The process of saliva production in the salivary glands requires transepithelial water transfer from the interstitium to the acinar lumen. There are two transepithelial pathways: the transcellular and paracellular. In the transcellular pathway, the aquaporin water channels induce passive water diffusion across the membrane lipid bilayer. It is well known that aquaporin-5 (AQP5) is expressed in the salivary glands, in which it is mainly localized at the apical membrane of the acinar cells. This suggests the physiological importance of AQP5 in transcellular water transfer. Reduced saliva secretion under pilocarpine stimulation in AQP5-null mice compared with normal mice further indicates the importance of AQP5 in this process, at least in stimulated saliva secretion. Questions remain therefore regarding the role and importance of AQP5 in basal saliva secretion. It has been speculated that there would be some short-term regulation of AQP5 such as a trafficking mechanism to regulate saliva secretion. However, no histochemical evidence of AQP5-trafficking has been found, although some of biochemical analyses suggested that it may occur. There are no reports of human disease caused by AQP5 mutations, but some studies have revealed an abnormal subcellular distribution of AQP5 in patients or animals with xerostomia caused by Sjögren’s syndrome and X-irradiation. These findings suggest the possible pathophysiological importance of AQP5 in the salivary glands.

The membrane water channel aquaporin (AQP) family is composed of 13 isoforms in mammals, eight of which are reportedly expressed in the kidney: AQP1, 2, 3, 4, 6, 7, 8, and 11. These isoforms are differentially expressed along the renal tubules and collecting ducts. AQP1 and 7 are distributed in the proximal tubules, whereas AQP2, 3, and 4 occur in the collecting duct system. They play important roles in the reabsorption of water and some solutes across the plasma membrane. In contrast to other aquaporins found in the kidney, AQP6, 8, and 11 are localized to the cytoplasm rather than to the apical or basolateral membranes. It is therefore doubtful that these isoforms are directly involved in water or solute reabsorption. AQP6 is localized in acid-secreting type A intercalated cells of the collecting duct. AQP8 has been found in the proximal tubule but its cellular location has not yet been defined by immunohistochemistry. AQP11 seems to be localized in the endoplasmic reticulum (ER) of proximal tubule cells. Interestingly, polycystic kidneys develop in AQP11-null mice. Many vacuole-like structures are seen in proximal tubule cells in kidneys of newborn AQP11-null mice. Subsequently, cysts are generated, and most of the mice die within a month due to severe renal failure. Although ER stress and impairment of polycystin-1, the product of the gene mutated in autosomal-dominant polycystic kidney disease, are possible causes of cystogenesis in AQP11-null mice, the exact mechanism of pathogenesis and the physiological function of AQP11 are yet to be resolved.

The transient receptor potential canonical 3 (TRPC3) channel is a member of the TRPC family that contributes to the entry of Ca2+through the plasma membrane or modulates the driving force for Ca2+entry channels.

Summary. Aquaporin-2 (AQP2) is a water channel protein that is trafficked between intracellular vesiclesand the plasma membrane of kidney collecting duct cells upon vasopressin stimulation. Vasopressin changes the phosphorylation states of the AQP2 C-terminal serines (Sers), Ser256, Ser261, Ser264, and Ser269, in rats and mice, which is thought to play a role in controlling trafficking. Here, we focused on Ser269. We developed a specific antibody to Ser269-phosphorylated AQP2. Using immunofluorescence microscopy, we examined its localization in the rat kidney following injection of vasopressin and a vasopressin type 2 receptor-specific antagonist (OPC-31260). Ser269-phosphorylated AQP2 was almost undetectable in the water-loaded rat kidney, but was detected intracellularly soon after vasopressin injection, and then highly accumulated on the apical membrane of connecting tubule and collecting duct principal cells. In addition to the apical membrane, Ser269-phosphorylated AQP2 was also detected on the basolateral membrane of connecting tubule cells and inner medullary collecting duct principal cells. OPC-31260 injection following vasopressin stimulation caused internalization of AQP2, a pool of which was phosphorylated at Ser269. These results suggest that 1) AQP2 is phosphorylated at Ser269 intracellularly upon vasopressin stimulation and is rapidly trafficked to the plasma membrane, and 2) AQP2 can be internalized from the plasma membrane even if it remains phosphorylated at Ser269.