Animal models of renal disease.

Mice have become a favored species to model disease. Many mouse strains have proven relatively resistant to some manipulations that have generated renal disease in other species. Kirchhoff et al. describe a means of producing hypertension, proteinuria, and glomerular sclerosis in a mouse strain.

Hypokalemic nephropathy in the rat. Role of ammonia in chronic tubular injury.

Chronic potassium deficiency results in progressive tubulointerstitial injury, associated with augmented renal ammoniagenesis. We investigated the role of elevated renal ammonia levels and the interaction of ammonia with the complement system in this injury. Potassium deficiency was induced in rats by feeding a low potassium diet. Experimental animals received 150 mM NaHCO3 or equimolar NaCl, as drinking water. After 3 wk, NaHCO3 supplemented rats demonstrated decreased ammonia production, less renal hypertrophy, less histologic evidence of injury, and less proteinuria. In in vitro studies on normal cortical tubular fragments, the addition of ammonia to serum in concentrations comparable to renal cortical levels in potassium-deficient animals significantly increased tubular deposition of C3 as quantitated by a radiolabeled antibody binding technique. Thus, alkali supplementation reduced chronic tubulointerstitial disease in a rat model of hypokalemic nephropathy. We propose that increased cortical ammonia levels contribute to hypokalemic nephropathy through ammonia-mediated activation of the alternative complement pathway.

Dietary protein restriction in established renal injury in the rat. Selective role of glomerular capillary pressure in progressive glomerular dysfunction.

Dietary protein restriction imposed before renal injury is established in the remnant kidney model in the rat reduces glomerular hypertension and hyperperfusion and renal injury. We demonstrate that dietary protein restriction (6% vs. 20%) imposed on a background of established renal injury in the remnant model leads to a greater preservation of renal function as measured by glomerular filtration rate and fractional clearances of albumin and IgG, despite the persistence of systemic hypertension. In similarly prepared rats, dietary protein restriction (6% vs. 20%) led to a lower glomerular capillary hydraulic pressure, a higher ultrafiltration coefficient, and similar single nephron filtration rates. In addition, less impairment of glomerular permselectivity was demonstrable after protein restriction. Our data demonstrate that the preservation of renal function with dietary protein restriction after established glomerular injury follows upon reduction of glomerular capillary hydraulic pressure, despite constancy of single nephron filtration rate and plasma flow and persistence of arterial hypertension.

Role of aldosterone in the remnant kidney model in the rat.

The renin-angiotensin-aldosterone system (RAAS) participates in the injury sustained by the remnant kidney. Our studies assessed the importance of aldosterone in that model and the response of aldosterone to drugs interfering with the RAAS. Initially, four groups of rats were studied: SHAM-operated rats, untreated remnant rats (REM), REM rats treated with losartan and enalapril (REM AIIA), and REM AIIA rats infused with exogenous aldosterone (REM AIIA + ALDO). The last group was maintained with aldosterone levels comparable to those in untreated REM rats by constant infusion of exogenous aldosterone. REM rats had larger adrenal glands and a > 10-fold elevation in plasma aldosterone compared to SHAM. REM AIIA rats demonstrated significant suppression of the hyperaldosteronism as well as marked attenuation of proteinuria, hypertension, and glomerulosclerosis compared to REM. REM AIIA + ALDO rats manifested greater proteinuria, hypertension, and glomerulosclerosis than REM AIIA rats. Indeed, by 4 wk of observation all of these features of the experimental disease were similar in magnitude in REM AIIA + ALDO and untreated REM. In separate REM rats spironolactone administration did not reduce glomerular sclerosis but did transiently reduce proteinuria, lowered arterial pressure, and lessened cardiac hypertrophy. In summary, aldosterone contributes to hypertension and renal injury in the remnant kidney model.

Effect of dietary protein on rat renin and angiotensinogen gene expression.

