Pflugers Arch - Eur J Physiol (2002) 445:405–412 DOI 10.1007/s00424-002-0940-8 EXOCRINE GLANDS Kwartarini Murdiastuti · Osamu Miki · Chenjuan Yao · Most. Nahid Parvin · Chisato Kosugi-Tanaka · Tetsuya Akamatsu · Norio Kanamori · Kazuo Hosoi Divergent expression and localization of aquaporin 5, an exocrine-type water channel, in the submandibular gland of Sprague-Dawley rats Received: 27 June 2002 / Revised: 30 August 2002 / Accepted: 30 August 2002 / Published online: 11 October 2002  Springer-Verlag 2002 Abstract By Western blot analysis, the expression level of aquaporin (AQP) 5 in the submandibular gland (SMG) was found to be different among individual rats of the Sprague-Dawley (SD) strain. Such differences were observed for AQP5 but not for AQP1 and consequently the SD strain was divided into two groups, one expressing a high level of AQP5 and the other a low one. The difference in average intensity of expression between the two groups was more than twofold. Immunohistochemical analysis of the SMG demonstrated that the AQP5 protein was localized in the basal and apical/lateral plasma membrane of acinar cells in rats expressing the high level of AQP5. In the rat expressing the low level, however, this channel protein was localized strongly in the apical/ lateral plasma membrane, but only very weakly in the basal membrane of the acinar cells. Such a diverse localization of AQP5 was confirmed by Western blotting as well. Breeding between brother and sister was repeated for two times within high expressers and low expressers to obtain the third generation progenies (F2); the AQP5 level of the SMG in the third generation (F2 rats) from high expressers was significantly higher than the F2 from low expressers. Our present study suggests the existence of genetic variation in the expression of a water channel protein, AQP5, in rats. Keywords Aquaporin 5 · Genetic variation · Sprague-Dawley (SD) rat · Submandibular gland Introduction Changes in water permeability across biological mem- branes under specific physiological conditions imply the existence of water channels. Identification of molecules of water channels has recently shown that they are members of a new family of membrane proteins termed aquaporin (AQP) [18]. The family of mammalian AQPs consists of 11 members, AQP0–10, which form pores in the plasma membranes of many cell types in tissues through which water selectively crosses [12, 13, 14, 18, 20]. AQP1 mediates proximal tubule fluid re-absorption and secretion of aqueous humor and cerebrospinal fluid, and is involved in lung water homeostasis [1, 18, 19, 26, 28]; whereas AQP2 mediates vasopressin-dependent water permeability in renal collecting ducts. Mutations or down regulation of the latter can cause nephrogenic diabetes insipidus [5, 7, 18, 27]. AQP3 in the basolateral membrane of collecting ducts provides an exit pathway for reabsorbed water, and is also found in the epithelial cells of digestive tract and in the conjunctiva epithelium of the eye [4, 11, 25, 29, 32]. AQP4 is abundant in the brain and participates in reabsorption of cerebrospinal fluid, osmoregulation, and regulation of brain edema, whereas it is less abundant in the eye, kidney, lung, and intestine [4, 8, 17, 29]. A cDNA for AQP5 was first cloned from the salivary gland, and is known as an exocrine-type water channel with a unique tissue expression [31]. By experiments using Northern blot analysis and in situ hybridization, strong expression of AQP5 mRNA was shown in many exocrine gland tissues, i.e., the salivary gland, eye, lacrimal gland, lung, and trachea [21, 31]. AQP5 is thought to play a fundamental role in the water movement for the formation of saliva, tears, and other secretions [18]. An AQP5 knockout experiment conducted in mice resulted in the production of saliva that was significantly hypertonic, viscous, and smaller in volume, directly indicating that AQP5 has an essential role in saliva K. Murdiastuti · O. Miki · C. Yao · M.N. Parvin · C. Kosugi-Tanaka · T. Akamatsu · N. Kanamori · K. Hosoi ( ) ) Department of Physiology and Oral Physiology, Tokushima University School of Dentistry, Tokushima-Shi, Tokushima 770-8504, Japan e-mail: hosoi@dent.tokushima-u.ac.jp Tel.: +81-88-6337323 Fax: +81-88-6337324 K. Murdiastuti Department of Periodontology, Gadjah Mada University Faculty of Dentistry, Yogyakarta 55281, Indonesia