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The transtubular potassium gradient (TTKG) is used to gauge renal potassium secretion by the cortical collecting duct, indirectly assessing mineralocorticoid bioactivity in patients who have hypo- or hyperkalemia. TTKG values <6 indicate an inappropriate renal response to hyperkalemia, whereas values >2 during hypokalemia point to renal loss. Hypokalemia is not addressed here. Studies supporting the usefulness of the TTKG in hyperkalemia are limited to case series. This calculation may be most useful in distinguishing hyperkalemic patients who have mineralocorticoid deficiency versus resistance by observing a change in TTKG values after physiologic or pharmacologic doses of mineralocorticoids.

For investigation of how the vascular endothelial growth factor (VEGF) system participates in the pathogenesis of diabetic kidney disease, type 2 diabetic db/db and control db/m mice were treated intraperitoneally with vehicle or 2 mg/kg of a pan-VEGF receptor tyrosine kinase inhibitor, SU5416, twice a week for 8 wk. Efficacy of SU5416 treatment in the kidney was verified by the inhibition of VEGF receptor-1 phosphorylation. Glomerular VEGF immunostaining, normally increased in diabetes, was unaffected by SU5416. Plasma creatinine did not change with diabetes or SU5416 treatment. The primary end point of albuminuria increased approximately four-fold in the diabetic db/db mice but was significantly ameliorated by SU5416. Correlates of albuminuria were investigated. Diabetic glomerular basement membrane thickening was prevented in the SU5416-treated db/db mice, whereas mesangial matrix expansion remained unchanged by treatment. The density of open slit pores between podocyte foot processes was decreased in db/db diabetes but was partly increased toward normal by SU5416. Finally, nephrin protein by immunofluorescence was decreased in the db/db mice but was significantly restored by SU5416. Paradoxically, total nephrin protein by immunoblotting was increased in diabetes, pointing toward a possible dysregulation of nephrin trafficking. Diabetic albuminuria is partially a function of VEGF receptor signaling overactivity. VEGF signaling was found to affect a number of podocyte-driven manifestations such as GBM thickening, slit pore density, and nephrin quantity, all of which are associated with the extent of diabetic albuminuria. By impeding these pathophysiologic processes, VEGF receptor inhibition by SU5416 might become a useful adjunct to anti-albuminuria therapy in diabetic nephropathy.

The critical role of hyperglycemia in the genesis of diabetic nephropathy has been established by cell culture studies, experimental animal models, and clinical trials. Certain cytokines and growth factors have been identified as likely mediators of the effects of high ambient glucose on the kidney, but prominent among these is TGF-beta, a prototypical hypertrophic and fibrogenic cytokine. Overexpression of TGF-beta has been demonstrated in the glomerular and tubulointerstitial compartments of experimental diabetic animals. The TGF-beta receptor signaling system is also triggered, as evidenced by upregulation of the TGF-beta type II receptor and activation of the downstream Smad signaling pathway. Treatment of diabetic mice with neutralizing anti-TGF-beta antibodies prevents the development of renal hypertrophy, mesangial matrix expansion, and the decline in renal function. Antibody therapy also reverses the established lesions of diabetic glomerulopathy. These studies argue strongly in support of the hypothesis that overactivity of the TGF-beta system in the kidney is a crucial mediator of diabetic renal hypertrophy and mesangial matrix expansion.

