Mucus Viscosity, Increased leads to MCC, Decreased

Mucus viscoelastic properties, whether altered by airway dehydration or mucus hypersecretion, directly influence MCC. In fact, there is an inverse relationship between mucus viscosity and CBF and mucus transport/MCC, as demonstrated in several in vivo and ex vivo studies. A large proportion of these studies have employed (bio)polymers or other large organic molecules to mimic the mucus layer in the airways and the increase in its viscosity. In addition, some of these studies have shown that decreased mucus viscosity may also result in impairment of MCC. Therefore, a causal link is only tentatively supported. Because cilia function, ASL height, and mucus properties are intricately linked to each other as evidenced by cystic fibrosis studies, we consider the biological plausibility of this KER moderate.

Exposure of primary cultures of hamster oviductal ciliated cells to increased viscous loading reduced the CBF, reaching a new stable value within the first 10 min (Fig. 1 a).The CBF dropped 35% within the range of 2–37 cP (2–15% dextran solutions), but no further decrease was observed at higher viscosities in the range of 37–200 cP (15–30% dextran solutions) (Andrade et al., 2005).

In tracheal and bronchial sections of mice, the CBF decreased with increasing viscosity up to about 32.0 mPa s, while it was nearly constant above 32.0 mPa s. The amplitude of the ciliary beat was kept at ~1.5 lm regardless of the viscosity (Kikuchi et al., 2017).

In control cultures of human nasopharyngeal pediatric airway ciliated cells, a decrease in CBF was observed immediately after the viscosity of the medium was increased, with a greater decrease in CBF in cultures exposed to 20% dextran (González et al., 2016).

CBF in bronchial biopsies from patients undergoing bronchoscopy for diagnostic purposes was inhibited by increased viscosity of the polyvinylpyrrolidone-supplemented medium (Luk and Dulfano, 1983).

In cultured ciliary cells from the frog esophagus photoelectric measurements were performed in the viscosity range of 1–2000 cP. In solutions of viscosity >30 cP. the decrease in CBF was more pronounced but the general trend of a relatively moderate decrease in the frequency at higher viscosity applies for the whole viscosity range (Gheber et al., 1998).

Sputum from chronic bronchitis patients placed on mucus-depleted frog palates moved slower if their Newtonian viscosity exceeded 1000-3000 P (Chen TM and Dulfano, 1978).

In human cystic fibrosis airway epithelial cultures grown at the air-liquid interface, ASL height was lower than that in non-cystic fibrosis cultures thereby increasing mucus viscosity, and mucus velocity was lower than that in non-cystic fibrosis cultures (Matsui et al., 1998).

In tracheas of 6-month old Cftr-deficient rats, mucus viscosity was higher than that of their wild-type littermates and mucus transport was significantly slowed down (Birket S. E. et al., 2018).

Polycarbophil and Carbopol 1342 polymers of increasing viscosity caused ever greater reductions MCT rates [cm/min] across bovine trachea (Shah and Donovan, 2007a). A similar observation was made for the transport of anionic polysaccharide gels of increasing viscosity and linear polyanionic samples across the frog palate (Lin et al., 1993; Shah and Donovan, 2007b).

In a study on children with cystic fibrosis, the mucus dry weights had a borderline significant inverse correlation with the average percent clearance through the first 60 minutes (MCC60) (Laube et al., 2020).

Mucus from patients with chronic bronchial inflammatory diseases treated with 600 mg/day of Sobrerol (decreases viscosity of sputum) traveled faster the pre-established distance on frog palate  (‘reference’ speed of MCT) than from mucus from untreated patients (Allegra et al., 1981).

Acetylcysteine reduced the viscosity of human tracheobronchial secretions in vitro (Sheffner et al., 1964). In non-smokers, treatment with 0.6 g N-acetylcysteine/day po. for 60 days increased mucociliary clearance rates that returned to baseline values after a washout period (Todisco et al., 1985).

