Aquaporin-5 Protein Is Selectively Reduced in Rat Parotid Glands under Conditions of Fasting or a Liquid Diet.

Aquaporin-5 (AQP5) water channel, transmembrane protein 16A (TMEM16A) Ca2+-activated Cl- channel, and Na+-K+-2Cl- cotransporter (NKCC1) are membrane proteins on salivary gland acinar cells that function in watery saliva secretion. We examined their expression changes in rat parotid glands under reduced mastication. Rats were either fed regular chow as a control group, fasted for 48 hr or fed a liquid diet for 48 hr or 1 week to reduce mastication. The parotid glands were then resected to analyze the protein and mRNA levels by immunofluorescence, immunoblotting, and reverse-transcription quantitative PCR (RT-qPCR). AQP5 protein was significantly decreased in both liquid diet groups and the fasting group but its mRNA levels showed no apparent changes compared with the control group. The protein and mRNA levels of TMEM16A and NKCC1 showed no significant changes between any of the groups other than an increase in NKCC1 mRNA in the 1-week liquid diet group. These results suggest that reduced mastication may increase the AQP5 protein degradation, but not that of other membrane proteins necessary for saliva secretion.

Long-term Pilocarpine Treatment Improves Salivary Flow in Irradiated Mice.

Radiation therapy for head and neck cancer frequently causes salivary gland dysfunction. Pilocarpine is a clinically approved and effective drug that induces saliva secretion, thereby keeping the oral mucosa moist and reducing discomfort in patients, but the effect is transient. We expected that this drug also has beneficial long-term effects that maintain the integrity of salivary glands by reducing, for instance, apoptosis. Here, we examined the effects of long-term pilocarpine administration in irradiated mice. The results indicated that long-term pilocarpine administration significantly improved salivary flow in irradiated mice, suggesting the potential beneficial effects of long-term administration. To elucidate the underlying mechanism, we analyzed the histology, apoptosis, and proliferation of acinar cells, and the expression of functional membrane proteins such as transmembrane member 16A, aquaporin-5, and Na-K-Cl cotransporter. Long-term pilocarpine treatment seemed to decrease irradiation-induced apoptosis, although the change was not statistically significant. The present results indicated that long-term administration of pilocarpine has beneficial effects on salivary flow in irradiated mice, and suggested that long-term administration possibly decreases apoptosis in irradiated salivary glands.