Aquaporin 2 (AQP2) is responsible for regulating the concentration of urine in the collecting tubules of the kidney under the control of vasopressin (Vp). Studies using Vp-deficient Brattleboro rats, however, indicated the existence of substantial Vp-independent mechanisms for membrane insertion, as well as transcriptional regulation, of this water channel. The Vp-independent mechanism(s) is clinically relevant to patients with X-linked nephrogenic diabetes insipidus (NDI) by therapeutically bypassing the dysfunctional Vp receptor. On the basis of studies with secretin receptor-null (SCTR ؊/؊ ) mice, we report here for the first time that mutation of the SCTR gene could lead to mild polydipsia and polyuria. Additionally, SCTR ؊/؊ mice were shown to have reduced renal expression of AQP2 and AQP4, as well as altered glomerular and tubular morphology, suggesting possible disturbances in the filtration and/or water reabsorption process in these animals. By using SCTR ؊/؊ mice as controls and comparing them with wild-type animals, we performed both in vivo and in vitro studies that demonstrated a role for secretin in stimulating (i) AQP2 translocation from intracellular vesicles to the plasma membrane in renal medullary tubules and (ii) expression of this water channel under hyperosmotic conditions. The present study therefore provides information for at least one of the Vp-independent mechanisms that modulate the process of renal water reabsorption. Future investigations in this direction should be important in developing therapeutic means for treating NDI patients.Secretin was originally isolated from upper intestinal mucosal extract, injection of which into the jugular vein of an anesthetized dog resulted in elevation of pancreatic and hepatic bile flow (4). The primary function of secretin in releasing bicarbonate, electrolytes, and water from pancreatic ductal epithelial cells is firmly established. Additionally, it can also stimulate electrolyte and water secretion in the epididymis, as well as bicarbonate-rich ductal bile secretion from cholangiocytes (1, 9). In cholangiocytes, a secretin-induced choleretic effect is associated with microtubule-dependent exocytotic insertion of cytoplasmic vesicles containing the water channel aquaporin 1 (AQP1) onto the apical plasma membrane (PM), leading to osmotic water movement (28). Interestingly, this type of regulated translocation of water channels has also been demonstrated in other cell types (5). For example, vasoactive intestinal polypeptide-induced translocation of AQP5 in Brunner's gland of the duodenum is associated with bicarbonate and mucin secretion, which is essential for mucosal protection (38), while Vp-and oxytocin-triggered translocation of AQP2 to and from the PM in renal collecting tubules (19, 36) is critical for renal water reabsorption. As there is considerable evidence indicating the involvement of Vp-independent mechanisms in the regulation of renal water reabsorption (19,21,25,44) and some of these mechanisms are associated with cyclic-AMP (cAMP...