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

Current understanding of subclinical diabetic retinopathy informed by histology and high-resolution in vivo imaging

The escalating incidence of diabetes mellitus has amplified the global impact of diabetic retinopathy. There are known structural and functional changes in the diabetic retina that precede the fundus photography abnormalities which currently are used to diagnose clinical diabetic retinopathy. Understanding these subclinical alterations is important for effective disease management. Histology and high-resolution clinical imaging reveal that the entire neurovascular unit, comprised of retinal vasculature, neurons and glial cells, is affected in subclinical disease. Early functional manifestations are seen in the form of blood flow and electroretinography disturbances. Structurally, there are alterations in the cellular components of vasculature, glia and the neuronal network. On clinical imaging, changes to vessel density and thickness of neuronal layers are observed. How these subclinical disturbances interact and ultimately manifest as clinical disease remains elusive. However, this knowledge reveals potential early therapeutic targets and the need for imaging modalities that can detect subclinical changes in a clinical setting.

The Influence of Indomethacin and Guanethidine on Experimental Streptozotocin Diabetic Neuropathy

In diabetic animals, reduced endoneurial perfusion and oxygen content have been linked to neuropathic abnormalities and might be amenable to pharmacological manipulation. In streptozotocin-induced diabetic rats, we studied the influence of guanethidine adrenergic sympathectomy, indomethacin treatment and a combined strategy on: serial in vivo motor and sensory conduction, resistance to ischemic conduction failure, in vitro myelinated and unmyelinated conduction, endoneurial perfusion and endoneurial oxygen tension. Unlike previous work diabetic animals had normal endoneurial perfusion but lower endoneurial oxygen tensions after six months of hyperglycemia. Guanethidine worsened sensory conduction despite lower microvascular resistance and an improvement in endoneurial oxygen tension. In contrast, indomethacin improved motor and sensory conduction but not oxygen tension. These studies do not support a linkage between conduction deficits and early endoneurial microangiopathy in experimental diabetes. Indomethacin, or related agents may offer a new therapeutic approach toward diabetic neuropathy through a mechanism independent of the endoneurial microvasculature.

Quantitative retinal blood flow mapping from fluorescein videoangiography using tracer kinetic modeling

Abnormal retinal blood flow (RBF) has been associated with numerous retinal pathologies, yet existing methods for measuring RBF predominantly provide only relative measures of blood flow and are unable to quantify volumetric blood flow, which could allow direct patient to patient comparison. This work presents a methodology based on linear systems theory and an image-based arterial input function to quantitatively map volumetric blood flow from standard fluorescein videoangiography data, and is therefore directly translatable to the clinic. Application of the approach to fluorescein retinal videoangiography in rats (4 control, 4 diabetic) demonstrated significantly higher RBF in 4-5 week diabetic rats as expected from the literature.

Effect of aerobic exercise training on regional blood flow and vascular resistance in diabetic rats

BACKGROUND

Hyperglycemia has been associated with decreased blood flow in various organs, leading to tissue damage and dysfunctions. Exercise training (ET) is known to promote beneficial changes in the autonomic nervous system and may have effects on circulation. The aim of this study was to evaluate coronary and renal blood flows and vascular resistances after ET in diabetic rats.

METHODS

Thirty-two rats were divided into four groups (n = 8): sedentary control (SC), trained control (TC), sedentary diabetic (SD), trained diabetic (TD). Diabetes was induced by an injection of streptozotocin (STZ, 50 mg/kg). The ET was performed on a treadmill for 10 weeks. The blood flows were measured using colored microspheres.

RESULTS

The diabetic groups presented hyperglycemia (blood glucose >350 mg/dL) and ET did not change this parameter. The SD group showed reduced renal blood flow when compared to SC group, and ET was able to normalize this parameter in TD rats (SC: 4.3 ± 0.5; TC: 2.9 ± 0.3; SD: 1.9 ± 0.4; TD: 3.2 ± 0.4 mL/min/g). TD group presented increased coronary blood flow in relation to SD group (SC: 2.3 ± 0.23; TC: 2.8 ± 0.5; SD: 1.2 ± 0.4; TD: 3.0 ± 0.4 mL/min/g). The heart and kidneys vascular resistance were increased in SD group when compared to SC group, and ET was able to reverse these changes.

CONCLUSIONS

Given the relevance of cardiomyopathy and nephropathy in mortality of diabetics, our results demonstrated that ET is effective in improving coronary and renal blood flows and vascular resistances in STZ-diabetic rats, reinforcing the positive role of this approach in preventing hyperglycemia-induced long-term organ damage.

Urinary Kim-1 is a sensitive biomarker for the early stage of diabetic nephropathy in Otsuka Long-Evans Tokushima Fatty rats

Although urinary albumin is the well-known non-invasive marker for diabetic nephropathy, its sensitivity is relatively low. To select more adequate marker, we examined whether urinary tubular markers were more sensitive than albumin using spontaneous type 2 diabetic Otsuka Long-Evans Tokushima Fatty (OLETF) rats. The OLETF rats exhibited histopathological alterations in glomeruli and tubules at 14 weeks of age, but there were no significant differences in the urinary albumin between OLETF and control, Long-Evans Tokushima Otsuka (LETO), rats at 10-16 weeks of age. In the OLETF rats, urinary excretions of N-acetyl-β-d-glucosaminidase and neutrophil gelatinase-associated lipocalin did not increase at least until 20 weeks of age, and urinary vanin-1 transiently increased at 18 weeks of age. On the other hand, urinary kidney injury molecule-1 (Kim-1) in the OLETF rats significantly increased at 14 weeks of age, and the elevation continued up to 22 weeks of age. In a clinical study, urinary KIM-1 levels tended to be higher in type 2 diabetic patients with and without albuminuria than in control subjects. These results suggest that compared to urinary albumin, urinary Kim-1 is a more sensitive biomarker for the detection of early stage of nephropathy in these type 2 diabetic animals. Merit of urinary KIM-1 in diabetic patients remains to be determined.

