Prevention of albuminuria by captopril in diabetic rats

Changes in cardiac heparan sulfate proteoglycan expression and streptozotocin-induced diastolic dysfunction in rats

Background

Changes in the proteoglycans glypican and syndecan-4 have been reported in several pathological conditions, but little is known about their expression in the heart during diabetes. The aim of this study was to investigate in vivo heart function changes and alterations in mRNA expression and protein levels of glypican-1 and syndecan-4 in cardiac and skeletal muscles during streptozotocin (STZ)-induced diabetes.

Methods

Diabetes was induced in male Wistar rats by STZ administration. The rats were assigned to one of the following groups: control (sham injection), after 24 hours, 10 days, or 30 days of STZ administration. Echocardiography was performed in the control and STZ 10-day groups. Western and Northern blots were used to quantify protein and mRNA levels in all groups. Immunohistochemistry was performed in the control and 30-day groups to correlate the observed mRNA changes to the protein expression.

Results

In vivo cardiac functional analysis performed using echocardiography in the 10-day group showed diastolic dysfunction with alterations in the peak velocity of early (E) diastolic filling and isovolumic relaxation time (IVRT) indices. These functional alterations observed in the STZ 10-day group correlated with the concomitant increase in syndecan-4 and glypican-1 protein expression. Cardiac glypican-1 mRNA and skeletal syndecan-4 mRNA and protein levels increased in the STZ 30-day group. On the other hand, the amount of glypican in skeletal muscle was lower than that in the control group. The same results were obtained from immunohistochemistry analysis.

Conclusion

Our data suggest that membrane proteoglycans participate in the sequence of events triggered by diabetes and inflicted on cardiac and skeletal muscles.

Glycosaminoglycans urinary excretion as a marker of the early stages of diabetic nephropathy and the disease progression

BACKGROUND

Diabetes mellitus affects the metabolism of several components of extra-cellular matrix, including glycosaminoglycans (GAG). Alterations in the metabolism of GAG may play an important role in the development of diabetic complications.

METHODS

Consequently, the relationship between diabetic nephropathy and urinary GAG excretion has been estimated in 86 diabetic patients (33 type 1 diabetes mellitus (DM), 53 type 2 DM) in comparison to 30 healthy controls (Figure 1). GAG concentration in 24-h urine samples has been determined by precipitation with cetylpyridinum chloride and potassium acetate in ethanol followed by a fluorometric test with 2-Hexadecyl-9H-pyrido(4,3b)indolium Bromide.

RESULTS

Diabetic subjects excrete significantly more GAG than the control group (66.47 mg/24 h vs 50.11 mg/24 h). A marked difference in urinary GAG excretion between diabetic patients with nephropathy (74.66 +/- 7.5 mg/24 h) and without nephropathy (50.13 +/- 5.37 mg/24 h) has been detected. With diabetic nephropathy, patients with a longer history of GAG excretion experience an increase. An increased urinary GAG excretion has been detected in patients with microalbuminuria or macroalbuminuria.

CONCLUSIONS

In conclusion, it can be stated that all patients with DM compared to the control group show an increase in GAG excretion independent of diabetes duration. Urinary GAG excretion positively correlates with the duration of diabetic nephropathy. The assessment of GAG excretion values may be useful for determining the early stages of diabetic nephropathy and the progression of the disease.

Effect of antihypertensive therapy on renal injury in type 2 diabetic rats with hypertension

In a previous study, we demonstrated that doxazosin (DZN), an alpha(1)-adrenergic blocker, prevented proteinuria in streptozotocin diabetic rats. In this study, we investigated whether DZN would lower established proteinuria by improving glomerular sclerosis in spontaneously hypertensive corpulent rats with type 2 diabetes mellitus. DZN treatment was compared with treatment with angiotensin-converting enzyme inhibitor, lisinopril (LIS) alone, and DZN in combination with LIS. Combination therapy was used to examine any additive effect of either drug alone in the reduction of proteinuria and glomerular sclerosis. Both male and female rats age 6 months with established proteinuria were used. The rats were allocated randomly to 1 of 4 groups: untreated, DZN treated, LIS treated, or a combination of DZN and LIS treatment. Drug treatment was continued for 16 weeks. The results show that (1) either drug alone or in combination significantly lowered systolic blood pressure; (2) DZN, LIS, or combination therapy reduced albuminuria at 16 weeks of treatment from baseline by 38.61+/-5.77%, 30.70+/-4. 21%, and 42.17+/-4.77% (mean+/-SE), respectively. No difference in albuminuria was observed among the 3 groups of rats; (3) the fractional mesangial area, which was 20.55+/-3.77% in untreated rats, was significantly reduced to 11.18+/-1.32% in DZN-treated rats, with a further reduction to 8.72+/-0.64% in LIS-treated rats and to 3.48+/-0.35% in rats treated with DZN+LIS; and (4) DZN but not LIS significantly improved plasma glucose levels in spontaneously hypertensive corpulent rats (untreated 21.06+/-0.97 mmol/L versus DZN treated 15.81+/-0.93 mmol/L or DZN+LIS treated 17.38+/-1.10 mmol/L; P<0.025 to 0.005). Thus, the data suggest that 16-week treatment with either DZN or LIS improves established proteinuria and glomerular sclerosis, but combination therapy is superior to either DZN or LIS alone in preventing glomerular sclerosis in type 2 diabetic rats with hypertension.

