Basement membrane abnormalities in diabetes mellitus: relationship to clinical microangiopathy

Extracellular matrix proteins as early markers in diabetic nephropathy

Basement membrane thickening and mesangial expansion characterize the renal involvement in diabetes mellitus and precede any symptoms of renal dysfunction, e.g., albuminuria and changes in glomerular filtration rate. Since the morphological changes can only be diagnosed by biopsy, this study was designed to investigate whether the urinary excretion of renal extracellular matrix proteins might reflect the morphological alterations. To specify the extent of renal involvement in diabetes, the patients, type I as well as type II diabetics, were classified according to their urinary albumin excretion: normal albumin excretion below 30 micrograms/min, microalbuminuria from 30 to 300 micrograms/min, and overt albuminuria above 200 micrograms/min. Laminin, collagen IV, and fibronectin, all intrinsic components of the renal extracellular matrix, were determined in serum and urine by radioimmunoassay or enzyme-linked-immunosorbent-assay, respectively. The results are given as median values (mean). Additionally, the urinary fragment pattern of fibronectin was analysed qualitatively by immunoblotting. Laminin concentrations in serum and in urine did not change in diabetics. Collagen IV decreased in serum of patients with increased albumin excretion (controls: mean = 255 micrograms/l, normoalbuminuric patients: mean = 56 micrograms/l, microalbuminuric patients: mean = 52 micrograms/l, and patients with overt albuminuria: mean = 70 micrograms/l; alpha < 0.01) and increased in urine (controls, normoalbuminuric and microalbuminuric patients: not detectable, patients with overt albuminuria: mean = 5 ng/12 h; apha < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Diabetic neuropathy and the microcirculation

There is a close relationship between the abnormal microcirculation in diabetic subjects and diabetic neuropathy. Neurogenic factors play a prominent role in the regulation of the microcirculation. In diabetic neuropathy, damage to these mechanisms results in a profound haemodynamic disturbance with increased arteriovenous shunting, abnormal postural regulation of blood flow, and abnormal inflammatory responses to tissue injury. Abnormal neurogenic regulation of microvascular haemodynamics may contribute to the development of microangiopathy manifest as increased basement thickening and both are undoubtedly implicated in the pathogenesis of diabetic foot ulceration. In turn it is now recognized that microvascular abnormalities may contribute to the ischaemic aetiology of diabetic neuropathy.

Hyperglycemia and non-enzymatic glycation of serum and tissue proteins in chickens

The objectives of these studies were to determine whether elevated plasma glucose concentrations in broiler breeder chickens (200-250 mg/dl) can result in the non-enzymatic attachment of glucose to serum proteins (fructosamine) and eventual cross-linking of tissue proteins (basement membrane thickness), and to investigate the effects of a factor that may influence this cross-linking process. In response to feeding the satiety factor calcium propionate (CaP, 1.7%), plasma glucose and fructosamine concentrations were increased (P < 0.05) from 1 to 9 weeks of age, whereas concentrations of plasma glucose and fructosamine in feed-restricted chicks were reduced for the first 7 weeks after hatch. In a second study, the age-related increase in kidney capillary basement membrane thickness was prevented (P < 0.05) by feeding the cross-linking inhibitor aminoguanidine (AG, 800 ppm) to 30-week-old broiler breeder hens for 34 weeks. The results from these studies suggest that concentrations of plasma glucose in chickens may, in fact, be exerting long-term detrimental effects on tissue proteins, which can be ameliorated by factors that limit the cross-linking reaction.

Microalbuminuria: prognostic and therapeutic implications in diabetes mellitus

Thirty years following the development of the first radioimmunoassay for albumin, microalbuminuria is widely acknowledged as an important predictor of overt nephropathy in patients with Type 1 diabetes and of cardiovascular mortality in Type 2 diabetes. In addition, there is accumulating evidence to suggest that diabetic patients with microalbuminuria may have more advanced retinopathy, higher blood pressure, and worse dyslipidaemia than patients with normal albumin excretion rates. Recent studies have focused on the role of intervention, principally with antihypertensive therapy and intensive glycaemic control, in reducing microalbuminuria. While successful in reducing urinary albumin excretion it remains to be established whether such therapies will be translated into a reduction in renal failure and decreased cardiovascular morbidity and mortality.