The relationship between blood pressure and sciatic nerve blood flow velocity in rats with insulin-treated experimental diabetes

Peripheral nerve blood flow (NBF) does not autoregulate but, instead, responds passively to changes in mean arterial pressure (MAP). How this relationship is impacted by insulin-treated experimental diabetes (ITED) is unknown. We tested the hypothesis that ITED will reduce NBF across a range of MAP in Sprague Dawley rats. Following 10 weeks of control or ITED conditions, conscious MAP (tail-cuff) was measured, and under anaesthesia, the MAP (carotid artery catheter, pressure transducer) and NBF (Doppler ultrasound, 40 MHz) responses to sodium nitroprusside (60 µg/kg) and phenylephrine (12 µg/kg) infusion were recorded (regression equations for MAP vs NBF were created for each rodent). Thereafter, motor nerve conduction velocity (MNCV) and nerve vascularization (haematoxylin and eosin stain) were determined. Conscious MAP was higher and MNCV was lower in the ITED group (p < 0.01). In response to drug infusions, the ΔMAP and ΔNBF were similar between groups (p ≥ 0.18). Estimated conscious NBF (based on substituting conscious MAP values into each individual regression equation) was greater in the ITED group (p < 0.01). Sciatic nerve vascularization was similar between groups (p ≥ 0.50). In contrast to the hypothesis, NBF was not reduced across a range of MAP. In spite of increased estimated conscious NBF values, MNCV was reduced in rats with ITED.

Exercise training enhances insulin-stimulated nerve arterial vasodilation in rats with insulin-treated experimental diabetes

Insulin stimulates nerve arterial vasodilation through a nitric oxide (NO) synthase (NOS) mechanism. Experimental diabetes reduces vasa nervorum NO reactivity. Studies investigating hyperglycemia and nerve arterial vasodilation typically omit insulin treatment and use sedentary rats resulting in severe hyperglycemia. We tested the hypotheses that 1) insulin-treated experimental diabetes and inactivity (DS rats) will attenuate insulin-mediated nerve arterial vasodilation, and 2) deficits in vasodilation in DS rats will be overcome by concurrent exercise training (DX rats; 75–85% V̇o2 max, 1 h/day, 5 days/wk, for 10 wk). The baseline index of vascular conductance values (VCi = nerve blood flow velocity/mean arterial blood pressure) were similar ( P ≥ 0.68), but peak VCi and the area under the curve (AUCi) for the VCi during a euglycemic hyperinsulinemic clamp (EHC; 10 mU·kg−1·min−1) were lower in DS rats versus control sedentary (CS) rats and DX rats ( P ≤ 0.01). Motor nerve conduction velocity (MNCV) was lower in DS rats versus CS rats and DX rats ( P ≤ 0.01). When compared with DS rats, DX rats expressed greater nerve endothelial NOS (eNOS) protein content ( P = 0.04). In a separate analysis, we examined the impact of diabetes in exercise-trained rats alone. When compared with exercise-trained control rats (CX), DX rats had a lower AUCi during the EHC, lower MNCV values, and lower sciatic nerve eNOS protein content ( P ≤ 0.03). Therefore, vasa nervorum and motor nerve function are impaired in DS rats. Such deficits in rats with diabetes can be overcome by concurrent exercise training. However, in exercise-trained rats (CX and DX groups), moderate hyperglycemia lowers vasa nervorum and nerve function.

Cadmium, diabetes and chronic kidney disease

Recent epidemiological studies suggest a positive association between exposure to the environmental pollutant cadmium (Cd) and the incidence and severity of diabetes. In this review, we examine the literature suggesting a relationship between Cd exposure, elevated blood glucose levels, and the development of diabetes. In addition we review human and animal studies indicating that Cd potentiates or exacerbates diabetic nephropathy. We also review the various possible cellular mechanisms by which Cd may alter blood glucose levels. In addition, we present some novel findings from our own laboratories showing that Cd elevates fasting blood glucose levels in an animal model of subchronic Cd exposure before overt signs of renal dysfunction are evident. These studies also show that Cd reduces insulin levels and has direct cytotoxic effects on the pancreas. Together, these findings indicate that Cd may be a factor in the development of some types of diabetes and they raise the possibility that Cd and diabetes-related hyperglycemia may act synergistically to damage the kidney.

Diabetes mellitus and the peripheral nervous system: manifestations and mechanisms

Diabetes targets the peripheral nervous system with several different patterns of damage and several mechanisms of disease. Diabetic polyneuropathy (DPN) is a common disorder involving a large proportion of diabetic patients, yet its pathophysiology is controversial. Mechanisms considered have included polyol flux, microangiopathy, oxidative stress, abnormal signaling from advanced glycation endproducts and growth factor deficiency. Although some clinical trials have demonstrated modest benefits in disease stabilization or pain therapy in DPN, robust therapy capable of reversing the disease is unavailable. In this review, general aspects of DPN and other diabetic neuropathies are examined, including a summary of recent therapeutic trials. A particular emphasis is placed on the evidence that the neurobiology of DPN reflects a unique yet common and disabling neurodegenerative disorder.

Growth factor antagonists for the treatment of diabetic vascular complications

Diabetic vascular disease is characterised by altered vascular reactivity and blood flow, hyperpermeability, hyperproliferative responses, and increased extracellular matrix deposition in tissues that are sites of complications. These vascular functional and structural changes have been linked to excessive glucose metabolism in target organs via at least three pathophysiological mechanisms, including increased sorbitol (polyol) pathway activity, increased nonenzymatic glycation of vascular wall proteins, and increased protein kinase C (PKC) activity. These potential mechanisms of glucose toxicity remain the subject of intense scientific investigation, and therapies targeting each of them are being evaluated in clinical trials. It is becoming increasingly clear that excessive production of growth factors provides a common denominator linking these diverse mechanisms of glucose toxicity to the functional and structural vascular alterations associated with diabetes. Increased expression of vascular endothelial growth factor (VEGF) has been linked to increased metabolism of glucose via the sorbitol pathway, to nonenzymatic glycation, and to increased PKC activity, and appears to modulate the hyperpermeability and hyperproliferative responses of diabetes. Consequently, because of the unmet medical need and market size, numerous pharmaceutical and biotechnology companies have initiated research programmes evaluating growth factor antagonists as a potential therapeutic approach for treating complications associated with diabetic vascular disease. However, before growth factor antagonists can enter clinical testing, a number of important issues must be clarified, including the physiological effect of chronic growth factor inhibition, which appears to be necessary for ameliorating chronic vascular deterioration of diabetes, and administration routes, especially for protein-based therapies.

