Water channel protein AQP3 is present in epithelia exposed to the environment of possible water loss

Aquaporins (AQPs) are membrane water channel proteins expressed in various tissues in the body. We surveyed the immunolocalization of AQP3, an isoform of the AQP family, in rat epithelial tissues. AQP3 was localized to many e pithelial cells in the urinary, digestive, and respiratory tracts and in th e skin. In the urinary tract, AQP3 was present at transitional epithelia. I n the digestive tract, abundant AQP3 was found in the stratified epithelia in the upper part, from the oral cavity to the forestomach, and in the simp le and stratified epithelia in the lower part, from the distal colon to the anal canal. In the respiratory tract, AQP3 was present in the pseudostrati fied ciliated epithelia from the nasal cavity to the intrapulmonary bronchi . In the skin, AQP3 was present in the epidermis. Interestingly, AQP3 was p resent at the basal aspects of the epithelia: in the basolateral membranes in the simple epithelia and in the multilayered epithelia at plasma membran es of the basal to intermediate cells. During development of the skin, AQP3 expression commenced late in fetal life. Because these AQP3-positive epith elia have a common feature, i.e., they are exposed to an environment of pos sible water loss, we propose that AQP3 could serve as a water channel to pr ovide these epithelial cells with water from the subepithelial side to prot ect them against dehydration.