The effects of hyper- and hypo-osmolarity on saliva secretion

Objectives: The salivary gland is water-handling tissue that is very sensitive to both hyper- and hypo-osmotic circumstances. The effects of osmotic challenge on fluid secretion and intracellular Ca²⁺ signaling were explored. Methods: An ex vivo mouse submandibular gland perfusion technique was used and the osmolality of the perfusion solution was changed from 50% hypotonic to hypertonic with up to 100 mM sucrose added. Then, the gland was stimulated with a muscarinic agonist, 0.3 μM carbachol, for 10 minutes at 37°C, and the secreted saliva was analyzed. For intracellular Ca²⁺ signaling, dispersed cells were prepared and the Fura2 fluorescence technique was used. The glands and cells were acclimated to the osmotic conditions for at least 5 minutes before stimulation. Student's t-test was used for the statistical analysis. Results: Fluid secretion was increased under hypotonic conditions with the maximum at 30% hypotonic, and fluid secretion was decreased markedly under hypertonic conditions with 30 mM (40% decrease) and 100 mM (70% decrease) sucrose added. The Ca²⁺ response was almost identical under hypotonic conditions, but decreased significantly under hypertonic conditions. Ion analysis of the secreted saliva revealed almost identical changes in Na⁺ and Cl^-, and the maximum release was observed under 30% hypotonic conditions. By contrast, the K⁺ release showed unique kinetics; the total release was the same for isotonic and 30% hypotonic conditions, and decreased slightly under hypertonic conditions. Interestingly, the Na⁺ and Cl^- concentrations in the secreted saliva were the highest with the isotonic solution. Conclusion: The changes observed under hypertonic conditions were attributed to the decrease in Ca²⁺ at the cellular level, which was not the case under hypotonic conditions.