Glomerular heparan sulfate alterations: mechanisms and relevance for proteinuria

Heparan sulfate (HS) is the anionic polysaccharide side chain of HS proteoglycans (HSPGs) present in basement membranes, in extracellular matrix, and on cell surfaces. Recently, agrin was identified as a major HSPG present in the glomerular basement membrane (GBM). An increased permeability of the GBM for proteins after digestion of HS by heparitinase or after antibody binding to HS demonstrated the importance of HS for the permselective properties of the GBM. With recently developed antibodies directed against the GBM HSPG (agrin) core protein and the HS side chain, we demonstrated a decrease in HS staining in the GBM in different human proteinuric glomerulopathies, such as systemic lupus erythematosus (SLE), minimal change disease, membranous glomerulonephritis, and diabetic nephropathy, whereas the staining of the agrin core protein remained unaltered. This suggested changes in the HS side chains of HSPG in proteinuric glomerular diseases. To gain more insight into the mechanisms responsible for this observation, we studied GBM HS(PG) expression in experimental models of proteinuria. Similar HS changes were found in murine lupus nephritis, adriamycin nephropathy, and active Heymann nephritis. In these models, an inverse correlation was found between HS staining in the GBM and proteinuria. From these investigations, four new and different mechanisms have emerged. First, in lupus nephritis, HS was found to be masked by nucleosomes complexed to antinuclear autoantibodies. This masking was due to the binding of cationic moieties on the N-terminal parts of the core histones to anionic determinants in HS. Second, in adriamycin nephropathy, glomerular HS was depolymerized by reactive oxygen species (ROS), mainly hydroxyl radicals, which could be prevented by scavengers both in vitro (exposure of HS to ROS) and in vivo. Third, in vivo renal perfusion of purified elastase led to a decrease of HS in the GBM caused by proteolytic cleavage of the agrin core protein near the attachment sites of HS by the HS-bound enzyme. Fourth, in streptozotocin-induced diabetic nephropathy and during culture of glomerular cells under high glucose conditions, evidence was obtained that hyperglycemia led to a down-regulation of HS synthesis, accompanied by a reduction in the degree of HS sulfation.

The effect of losartan and captopril on glomerular basement membrane anionic charge in a diabetic rat model

OBJECTIVE

Although angiotensin-converting enzyme inhibitors are known to reduce albuminuria by preserving glomerular basement membrane anionic content, the effects of angiotensin II receptor blockage are currently not known. The aim of this study was to evaluate the effects of captopril and losartan on glomerular basement membrane anionic charges in a diabetic rat model.

DESIGN

After diabetes induction with streptozotocin, female Wistar rats were divided into three groups: group A, losartan 10 mg/kg by gavage (n = 8); group B, captopril 50 mg/l in drinking water (n = 6); group C, diabetic control rats (n = 8) given only tap water. Group D (eight rats) served as non-diabetic controls. At the end of 8 weeks, erythrocyte membrane charge, serum sialic acid, urinary glycosaminoglycan and albumin were measured and kidney specimens stained with Alcian blue in order to assess basement membrane glycosaminoglycans.

