Aquaporin 2 is a vasopressin-independent, constitutive apical membrane protein in rat vas deferens

Aquaporin 2 (AQP2), the vasopressin-regulated water channel, was originally identified in renal collecting duct principal cells. However, our recent d escription of AQP2 in the vas deferens indicated that this water channel ma y have extrarenal functions, possibly related to sperm concentration in the male reproductive tract. In this study, we have examined the regulation an d membrane insertion pathway of AQP2 in the vas deferens. The amino acid se quence of vas deferens AQP2 showed 100% identity to the renal protein. AQP2 was highly expressed in the distal portion (ampulla) of the vas deferens, but not in the proximal portion nearest the epididymis. It was concentrated on the apical plasma membrane of vas deferens principal cells, and very li ttle was detected on intracellular vesicles. Protein expression levels and cellular localization patterns were similar in normal rats and vasopressin- deficient Brattleboro homozygous rats, and were not changed after 36 h of d ehydration, or after 3 days of vasopressin infusion into Brattleboro rats. AQP2 was not found in apical endosomes (labeled with Texas Red-dextran) in vas deferens principal cells, indicating that it is not rapidly recycling i n this tissue. Finally, vasopressin receptors were not detectable on vas de ferens epithelial cell membranes using a [H-3]vasopressin binding assay. Th ese data indicate that AQP2 is a constitutive apical membrane protein in th e vas deferens, and that it is not vasopressin-regulated in this tissue. Th us AQP2 contains targeting information that can be interpreted in a cell-ty pe-specific fashion in vivo.