When hypertonic saline is added to mucus/sputum, its viscosity is markedly reduced (Elkins and Bye, 2011), and this greatly enhances its transportability in bovine trachea (King et al., 1997; Wills and Cole, 1995; Wills et al., 1998). Inhalation of hypertonic saline solution increased MCC in asthmatic and healthy subjects (Daviskas et al., 1996), adult patients with cystic fibrosis (Robinson et al., 1997; Robinson et al., 1996) and improved MCT times in patients with acute/chronic sinusitis and those having undergone sinus surgery (Talbot et al., 1997).

Inhaled mannitol reduced the viscosity of the sputum in asthmatics with mucus hypersecretion (Daviskas et al., 2007). Inhaled mannitol also improved MCC in asthmatics and patients with cystic fibrosis and bronchiectasis (Daviskas et al., 1997; Daviskas et al., 2008; Daviskas et al., 2010).

Poly(acetyl, arginyl) glucosamine (PAAG) improved the viscoelasticity of sputum. The treatment of cystic fibrosis bronchial epithelial cell culture monolayers with sputum samples that were treated with PAAG had MCT rates significantly faster than those treated with PBS (Fernandez-Petty et al., 2019).

A three times daily dose of 16 mg bromhexine, which modifies the physicochemical characteristics of mucus/is a mucolytic agent (Zanasi et al., 2017), for 14 days resulted in improved MCC in subjects with chronic bronchitis (Thomson et al., 1974).

Studies interrogating the link between CBF and/or mucus viscosity and MCC found the optimal range of viscoelastic mucus properties to be between 10 and 30 cP and 11 to 25 dyn/cm2 (Chen and Dulfano, 1978; King , 1979; King, 2006; King et al., 1997). These studies also documented that both increases and decreases in mucus viscosity beyond that optimal range impact CBF and decrease and increases, respectively, MCC. A large proportion of these studies utilize (bio)polymers or other large organic molecules to mimic the mucus layer in the airways and increases in its viscosity. Therefore, there may be limitations to the translatability of these findings.  There is at least one study showing that increased mucus viscosity not only slows CBF, but also alters cilia beat metachrony, with medium viscosities in the range of 30–1500 cP increasing metachronal wave velocities by up to 50% and changes in wave direction in cultured frog esophagus (Gheber et al., 1998; Stafanger et al., 1987). CBF also appears to be, at least in part, autoregulated by ciliated respiratory cells, which adjust cilia beating to differences in viscous load via a mechanosensory mechanism (Johnson et al., 1991).

In tracheas of 6-month old Cftr-deficient rats, mucus was 20-fold more viscous than that of their wild-type littermates (2.91 ± 0.9 cP WT vs. 65.09 ± 3.6 cP KO) and mucus transport rate was significantly slowed down (ca. 0.8 mm/min WT vs ca. 0.3 mm/min KO). Following addition of sodium bicarbonate, which is known to improve airway hydration and hence decrease mucus viscosity, at concentrations of 23 mM, 69 mM, 92 mM, and 115 mM to the apical surface of Cftr-deficient rat tracheas ex vivo (6-month old animals) increased MCT rates in a dose-dependent manner, up to ca. 2.5 mm/min (Birket et al., 2018).

Sputum dry weights more than 151.0 mg/mL were linked with 6.8 (3.3‐17.7) median percent MCC60, and sputum dry weights less than 151.0 mg/mL were linked with 13.2 (6.2‐28.6) median percent MCC60. Percent MCC60 was borderline significantly higher in children with dry weights less than 151.0 mg/mL than in children with dry weights more than 151.0 mg/mL (Laube et al., 2020).

Adult CF patients receiving hypertonic saline via Omron-NE-U06 ultrasonic nebulizer exhibited increased MCC as evidenced by increased Tc particle clearance rates. The amount cleared at 90 min on the control day was 12.7% (95% confidence interval (CI) 9.8 to 17.2) compared with 19.7% (95% CI 13.6 to 29.5) for 3% hypertonic saline, 23.8% (95% CI 15.9 to 36.7) for 7% hypertonic saline and 26.0% (95% CI 19.8 to 35.9) for 12% hypertonic saline (Robinson et al., 1997).