Plasma renin activity varies with the level of dietary protein, being higher on a high protein diet. To explore the molecular mechanisms underlying this relationship we first examined the effect of dietary protein on renin and angiotensinogen gene expression at the level of steady state mRNA in male Sprague-Dawley rats. Renal renin mRNA was higher on a 50% (high) compared to a 6% (low) protein diet both 3 d (9.4 +/- 1.1 vs. 5.3 +/- 0.4 pg/micrograms of total RNA; P less than 0.02) and 21 d (6.8 +/- 1.0 vs. 3.5 +/- 0.4 pg/micrograms of total RNA; P less than 0.02) after dietary change. No change occurred in either renal or liver angiotensinogen mRNA. When three levels of dietary protein were examined, renal renin mRNA was elevated on a 50% and lowered on a 6% protein diet compared to a more standard 20% protein diet. Kidney weights and renal protein, RNA, and RNA/DNA increased with the level of dietary protein reflecting protein-induced renal hypertrophy. Uninephrectomy resulted in no change in renin mRNA compared to sham operation (3.7 +/- 0.1 vs. 3.4 +/- 0.1 pg/micrograms RNA; P = NS) despite renal growth in the uninephrectomy group implicating dietary protein and not hypertrophy as the major factor for stimulating renin mRNA. In conclusion, the level of dietary protein is a novel and specific stimulus for changes in renal renin mRNA. The increased plasma renin activity on a high protein diet is due at least in part to increased renin synthesis.

Pathophysiology of chronic tubulo-interstitial disease in rats. Interactions of dietary acid load, ammonia, and complement component C3.

The human end-stage kidney and its experimental analogue, the remnant kidney in the rat, exhibit widespread tubulo-interstitial disease. We investigated whether the pathogenesis of such tubulo-interstitial injury is dependent upon adaptive changes in tubular function and, in particular, in ammonia production when renal mass is reduced. Dietary acid load was reduced in 1 3/4-nephrectomized rats by dietary supplementation with sodium bicarbonate (NaHCO3), while control rats, paired for serum creatinine after 1 3/4 nephrectomy, were supplemented with equimolar sodium chloride. After 4-6 wk, NaHCO3-supplemented rats demonstrated less impairment of tubular function as measured by urinary excretory rates for total protein and low molecular weight protein and higher transport maximum for para-aminohippurate per unit glomerular filtration rate, less histologic evidence of tubulo-interstitial damage, less deposition of complement components C3 and C5b-9, and a lower renal vein total ammonia concentration. Such differences in tubular function could not be accounted for simply on the basis of systemic alkalinization, and differences in tubular injury could not be ascribed to differences in glomerular function. Because nitrogen nucleophiles such as ammonia react with C3 to form a convertase for the alternative complement pathway, and because increased tissue levels of ammonia are associated with increased tubulo-interstitial injury, we propose that augmented intrarenal levels of ammonia are injurious because of activation of the alternative complement pathway. Chemotactic and cytolytic complement components are thereby generated, leading to tubulo-interstitial inflammation. Thus, alkali supplementation reduces chronic tubulo-interstitial disease in the remnant kidney of the rat, and we propose that this results, at least in part, from reduction in cortical ammonia and its interaction with the alternative complement pathway.

Glomerular permeability barrier in the rat. Functional assessment by in vitro methods.

The formation of glomerular ultrafiltrate is dependent on the prevailing hemodynamic forces within the glomerular microcirculation and the intrinsic properties of the filtration barrier. However, direct assessment of the permeability barrier is difficult with most available techniques. We used confocal microscopy to image 1-micron thick optical cross-sections of isolated intact glomeruli and glomeruli denuded of cells and quantitated dextran (70,000 mol wt) diffusion from the capillary lumen. Dextran permeance was 11 times greater for the acellular filtration barrier than the intact peripheral capillary. Consideration of the basement membrane and cells as series resistors demonstrated that cells of the filtration barrier contribute 90% of the total resistance to macromolecular permeance. Using a different approach, dextran sieving coefficients for acellular glomeruli consolidated as a multilayer sheet in a filtration cell were similar to those for intact glomeruli in vivo at radii 30-36 A and approximately 50 times greater at a dextran radius of 60 A. The presence of cells significantly reduced hydraulic permeability determined on consolidated intact or acellular glomeruli in an ultrafiltration cell with 50 mmHg applied pressure. The glomerular basement membrane does restrict macromolecular permeability but cells are important determinants of the overall macromolecular and hydraulic permeability of the glomerulus.

Hemodynamic basis for glomerular injury in rats with desoxycorticosterone-salt hypertension.

