Urine concentration is regulated by vasopressin. Congenital nephrogenic diabetes insipidus (NDI) is caused by vasopressin type 2 receptor (V2R) mutations. We studied whether metformin could improve urine concentration in rodent models of congenital NDI by stimulating AMPK. To block the V2R in rats, tolvaptan (10 mg/kg/d) was given by oral gavage with or without metformin (800 mg/kg/d). Control rats received vehicle with or without metformin. Tamoxifen-induced V2R KO mice were given metformin (600 mg/kg) or vehicle twice daily. Urine osmolality in tolvaptan-treated rats (1,303 ± 126 mOsM) was restored to control levels by metformin (2,335 ± 273 mOsM) within 3 days and was sustained for up to 10 days. Metformin increased protein abundance of inner medullary urea transporter UT-A1 by 61% and aquaporin 2 (AQP2) by 44% in tolvaptan-treated rats, and immunohistochemistry showed increased membrane accumulation of AQP2 with acute and chronic AMPK stimulation. Outer medullary Na+-K+-2Cl– cotransporter 2 (NKCC2) abundance increased (117%) with AMPK stimulation in control rats but not in V2R-blocked rats. Metformin increased V2R KO mouse urine osmolality within 3 hours, and the increase persisted for up to 12 hours. Metformin increased AQP2 in the V2R KO mice similar to the tolvaptan-treated rats. These results indicate that AMPK activators, such as metformin, might provide a promising treatment for congenital NDI.
Orhan Efe, Janet D. Klein, Lauren M. LaRocque, Huiwen Ren, Jeff M. Sands
Influence of metformin on urine-concentrating transporters.
The nephron is shown with 2 sections highlighted: the thick ascending limb and the inner medullary collecting duct (IMCD). In the thick ascending limb, metformin increases phosphorylation of the Na+-K+-2Cl– cotransporter 2 (NKCC2), activating it’s reabsorption of Na, K, and Cl into the cell. The Na-K-ATPase moves sodium into the interstitium at the basolateral membrane, ROMK returns K to the lumen. The net result is a movement of Na to the interstitium to increase interstitial hyperosmolality. In the IMCD, aquaporin 2 (AQP2) and urea transporter A1 (UT-A1) are phosphorylated and activated by AMPK and stimulated by metformin. They move water and urea into the IMCD cells, and AQP3 (and AQP4, not shown) and UT-A3 complete the transfer out of the cell into the interstitium.