Diabetes mellitus: a disease of abnormal cellular calcium metabolism?

Although the pathogenesis of the diabetes mellitus syndrome remains poorly understood, both insulin-dependent diabetes mellitus and non-insulin-dependent diabetes mellitus predispose the individual to a similar spectrum of complications, including hypertension, macrovascular and microvascular disease, cataracts cardiomyopathy, neuropathy, and premature aging, suggesting that these complications develop along a pathway common to both diabetic conditions. Yet not all diabetic persons are affected by all of these complications or to the same degree. What causes this marked variability in the clinical manifestations of the diabetes syndrome remains an enigma. Accumulating data from animal models of diabetes and from studying patients with diabetes reveal that intracellular calcium levels are increased in most tissues. The activities of the membrane, adenosine triphosphatase (ATPase) associated cation pumps, which determine intracellular calcium level (i.e., calcium-ATPase and [sodium + potassium]-ATPase), are also altered. The nature of the alteration is often tissue specific and may depend on the level of blood glucose or insulin, or both. In this review we discuss the potential contribution of these changes in intracellular calcium regulation, whether acquired or genetically determined, to the pathogenesis of the diabetes syndrome, to the abnormalities in insulin secretion and action (mainly in non-insulin-dependent diabetes), and to the complications of both diabetes syndromes. Altered intracellular calcium metabolism may represent a common, underlying abnormality linking the metabolic, cardiovascular, ocular, and neural manifestations of the diabetic disease process.

Connective tissue metabolism in diabetic peripheral nerves

Diabetes mellitus is associated with multiple connective tissue changes, such as generalized thickening of basement membranes. These alterations are suspected of contributing to the development of diabetic long-term complications encountered in many organs, including kidney, eye and peripheral nerves. The latter tissue, however, has gained relatively little attention with respect to connective tissue changes associated with diabetes. The morphological alterations of connective tissue in the diabetic peripheral nerve include thickening of basement membranes, increased diameter of endoneurial collagen fibrils, and accumulation of microfibrillar material. Recent studies have further elucidated the changes in the extracellular matrix of diabetic nerves and the molecular mechanisms underlying these alterations. For instance, elevated glucose concentrations modulate the expression of several proteins of the extracellular matrix in cultured nerve-derived connective tissue cells. In this article, we review the recent progress in the field of connective tissue alterations in diabetes and particularly in the diabetic peripheral nerve.

Investigating the capillary circulation of the foot with 99mTc-macroaggregated albumin: a prospective study in patients with diabetes and foot ulceration

Macroaggregated albumin has been used to assess capillary circulation for over 20 years. The aim of the present study was to use 99mTc-macroaggregated albumin (99mTc-MAA) perfusion scanning to assess capillary circulation in feet with ischaemic ulcers in patients with diabetes, and relate the appearances to outcome. Twenty-three patients with diabetes presented with a total of 41 neuroischaemic foot ulcers. Perfusion scanning was performed by direct femoral artery injection of approximately 400,000 particles of 99mTc-MAA and imaging with a gamma camera. Patients were followed prospectively for 3 months. Scans were graded independently by a radiologist who was unaware of the site of the ulcers. Significant associations were detected between a radiologist's assessment of increased tissue perfusion and complete healing after 3 months (p = 0.047) and poor tissue perfusion on the image and failure to heal (p = 0.0005). This technique may be useful in deciding whether to persist with conservative treatment in patients with diabetes and foot ulceration.

