AQP2 and Central diabetes insipidus: Metformin increases urine osmolality in a rat model of congenital diabetes insipidus by stimulating urea transporter (UT-A1), aquaporin 2 (AQP2), and Na+-K+-2Cl- cotransporter 2 (NKCC2) (Efe et al., 2016; Klein et al., 2016).