High ambient glucose exerts its injurious effects on renal cells through nonenzymatic and enzymatic pathways, including altered signal transduction and upregulation of the transforming growth factor-beta (TGF-beta) system. Extracellular signal-regulated kinase (ERK), a member of the mitogen-activated protein kinase (MAPK) cascade, is activated in mesangial cells cultured in high glucose and in glomeruli of diabetic rats. However, the biologic consequences of ERK activation in the kidney have not been investigated. To clarify the role of ERK activation, mouse mesangial cells were exposed to normal (5.5 mM) or high (25 mM) glucose with or without addition of PD98059, a specific inhibitor of MAPK/ERK kinase (MEK), an upstream kinase activator of ERK. Cells that were exposed to high glucose exhibited significant increases in ERK activity, TGF-beta1 expression (total protein, mRNA levels, and promoter activity), [(3)H]-proline uptake, and alpha1(I) collagen and fibronectin mRNA levels. Treatment with PD98059 (up to 25 microM) significantly inhibited these parameters. In contrast, 25 microM PD98059 had no significant effect on any of the parameters measured in cells that were exposed to normal glucose. Overexpression of MAPK phosphatase CL 100 prevented TGF-beta1 promoter activation by high glucose, confirming the involvement of the MEK-ERK pathway in response to high glucose. The conclusion is that activation of ERK in mesangial cells is responsible for high-glucose-induced stimulation of TGF-beta1 and contributes to the increased extracellular matrix expression.

Renal tubular epithelial cells and interstitial fibroblasts are active participants in tubulointerstitial fibrosis, the best correlate of decreased glomerular filtration in diabetic nephropathy. It was reported previously that high ambient glucose stimulates transforming growth factor-beta (TGF-beta) mRNA and bioactivity, promotes cellular hypertrophy, and increases collagen synthesis in proximal tubular cells. This study evaluates the effects of high glucose and TGF-beta on the behavior of murine renal cortical fibroblasts (TFB) in culture. High glucose (450 mg/dl) significantly increased [3H]-thymidine incorporation (by 60 to 80% after 24 to 72 h) and cell number, without significantly increasing cell death when compared with normal glucose (100 mg/dl). There also was a transient increase in the mRNA of the c-myc and egr-1 early-response genes. Exogenous TGF-beta1 was promitogenic rather than antiproliferative in contrast to other renal cell types. Northern blot analysis demonstrated constitutive expression of TGF-beta1, -beta2, and -beta3 transcripts. Exposure to high glucose increased all three TGF-beta isoforms in a time-dependent manner. High glucose as well as exogenous TGF-beta1 also increased [3H]-proline incorporation, alpha2(I) collagen mRNA, and type I collagen protein (measured by immunoassay). Treatment with a neutralizing pan-selective monoclonal anti-TGF-beta antibody markedly attenuated the stimulation by high ambient glucose of thymidine incorporation, TGF-beta1 mRNA, and type I collagen mRNA and protein levels. It is concluded that high ambient glucose and exogenous TGF-beta1 share similar actions on renal fibroblasts. Moreover, the stimulation of cell proliferation and collagen type I synthesis in these cells by high ambient glucose are mediated by activation of an autocrine TGF-beta system.

Expression of the inducible isoform of nitric oxide synthase (iNOS) and generation of nitric oxide (NO) have been recently described, in addition to mesangial and medullary thick ascending limb cells, in proximal tubular cells, including MCT, a mouse proximal tubular epithelium cell line. Because vasoconstrictors may interfere with the induction of iNOS and the subsequent generation of NO, in the study presented here, whether exogenous angiotensin II (ANG II) influences bacterial lipopolysaccharide (LPS)/gamma-interferon (gamma-IF)-stimulated NO synthesis and iNOS protein and mRNA expression in MCT cells was tested. LPS/gamma-IF readily stimulated nitrite synthesis in MCT cells, as one measured parameter of NO synthesis. Coincubation of cells with 10(-9)-10(-6) M ANG II attenuated this LPS/gamma-IF-stimulated induction of nitrite. This effect was reversed by the AT1-receptor blocker losartan, but not by an AT2-receptor antagonist, indicating signal transduction through AT1-receptors. Western blot analysis applying a specific monoclonal antibody generated against mouse iNOS revealed that 10(-8)-10(-6) M ANG II significantly reduced LPS/gamma-IF-induced iNOS protein expression. However, ANG II had no effect on LPS/gamma-IF-induced iNOS mRNA as assessed by Northern blots. Moreover, transient transfection studies using a chimeric gene construct, in which iNOS regulatory elements are linked to the CAT reporter gene, showed no effect of ANG II on the LPS/gamma-IF-stimulated transcriptional activity. The study presented here demonstrates that ANG II influences LPS/gamma-IF-stimulated NO generation in MCT cells, most likely at a posttranscriptional level, by influencing iNOS protein expression. Whether proximal tubular cells in vivo express iNOS remains to be established, but this study suggests a mechanism for how iNOS activity is influenced by ANG II in cultured proximal tubular cells.