Micropuncture and/or morphologic studies were performed in seven groups of uninephrectomized (UNX) adult male Munich-Wistar rats. Control groups 1, 3, and 6 received standard (24% protein) chow and tap water. Groups 2, 4, and 5 received weekly injections of desoxycorticosterone pivilate (DOC) and 1% saline for drinking, groups 2 and 4 were fed standard chow, and Group 5 a diet containing 6% protein. Group 7 received DOC, salt, and standard chow for 3 wk followed by withdrawal of DOC and salt for an additional 6 wk. 10-14 d after UNX, groups 1 and 2 exhibited similar single nephron glomerular filtration rates (SNGFR) and initial glomerular plasma flow rates (QA). Group 2 had higher mean arterial pressure (AP) and glomerular capillary hydraulic pressure (PGC) than group 1. 3-4 wk after UNX, group 4 exhibited further elevations in AP and PGC as compared with groups 2 and 3. SNGFR and QA were similar in groups 3 and 4, but these average values were greater than typical for normal rats. Group 4 also demonstrated increased urinary protein excretion. Morphologic evaluation of glomeruli in groups 2 and 4 revealed mesangial expansion and focal intraglomerular hemorrhage whereas glomeruli of groups 1 and 3 were essentially normal. Values for AP and PGC in group 5 were not different than group 3 but significantly lower than group 4. QA and SNGFR were lower in group 5 (low protein) than in groups 3 and 4. Furthermore, proteinuria and glomerular structural lesions were abolished in group 5. Morphologic studies performed in groups 6 and 7 showed that early DOC-SALT lesions progress to focal glomerular sclerosis. These studies suggest that continued elevations in glomerular capillary flows and pressures predispose to glomerular injury in this model of systemic arterial hypertension.

Aldose reductase inhibition and glomerular abnormalities in diabetic rats.

We examined the effects of aldose reductase inhibition (ARI) on glomerular filtration rate (GFR), albuminuria, and kidney histology in partially insulin-treated streptozocin-induced diabetic (STZ-D) rats. After 1 mo of diabetes, GFR was elevated over control values in the STZ-D rats but was not affected by treatment with statil (an aldose reductase inhibitor). In another set of rats maintained for 7 mo, albuminuria was significantly increased in the diabetic rats from 2 mo on but was also not affected by statil treatment. Similarly, histological glomerular damage and diabetes-induced kidney hypertrophy were also greater in diabetic animals but were not altered by statil treatment. The frequency of diabetic cataracts was reduced by statil, and erythrocyte and kidney sorbitol levels were normalized, confirming the efficacy of ARI. Thus, inhibition of the aldose reductase pathway with statil does not ameliorate the hemodynamic, proteinuric, histological, or growth abnormalities in this model of diabetic nephropathy.

Kidney complications.

Diabetic glomerulosclerosis in man and in all spontaneous-onset and chemically induced diabetes in experimental models is characterized by diffuse increase in mesangial matrix and glomerular basement membrane thickening. The most prominent features of the biochemical changes in the glomerular basement membrane are increase in the collagen-like components, decreased sialic acid, and increased glucosylation. However, the heterogeneity of the various glycoprotein components of the glomerular basement membrane and related components of the mesangium make comparative biochemistry difficult. Increased glomerular blood flow with no apparent alterations in the glomerular filtration coefficient in diabetes may be attributed to altered vascular control mechanisms which may include both hormonal mediation as well as changes in end-organ responsiveness. Although proteinuria is a common manifestation of diabetic involvement of the glomerulus, there is little biochemical or physiologic evidence as to the specific causes of increased glomerular filtration apparatus permeability. Further information as to the pathogenesis of diabetic vascular disease of the kidney and the ability to reverse pathologic changes by correction of the metabolic milieu will require analysis of carefully selected animal models. Particular care in experimental design must include the ability to integrate pathology, physiology, and biochemistry in each model in order to relate the information to human renal diabetic complications.

Diabetic nephropathy. Metabolic versus hemodynamic considerations.

Not all patients with diabetes develop clinically significant nephropathy and, for this reason, attention has begun to focus on the risk factors for development of this serious complication. These risk factors have not been quantified to the same degree as those factors associated with more common progressive vascular diseases, such as atherosclerosis. However, studies of pathogenesis and clinical and epidemiological surveys of diabetic nephropathy point to numerous risk categories. Glycemic control, genetic and familial predispositions, renal and glomerular enlargement, glomerular hyperfiltration, and capillary and systemic hypertension can be invoked as contributors to this disease process. This review focuses on hemodynamic alterations and their role in the development and progression of diabetic nephropathy. Increases in GFR, largely driven by increases in plasma flow and capillary pressure, appear in early IDDM and NIDDM. This abnormality of renal vascular control probably is derived from alterations in several vasoactive control systems. In addition, the elevations in capillary pressure may be damaging to the glomerular capillaries. Arterial hypertension is not necessarily present before clinical nephropathy appears; however, it is a usual concomitant of progressive diabetic renal disease. The strongest evidences for the roles of altered systemic and renal hemodynamics in the progression of diabetic renal disease are clinical and experimental studies demonstrating attenuation of the disease process by lowering systemic and capillary pressures with antihypertensive agents, and dietary and glycemic modifications. Thus, although multiple factors probably interact to determine risk for the development of diabetic nephropathy, hemodynamic forces are a particularly important contributor and are especially amenable to therapeutic intervention.

