Abstract:
For in vivo experiments regarding the regulation of Ca 2+ channel the sgk1-/- mice under control diet immunohistochemistry revealed lower abundance of TRPV5 and calbindin D-28K protein than in sgk1+/+ mice (colocalized in distal convoluted tubules and connecting tubules). Renal Ca2+ excretion under cd was significantly lower in sgk1-/- than in sgk1+/+ mice. Under a Ca2+-deficient diet TRPV5 protein abundance markedly increased in both genotypes, clearly pointing to SGK1-independent regulation of TRPV5, but renal excretion of Ca2+ decreased to the same levels in both phenotypes.
Under a Ca2+-deficient diet, renal elimination of Ca2+ decreased significantly in both sgk1-/- and sgk1+ mice. The ability of the sgk1-/- mice to decrease urinary Ca2+ output again points to SGK1- independent regulation of renal tubular Ca2+ reabsorption. However, the decrease of absolute and fractional excretion upon exposure to the Ca2+-deficient diet was blunted in the sgk1-/- mice. Replacement of tap water in the drinking bottle with 1% saline led to the expected increase in urinary Na+ output, paralleled by a moderate increase in urinary Ca2+ output, but the urinary excretion of Ca2+ remained slightly lower in sgk1-/-than in sgk1+/+ mice. To explore the sensitivity of renal Ca2+ excretion to inhibition of Na+ reabsorption, the NKCC2 inhibitor furosemide (20 µg/g bw) was applied with or without the carboanhydrase inhibitor acetazolamide (50 µg/g bw). Furosemide alone increased renal Ca2+ excretion in sgk1-/- and sgk1+/+ mice and abolished the difference of Ca2+ excretion between genotypes. The additional administration of acetazolamide did not further increase natriuresis or calciuria. SGK1 knockout mice are anticalciuric presumably due to a decrease in extracellular volume and enhancement of Na+ and Ca2+ reabsorption upstream of the aldosterone-sensitive distal nephron including the loop of Henle.
To study the role of SGK1 in renal fibrosis, sgk1-/- and their wild-type littermates sgk1+/+ were implanted with desoxycorticosterone acetate (DOCA)-release pellets and offered 1% saline as drinking water for 12 weeks.
Treatment with DOCA/high salt led to a marked increase of fluid and Na+ intake in both genotypes. Three weeks after the initiation of DOCA/high salt treatment, the fluid intake increased by 3 fold in sgk1-/- mice, and by 11 fold in sgk1+/+ mice increasing even more during the whole study.
After 12 weeks of treatment, body weight increased significantly (p<0.05) more in sgk1+/+ mice than in sgk1-/- mice and the urinary volume augmented significantly by 25 fold in sgk1+/+ mice and 4 fold in sgk1-/- mice with further increase until 12 weeks. Urinary albumin excretion was markedly increased following treatment of sgk1+/+ mice with DOCA/high salt, the increase was almost absent in sgk1-/- mice.
Histological analysis revealed a marked influence of treatment on the morphology of kidneys from sgk1+/+ mice. The treatment was followed by marked glomerular enlargement with segmental glomerulosclerosis and tubulointerstitial fibrosis in sgk1+/+ mice, effects clearly blunted in sgk1-/- mice.
In conclusion, SGK1 is required for the development of severe albuminuria following DOCA/high salt treatment whereas lack of SGK1 protects against DOCA/high-salt-induced albuminuria and renal fibrosis.
To study the possibility that SGK1 and SGK3 could mutually replace each other, thus preventing severe NaCl loss in sgk1-/- and sgk3-/- mice, double knock-out mutants (sgk1-/-/sgk3-/-) were compared with their wildtype littermates (sgk1+/+/sgk3+/+). The sgk1-/-/sgk3-/- mice share the delayed hair growth with sgk3-/- mice and the modestly impaired renal salt retention with sgk1-/- mice. Plasma aldosterone concentrations were significantly (p <0.01) higher in sgk1-/-/sgk3-/- than in sgk1+/+/sgk3+/+ mice fed control and low-salt diets. During salt depletion, absolute and fractional excretions of Na+ were significantly higher in sgk1-/-/sgk3-/- than in sgk1+/+/sgk3+/+ mice. The fractional Na+ excretion was significantly decreased by low salt diet in both sgk1+/+/sgk3+/+ mice and sgk1-/-/sgk3-/-mice.
Fluid intake in sgk1-/-/sgk3-/- mice similarly tended to increase following salt deficient diet and tended to be larger in sgk1-/-/sgk3-/- than in sgk1+/+/sgk3+/+ mice. Plasma K+ concentration tended to augment upon treatment with low salt diet and tended to be higher in sgk1+/+/sgk3+/+ than in sgk1-/-/sgk3-/- mice. Under low salt diet urinary sodium excretion decreased significantly in both sgk1+/+/sgk3+/+ mice and sgk1-/-/sgk3-/- mice. Under control diet blood pressure was significantly lower in sgk1-/-/sgk3-/- than in sgk1+/+/sgk3+/+ mice. Under both low and high salt diet blood pressure remained significantly lower in sgk1-/-/sgk3-/-than in sgk1+/+/sgk3+/+ mice.
Renal function , Sgk1 , Calcium , Kidney , Knock-out , Wildtype