Treatment with 100-500 μg/mL poly(acetyl, arginyl) glucosamine (PAAG) for 2 h significantly reduced the dynamic viscosity of CF sputum at low shear rates. Effective viscosities measured at a shear rate of 0.8 s^–1 indicated that the treatment effect was particularly prominent in CF sputum samples that exhibited high dynamic viscosity at baseline (359 ± 561 Pa•s for sputum treated with PBS compared with 62 ± 97 Pa•s for sputum treated with PAAG). When PAAG-treated (250 μg/mL) sputum was added to trachea sputum, it was more rapidly transported in a homogenous fashion than untreated sputum (3.91 ± 1.89 mm/min PAAG vs 1.62 ± 0.56 mm/min PBS) (Fernandez-Petty et al., 2019).

In bronchial epithelial cultures grown at the air-liquid interface, treatment with between 250 and 500 μg/mL PAAG caused a 2-log reduction in effective viscosity across all frequency ranges (597.0 ± 56.1 cP for PBS control versus 2.84 ± 0.11 cP PAAG at 1.0 Hz) and a 57% increase in MCT rate in cells treated with PAAG (250 μg/mL) compared with the PBS control (Fernandez-Petty et al., 2019).

Using Cftr^–/– rats aged 6 months in which delayed MCT occurs due to abnormally viscous mucus (Birket et al., 2018), PAAG (250 μg/mL × 20 mL over 45 minutes) or glycerol vehicle control was nebulized once daily for 14 days. Mucus transport increased approx. 3.5-fold in PAAG-treated rats compared to vehicle-treated rats, achieving approximately 44% of MCT rates in normal rats (Fernandez-Petty et al., 2019).

At 24 h following environmental challenge (i.e., stabling in stalls with straw as bedding and hay as feed) of horses with recurrent airway obstruction, the viscoelasticity of mucus increased ca. 3-fold (to log G* averaging 3.0 ± 0.3 dyn/cm² vs 2.38 ± 0.11 dyn/cm² in controls) and mucociliary clearance index (MCI) decreased to 0.69 ± 0.05 (vs 0.92 ± 0.06 in controls) (Gerber et al., 2000).

In dogs receiving a high dose of methacholine chloride (16-32 mg/mL) acutely, mucus viscosity increased by 203 ± 23% from control, while mucus transport rate on frog palates decreased to 79 ± 6% of control (King, 1979).  

Within the first 10 min following addition of 2–15% dextran, the CBF of human oviductal cells dropped ~35% within the range of 2–37 cP. No further decrease was observed at higher viscosities (15–30% dextran solutions) in the range of 37–200 cP (Andrade et al., 2005).

Following addition of sodium bicarbonate, which is known to improve airway hydration and hence decrease mucus viscosity, at concentrations of 23 mM, 69 mM, 92 mM, and 115 mM to the apical surface of Cftr-deficient rat tracheas ex vivo (6-month old animals) increased MCT rates in a dose-dependent manner, up to ca. 2.5 mm/min. The peak effect of bicarbonate addition occurred at 20 minutes after addition and returned to baseline by 35 minutes after addition, consistent with the short half-life of bicarbonate at the surface of the airway (Birket et al., 2018).

At 24 h following environmental challenge (i.e., stabling in stalls with straw as bedding and hay as feed) of horses with recurrent airway obstruction, the viscoelasticity of mucus increased ca. 3-fold (to log G* averaging 3.0 ± 0.3 dyn/cm2 vs 2.38 ± 0.11 dyn/cm2 in controls) and mucociliary clearance index (MCI) decreased to 0.69 ± 0.05 (vs 0.92 ± 0.06 in controls). Significant changes in mucus viscoelasticity and MCI were only observed at 24 and 48 h, but not at 6 h post-challenge (Gerber et al., 2000).

In dogs receiving a high dose of methacholine chloride (16-32 mg/mL) acutely, mucus viscosity increased, while mucus transport rate on frog palates decreased at approx. 2 to 5 min post-treatment (King, 1979).

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