A new mechanism for the perception of thirst

Objectives: Thirst is a major driver to drink and is critical to maintaining homeostasis. The perception of thirst is often associated with a dry mouth but it is unclear if any changes occur in the mouth to cause dry mouth. Previously we have shown no change in salivary flow rate occurs with the onset of thirst even in exercise-induced dehydration. In contrast, salivary osmolality closely correlated with thirst and is mostly reflects salivary levels of sodium, potassium and urea. In this study we examined changes in the mucosal pellicle - - the layer of salivary mucins adhered to the mucosa.

Methods: Healthy subjects were assessed for thirst perception using a labelled magnitude scale, salivary flow rates by weight, sodium and potassium levels were measure by ICP-MS, urea by colourmetric assay. Samples of mucosal pellicle were collected using sterile filter paper applied to the mucosa for set time (10 seconds). Filter papers were placed into a tube, 100 ul of water added and eluted by centrifugation (1000g x 3 mins).

Results: Sodium ions, in contrast to potassium or urea, accumulated in the mucosal pellicle. Resting saliva contains low levels of sodium (5-10 mM), which were much higher in the mucosal pellicle (90 mM). In contrast, potassium ions were at a similar level in saliva (50 mM) and the pellicle (65 mM). After a mouthrinse of water the concentrations of urea and potassium decreased whereas sodium remained unchanged. After a mouthrinse of sodium chloride, sodium ions increased in the pellicle.

Conclusions: The mucosal pellicle appears capable of specifically binding and concentrating sodium ions. This is likely to affect the rheological properties of the adhered salivary mucins which would affect the mouthfeel, leading to the perception of dryness or thirst.