Mucus dehydration may contribute to chronic bronchitis

By Eleanor McDermid, Senior medwireNews Reporter

Mucus dehydration may lead to reduced mucociliary clearance and symptoms of chronic bronchitis, research suggests.

Wayne Anderson and colleagues from the University of North Carolina at Chapel Hill, USA, found that mucus concentration was increased in patients with chronic bronchitis compared with in healthy controls, and this correlated with disease severity.

“These data suggest airway mucus concentration could serve as a biomarker to complement the symptom-based diagnosis of [chronic bronchitis]”, they write in the American Journal of Respiratory and Critical Care Medicine.

The 68 chronic bronchitis patients were aged an average of 59 years and had a smoking history of at least 20 pack–years; 44% were current smokers.

The average % solids of bronchoscopically obtained mucus was increased about threefold in these patients, compared with in 29 nonsmoking healthy controls and in 34 smokers with no respiratory symptoms. The same was true for induced sputum, which the team notes would make a more practical diagnostic test.

In line with this, bronchitis patients had significantly reduced levels of ATP and ADO – the active nucleotides/nucleosides that regulate hydration of airway surfaces.

The mucus of bronchitis patients had significant increases in partial osmotic pressures, in association with rising mucus concentration. When mucus concentration rose to about 3%, the partial osmotic pressure rose to a point that the researchers predicted would slow mucociliary clearance.

Indeed, Anderson and team found mucus concentration to be inversely associated with mucociliary clearance both in cultured human airway epithelial cells and on gamma scintigraphy in chronic bronchitis patients.

They stress that the similar relationship between mucus hydration and clearance in cultured cells and in the patients suggests that other factors present in the patients’ sputum, such as cells and proteases, did not notably affect the association between mucus concentration and partial osmotic pressure.

“Thus, we conclude that increased mucus layer partial osmotic pressure, via [periciliary] osmotic compression, provides a key link between sputum % solids and reduced [mucociliary clearance]”, say the researchers.

They also found that both increased mucus concentration and slower mucociliary clearance were associated with more severe disease, ie, lower forced expiratory volume in 1 second (FEV1) % predicted.

“An unresolved question from our study is whether increased mucus concentration (% solids), reflecting in part cigarette smoke induced effects on hydration and/or mucin hypersecretion, drives the progression of disease or results from disease progression”, says the team.

They say that therapies directly targeting mucus concentration may be needed to further define the role of mucus concentration in chronic obstructive pulmonary disease pathogenesis.

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