Expression and localization of aquaporin water channels in adult pig urinary bladder

The bladder is lined by urothelium, a transitional epithelium, which is generally considered to be a poorly permeable urine‐blood barrier with a crucial function to separate tissues of the urinary tract from the noxious composition of urine.1 Although it is believed that kidneys are responsible for the final concentration and volume of urine, significant in vivo reabsorption and secretion of Na+, K+, urea and creatinine has been measured in rabbit and rat bladders,1, 2 as well as difference in urine composition between the renal pelvis and voided urine in human subjects, indicating net water uptake.3

Recent studies have shown that the urothelium expresses transmembrane water channels, aquaporins (AQPs). Currently 13 AQP (0‐12) subtypes have been identified in mammalian tissues, and from these subtypes, AQP3, AQP4, AQP7 and AQP9 have been found in the human urothelium4 and AQP1, AQP2 and AQP3 in rat urothelium,5 indicating that AQPs could regulate urothelial cell volume and osmolarity, determining the final composition of urine.

Although AQPs have been identified in normal human urothelial cells, their exact functional role requires further investigation. The adult pig bladder offers a viable animal model as it has comparable structural and physiological properties to the human bladder.6, 7 We report the initial stage to characterize the expression and localization of AQPs in adult pig urinary bladder.