Dietary myo-inositol supplementation does not prevent retinal and glomerular vascular structural changes in chronically diabetic rats

To assess effects of dietary myo-inositol supplementation on diabetes-induced vascular structural lesions, diabetes was induced in Sprague-Dawley rats with streptozotocin; one-third of these rats was fed a 2% myo-inositol diet for 9 months, one-third was left untreated for 5 months then treated with myo-inositol for the last 4 months, and one-third was untreated for the entire 9 months. Controls included untreated and myo-inositol-treated groups. Weight gain was impaired and plasma glucose, glycosylated hemoglobin, food consumption, urine volume, and albuminuria were increased significantly in diabetic versus age-matched control rats. Plasma myo-inositol levels were increased approximately fivefold in controls and approximately six- to eightfold in diabetic rats treated with myo-inositol. In general, myo-inositol did not affect any of the above parameters in control or diabetic rats. Retinal capillary basement membrane width (CBMW) was increased significantly (approximately 50% versus controls) after 9 months of diabetes. In the control group myo-inositol increased CBMW to the level of untreated diabetic rats; myo-inositol had no effect on CBMW in each diabetic group. The number of retinal capillaries containing pericyte nuclei and pericyte capillary coverage were increased in untreated as well as myo-inositol-treated diabetic rats and in the myo-inositol-treated control group. Glomerular CBMW was increased after 5 and 9 months of diabetes versus age-matched controls, and was increased even more by myo-inositol. Mesangial fractional volume of the glomerulus was increased 36% by diabetes and was decreased slightly but significantly by myo-inositol. These results indicate that diets supplemented with 2% myo-inositol (1) cause capillary basement membrane (CBM) thickening and pericyte changes in retinal capillaries of normal rats, (2) are ineffective in preventing or reversing diabetes-induced retinal CBM thickening, and (3) cause further thickening of glomerular CBM in diabetic rats.

Pericyte growth and contractile phenotype: modulation by endothelial-synthesized matrix and comparison with aortic smooth muscle

We compared the effects of endothelial-synthesized matrix and purified matrix molecules on pericyte (PC) and aortic smooth muscle cell (SMC) growth, heparin sensitivity, and contractile phenotype in vitro. When PC are plated on endothelial-synthesized (EC) matrix, cell number is, on average, 3.1-fold higher than identical populations grown on plastic. Under the same conditions, SMC proliferation is stimulated 1.6-fold. Purified matrix molecules, such as collagen type IV (COLL) or fibronectin (FN), both major components of the EC matrix, stimulate PC/SMC growth 1.2-1.7-fold. Heparin (100 micrograms/ml), which inhibits the growth of early passage SMC by 60%, inhibits PC growth approximately 50%, when cells were plated on plastic. However, PC plated on EC matrix in the presence of heparin (100 micrograms/ml) grow as well as parallel cultures grown on plastic (in the absence of heparin). Concomitant with matrix-stimulated proliferation, we observed a marked reduction in PC containing alpha vascular smooth muscle actin (alpha VSMA), as seen by immunofluorescence using affinity-purified antibodies (173/615 positive pericytes on DOC matrix (28%) vs. 221/285 (77%) positive on glass). SMC respond similarly. Whereas alpha VSMA protein is markedly altered when PC and SMC are cultured on EC matrix, similar reductions in mRNA are not observed. However, Northern blotting does reveal that PC contain 17-30 times the steady-state levels of alpha VSMA mRNA compared to SMC. When SMC and PC cultures on plastic are treated with heparin, the steady-state levels of vascular smooth muscle actin mRNA increase 5 and 1.5 fold, respectively. Similarly, heparin treatment of PC grown on plastic induces a 1.8 fold increase in nonmuscle actin mRNA. These heparin-induced alterations in isoactin mRNA levels are not seen when PC are cultured on EC matrix. We also observed reductions in alpha VSMA and beta actin mRNA levels when PC are plated on FN, where they maintain a ratio of 13:1 (alpha:beta). Similar ratios are found in SMC present in rat and bovine aortae in vivo. These steady-state isoactin mRNA ratios are slightly different from those seen in cultured PC (8-10:1; alpha:beta). These results suggest that selective synthesis and remodelling of the endothelial basal lamina may signal alterations in pericyte growth and contractile phenotype during normal vascular morphogenesis, angiogenesis, or during the microvascular remodelling that accompanies hypertensive onset.