Angiotensin II blockade restores albumin reabsorption in the proximal tubules of diabetic rats

The kidney plays an important role in protein metabolism. The albumin reabsorption in the proximal tubule is disturbed in the early stage of diabetic nephropathy. We evaluated the effects of angiotensin converting enzyme inhibitor (ACEI) and angiotensin III type 1 receptor blocker (ARB) on albumin reabsorption and expression of megalin, an endocytosis receptor for albumin, in proximal tubules of streptozotocin (STZ)-induced diabetic-rats. Diabetic rats at the second week after STZ injection were treated with quinapril (3 mg/kg/day) or candesartan (0.05 mg/kg/day) for 2 weeks. The tubular reabsorption of fluorescein isothiocyanate (FITC)-labeled albumin was evaluated by immunogold electron microscopy, and megalin expression was investigated by immunohistochemistry and Western blotting. Reabsorption of FITC-labeled albumin and megalin expression were prominently inhibited in the proximal convoluted tubules of diabetic rats compared to the controls. Both quinapril and candesartan restored albumin reabsorption in the proximal tubule due to normalization of megalin expression. Urinary albumin excretion was significantly reduced by both ACEI and ARB treatment. Angiotensin II infusion decreased megalin expression and albumin reabsorption in the proximal tubule. In conclusion, angiotensin II blockade restored albumin reabsorption via amelioration of megalin expression in the proximal tubules of early stage diabetic rats.

Nerve and ganglion blood flow in diabetes: an appraisal

Vasa nervorum, the vascular supply to peripheral nerve trunks, and their associated cell bodies in ganglia have unique anatomical and physiological characteristics. Several different experimental approaches toward understanding the changes in vase nervorum following injury and disease have been used. Quantative techniques most widely employed have been microelectrode hydrogen clearance palarography and [14C]iodoantipyrine autoradiographic distribution, whereas estimates of red blood cell flux using a fiber-optic laser Doppler probe offer real time data at different sites along the nerve trunk. There are important caveats about the use of these techniques, their advantages, and their limitations. Reports of nerve blood flow require careful documentation of physiological variables, including mean arterial pressure and nerve temperature during the recordings. Several ischemic models of the peripheral nerve trunk have addressed the ischemic threshold below which axonal degeneration ensues (< 5ml/100 g/min). Following injury, rises in local blood flow reflect acitons of vasoactive peptides, nitric oxide, and the development of angiogenesis. In experimental diabetes, a large number of studies have documented reductions in nerve blood flow and tandem corrections of nerve blood flow and conduction slowing. A significant proportions, however, of the work can be criticized on the basis of methodology and interpretation. Similarly, not all work has confirmed that reductions of nerve blood flow are an invariable feature of experimental or human diabetic polyneuropathy. Therefore, while there is disagreement as to whether early declines in nerve blood flow "account" for diabetic polyneuropathy, there is unquestioned eveidence of early microangiopathy. Abnormalities of vase nervorum and micorvessels supplying ganglia at the very least develop parallel to and together with changes in neurons, Schwann cells, and axons.

Influence of experimental diabetes on the microcirculation of injured peripheral nerve: functional and morphological aspects

Regeneration of diabetic axons has delays in onset, rate, and maturation. It is possible that microangiopathy of vasa nervorum, the vascular supply of the peripheral nerve, may render an unfavorable local environment for nerve regeneration. We examined local nerve blood flow proximal and distal to sciatic nerve transection in rats with long-term (8 month) experimental streptozotocin diabetes using laser Doppler flowmetry and microelectrode hydrogen clearance polarography. We then correlated these findings, using in vivo perfusion of an India ink preparation, by outlining the lumens of microvessels from unfixed nerve sections. There were no differences in baseline nerve blood flow between diabetic and nondiabetic uninjured nerves, and vessel number, density, and area were unaltered. After transection, there were greater rises in blood flow in proximal stumps of nondiabetic nerves than in diabetic animals associated with a higher number, density, and caliber of epineurial vessels. Hyperemia also developed in distal stumps of nondiabetic nerves but did not develop in diabetic nerves. In these stumps, diabetic rats had reduced vessel numbers and smaller mean endoneurial vessel areas. Failed or delayed upregulation of nerve blood flow after peripheral nerve injury in diabetes may create a relatively ischemic regenerative microenvironment.

Increased retinal endothelial cell monolayer permeability induced by the diabetic milieu: role of advanced non-enzymatic glycation and polyol pathway activation

BACKGROUND

Increased vascular permeability could be involved in the pathogenesis of diabetic retinopathy. The present study was aimed at assessing whether high glucose concentrations can impair retinal endothelial cell barrier function directly, irrespective of changes in other determinants of permeability, and the role of non-enzymatic glycation and polyol pathway activation in these alterations.

METHODS

Bovine retinal endothelial cells (BREC) were exposed for various periods to high glucose vs iso-osmolar mannitol and normal glucose containing media+/-agents mimicking or inhibiting advanced glycation end product (AGE) formation and polyol pathway activation. Monolayer permeability was assessed by measuring the transendothelial passage of (125)I-labeled proteins.

