Compensatory increase in AQP2, p-AQP2, and AQP3 expression in rats with diabetes mellitus

Diabetes mellitus (DM) is associated with osmotic diuresis and natriuresis. At day 15, rats with DM induced by streptozotocin (n = 13) had severe hype rglycemia (27.1 +/- 0.4 vs. 4.7 +/- 0.1 mM in controls) and had a fivefold increase in water intake (123 +/- 5 vs. 25 +/- 2 ml/ day) and urine output. Semiquantitative immunoblotting revealed a significant increase in inner m edullary AQP2 (201 +/- 12% of control rats, P < 0.05) and phosphorylated (S er(256)) AQP2 (p- AQP2) abundance (299 +/- 32%) in DM rats. Also, the abund ance of inner medullary AQP3 was markedly increased to 171 +/- 19% of contr ol levels (100 +/- 4%, n = 7, P < 0.05). In contrast, the abundance of whol e kidney AQP1 (90 +/- 3%) and inner medullary AQP4 (121 +/- 16%) was unchan ged in rats with DM. Immunoelectron microscopy further revealed an increase d labeling of AQP2 in the apical plasma membrane of collecting duct princip al cells (with less labeling in the intracellular vesicles) of DM rats, ind icating enhanced trafficking of AQP2 to the apical plasma membrane. There w as a marked increase in urinary sodium excretion in DM. Only Na+/H+ exchang er NHE3 was downregulated (67 +/- 10 vs. 100 +/- 11%) whereas there were no significant changes in abundance of type 2 Na- phosphate cotransporter (12 8 +/- 6 vs. 100 +/- 10%); the Na- K- 2Cl cotransporter (125 +/- 19 vs. 100 +/- 10%); the thiazide- sensitive Na- Cl cotransporter (121 +/- 9 vs. 100 /- 10%); the alpha (1)-subunit of the Na- K- ATPase (106 +/- 7 vs. 100 +/- 5%); and the proximal tubule Na-HCO3 cotransporter (98 +/- 16 vs. 100 +/- 7 %). In conclusion, DM rats had an increased AQP2, p- AQP2, and AQP3 abundan ce as well as high AQP2 labeling of the apical plasma membrane, which is li kely to represent a vasopressin- mediated compensatory increase in response to the severe polyuria. In contrast, there were no major changes in the ab undance of AQP1, AQP4, and several major proximal and distal tubule Na+ tra nsporters except NHE3 downregulation, which may participate in the increase d sodium excretion.