Renin expression in renal ablation.

To determine whether expression of the renin-angiotensin system (RAS) is influenced by the degree of renal ablation, male Sprague-Dawley rats underwent uninephrectomy, 1 1/3 nephrectomy, or sham operation. Renin and angiotensinogen messenger RNA (mRNA) were not different among the three groups 2 weeks after surgery. The time course of expression of renin mRNA after 1 1/3 nephrectomy showed no difference versus controls at 2 and 4 weeks and a decrease at 6 weeks after surgical ablation. Because nephrons adjacent to the infarcted area in the 1 1/3 nephrectomy may be hypoperfused and a source of increased renin synthesis, intrarenal distribution of tissue renin content, renin mRNA, and immunostainable renin were examined in separate groups of rats subjected to 1 1/3 nephrectomy. The kidney was divided into two pieces, one containing the scar and scar-adjacent tissue and the other portion the tissue distant from the scar. Tissue renin content, renin mRNA, and immunostainable renin were significantly greater in the scar-adjacent tissue compared with the nonscar tissue. Immunoreactive renin was seen in the juxtaglomerular apparatuses as well as in vascular elements proximal to the juxtaglomerular apparatus and within mesangial cells of some glomeruli of the scar-adjacent tissue. In conclusion, immunostainable renin, tissue renin content, and renin mRNA were increased in scar-adjacent tissue after 1 1/3 nephrectomy. We speculate that this unique scar-associated redistribution of renin may play a pathophysiological role in the progression of renal disease.

Progression of kidney disease in chronic renal transplant rejection.

The rate of progression of renal insufficiency was quantitated from reciprocal of serum creatinine versus time plots in patients with clinical and histologic evidence of chronic renal transplant rejection. The plots were evaluated by the breakpoint test. This method identifies breakpoints in a linear plot and compares the statistical significance of the fit provided by two intersecting lines with that of a single straight line. The breakpoint test when applied to the 22 patients with a significant linear correlation between the reciprocal of serum creatinine versus time detected a change in the slope in 20 cases (90.9%) indicating the presence of a breakpoint. The average diastolic, systolic, and mean arterial pressures before the breakpoint were significantly correlated with the value of the serum creatinine at the time of the change of the slope (r = 0.45, P less than 0.05; r = 0.58, P less than 0.01; r = 0.56, P less than 0.05, respectively) demonstrating more severe hypertension in those patients with the more severe renal dysfunction. The slope after the breakpoint was significantly correlated with the mean diastolic blood pressure values after the breakpoint (r = 0.48, P less than 0.05) with higher pressures being found in those patients with faster rates of decline in renal function. Both before and after the breakpoint occurred, the rate of progression of the renal disease, as estimated by the reciprocal of serum creatinine versus time plot, was greater when the mean diastolic blood pressure was higher than 90 mmHg. In conclusion, the vast majority of patients with proven chronic rejection progress linearly although a change in the rate of progression was frequent. Higher levels of blood pressure correlate with greater rates of progression of renal insufficiency, and a faster progression associates with a diastolic blood pressure greater than 90 mmHg.

The renal hemodynamic basis of diabetic nephropathy.

Diabetic nephropathy occurs in approximately one third of individuals with insulin-dependent diabetes mellitus (IDDM), recent studies suggest that a similar proportion of non-insulin-dependent diabetes mellitus (NIDDM) patients develop this serious complication as well. Of the many risk factors identified in the pathogenesis of nephropathy, hemodynamic alterations have been particularly well studied. Increases in glomerular filtration rate (GFR), largely driven by increases in plasma flow and glomerular capillary pressure, are apparent in early IDDM and NIDDM. Furthermore, the elevation in capillary pressure may be damaging to glomerular endothelial, epithelial and mesangial cells, thereby initiating and contributing to the progression of diabetic nephropathy. Numerous mediators of diabetic hyperfiltration have been proposed, and this phenomenon likely reflects a mutilfactorial etiology. The purpose of this article is to examine the hemodynamic alterations characteristic of diabetic nephropathy, their etiology, and their role in the development and progression of diabetic nephropathy.

Infection-related chronic interstitial nephropathy.