RESULTS

Permeability increased significantly (up to +70%) in BREC exposed to high glucose, but not to mannitol, for 1-30 days, vs normal glucose control cells. Exposure to AGE-modified bovine serum albumin (BSA) (> or = 90%) and, to a lesser extent, sorbitol (+28%) mimicked the high glucose effect. The AGE formation and nitric oxide synthase (NOS) inhibitor aminoguanidine significantly reduced (by 60%) changes induced by 30-day exposure to high glucose, whereas methylguanidine, which inhibits only NOS activity, did not affect permeability. Aldose reductase or sorbitol dehydrogenase inhibitors decreased (by approximately 40%) the enhanced leakage produced by 1-day, but not 30-day, incubation in high glucose.

CONCLUSIONS

The present results indicate that high glucose is capable of impairing retinal endothelial cell barrier function directly and that non-enzymatic glycation and polyol pathway activation may mediate these changes, with AGEs participating in the long-term alterations and increased flux through the sorbitol pathway in the short-term effect.

New trends in the etiopathogenesis of diabetic peripheral neuropathy

Neuropathy is well recognized as a major complication of insulin-dependent diabetes mellitus in adults, resulting in significant morbidity and possibly an increased mortality. Both the peripheral and autonomic nervous systems can be involved, and adolescents with diabetes can show early evidence of neuropathy. The pathogenesis of diabetic neuropathy remains unclear but is thought to involve various mechanisms. This complication can be traced to the metabolic effects of hyperglycemia and/or other effects of insulin deficiency on the various constituents of the peripheral nerve. The polyol pathway and/or nonenzymatic glycation affecting one or more cell types in the multicellular constituents of the peripheral nerve appear likely to have an inciting role. The role of other factors, such as possible direct neurotrophic effects of insulin and insulin-related growth factors, seems to be relevant.

Nitric oxide synthase activity and expression in experimental diabetic neuropathy

The changes of nitric oxide synthase (NOS) activity and expression in experimental diabetic neuropathy have not been examined. Increases in ganglia NOS might be similar to those that follow axotomy, whereas declines in endothelial NOS (eNOS) and immunological NOS (iNOS) might explain dysfunction of microvessels or macrophages. In this work, we studied NOS activity in lumbar dorsal root ganglia (DRG) of rats with both short- and long-term experimental streptozotocin-induced diabetes and correlated it with expression of each of the 3 NOS isoforms. NOS enzymatic activity in DRG increased after 12 months of diabetes. This increase, however, was not accompanied by an increase in neuronal NOS immunohistochemistry or mRNA. Immunohistochemical and RT-PCR studies did not identify changes of eNOS expression in 12-month sciatic nerves or DRG from diabetics. Two-month diabetic DRG had increased eNOS mRNA and there was novel eNOS labeling of capsular DRG and perineurial cells. iNOS mRNA levels were lower in diabetics at both time points in peripheral nerves but were unchanged in DRG. Diabetic ganglia showed an increase in NOS activity not explained by novel NOS isoform synthesis. The increases may compensate for NO "quenching" by endproducts of glycosylation. Declines in iNOS may indicate impaired macrophage function.

Ocular haemodynamics and colour contrast sensitivity in patients with type 1 diabetes

BACKGROUND

There is evidence that altered ocular blood flow is involved in the development and progression of diabetic retinopathy. However, the nature of these perfusion abnormalities is still a matter of controversy. Ocular haemodynamics were characterised with two recently introduced methods.

METHODS

The cross sectional study was performed in 59 patients with type 1 diabetes with a diabetes duration between 12 and 17 years and an age less than 32 years and a group of 25 age matched healthy controls. Scanning laser Doppler flowmetry and laser interferometric measurement of fundus pulsation amplitude were used to assess retinal and pulsatile choroidal blood flow, respectively. In addition, colour contrast sensitivity along the tritan axis was determined.

RESULTS

Fundus pulsation amplitude, but not retinal blood flow, increased with the progression of diabetic retinopathy. Retinal blood flow was influenced by plasma glucose levels (r = 0.32), whereas fundus pulsation amplitude was associated with HbA(1c) (r = 0.30). In addition, a negative correlation between the colour contrast sensitivity along the tritan axis and retinal blood flow was observed.

CONCLUSIONS

The present study indicates that pulsatile choroidal blood flow increases with the progression of diabetic retinopathy. Increased retinal blood flow appears to be related to loss of colour sensitivity in patents with type 1 diabetes.

Diabetic Neuropathies

There are currently no treatments available (beyond optimal control of hyperglycemia) that arrest or reverse progressive diabetic polyneuropathy. Consultation with a diabetologist is indicated for patients with poorly controlled disease and polyneuropathy. Immunotherapy for diabetic lumbosacral plexopathy has been advocated but is not supported to date by class 1 clinical trial evidence. Pharmacologic treatment for painful neuropathy may include topical anesthetics, capsaicin cream, anticonvulsants, tricyclic antidepressants, mexiletine, and opioids. Gabapentin, a newer anticonvulsant, has an attractive side-effect profile (a consideration in older patients) and has fewer interactions with other drugs. Amitriptyline may cause excessive sedation, postural hypotension, constipation, and urinary retention, but low evening doses (10 to 25 mg), slowly titrated upward, may offer relief from nocturnal pain. Opioids should not be withheld from patients with severe pain or with intolerance of or contraindications to other agents. A single physician should supervise their use.

Increased peripheral nerve microvessels in early experimental diabetic neuropathy: quantitative studies of nerve and dorsal root ganglia

Microangiopathy is an important complication of diabetes mellitus and neovascularity is a feature of human diabetic retinopathy. The objective of this work was to evaluate numbers, areas and size distributions of whole nerve, endoneurial and dorsal root ganglia perfused microvessels in a detailed fashion using unfixed tissues from rats with experimental diabetes. Experimental neuropathy was studied in male Sprague-Dawley rats 12 weeks after streptozotocin or citrate buffer injection. Electrophysiological recordings of sciatic-tibial motor and caudal sensory fibers identified conduction slowing in diabetes indicating neuropathy. Diabetics had a rise in the numbers of whole nerve microvessels and endoneurial microvessels with associated rises in vessel densities and total vessel luminal areas. Increased vessel numbers in 15-30 microm diameter size ranges were particularly prominent. There was a rise in summed vascular areas in diabetics but the mean luminal area of vessels was not increased. Similar, but not significant trends were observed in a selective analysis of endoneurial vessels alone. In contrast, dorsal root ganglia microvessels were not increased in number. Early experimental diabetic neuropathy is associated with increased numbers of microvessels supplying the peripheral nerve trunk, likely representing neovascularity.