RESULTS

Red blood cell anionic charges (ng Alcian blue/ 10(6) red blood cells) were 371.5+/-9.9 for group A, 443.5+/-7.1 for group B, 400.1+/-14.7 for group C, 468.7+/-4 for group D (A<D, C<D, P<0.01). Serum sialic acid levels (mg/dl) were 90.6+/-14.1 for group A, 45.6+/-6.8 for group B, 89.1+/-8.5 for group C, 50.8+/-6.4 for group D (A, C>D, P<0.01; A>B P<0.01). Albuminuria (microg/day) was 778+/-221 for group A, 719+/-314 for group B, 1724+/-945 for group C, 393+/-263 for group D (A, B<C, P<0.05). Urinary glycosaminoglycan/creatinine ratio was 14.2+/-1.1 for group A, 9.9+/-1 for group B; 28.3+/-8 for group C, 5.5+/-1.7 for group D (B<C, P<0.001; B<A, P<0.003). Alcian blue staining was 1.8+/-0.2, 2.2+/-0.4, 1.5+/-0.16, 2.8+/-0.2 for groups A, B, C and D respectively (C, A<D P<0.05).

CONCLUSION

Losartan and captopril have comparable effects on reducing albuminuria in a diabetic rat model. While captopril preserves basement membrane anionic charges, losartan has no effect. The anti-proteinuric effects of these drugs seem to have different mechanisms.

Protective effects of a small dose of captopril on the reduction of glomerular basement membrane anionic sites in spontaneously hypertensive rats with streptozotocin-induced diabetes

Angiotensin-converting enzyme (ACE) inhibitors have been used in several clinical trials to slow a progressive decline in glomerular function in patients with diabetic nephropathy independent of their effects on blood pressure. The purpose of this study was to clarify the mechanisms(s) through which an ACE inhibitor, captopril, exerts its protective effect on renal function using spontaneously hypertensive rats (SHR) with streptozotocin (STZ)-induced diabetes. Male SHRs were made diabetic by intravenous injection of STZ (45 mg/kg). One hundred or 25 mg/kg of captopril was administered daily for 4 weeks to them. Urine albumin excretion (UAE) rate was markedly increased in diabetic SHRs, while captopril treatment resulted in a significant suppression of UAE in diabetic SHRs, independent of both its daily dose and effects on blood pressure as well as glycemic control. Examination by electron microscope revealed that the number of anionic sites (AS) in the lamina rara externa per 1000 nm of glomerular basement membrane (GBM) was significantly decreased (22.9+/-0.2 to 16.1+/-0.3, p < 0.001), after induction of diabetes, whereas, significant recovery (18.2+/-0.1, p < 0.001) could be obtained even by the smaller dose (25 mg/kg) of captopril which did not exert either antihypertensive or antidiabetic effect on diabetic SHRs. Thus, we demonstrate here the direct evidence that captopril, an ACE inhibitor, can protect against damage on GBM of diabetic SHR without controlling blood pressure as well as blood glucose level.

Heparan sulfate proteoglycan synthesis and its expression are decreased in the retina of diabetic rats

PURPOSE

Heparan sulfate proteoglycan (HSPG) is an integral component of all basement membranes and is implicated in the charge-selective properties of these basement membranes. Studies have shown a decrease in the number of HSPG anionic sites in the retinal capillary and glomerular basement membrane of diabetic animals. This study examined whether a decrease in both HSPG synthesis and its expression as perlecan are reduced in the retina of streptozotocin-diabetic rats.

METHODS

Diabetes was induced in male Wistar rats by intraperitoneal injection of streptozotocin and the rats were sacrificed 20 weeks later. Retinas were isolated and HSPG synthesis was assessed by incorporation of 35S-sulfate into heparan sulfate. The expression of mRNA for perlecan was quantified by RNase protection assay.

RESULTS

Both the synthesis of HSPG and mRNA expression for perlecan were decreased in the retina of diabetic compared to normal rats.

CONCLUSIONS

The decrease in HSPG synthesis may account for the reported decrease in retinal basement membrane anionic sites and increased capillary permeability that occurs in diabetes.

Doxazosin prevents proteinuria and glomerular loss of heparan sulfate in diabetic rats

We examined whether blood pressure reduction or good glycemic control equally lower albuminuria by preventing glomerular loss of heparan sulfate and progression of glomerulosclerosis in streptozotocin-induced diabetic rats. We used doxazosin, and alpha 1-adrenergic blocker, to lower systemic blood pressure, and good glycemic control was achieved by insulin treatment. Rats were killed after 20 weeks of treatment. Doxazosin significantly lowered systolic pressure in diabetic rats; however, it had no effect in normal rats. Good glycemic control also lowered systolic pressure. In diabetic rats with good glycemic control, doxazosin had an additive effect on blood pressure. Glomerular heparan sulfate synthesis was significantly lower and urinary albumin excretion higher in diabetic than in normal rats. Both doxazosin treatment and good glycemic control normalized these abnormalities in diabetic rats. Insulin normalized plasma glucose and glycosylated HbA1 concentrations in diabetic rats, as did doxazosin. Significant increases in mesangial area and glomeruloscelerosis were observed in diabetic rats. Only good glycemic control normalized these pathological changes in all diabetic rats. Two-way factorial ANOVA showed an interaction between the effects of doxazosin and insulin on systolic pressure and plasma glucose. The data show that after 20 weeks of doxazosin treatment, albuminuria was reduced by 80%; however, this treatment had no significant effect on mesangial expansion or progression to glomerulosclerosis. Conversely, good glycemic control prevented all three of the preceding sequelae.