Kidney infection, when uncomplicated by anatomic abnormalities such as reflux or obstructing lesions, does not appear to lead to renal damage or hypertension. In children in whom predisposing anatomic causes (reflux) are particularly prominent or in adults who have or develop these abnormalities, progressive renal injury with hypertension can occur with infection. Infection appears to enhance the damaging effects of the underlying anatomic abnormalities. The mechanisms of progressive damage include the inflammatory effects of the infection itself, potentially "autoimmune" effects, and the inflammatory and infection-promoting effects of bacterial products, especially ammonia. After initial renal damage, the hemodynamic effects of systemic hypertension and of intrarenal hyperperfusion of residual nephrons may further promote progressive injury.

Role of the renin-angiotensin-aldosterone system in the progression of renal disease: a critical review.

Interruption of the renin-angiotensin-aldosterone system (RAAS) by converting enzyme inhibition or angiotensin II (ANG II) receptor antagonism dramatically reduces injury in the remnant kidney model. Furthermore, converting enzyme inhibition reduces proteinuria and slows the decline in renal function in clinical disease. Hemodynamic actions of ANG II in the kidney in conjunction with a more poorly defined effect of the RAAS on systemic hypertension have been posited as the major mechanisms for maintenance of elevated glomerular pressure. Reductions in glomerular pressure have been attributed, at least in part, to removal of intrarenal effects of ANG II. Growth and fibrotic actions of ANG II may also contribute to progressive renal injury and relief from them reduce injury. The participation of circulating aldosterone in the remnant kidney model has been recently raised. Hyperaldosteronism and adrenal hypertrophy attend the hypertension, proteinuria, and glomerulosclerosis of this model. Although the hemodynamic actions of aldosterone probably account for some of the adverse effects it has in this model, other direct cellular actions may participate in its renal, as well as cardiac and fibrotic consequences. Thus, the RAAS, working through both ANG II and aldosterone, contributes to chronic progressive renal injury.

Aldosterone in progressive renal disease.

Blockade of the renin-angiotensin-aldosterone system has proven effective in retarding progression of renal disease in the remnant kidney model, as well as other experimental diseases, and, most importantly, in a range of progressive human renal diseases. Attention has focused on the role of angiotensin II (Ang II) in propagating progression both by its hemodynamic and nonhemodynamic actions. Recent evidence, predominately in the remnant kidney model, indicates that the drugs used to block this hormone system, angiotensin-converting enzyme inhibitors and angiotensin II receptor blockers, also lower aldosterone levels. Thus, aldosterone, as well as angiotensin II, appears to be instrumental in sustaining the hypertension and fibroproliferative destruction of the residual kidney.

Dietary protein and the renin-angiotensin system in chronic renal allograft rejection.

We examined the effects of dietary protein restriction in fourteen patients with chronic kidney rejection. The patients were randomly assigned, using a crossover design to two 11-day periods, one on a low-protein diet (0.55 g/kg/day) and the other on a high-protein diet (2 g/kg/day). The low protein diet was associated with a significant improvement in glomerular permselectivity without any change in blood pressure, glomerular filtration rate, or renal plasma flow. The low protein diet was also associated with a significant reduction in plasma renin activity. Acute converting enzyme inhibition decreased proteinuria when administered at the end of the high protein diet, but had no additional antiproteinuric effect when given at the end of the low protein diet. Comparable reductions in blood pressure with hydralazine had no effect on proteinuria. Protein restriction was also associated with modest but significant fall in serum proteins. In conclusion, dietary protein restriction may improve the course of renal failure in chronic rejection partly by suppressing the renin-angiotensin system. Studies are needed to establish the safe level of dietary protein restriction in these patients and to assess the efficacy of such restriction in slowing the progression of renal failure.

Aldosterone is a major factor in the progression of renal disease.

There is compelling evidence supporting the renin-angiotensin-aldosterone system contribution in experimental and human renal disease. Interruption of this system by converting enzyme inhibition or angiotensin II receptor antagonism reduces injury. Angiotensin II contributes to the progression of renal disease through its direct vascular effects and proliferative properties. The mediators of angiotensin II induced renal injury are many and include TGF-beta, PDGF, bFGF, and endothelin. Though the mechanisms involved in its contribution to progressive renal disease are not well delineated, aldosterone seems to be an overlooked contributor to the progression of kidney disease and its effects may also depend on both its hemodynamic and more direct cellular actions.

The hyperfiltering glomerulus.

The observation has been made that after any of a variety of initial renal injuries, nephron units that have been spared undergo structural and functional compensations. The functional compensation of increased perfusion of residual nephrons may present deleterious, maladaptive stresses to those surviving nephrons and lead to their ultimate destruction. This hypothesis provides a final common pathway for the progression of many different renal diseases to renal failure and explains the tendency of renal insufficiency to progress rather than stabilize.