Effects of heparin on vascular dysfunction in diabetic rats

1. Treatment with heparin has beneficial effects in diabetic nephropathy. The occurrence of increased urinary albumin excretion in diabetic patients reflects general vascular dysfunction, including increased transcapillary permeability of macromolecules. The aim of the present study was to evaluate the effects of heparin on vascular dysfunction in diabetic rats. 2. Male Sprague-Dawley rats were used in two studies. Diabetes was induced by 65 mg/kg, i.v., streptozotocin. In one study, diabetic rats were dosed subcutaneously with different heparin fractions for 8 months and the transcapillary escape rate of albumin (TERalb) was measured in anaesthetized animals. In the other study, heparin was given for 6 weeks, followed by tissue albumin clearance measurements in awake rats. Normal and diabetic rats receiving saline served as controls. 3. Blood glucose did not differ among the diabetic groups and ranged from 22 to 26 mmol/L. The mean (+/- SD) TERalb was increased by diabetes compared with values in normal rats (17.5 +/- 3 vs 14.1 +/- 3.3%/h, respectively). Neither unfractionated nor low molecular weight heparin significantly affected this increase. [131I]-Albumin clearance was significantly increased in diabetic rats in the eye, skin and skeletal muscle tissues compared with normal rats (0.17-0.40 vs 0.1-0.23 microL plasma/g per min). Low molecular weight heparin treatment did not affect the increased organ albumin clearance. 4. In conclusion, heparin treatment does not affect diabetes-induced vascular dysfunction as expressed by increased TERalb and clearance of albumin in rats.

Diabetic neuropathies: features and mechanisms

Diabetic neuropathies include both focal neuropathies and diffuse polyneuropathy. Polyneuropathy, the most common of the diabetic neuropathies excluding focal entrapment, has not yet been explained by a single disease mechanism despite intensive investigation. A number of abnormalities appear to cascade into a 'vicious cycle' of progressive microvascular disease associated with motor, sensory and autonomic fiber loss. These abnormalities include excessive polyol (sugar alcohol) flux through the aldose reductase pathway, functional and structural alterations of nerve microvessels, nerve and ganglia hypoxia, oxidative stress, nonspecific glycosylation of axon and microvessel proteins, and impairment in the elaboration of trophic factors critical for peripheral nerves and their ganglia. While an initiating role for nerve ischemia in the development of polyneuropathy has been proposed, the evidence for it can be questioned. The role of sensory and autonomic ganglia in the development of polyneuropathy has had relatively less attention despite the possibility that they may be vulnerable to a variety of insults, particularly neurotrophin deficiency. Superimposed on the deficits of polyneuropathy is the failure of diabetic nerves to regenerate as effectively as nondiabetics. Polyneuropathy has not yet yielded to specific forms of treatment but a variety of new trials addressing plausible hypotheses have been initiated. This review will summarize some of the clinical, pathological and experimental work applied toward understanding human diabetic neuropathy and will emphasize ideas on pathogenesis.

Effects of Magnesium Dobesilate on Nitric Oxide Synthase Activity in Endothelial Cells

This study was done to determine the effects of the angioprotective agent dobesilate on expression and activity on the constitutive nitric oxide synthase (ecNOS) in resident endothelial cells, as well as of the inducible nitric oxide synthase (iNOS) in cytokine-activated endothelial cells, by recording, in culture supernatants, the concentrations of citrulline as a reaction product of both enzymes. In capillary, microvascular, and macrovascular endothelial cells, Mg dobesilate incubation (0.25-1 mM) for 24 hours led to a highly significant concentration-correlating increase in ecNOS activities. These increases were not due to iNOS expression, and with cytokine-activated endothelial cell cultures that do express iNOS only moderate effects with little or no concentration dependency were seen. Addition of the NOS inhibitor NG-monomethyl-L-arginine (NMA) significantly suppresses citrulline formation in all cultures as evidence for the enzyme specificity.

Blood coagulability and fibrinolysis in streptozotocin-induced diabetic rats

Changes in coagulation and fibrinolysis in the plasma (in vivo) and hepatocytes (ex vivo) were studied using hyperglycemic rats. Hyperglycemia was induced by intravenous injection of 50 mg/kg streptozotocin (STZ). Eight weeks after the injection, we observed increases in thrombin-antithrombin III complex and tissue type plasminogen activator activity, decreases in plasma levels of antithrombin III, plasminogen and alpha2-plasmin inhibitor, and significant shortening of activated partial thromboplastin time. In freshly isolated or cultured hepatocytes from STZ-induced hyperglycemic rats, concentrations of proteins related to coagulation were increased. An increase in alanine-aminotransferase leakage and decreases in the levels of amylase, triglycerides and phospholipids were observed in the culture medium of hepatocytes from STZ treated rats. In vivo study revealed that STZ-induced subchronic diabetes induced imbalance between coagulation and fibrinolysis, and ex vivo study in hepatocytes from STZ-treated rats showed membrane degeneration and reduction in amylase synthesis, while protein synthesis related to coagulation was not inhibited. These results suggest that, despite vulnerability of liver cells from STZ treated rats, coagulation activity in the liver is retained and rather enhanced in STZ-induced hyperglycemic rats, which may contribute to the promotion of atherosclerosis.