The effect of acarbose on diabetes- and age-related basement membrane thickening in retinal capillaries of the BB/W-rat

The effect of the alpha-glucosidase inhibitor acarbose on retinal capillary basement membrane thickening was examined in the spontaneously diabetic BB/W-rat. Four months of diabetes resulted in significant thickening of the basement membranes of both the superficial and deep capillary nets of the retina. This characteristic change of the retinal microvasculature in diabetes was completely prevented by acarbose treatment that substantially reduced postprandial hyperglycemia. A similar but less pronounced effect was seen on the age-related increase in basement membrane thickening in acarbose-treated non-diabetic control rats who demonstrated decreased glycated hemoglobin levels compared to non-treated control rats. Significant positive correlations between basement membrane thickness and glycated hemoglobin area suggest that diabetic retinal microangiopathy may be prevented by lowering the cumulative glucose exposure to the microvasculature, and that age-related basement membrane thickening is mediated by long-term exposure to normal glucose levels.

Aortic collagen alterations in human diabetes mellitus. Changes in basement membrane collagen content and in the susceptibility of total collagen to cyanogen bromide solubilisation

Structural alterations in arterial extracellular matrix components have been suggested to play a role in the development of arterial disease among patients with diabetes mellitus. This study examines the quantity and quality of collagenous components in aortas from diabetic patients. In order to obtain data about the arterial tissue concentration of type IV and V collagen in diabetic and non-diabetic patients, aortas from 21 patients with diabetes (9 with Type 1 (insulin-dependent) diabetes and 12 with Type 2 (non-insulin-dependent) diabetes), were collected at autopsy together with aortas from groups of sex- and age-matched patients. Intima and media samples from normal and fibrous plaque areas from the individual vessels were evaluated. Pulverized, dried and defatted tissue samples were subjected to chemical solubilization with cyanogen bromide and subsequent immuno-chemical quantitation of the dissolved type IV and V collagen in an ELISA. It was found that the concentration of type IV collagen was increased in the tunica media both in plaque and non-plaque areas in the samples from the diabetic patient groups as compared to the non-diabetic groups. No consistent differences in type IV collagen concentrations were found between diabetic and non-diabetic patients in tunica intima. The type V collagen concentrations and the total collagen content were not altered in the diabetic samples. The fraction of the total collagen that was solubilized during cyanogen bromide treatment was determined, and it was found that this fraction was decreased in most tissue areas in the diabetic patient groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Astemizole reduces blood-retinal barrier leakage in experimental diabetes

We examined the potential of astemizole, a histamine H1-receptor antagonist that does not cross the blood-brain barrier, to reverse blood-retinal barrier leakage to albumin in streptozotocin diabetic rats. Four groups of nondiabetic and four groups of diabetic rats received vehicle or astemizole at dosages of 5, 10, or 20 mg/kg body weight for days 22-28 of a 28-day holding period. There were no significant differences in nondiabetic plasma-vitreous albumin ratios between animals receiving vehicle or any of the three astemizole dosages. Only diabetic rats receiving vehicle showed a significant (p < 0.05) 100% increase in the plasma-vitreous albumin ratio over their nondiabetic counterparts. Diabetic rats receiving either 5, 10, or 20 mg/kg astemizole exhibited total normalization of vitreous albumin accumulation, despite persistence of diabetes. These data indicate that astemizole, an H1-receptor antagonist that does not cross the blood-retinal barrier, is effective in reversing blood-retinal barrier leakage of albumin in experimental diabetes.

Aetiology of diabetic foot ulceration: a role for the microcirculation?

Neuropathy, mechanical stress, and macrovascular disease are involved in the pathogenesis of diabetic foot ulceration. Implicit in the development of gangrene and ulceration is the recognition that these factors interact with the microcirculation, resulting in the failure of skin capillary flow to meet nutritive requirements. There is little evidence to associate structural microangiopathy with foot microcirculatory failure. Significant functional abnormalities of the microcirculation have been defined. In accord with the haemodynamic hypothesis early hyperaemia and capillary hypertension promote more sinister late functional abnormalities with increasing duration of diabetes. These late functional abnormalities include loss of autoregulation and reduced hyperaemic responses which interact with loss of neurogenic flow regulation, disturbed endothelial function, and abnormal rheology to produce the familiar clinical picture of the diabetic foot. Ischaemia secondary to multi-segment arterial disease induces additional abnormalities of microcirculatory function which are superimposed on the pre-existing diabetic microvascular structural and functional microangiopathy.

