Endocrine Abstracts

Alterations in sodium reabsorption in collecting duct epithelium can have severe implications for the normal function of the nephron and may precipitate the development of secondary hypertension. One of the principal protein kinases involved in sodium transport in the nephron is the serum and glucocorticoid regulated kinase (SGK1) that mediates translocation and insertion of the epithelial sodium channel (ENaC) into the plasma membrane. SGK1 appears to be increased in the kidneys of diabetic mice and humans and recent reports have identified SGK1 as a possible key signalling component in the development of hypertension associated with diabetes.

RT-PCR, immunocytochemistry and western blot analysis demonstrated that cells derived from human cortical collecting duct (HCD) cells express SGK1 and alpha-ENaC mRNA and protein. Incubation of HCD cells with high glucose (25 mM) for 24 and 48 hours increased expression of SGK1 and alpha-ENaC proteins (SGK, 262±14.9% of control (5 mM) at 48 hours; alpha-ENaC, 170±10.9% of control (5 mM) at 24 hours; n=3–6, P<0.01). Application of TGF-beta1 (2 nM) or the calcium ionophore, ionomycin, (1 μM) induced significant and time-dependent increases in SGK1 and alpha-ENaC protein expression. Alpha-ENaC expression was maximally induced at 6 hours by both TGF-beta1 (160±9.8% of control) and ionomycin (175±5.5% of control) (n=3, P<0.01). Likewise SGK1 expression was maximally induced at 8 hours by TGF-beta1 (128±13.0% of control) and 6 hours by ionomycin (262±6.0% of control) (n=3–4, P<0.01). These molecular changes correlated with increased sodium uptake following incubation of HCD cells in 25 mM glucose (113±1.0% of control (5 mM) at 24 hours; n=3, P<0.01).

These data suggest that increases in intracellular sodium in response to high glucose, TGF-beta1 or ionomycin may be mediated via induced expression of both SGK1 and alpha-ENaC. Therefore highlighting a link between increased glucose levels and the altered sodium reabsorption observed in diabetic nephropathy.