Reduction of heparan sulphate-associated anionic sites in the glomerular basement membrane of rats with streptozotocin-induced diabetic nephropathy

Heparan sulphate-associated anionic sites in the glomerular basement membrane were studied in rats 8 months after induction of diabetes by streptozotocin and in age- adn sex-matched control rats, employing the cationic dye cuprolinic blue. Morphometric analysis at the ultrastructural level was performed using a computerized image processor. The heparan sulphate specificity of the cuprolinic blue staining was demonstrated by glycosaminoglycan-degrading enzymes, showing that pretreatment of the sections with heparitinase abolished all staining, whereas chondroitinase ABC had no effect. The majority of anionic sites (74% in diabetic and 81% in control rats) were found within the lamina rara externa of the glomerular basement membrane. A minority of anionic sites were scattered throughout the lamina densa and lamina rara interna, and were significantly smaller than those in the lamina rara externa of the glomerular basement membrane (p<0.001 and p<0.01 for diabetic and control rats, respectively). Diabetic rats progressively developed albuminuria reaching 40.3 (32.2-62.0) mg/24 h after 8 months in contrast to the control animals (0.8 (0.2-0.9) mg/24 h, p<0.002). At the same time, the number of heparan sulphate anionic sites and the total anionic site surface (number of anionic sites x mean anionic site surface) in the lamina rara externa of the glomerular basement membrane was reduced by 19% (p<0.021) and by 26% (p<0.02), respectively. Number and total anionic site surface in the remaining part of the glomerular basement membrane (lamina densa and lamina rara interna) were not significantly changed. We conclude that in streptozotocin-diabetic rats with an increased urinary albumin excretion, a reduced heparan sulphate charge barrier/density is found at the lamina rara externa of the glomerular basement membrane.

Long-term effects of antihypertensive treatment and good glycemic control on plasma lipids in diabetic rats

The long-term effects of angiotensin-converting enzyme inhibitors (captopril and enalapril), calcium-entry blockers (diltiazem and nicardipine), and good glycemic control on plasma lipids and lipoproteins were studied in streptozotocin diabetic rats. Diabetic rats had increased plasma cholesterol, tryiglycerides, very-low-density lipoprotein (VLDL), and low-density lipoprotein (LDL) cholesterol, and decreased levels of high-density lipoprotein (HDL) cholesterol than in normal rats. Compared to other antihypertensives, nicardipine seems to have a less beneficial effect on lipids and lipoproteins. However, it is only the good glycemic control that normalized these plasma lipids and lipoproteins in diabetic rats. This suggests that good glycemic control prevents dyslipidemia in diabetic rats. The observed beneficial effects of antihypertensives were unrelated to either food or water intake.

Possible effect of angiotensin-converting enzyme inhibition on glomerular charge selectivity

Angiotensin-converting enzyme (ACE) inhibitors are known to reduce urinary albumin excretion (UAE) in diabetic patients. Animal studies have shown that, besides diminishing the glomerular capillary pressure, ACE inhibitors might reduce albuminuria by influencing glomerular charge selectivity through glomerular preservation of heparan sulphate proteoglycan. In humans, an indirect measurement of glomerular charge selectivity can be obtained by calculating the glomerular charge selectivity index (SI), a clearance ratio of IgG/IgG4, two identically sized but differently charged molecules. The aim of the present study was to evaluate the effect of ACE inhibition on charge selectivity by comparing SI in type I (insulin-dependent) diabetic patients with microalbuminuria after 6 years of treatment either with or without captopril. Thirty-five of 45 patients participating in a prospective randomized study evaluating the effect of captopril in preventing the development of diabetic nephropathy were included in the present study, 17 being treated with captopril, 18 left as untreated controls. The selectivity index was calculated after measuring s-IgG, u-IgG, s-IgG4, and u-IgG4. The results demonstrated a higher selectivity index in the captopril-treated group [1.21 (0.51-1.94) median (range)] compared to the control group [0.94 (0.31-1.87)], however, the difference was not statistically significant (p = 0.16). A negative correlation between the selectivity index and UAE was demonstrated in the captopril-treated group (r = -0.77; p = 0.0004), whereas the correlation in the control group did not reach statistical significance (r = -0.3; p = 0.2).(ABSTRACT TRUNCATED AT 250 WORDS)