Capillary pericytes: perspectives and future trends

A complete understanding of the microcirculation requires full knowledge of the structure and function of each of the constituent cells, including pericytes. Vascular endothelium and smooth muscle cells have been investigated intensively during the last two decades, but much less is known about the metabolism and function of capillary pericytes. However, the development of new electron microscopy techniques and the application of new cell culture and molecular biology techniques should allow for the rapid elucidation of the cellular biochemistry and the microvascular function and pathology of this ubiquitous capillary cell.

Anionic sites in diabetic basement membranes and their possible role in diffusion barrier abnormalities in the BB-rat

Basement membrane anionic sites, thought to be responsible for charge selective permeability barriers, were investigated in retinal, endoneurial, and muscle capillary basement membranes and in Bruch's membrane of diabetic, and age- and sex-matched non-diabetic BB-rats using an ultrastructural quantitative histochemical technique. Six months of diabetes was associated with significant basement membrane thickening which was linearly related to a decrease in anionic site density suggesting a relative loss of proteoglycans. Calculation of anionic sites per unit length of basement membrane, reflecting their absolute number, revealed a significant loss in basement membrane, constituting part of normal blood-tissue barrier systems such as retinal and endoneurial capillary basement membranes, and the basement membrane of the retinal pigment epithelium. The absolute number of anionic sites in normally permeable microvessels, such as those of muscle and choriocapillaries, was unaltered by diabetes. We conclude that this specific loss of anionic sites in basement membranes of tissues affected by chronic diabetic complications may in part be responsible for permeability abnormalities seen in these tissues.

Diacylglycerol accumulation and microvascular abnormalities induced by elevated glucose levels

The present experiments were undertaken to examine the hypothesis that glucose-induced increased de novo synthesis of 1,2-diacyl-sn-glycerol (which has been observed in a number of different tissues, including retinal capillary endothelial cells exposed to elevated glucose levels in vitro) and associated activation of protein kinase C may play a role in mediating glucose-induced vascular functional changes. We report here that twice daily instillation of 30 mM glucose over 10 d in a rat skin chamber granulation tissue model induces approximately a 2.7-fold increase in diacylglycerol (DAG) levels (versus tissues exposed to 5 mM glucose) in association with marked increases in vascular clearance of albumin and blood flow. The glucose-induced increase in DAG levels as well as the vascular functional changes are prevented by addition of 3 mM pyruvate. Pharmacological activation of protein kinase C with the phorbol ester TPA in the presence of 5 mM glucose increases microvascular albumin clearance and blood flow, and similar effects are observed with 1-monoolein (MOG), a pharmacological inhibitor of the catabolism of endogenous DAG. A pharmacological inhibitor of protein kinase C (staurosporine) greatly attenuates the rise in microvascular albumin clearance (but not the rise in blood flow) induced by glucose or by MOG. These findings are compatible with the hypothesis that elevated concentrations of glucose increase tissue DAG content via de novo synthesis, resulting in protein kinase C activation, and that these biochemical events are among the factors that generate the increased microvascular albumin clearance.

Vascular filtration function in galactose-fed versus diabetic rats: the role of polyol pathway activity

These studies were undertaken to assess the effects of increased galactose (v increased glucose) metabolism via the polyol pathway on vascular filtration function in the kidneys, eyes, nerves, and aorta. Quantitative radiolabeled tracer techniques were used to assess glomerular filtration rate (GFR) and regional tissue vascular clearance of plasma 131I-bovine serum albumin (BSA) in five groups of male Sprague-Dawley rats: nondiabetic controls, streptozotocin-diabetic rats, nondiabetic rats fed a 50% galactose diet, diabetic rats treated with sorbinil (an aldose reductase inhibitor), and galactose-fed rats treated with sorbinil. Sorbinil was added to the diet to provide a daily dose of approximately .2 mmol/kg body weight. After 2 months of diabetes or galactose ingestion, albumin clearance was increased twofold to fourfold in the eye (anterior uvea, choroid, and retina), sciatic nerve, aorta, and kidney; GFR was increased approximately twofold and urinary excretion of endogenous albumin and IgG were increased approximately 10-fold. Sorbinil treatment markedly reduced or completely prevented all of these changes in galactose-fed, as well as in diabetic rats. These observations support the hypothesis that increased metabolism of glucose via the sorbitol pathway is of central importance in mediating virtually all of the early changes in vascular filtration function associated with diabetes in the kidney, as well as in the eyes, nerves, and aorta. On the other hand, renal hypertrophy in diabetic rats and polyuria, hyperphagia, and impaired weight gain in galactose-fed and in diabetic rats were unaffected by sorbinil and therefore are unlikely to be mediated by increased polyol metabolism.

