Thirst and the influence of ionic concentration; an investigation into the effect on salivary lubrication and the role of MUC5B

Saliva plays a crucial role in lubricating the oral cavity and maintaining a normal hydrated mouthfeel, with the main macromolecule in saliva, MUC5B, playing a significant role in its lubricating properties. Mucins form a hydrated biogel that covers the oral epithelium and forms part of the mucosal pellicle, maintaining proper hydration. This study investigates the relationship between ionic concentration, saliva lubrication, and the perception of oral dryness. At baseline, sodium was found to be ten times more concentrated in the mucosal pellicle compared to bulk saliva, in contrast to potassium which was only 1.5 times more concentrated in the pellicle. Increasing concentrations of mouthwashes containing sodium or potassium indicated a preferential sequestering of sodium in the mucosal pellicle with sodium chloride rinses being associated with an increased thirst perception. We therefore propose that sodium binds to MUC5B, reducing the electrostatic repulsions between its negatively charged moieties. This interaction subsequently leads to a compaction of the mucin structure, which decreases the hydration of MUC5B and reduces its lubricating properties. We determined that increased sodium concentrations compact the mucin structure as indicated by transmission electron microscopy. However, we found that increased sodium concentrations were associated with improved the lubrication of salivary and MUC5B films in an oral tribology mimic, decreasing the coefficient of friction from 0.044 to 0.018, and 0.416 to 0.100 with the addition of 200 mM sodium chloride for saliva and MUC5B, respectively. MUC5B was also demonstrated to become more hydrated with increasing ionic concentrations, as indicated by Quartz Crystal Microbalance with Dissipation. We therefore propose that increased hydration shells surrounding the sodium counterions are responsible for this improved lubrication via the hydration lubrication mechanism. Lastly, it was observed that the spinnbarkeit of saliva decreased with increasing ionic concentrations, indicating a reduced stretchiness in its rheological properties. However, the concentration of MUC5B alone did not appear to affect spinnbarkeit, highlighting that many factors influence the biophysical properties of saliva.

Characterisation

Tribology was used to determine the coefficient of friction between 2 p.m. surfaces with a Young’s modulus of 1.84 MPa at 23 °C. Indentation and surface roughness measurements were determined as previously described. The elastic modulus of the PDMS was measured using an indention test with a Mach 1 rig (Biomomentum Inc., Laval, Canada). The roughness of the PDMS was measured at three different locations on the surface, using a WYKO NT9100 optical profilometer (Veeco, New York, USA). Quartz crystal microbalance with dissipation was used to determine the adsorption and film properties of mucin with increasing ionic concentrations using PDMS spin-coated QSensor QSX 301 gold-coated quartz crystals sensors (Biolin Scientific). To achieve this a prepolymer and cross-linker SYGLARD® 184 (DowCorning) were mixed in a ratio of 10:1 and diluted in n-hexane to 1 % (v/v). The sensor was coated using a WS-400B-6NPP/LITE Spin Processor (Laurell, Pennsylvania, USA) at 3000 rpm for 1 min and curing 80  °C for 4 hours. The thickness of the PDMS layer is estimated to be approximately 3 μm. Transmission electron microscopy was used to determine the structural conformation of MUC5B adsorbed to a carbon surface in high and no salt conditions using a JEM1400Flash microscope (JEOL Ltd, Hertfordshire, UK) at 20k magnification operating at a voltage of 120 kV.

Mucin; Tribology; MUC5B; Saliva; Dry mouth