Selective proteinuria in diabetic nephropathy in the rat is associated with a relative decrease in glomerular basement membrane heparan sulphate

In the present study we investigated whether glomerular hyperfiltration and albuminuria in streptozotocin-induced diabetic nephropathy in male Wistar-Münich rats are associated with changes in the heparan sulphate content of the glomerular basement membrane. Rats with a diabetes mellitus duration of 8 months, treated with low doses of insulin, showed a significant increase in glomerular filtration rate (p < 0.01) and effective renal plasma flow (p < 0.05), without alterations in filtration fraction or mean arterial blood pressure. Diabetic rats developed progressive albuminuria (at 7 months, diabetic rats (D): 42 +/- 13 vs control rats (C): 0.5 +/- 0.2 mg/24 h, p < 0.002) and a decrease of the selectivity index (clearance IgG/clearance albumin) of the proteinuria (at 7 months, D: 0.20 +/- 0.04 vs C: 0.39 +/- 0.17, p < 0.05), suggesting loss of glomerular basement membrane charge. Light- and electron microscopy demonstrated a moderate increase of mesangial matrix and thickening of the glomerular basement membrane in the diabetic rats. Immunohistochemically an increase of laminin, collagen III and IV staining was observed in the mesangium and in the glomerular basement membrane, without alterations in glomerular basement membrane staining of heparan sulphate proteoglycan core protein or heparan sulphate. Glomerular basement membrane heparan sulphate content, quantitated in individual glomerular extracts by a new inhibition ELISA using a specific anti-glomerular basement membrane heparan sulphate monoclonal antibody (JM403), was not altered (median (range) D: 314 (152-941) vs C: 262 (244-467) ng heparan sulphate/mg glomerulus). However, the amount of glomerular 4-hydroxyproline, as a measure for collagen content, was significantly increased (D: 1665 (712-2014) vs C: 672 (515-1208) ng/mg glomerulus, p < 0.01). Consequently, a significant decrease of the heparan sulphate/4-hydroxyproline ratio (D: 0.21 (0.14-1.16) vs C: 0.39 (0.30-0.47), p < 0.05) was found. In summary, we demonstrate that in streptozotocin-diabetic rats glomerular hyperfiltration and a progressive, selective proteinuria are associated with a relative decrease of glomerular basement membrane heparan sulphate. Functionally, a diminished heparan sulphate-associated charge density within the glomerular basement membrane might explain the selective proteinuria in the diabetic rats.

Effect of diltiazem on glomerular heparan sulfate and albuminuria in diabetic rats

Calcium entry blockers, particularly diltiazem, have been shown to lower not only systemic blood pressure but also improve proteinuria in non-insulin-dependent diabetic patients. The presence of proteinuria is attributed to the loss of glomerular heparan sulfate, which confers a negative charge on the basement membrane. In the present study, we evaluated the efficacy of diltiazem in lowering blood pressure and proteinuria in diabetic rats and also examined the possibility that diltiazem prevents proteinuria through glomerular preservation of heparan sulfate. Diabetes was induced in male Wistar rats by streptozotocin (60 mg/kg). One group of diabetic rats was treated with diltiazem (25 mg/L) in drinking water for 20 weeks. Another group of diabetic rats and a group of nondiabetic rats were given tap water only. Systolic blood pressure was measured at 4, 8, 12, and 20 weeks. Urinary excretion of albumin was done at 4, 8, 12, 16, and 20 weeks. At the end of 20 weeks, all rats were killed, kidneys were removed, and glomeruli were isolated. Total glycosaminoglycan and heparan sulfate synthesis were determined by incubating glomeruli in the presence of [35S]sulfate. Diltiazem lowered blood pressure significantly in diabetic rats at 8, 12, and 20 weeks. Diabetic glomeruli synthesized less total glycosaminoglycan and heparan sulfate than glomeruli from normal rats. Characterization of heparan sulfate by ion-exchange chromatography showed that the fraction eluted with 1 M NaCl was significantly lower and the fraction eluted with 1.25 M NaCl significantly higher in diabetic than in normal rats. Diltiazem therapy returned not only glomerular synthesis but also various fractions of heparan sulfate to normal.(ABSTRACT TRUNCATED AT 250 WORDS)