Glucose-induced microvascular functional changes in nondiabetic rats are stereospecific and are prevented by an aldose reductase inhibitor

Exposure of skin chamber granulation tissue vessels in nondiabetic rats to 11 or 15 mM D-glucose (but not L-glucose or 3-O-methylglucose) twice daily for 10 d induces vascular functional changes (increased albumin permeation and blood flow) identical to those in animals with mild or severe streptozotocin diabetes, respectively. These vascular changes are strongly linked to increased metabolism of glucose via the sorbitol pathway and are independent of nonenzymatic glycosylation as well as systemic metabolic and hormonal imbalances associated with the diabetic milieu. (J. Clin. Invest. 1990. 85:1167-1172.)

Immunogold localization of basal laminar heparan sulfate proteoglycan in rat brain and retinal capillaries

Heparan sulfate proteoglycan was localized in the basal lamina of the brain and retinal capillaries, using an antibody to the core protein of the proteoglycan and a postembedding immunogold labeling method at the ultrastructural level. Gold particles appeared to be randomly distributed in the basal lamina of both types of capillaries, in contrast to discrete ruthenium red-staining sites near the endothelial plasma membrane, as shown in previous studies of the retinal vessel. The present study not only agrees with previous biochemical and immunofluorescence studies, but also provides a more precise localization of the antigen. Furthermore, the method can be useful for quantitative studies.

Effects of very mild versus overt diabetes on vascular haemodynamics and barrier function in rats

Rats injected i.p. with a single dose of nicotinamide (250 mg/kg) 15 min prior to i.v. injection of streptozotocin (65 mg/kg) develop a very mild form of diabetes characterized by slight elevations of plasma glucose, increased levels of HbA1, and reduced insulin secretion in response to an i.v. glucose tolerance test. These rats gain weight normally and they are not hyperphagic, glycosuric, or polyuric. The effects of this very mild form of diabetes vs overt streptozotocin diabetes of three months duration on regional vascular 131I-albumin clearance, blood flow (assessed by 15 microns 85Sr-microspheres), and renal filtration function were examined in male Sprague-Dawley rats. Plasma glucose levels of rats with mild diabetes were 7.4 +/- 0.9 (mean +/- SD) (mmol/l) vs 6.5 +/- 0.6 for control rats and 31.3 +/- 6.0 for overtly diabetic rats. HbA1 levels were increased 1.4 fold in mildly diabetic and 2.3 fold in overtly diabetic rats. Vascular clearance of 131I-albumin was markedly increased in ocular tissues (anterior uvea, retina, and choroid), sciatic nerve, aorta, new (subcutaneous) granulation tissue, and kidney of both diabetic groups, although increases in overtly diabetic rats exceeded those in the mildly diabetic group (2.2-4.6 times control animals vs 1.6-3.3 times, respectively). Likewise, both overt and very mild diabetes markedly increased glomerular filtration rate (approximately 1.8 times and 1.2 times control animals, respectively), urinary excretion of endogenous albumin (approximately 9 times and 4 times) and IgG (approximately 15 times and 4 times), as well as regional blood flow in the anterior uvea, choroid, and sciatic nerve. Increases in tissue sorbitol levels were much larger in overtly diabetic rats (generally 10-20 times control animals) than in mildly diabetic rats (1.5-3 times controls). myo-Inositol levels were significantly decreased only in lens and sciatic nerve of overtly diabetic rats. These observations indicate that even very mild diabetes is associated with vascular functional changes which develop more slowly than in overtly diabetic rats, but are disproportionately large (in view of the minimal increases in glycaemia and tissue polyol levels) compared to those in overtly diabetic rats.