Atrial natriuretic peptide (ANP) is a growth suppressor for a variety of different cell types including cultured mesangial cells. This study examined the effects of ANP on the expression and synthesis of transforming growth factor-beta (TGF-beta) in a cultured murine mesangial cell line (MMC). Full-length rat ANP as well as 8-bromo-cGMP, but not ring-deleted ANP analogues, induced, in a dose-dependent manner, TGF-beta 1 mRNA expression in MMC. Moreover, full-length ANP and 8-bromo-cGMP stimulated the synthesis and release of TGF-beta 1 into cell culture supernatants as measured by a specific enzyme-linked immunosorbent assay and mink cell bioassay. The induction of TGF-beta 1 by 10(-6) M ANP and 10(-4) M 8-bromo-cGMP for 24 h was also documented by western blotting of MMC culture supernatants. However, transient transfection studies in MMC with three different murine TGF-beta 1 reporter gene constructs revealed only little stimulation of activity after ANP treatment, suggesting that posttranscriptional activation may mainly contribute to TGF-beta mRNA induction. Functional studies demonstrated that the ANP-mediated inhibition of mitosis, induced by 5% fetal calf serum, is partly abolished in the presence of a neutralizing anti-TGF-beta 1-3 antibody. In addition, the antiproliferative effects of exogenous 8-bromo-cGMP are also attenuated by anti-TGF-beta 1-3 antibody. These findings indicate that ANP stimulates TGF-beta expression in MMC and that the antiproliferative effects of ANP in this cell line may be mediated, at least to some extent, by the endogenous induction of TGF-beta. An increase in intracellular cGMP is most likely the mediator of this TGF-beta 1 induction. Considering the fibrogenic effects of TGF-beta, these findings may be important in the development of glomerulosclerosis in situations with an increase in local or systemic ANP.

Tubulointerstitial lesions often develop in the kidneys of patients and experimental animals with diabetes mellitus. In an in vitro model of diabetic renal disease, it has been previously demonstrated in this laboratory that elevated glucose levels stimulate procollagen transcription and secretion in proximal tubule cells in culture while inducing cellular hypertrophy and reducing cellular proliferation. Previous experiments in other tissues have suggested that myo-inositol supplementation, probably by reversing a disturbance in cell myo-inositol metabolism related to increased activity of the polyol pathway, reverses the effects of glucose on cell function. We tested the effect of myo-inositol supplementation on proximal tubule cells in culture in the presence of elevated medium glucose level. Incubation in 450 mg/dL of glucose media reduced cell proliferation; 450 mg/dL of glucose plus myo-inositol (800 microM) increased proliferation, returning the value to that seen in cells incubated in 100 mg/dL of glucose. Incubation in 450 mg/dL of glucose media increased type IV and type I procollagen mRNA levels and peptide secretion rates compared with those seen in cells incubated in medium containing 100 mg/dL of glucose. This glucose-induced stimulation of procollagen mRNA levels and procollagen secretion was not observed when the elevated glucose medium was supplemented with 800 microM myo-inositol. On the other hand, myo-inositol supplementation did not prevent the glucose-induced cellular hypertrophy: there was no reduction in the increased leucine incorporation and cellular protein content. Cell incubation in 450 mg/dL of glucose media did not lead to a measurable decrease in total cellular myo-inositol.(ABSTRACT TRUNCATED AT 250 WORDS)