The Relationship of Mucus Concentration (Hydration) to Mucus Osmotic Pressure and Transport in Chronic Bronchitis

Abstract

Rationale: Chronic bronchitis (CB) is characterized by persistent cough and sputum production. Studies were performed to test whether mucus hyperconcentration and increased partial osmotic pressure, in part caused by abnormal purine nucleotide regulation of ion transport, contribute to the pathogenesis of CB.

Objectives: We tested the hypothesis that CB is characterized by mucus hyperconcentration, increased mucus partial osmotic pressures, and reduced mucus clearance.

Methods: We measured in subjects with CB as compared with normal and asymptomatic smoking control subjects indices of mucus concentration (hydration; i.e., percentage solids) and sputum adenine nucleotide/nucleoside concentrations. In addition, sputum partial osmotic pressures and mucus transport rates were measured in subjects with CB.

Measurements and results: CB secretions were hyperconcentrated as indexed by an increase in percentage solids and total mucins, in part reflecting decreased extracellular nucleotide/nucleoside concentrations. CB mucus generated concentration-dependent increases in partial osmotic pressures into ranges predicted to reduce mucus transport. Mucociliary clearance (MCC) in subjects with CB was negatively correlated with mucus concentration (percentage solids). As a test of relationships between mucus concentration and disease, mucus concentrations and MCC were compared with FEV1, and both were significantly correlated.

Conclusions: Abnormal regulation of airway surface hydration may slow MCC in CB and contribute to disease pathogenesis.

Keywords: COPD; mucociliary clearance; mucus hyperconcentration.

Figure 1.

Mucus hydration and mucin concentrations in normal subjects (NS), normal smokers (CS-N), and subjects with chronic bronchitis (CB). (A) Percentage solids measured in mucus samples obtained by bronchoscopy. The data are presented as means ± SE for each of the three subject groups: NS (n = 24), CS-N (n = 22), and CB (n = 43). *CB versus NS and versus CS-N, P < 0.0001. (B) Sputum percentage solids as means ± SE NS (n = 29), CS-N (n = 29), and CB (n = 65); and total sputum mucin concentration as means ± SE NS (n = 15), CS-N (n = 15), and CB (n = 42). *The sputum percentage solids of the CB group were significantly higher than the NS (P = 0.006) and the CS-N (P = 0.0007) groups. ⁺Similarly, the total mucin concentration was greater in the CB group versus the CS-N group (P = 0.032) and trended higher than the NS group (P = 0.15).

Figure 2.

Sputum nucleotide/nucleoside concentrations. (A) Sputum adenyl nucleotide/nucleoside concentrations (nM) are plotted as means ± SE for the normal subjects (NS) (n = 21), normal smokers (CS-N) (n = 21), and chronic bronchitis (CB) (n = 45) groups. Comparisons for all pairs (Tukey-Kramer honestly significant difference) for ATP: *NS versus CS-N, P = 0.002; NS versus CB, P < 0.0001; ADP, no significant differences; AMP, no significant differences; ADO, ^#CS-N versus CB, P = 0.054. (B) Sum of ATP plus ADO concentrations presented as means ± SE for NS (n = 21), CS-N (n = 21), and CB (n = 45) groups. *The ATP/ADO sum (nM) in the CB group was significantly lower than the NS (P = 0.0006) and CS-N (P = 0.02) groups. There were no significant differences between the control groups. (C) Sputum Ecto-ATPase Activity (percentage hydrolysis): data are shown as means ± SE for the NS (n = 12), CS-N (n = 10), and CB (n = 17) groups. *Percentage hydrolysis was significantly greater in the CB group versus the NS (P = 0.005) and the CS-N (P = 0.006) groups. ADO = adenosine; ADP = adenosine diphosphate; AMP = adenosine monophosphate.

Figure 3.

Sputum percentage solids and partial osmotic pressure in subjects with chronic bronchitis and the relationship of percentage solids to mucus transport rates in vitro. (A) Correlation between percentage solids and partial osmotic pressure. Osmotic pressure was measured from a subset of the subjects on sputum samples collected at multiple visits, squares (n = 6; 34 samples). Solid line is Spearman correlation, ρ = 0.690, P < 0.0001. Dashed line is the osmotic pressure of the periciliary layer, and the dotted line is the relationship between percentage solids and mucus produced by airway epithelial cells in culture (16). The single open diamond represents the mean percentage solids ± SD (1.15 ± 0.3) and mean ± SD partial osmotic pressure (74.6 ± 24.3) for nine normal subjects reported previously (20). (B) Relationship between mucus concentration and mucociliary transport in vitro. Data are presented as means ± SEM, n = 4–6 per group.

Figure 4.

Relationship between mucus percentage solids, mucus clearance, and FEV₁ in subjects with chronic bronchitis (CB). (A) Relationship between percentage solids (bronchoscopy) and average mucociliary clearance in the central lung region of subjects with CB (n = 43). The Spearman correlation is ρ = −0.387, P = 0.01. A simple linear regression line is added for visualization. (B) Relationship between FEV₁% predicted and central lung TB average clearance for the subjects with CB. Spearman correlation coefficient, ρ = −0.338, P = 0.005. A simple linear regression line is included for visualization (n = 67). (C) Correlation of FEV₁% predicted and percentage solids (bronchoscopic sample). FEV₁% predicted is plotted against the percentage solids determined from the bronchoscopic samples for the subjects with CB (n = 43). ρ = −0.708; P < 0.0001. A simple linear regression line is included for visualization. TB = tracheobronchial.

Figure 5.

Schema depicting relationships between mucus percentage solids, partial osmotic pressures of the mucus layer (π_mucus), the periciliary layer (PCL) in the basal state (bπ_PCL), and mucus clearance rates. At normal mucus hydration (∼2% solids), the partial osmotic pressure of the mucus layer is below basal PCL values (bπ_PCL), the PCL is fully hydrated, and mucus transport is efficient. At modest levels of mucus dehydration (i.e., percentage solids ∼3–4%) the osmotic pressure of the mucus layer slightly exceeds the basal PCL level, modest compression of the PCL results, and mucus transport slows. When mucus dehydration is severe (i.e., percentage solids ∼7–8%) the mucus layer osmotically compresses and/or traps the cilia, producing mucus stasis and adhesion.