Dobesilate enhances endothelial nitric oxide synthase-activity in macro- and microvascular endothelial cells

1. Dobesilate is used for normalizing vascular dysfunction in a number of diseases. In search for an effect on endothelial NO production, macrovascular endothelial cells from rat aorta, microvascular endothelial cells from rat exocrine pancreatic tissue, and capillary endothelial cells from rat islets, were cultured in the presence or absence of Mg-Dobesilate. The activity of constitutive nitric oxide synthase (ecNOS) in resident cells as well as of inducible nitric oxide synthase (iNOS) in cytokine-activated cells was measured indirectly by recording the citrulline concentrations in culture supernatants. 2. In each of the different endothelial cells Mg-Dobesilate incubation (0.25-1 mM) for 24 h led to a significant and concentration-dependent increase in ecNOS-activities. With cytokine-activated endothelial cell cultures only moderate effects were seen with little or no concentration-dependency. Addition of the NOS-inhibitor N(G)-monomethyl-L-arginine led to a significant suppression of citrulline formation in all cultures as an evidence for the enzyme specificity of these effects. 3. iNOS- and ecNOS-specific reverse transcription and semi-quantitative polymerase chain reaction (RT-PCR) with RNA from resident or cytokine-activated endothelial cells gave no evidence for an increase in NOS-specific mRNA after Mg-Dobesilate-treatment. Furthermore, Dobesilate-mediated enhancement of NO synthesis in resting endothelial cells was not due to iNOS induction in these cells, as no iNOS-specific signal was found by RT-PCR.

Diabetes impairs sciatic nerve hyperemia induced by surgical trauma: implications for diabetic neuropathy

The most widely used methods to assess nerve blood flow in diabetics rats are hydrogen clearance polarography and laser Doppler flowmetry, techniques requiring surgical exposure of the nerve. In these experiments, we examined the hypothesis that the trauma of surgical exposure introduces an important and hitherto largely unrecognized variable that could account for discordant reports on nerve blood flow changes induced by diabetes. We used the noninvasive (for sciatic nerve) reference sample microsphere method to quantify sciatic nerve blood flow in unexposed va. surgically exposed nerves in rats with streptozotocin-induced diabetes (at different temperatures and after curarization) and in unexposed vs. surgically exposed nerves in galactose-fed rats. Baseline resting blood flow in unexposed nerves in both animal models of diabetes was either normal or increased (but was decreased in diabetic rats given d-tubocurarine). Furthermore, the normal brisk hyperemic nerve blood flow response to the minimal trauma associated with surgical exposure of the nerve was markedly impaired in diabetic and in galactose-fed rats. Normalization of the blood flow response to trauma in galactose-fed rats by an aldose reductase inhibitor suggests that the impairment is linked to increased polyol pathway metabolism. These findings 1) confirm our previous findings that sciatic nerve blood flow in diabetic rats is increased or unchanged in unexposed nerves, while also confirming reports that in surgically exposed nerves blood flow is higher in control than in diabetic rats, and 2) indicate that blood flows in surgically exposed nerves are largely a measure of vascular responses to injury rather than (patho)physiological blood flow in undisturbed nerves.

Increased sciatic nerve blood flow in diabetic rats: assessment by "molecular" vs. particulate microspheres

Sciatic nerve blood flow in diabetic rats in typically increased or unchanged when assessed by the reference sample microsphere method in our laboratory. In contrast, blood flow is generally reported to be decreased approximately 50% when assessed with laser Doppler flowmetry or hydrogen clearance polarography. To address concerns that increased blood flow observed with microspheres might be anomalous because of their particulate nature and/or because insufficient numbers of microspheres are captured in the nerve, a plasma-soluble "molecular microsphere" ([3H]desmethylimipramine, mol wt = 266) and 11.3-micron 153Gd-labeled microspheres were injected sequentially to assess blood flow in rats with streptozotocin diabetes of 2-4 wk duration. Nerve blood flows in diabetic rats were increased 1.5- to 2-fold (vs. control rats) with both tracers; these increases were prevented by tolrestat, an inhibitor of aldose reductase. These observations indicate that blood flow in sciatic nerve (like that in retina and kidney) is increased early after the onset of diabetes and is 1) demonstrable with a plasma-soluble tracer as well as with particulate microspheres and 2) linked to increased metabolism of glucose via the sorbitol pathway.

Increased activity of the insulin-like growth factor system in mesangial cells cultured in high glucose conditions. Relation to glucose-enhanced extracellular matrix production

Recent evidence suggests that several growth factors participate in diabetic glomerular disease by mediating increased extracellular matrix accumulation and altered cell growth and turnover leading to mesangial expansion. Transforming growth factor (TGF)-beta has been demonstrated to be upregulated both in vivo and in vitro, whereas studies on the activity of the renal insulin-like growth factor (IGF) system in experimental diabetes have provided conflicting results. We investigated the effects of prolonged exposure (4 weeks) of cultured human and rat mesangial cells to high (30 mmol/l) glucose vs iso-osmolar mannitol or normal (5.5 mmol/l) glucose levels on: 1) the autocrine/paracrine activity of the IGF system (as assessed by measuring IGF-I and II, IGF-I and II receptors, and IGF binding proteins); and, in parallel, on 2) TGF-beta 1 gene expression; 3) matrix production; and 4) cell proliferation. High glucose levels progressively increased the medium content of IGF-I and the mRNA levels for IGF-I and IGF-II, increased IGF-I and IGF-II binding and IGF-I receptor gene expression, and reduced IGF binding protein production. TGF-beta 1 transcripts and matrix accumulation and gene expression were increased in parallel, whereas cell proliferation was reduced. Iso-osmolar mannitol did not affect any of the above parameters. These experiments demonstrated that high glucose levels induce enhanced mesangial IGF activity, together with enhanced TGF-beta 1 gene expression, increased matrix production, and reduced cell proliferation. It is possible that IGFs participate in mediating diabetes-induced changes in matrix turnover leading to mesangial expansion, by acting in a paracrine/autocrine fashion within the glomerulus.

Expression of nitric oxide synthase in macula densa in streptozotocin diabetic rats

Renal haemodynamic changes are suggested to be an early sign of diabetic glomerulopathy. The juxtaglomerular apparatus relevant to the renin angiotensin system, known to be the site of nitric oxide (NO) production, is considered to play a role in the regulation of glomerular blood flow. This study was therefore designed to clarify whether in situ expression of nitric oxide synthase (NOS) is altered in the kidney of diabetic rats. Streptozotocin-induced diabetic rats with 6, 8, 12 and 32 weeks diabetes duration and age-matched normal control rats were used. The expression of a constitutive form of NOS (cNOS, neural type) and NADPH diaphorase activity in the renal cortex were studied immunohistochemically and histochemically. Diabetic rats had lower body weight and heavier kidney mass compared to control rats at each time point examined. Mean glomerular surface area was greater in 6, 8 and 12-week diabetic rats compared to age-matched control rats. cNOS reaction was localized in the macula densa and appeared less intense in diabetic rats compared to age-matched control rats. The mean number of macula densa cells positive for cNOS in each glomerulus and in each glomerular area was significantly lower in diabetic rats compared to control rats at any time examined. In contrast, NADPH diaphorase activity was detected in both juxtaglomerular arterioles and macula densa cells. The staining reaction of NADPH diaphorase in the arterioles remained positive but appeared less intense in macula densa cells in diabetic rats. These results suggest that NO production in macula densa cells may be reduced in diabetic rats, modulating the vasodilatory function of afferent arterioles. Further investigation on the changes in inducible NOS as well as endothelial cNOS are necessary to clarify mechanisms of haemodynamic changes in the diabetic kidney.

Increased endocytosis in retinal vascular endothelial cells grown in high glucose medium is modulated by inhibitors of nonenzymatic glycosylation

We sought to determine if hyperglycaemia is responsible for increased retinal vascular endothelial-cell (RVEC) endocytosis in diabetes and to assess the role of nonenzymatic glycosylation in mediation of this novel endothelial-cell pathology. RVECs were propagated in media containing either 5 or 25 mmol/l glucose for up to 10 days after which they were exposed to the protein tracer horseradish peroxidase for 30 min. The level of RVEC endocytosis was quantified in intact cell monolayers by electron microscopic stereology, and in cell lysates by a simple spectrophotometric method. The effect of the nonenzymatic glycosylation inhibitors, aminoguanidine and D-lysine, on high-glucose medium induced changes in RVEC endocytosis was tested by inclusion of these agents in the culture medium. RVECs exposed to 25 mmol/l glucose showed a stepwise increase in endocytosis of horseradish peroxidase culminating in a two- to threefold increase after 10 days. Endocytosis returned to normal levels after a further 10 days in 5 mmol/l glucose medium. The increase in RVEC endocytosis was markedly reduced, but not completely normalised, by aminoguanidine and D-lysine. Exposure of cultured RVECs to 25 mmol/l glucose causes an increase in endocytosis of similar magnitude to that experienced by RVEC in early diabetes, and implicates hyperglycaemia in the latter situation. A significant component of the increase in RVEC endocytosis appears to be mediated by nonenzymatic glycosylation.

Dorsal root ganglia microenvironment of female BB Wistar diabetic rats with mild neuropathy

Abnormalities in the microenvironment of dorsal root ganglia (DRG) might play a role in the pathogenesis of sensory abnormalities in human diabetic neuropathy. We examined aspects of DRG microenvironment by measuring local blood flow and oxygen tension in the L4 dorsal root ganglia of female BB Wistar (BBW) diabetic rats with mild neuropathy. The findings were compared with concurrent measurements of local sciatic endoneurial blood flow and oxygen tension. Diabetic rats were treated with insulin and underwent electrophysiological, blood flow and oxygen tension measurements at either 7-11 or 17-23 weeks after the development of glycosuria. Nondiabetic female BB Wistar rats from the same colony served as controls. At both ages, BBW diabetic rats had significant abnormalities in sensory, but not motor conduction compared to nondiabetic controls. Sciatic endoneurial blood flow in the diabetic rats of both ages was similar to control values, but the older (17-23 week diabetic) BBW diabetic rats had a selective reduction in DRG blood flow. Sciatic endoneurial oxygen tensions were not significantly altered in the diabetic rats. DRG oxygen tension appeared lowered in younger (7-11 week diabetic) but not older (17-23 week diabetic) BBW rats. Our findings indicate that there are important changes in the DRG microenvironment of diabetic rats with selective sensory neuropathy.

The influence of sulindac on experimental streptozotocin-induced diabetic neuropathy

We studied the influence of sulindac, a nonsteroidal anti-inflammatory agent on experimental streptozotocin-induced diabetic neuropathy. Untreated diabetic rats were compared with nondiabetic rats, diabetic rats treated with low dose insulin and diabetic rats given sulindac (6.0 mg/kg by gavage 5 of 7 days weekly). Neuropathy was assessed by following serial in vivo motor and sensory caudal conduction, resistance to ischemic conduction failure, and in vitro conduction in sural myelinated and unmyelinated sensory fibers. The impact of low dose insulin and sulindac treatment on the microenvironment of the L4 dorsal root ganglion and sciatic endoneurium was assessed by measuring local perfusion and oxygen tension after 16 weeks of diabetes. Sulindac normalized conduction velocity in caudal sensory fibers, sural myelinated fibers and sural unmyelinated fibers, and reduced the number of diabetic cataracts. Sulindac also normalized a deficit in dorsal root ganglion blood flow and a reduction in sciatic endoneurial oxygen tension in diabetic rats. Low dose insulin improved neuropathy as well but the pattern of benefits was less robust than that of sulindac. Sulindac may be a candidate for a clinical trial in human diabetic polyneuropathy.

Effects of combined insulin and sorbinil treatment on diabetes-induced vascular dysfunction in rats

These experiments were undertaken to assess the effects of combined treatment with insulin (designed to partially restore metabolic control) and sorbinil (an aldose reductase inhibitor [ARI]) versus the effects of sorbinil alone or of two insulin regimens providing different degrees of glycemic control on diabetes-induced metabolic derangements and vascular function. Streptozocin-diabetic rats were divided into the following five groups: (1) untreated (D); (2) treated with approximately 1 U NPH insulin/100 g body weight/d administered in one subcutaneous (SC) injection (DI-1); (3) treated with the same total daily dose of insulin administered in two SC injections (DI-2); (4) treated with approximately 0.2 mmol sorbinil in the diet/kg body weight/d (DS); and (5) treated with once-daily insulin plus sorbinil (DSI-1). Two groups of nondiabetic rats, untreated (C) and sorbinil-treated (CS), served as controls. Metabolic parameters were unaffected by sorbinil treatment in controls and diabetics, whereas insulin administration in the diabetics virtually normalized body growth, food consumption, urine volume, and plasma glucose levels, and markedly decreased hemoglobin A1 (HbA1) levels. Two daily injections were more effective than one in improving metabolic control as measured by HbA1 levels. Regional vascular 131I-albumin permeation was increased about twofold to threefold by diabetes in ocular tissues, sciatic nerve, aorta, diaphragm, and new granulation tissue; it was decreased (but not normalized) by insulin treatment in accordance with improved metabolic control, and was completely normalized by sorbinil. 131I-albumin kidney clearance, as well as urinary albumin and IgG excretion, were markedly increased in diabetic rats and were significantly decreased but not completely normalized by sorbinil and by twice-daily insulin treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased aortic endothelial death and enhanced transendothelial macromolecular transport in streptozotocin-diabetic rats

Hypertension, cigarette smoking and diabetes mellitus are well-known risk factors for atherosclerosis and coronary heart disease. Repeated endothelial cell injury and increased lipid entry have been suggested as initiating events in atherogenesis. Our previous studies have demonstrated that the frequency of endothelial cell death and associated endothelial permeability were significantly increased in the aorta of spontaneously hypertensive rats and chronic oral nicotine-treated rats. In the present investigation, we examined the hypothesis that diabetes also increases the frequency of arterial endothelial cell death and hence transendothelial macromolecular transport, which may have some implications in increasing lipid entry and thus accelerating atherogenesis. Diabetes was induced in 15 male Sprague-Dawley rats by intraperitoneal injection of 60 mg streptozotocin per kg body weight. The duration of diabetes was 6 weeks. A group of 15 age-matched rats, injected only with the buffer and maintained over the same time period, served as the controls. In en face preparations of the thoracic aorta, IgG-containing dead endothelial cells were identified by an indirect immunoperoxidase method, and endothelial leakage to Evans blue-albumin complexes was quantified by fluorescence microscopy. Diabetic rats, compared to control rats, had significantly higher values for the frequency of endothelial cell death (0.77 +/- 0.10% vs 0.38 +/- 0.04%; p < 0.005 by two-tailed, unpaired Student's t-test) and the number density of Evans blue-albumin leaky foci (4.33 +/- 0.48/mm2 vs 2.99 +/- 0.38/mm2; p < 0.05 by two-tailed, unpaired t-test) in the aorta.(ABSTRACT TRUNCATED AT 250 WORDS)

Diabetes mellitus prevents capsaicin from inducing hyperaemia in the rat sciatic nerve

Loss of neurogenic inflammation in response to tissue injury may be an important complication of diabetes mellitus. We studied local neurogenic inflammation in the peripheral nerve trunk of Sprague-Dawley rats 4 months following the induction of diabetes by streptozotocin injection. To assess neurogenic inflammation, the epineurial plexus of the sciatic nerve was exposed to topical capsaicin, an agent that releases vasoactive neuropeptides from perivascular afferent terminals. Under normal circumstances, local vasodilation results in endoneurial hyperaemia or a 'flare'. We evaluated the influence of capsaicin in diabetic sciatic nerve by making serial measurements of endoneurial blood flow using microelectrodes sensitive to hydrogen clearance. After 4 months of hyperglycaemia (glucose > 16.0 mmol/l), diabetic animals had slowing of unmyelinated and myelinated sural sensory conduction velocity compared to citrate buffer injected controls. Baseline sciatic endoneurial blood flow was unaltered by diabetes, and was comparable to controls. There was an expected hyperaemic response of endoneurial blood flow to capsaicin in control rat sciatic endoneurium but no consistent 'flare' response in diabetic rats. Our findings indicate that there is loss of capsaicin-related neurogenic inflammation in the vasa nervorum of experimental diabetes. It is possible that a similar deficit following nerve injury could impair the milieu for axonal regeneration in diabetes.

The effect of aldose reductase inhibitors on glomerular prostaglandin production and urinary albumin excretion in experimental diabetes mellitus

The effect of two structurally unrelated aldose reductase inhibitors, sorbinil and ponalrestat, on glomerular prostaglandin production and urinary albumin excretion was investigated in rats with diabetes induced by streptozotocin. It was found that both aldose reductase inhibitors, when administered from the time of induction of the diabetes, significantly decreased the raised urinary albumin excretion in the diabetic rats, although it remained elevated compared with non-diabetic rats. Glomerular prostaglandin E and 6-keto-prostaglandin F1 alpha production was significantly increased in glomeruli obtained from the diabetic rats. Inhibition of aldose reductase caused a reduction in the raised glomerular prostaglandin production, although this remained above that observed in the non-diabetic rats. Subsequent experiments were performed to determine whether the effects of the aldose reductase inhibitors could be explained by effects on glomerular filtration rate. It was found that ponalrestat, at a dose which markedly reduced urinary albumin excretion, did not significantly affect glomerular filtration rate in non-diabetic rats, rats with untreated streptozotocin-induced diabetes and rats with diabetes partially treated with low dose insulin. Glomerular sorbitol concentrations were significantly elevated in untreated diabetic rats as early as two weeks after the induction of diabetes. It is concluded that the administration of aldose reductase inhibitors from the time of induction of diabetes significantly reduces glomerular prostaglandin production and urinary albumin excretion. The latter effect is not due to an effect on glomerular filtration rate. Increased polyol pathway activity may account in part for the increased glomerular prostaglandin production and urinary albumin excretion in early experimental diabetes.

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.)