome.