A signaling pathway for stimulation of Na+ uptake induced by angiotensin II in primary cultured rabbit renal proximal tubule cells

Immunochemically unreactive albumin in urine: fiction or reality?

Urinary albumin measurements are currently not standardized due to a lack of a reference method and reference (primary and secondary [matrix]) material. Multiple molecular forms of albumin in urine are identified. Modification of albumin by proteolysis during passage through the urinary tract and chemical modification during specimen storage leads to the formation of albumin fragments. Multiple methods have been developed to quantify albuminuria and significant different results are reported dependent on the available assay. The current point of view of the National Kidney Disease Education Program - IFCC Working Group on Standardization of Albumin considers the immunoassay with polyclonal sera as the primary method of quantifying urine albumin. This article reviews the process of albumin fragmentation and focuses on the controversial topic of immuno-unreactive, nonimmunoreactive, or immunochemically nonreactive albumin fractions and its consequences for albumin analysis. We conclude that at present there are no hard arguments for measuring immunochemically unreactive albumin in urine. Immunoassays using polyclonal antisera for the detection of urinary albumin remain the gold standard. The development of a reference measurement procedure remains one of the challenges for the future.

Angiotensin II enhances connecting tubule glomerular feedback

Increasing Na delivery to epithelial Na channels (ENaCs) in the connecting tubule (CNT) causes dilation of the afferent arteriole (Af-Art), a process we call CNT glomerular feedback (CTGF). Angiotensin II (Ang II) stimulates ENaC in the collecting duct via Ang II type 1 receptors. We hypothesized that Ang II in the CNT lumen enhances CTGF by activation of Ang II type 1 receptors, protein kinase C and ENaC. Rabbit afferent arterioles and their adherent CNT were microperfused and preconstricted with norepinephrine. Each experiment involved generating 2 consecutive concentration-response curves by increasing NaCl in the CNT lumen. During the control period, the maximum dilation of the afferent arteriole was 7.9±0.4 μm, and the concentration of NaCl in the CNT needed to achieve half maximal response (EC(50)) was 34.7±5.2 mmol/L. After adding Ang II (10(-9) mol/L) to the CNT lumen, the maximal response was 9.5±0.7 μm and the EC(50) was 11.6±1.3 mmol/L (P=0.01 versus control). Losartan, an Ang II type 1 antagonist (10(-6) mol/L) blocked the stimulatory effect of Ang II; PD123319, an Ang II type 2 antagonist (10(-6) mol/L), did not. The protein kinase C inhibitor staurosporine (10(-8) mol/L) added to the CNT inhibited the stimulatory effect of Ang II. The ENaC inhibitor benzamil (10(-6) mol/L) prevented both CTGF and its stimulation by Ang II. We concluded that Ang II in the CNT lumen enhances CTGF via activation of Ang II type 1 and that this effect requires activation of protein kinase C and ENaC. Potentiation of CTGF by Ang II could help preserve glomerular filtration rate in the presence of renal vasoconstriction.

Evolving concepts in advanced glycation, diabetic nephropathy, and diabetic vascular disease

Advanced glycation endproducts (AGEs) have been postulated to play a role in the development of both nephropathy and large vessel disease in diabetes. However, it is still not clear which AGE subtypes play a pathogenetic role and which of several AGE receptors mediate AGE effects on cells. This review summarises the renoprotective effect of inhibitors of AGE formation, including aminoguanidine, and of cross-link breakers, including ALT-711, on experimental diabetic nephropathy and on mesenteric vascular hypertrophy. It also demonstrates similar effects of aminoguanidine and ramipril (an angiotensin converting enzyme inhibitor) on fluorescent and immunoassayable AGE levels, renal protein kinase C activity, nitrotyrosine expression, lysosomal function, and protein handling in experimental diabetes. These findings indicate that inhibition of the renin angiotensin system blocks both upstream and downstream pathways leading to tissue injury. We postulate that the chemical pathways leading to advanced glycation endproduct formation and the renin angiotensin systems may interact through the generation of free radicals, induced both by glucose and angiotensin II. There is also evidence to suggest that AGE-dependent pathways may play a role in the development of tubulointerstitial fibrosis in the diabetic kidney. This effect is mediated through RAGE and is TGF-beta and CTGF-dependent.

Protein kinase C beta isoenzymes in diabetic kidneys and their relation to nephroprotective actions of the ACE inhibitor lisinopril

Inhibitors of angiotensin-converting enzyme (ACE) or beta isoforms of protein kinase C (PKC) are nephroprotective in diabetes mellitus. We investigated the influence of streptozotocin (STZ)-induced diabetes mellitus and of treatment with the ACE inhibitor lisinopril (4 mg/kg p.o. twice daily for 4 weeks) on the expression of PKC beta 1 and PKC beta 2 in the renal cortex of female Sprague-Dawley rats. Immunohistochemistry indicated an enhanced renocortical accumulation of macrophages expressing both MHC II, a marker for antigen-presenting cells, as well as PKC beta 2 in STZ diabetes which was confirmed by Western blotting demonstrating an enhanced renocortical expression of MHC II (1.8-fold) as well as of membrane-associated PKC beta 2 (1.9-fold). Whereas immunohistochemistry could not detect unequivocal alterations, Western blotting showed a rise in the renocortical expression of membrane-associated PKC beta 1 (1.7-fold) in STZ diabetes. Lisinopril lowered renocortical albumin content and proteinuria in STZ diabetes and attenuated the enhanced accumulation of macrophages expressing PKC beta 2 as well as the increase of membrane-associated expression of PKC beta 1 and PKC beta 2 in the renal cortex. The data suggest that the nephroprotective actions of the ACE inhibitor lisinopril in experimental diabetes mellitus were associated with and thus could be mediated in part by inhibition of diabetes-induced activation of PKC beta isoenzymes in the renal cortex.

Role of PKC and calcium in modulation of effects of angiotensin II on sodium transport in proximal tubule

It has been well documented that low concentrations of ANG II (10(-11) to 10(-10) M) stimulate, whereas high concentrations of ANG II (10(-8) to 10(-5) M) inhibit Na(+) transport in proximal tubules of rat and rabbit kidneys. Measured ANG II concentration in proximal tubular fluid is in the nanomolar range. In the present study, we investigated the role of PKC, intracellular Ca(2+), and cAMP in modulating the effects of luminal ANG II on Na(+) absorption by microperfusion techniques in rabbit superficial segment of proximal tubules in vitro. We confirmed that ANG II (10(-9) M) had no change on fluid absorption (J(v)); however, fluid absorption increased significantly when 10(-9) M ANG II and 3,4,5-trimethoxybenzoic acid-8-(diethylamino)octyl ester (TMB-8), a blocker of intracellular calcium mobilization, were added together. In contrast, ANG II significantly decreased J(v) when PKC was inhibited. When 10(-9) M ANG II was present together with 1-(5-isoquinolinesulfonyl)-2-mehtylpiperazine and TMB-8, no significant change of J(v) occurred. Inhibition of endogenous cAMP activity by a PKA inhibitor did not change either basal or ANG II-stimulated fluid absorption. Our results indicate that ANG II regulates Na(+) absorption by a cAMP-independent mechanism and that PKC and intracellular calcium both play a critical role in modulating the effects of physiological concentration of ANG II on proximal tubule transport. Balance between these two cytosolic messengers modulates the effects of ANG II on fluid absorption in the proximal tubule.

Renal handling of albumin: a critical review of basic concepts and perspective

Biochemical and physiological processes that underlie the mechanism of albuminuria are completely reassessed in this article in view of recent discoveries that filtered proteins undergo rapid degradation during renal passage and the resulting excreted peptide fragments are not detected by conventional urine protein assays. This means that filtered protein and/or albumin levels in urine have been seriously underestimated. The concept that albuminuria is a result of changes in glomerular permeability is questioned in light of these findings and also in terms of a critical examination of charge selectivity, shunts, or large-pore formation and hemodynamic effects. The glomerulus appears to function merely in terms of size selectivity alone, and for albumin, this does not change significantly in disease states. Intensive albumin processing by a living kidney occurs through cellular processes distal to the glomerular basement membrane. Failure of this cellular processing primarily leads to albuminuria. This review brings together recent data about urinary albumin clearance and current knowledge of receptors known to process albumin in both health and disease states. We conclude with a discussion of topical and controversial issues associated with the proposed new understanding of renal handling of albumin.

Apoptosis and angiotensin II: yet another renal regulatory system?

Apoptosis plays a key role in the regulation of normal renal structure and kidney remodeling in various renal diseases. Angiotensin II plays a prominent role in renal injury through its receptor subtypes, the type 1 (AT1) receptor and the type 2 (AT2) receptor, which involve different molecular mechanisms. In addition to its haemodynamic actions, angiotensin II induces apoptosis. Angiotensin II also increases proliferation in the kidney. A close correlation between renal cell proliferation and apoptosis has been shown in renal diseases as well as in the angiotensin II infusion model. Angiotensin induces upregulation of p53 and other pro-apoptotic proteins. Recent studies suggest that both AT1 and AT2 receptors influence the apoptotic process in the kidney. These apoptotic effects of angiotensin II should be considered as representing another regulatory mechanism that may modulate the balance between cell growth and proliferation within the kidney.

High glucose levels alter angiotensin II-induced Ca(2+) uptake via PKC and cAMP pathways in renal proximal tubular cells

Although a dysfunction of the calcium metabolism occurs in diabetes mellitus, alterations of Ca(2+) uptake induced by angiotensin II (ANG II) in renal proximal tubular cells (PTCs) grown in high-glucose medium are not fully elucidated. Thus, we examined whether high glucose concentrations can induce an alteration of the ANG II effect on the Ca(2+) uptake and its action mechanism in primary cultured renal PTCs. PTCs were exposed to different glucose concentrations (5-100 mM) and time intervals (0-48 h). There was a sustained increase of Ca(2+) uptake at glucose concentrations >15 mM. Thus, we selected 25 mM glucose and incubation for 48 h to maintain a hyperglycemic condition in vitro, unlike short-time regulatin. ANG II significantly inhibited the Ca(2+) uptake in a dose-dependent manner in a 5-mM glucose medium. In addition, downregulation of ANG II receptors appeared in a glucose dose dependent manner. However, PTCs treated with 25 mM glucose for 48 h, not 12 h, did not exhibit the inhibitory effect of ANG II (10(-7) M) on Ca(2+) uptake, although the inhibitory effect of ANG II on Ca(2+) uptake occurred in the presence of 25 mM mannitol or L-glucose. Staurosporine, bisindolylmaleimide I (protein kinase C, PKC, inhibitors), 12-o-tetradecanoylphorbol 13-acetate pretreatment, SQ 22536 (an adenylate cyclase inhibitor), and myristoylated protein kinase A inhibitor amide 14-22 (a protein kinase A inhibitor) blocked the 25-mM-glucose-induced alteration of ANG II effect on Ca(2+) uptake. These results suggest that both PKC and cyclic adenosine monophosphate (cAMP) pathways are involved in the high-glucose-induced alteration of ANG II effect on Ca(2+) uptake. Indeed, 25 mM glucose increased PKC activity and cAMP contents. In conclusion, a high glucose concentration altered ANG II induced inhibition of Ca(2+) uptake via PKC and cAMP pathways in the PTCs.