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.

Antineutrophil cytoplasmic antibodies are present in long standing type 1 diabetics but do not correlate with selective proteinuria

Antineutrophil cytoplasmic antibodies (ANCA) are present in systemic vasculitis with or without renal involvement and in inflammatory bowel diseases, conditions which share damage in proteoglycan content of basal membrane. In diabetes, there is a reduction in proteoglycans in the kidney basal membrane, responsible for the decrease in fixed anionic charges and, consequently, for the prevalent anionic proteinuria (albumin, IgG4) even in the early preclinical stage of nephropathy. The aims of this study were to search for the presence of ANCA in long-standing type 1 diabetic patients and to evaluate possible correlations with size- and/or charge-selective proteinuria. Twenty-two type 1 diabetic patients (duration of diabetes 24 years, range 9-30) selected and grouped according to albumin excretion rate values, were studied together with 21 age and sex comparable normal subjects. ANCA, albumin excretion rate, and the clearances of albumin, of prevalently cationic total IgG (IgG) and of anionic IgG4 were evaluated. ANCA were measured using ELISA and indirect immunofluorescence methods; albumin, IgG and IgG4 were tested with RIA or ELISA methods developed in our laboratory. ANCA were found in five patients, three of whom showed proteinuria. 33.3% and 18.2% of patients with normal IgG and albumin clearances respectively had elevated IgG4 clearance. This study shows for the first time the presence of ANCA in long-standing type 1 diabetic patients and confirms a prevalent anionic protein excretion in these patients, but does not show a correlation between the presence of ANCA and proteinuria, even if the presence of ANCA in diseases sharing alterations in proteoglycan content of vascular basal membrane is noteworthy.

Abnormalities of cognitive functions in IDDM revealed by P300 event-related potential analysis. Comparison with short-latency evoked potentials and psychometric tests

The possible influence of diabetes on the higher mnestic and cognitive functions has been investigated. The P300 wave latency, an endogenous electrophysiological event, was explored and compared with the multimodal short-latency evoked potential (EP) recordings (visual [VEP], brainstem auditory [BAEP], and median and tibial nerve somatosensory EPs [mSEP and tSEP, respectively]) and psychometric test measures in 16 insulin-dependent diabetic (IDDM) patients, in 16 age- and (IDDM) sex-matched nondiabetic subjects, and in a large normal reference population. The age of subjects, the duration of IDDM, and the metabolic control of patients were taken into account. P300 values were significantly increased in IDDM versus matched control subjects (P less than 0.001), and 3 patients showed values above the reference value range. Abnormal VEP recordings were present in 1 of 16 patients, BAEP in 3 of 16, mSEP in 7 of 16, and tSEP in 6 of 16. Digit-span backward test results were significantly (P less than 0.02) modified in the diabetic cohort. There was no tendency for anomalies of P300, short-latency EPs, and psychometric test values to be contemporarily present, except in 1 patient. Electrophysiological or psychometric abnormalities were not clearly correlated with the duration of IDDM or the degree of short-term metabolic control. These findings give evidence that 1) higher cognitive functions may be affected in diabetes as documented by P300 analysis and short-term memory tests, 2) endogenous electrophysiological analysis highlights neuropsychological changes not detectable by psychometric tests, 3) an alteration of evoked potentials was present in half of the IDDM subjects studied, and 4) anomalies of the CNS are patchily distributed in diabetes.

Influence of insulin antibodies on pharmacokinetics and bioavailability of recombinant human and highly purified beef insulins in insulin dependent diabetics

Sixteen insulin dependent diabetics of long standing, with undetectable fasting plasma C peptide concentrations, and eight non-diabetic controls were each infused intravenously with biosynthetic human and highly purified beef insulin (1 mU/kg/min) while euglycaemia was maintained by a Biostator. No difference was observed between the two insulins in respect of insulin pharmacokinetics or biological action. The diabetics showed appreciable insulin resistance, manifested by a 40% reduction in the rate of insulin mediated glucose disposal, which was unrelated to the presence of insulin antibodies. Insulin binding antibodies, however, increased insulin's clearance rate and distribution space and prolonged its pharmacological and biological half lives. The rate at which insulin action was lost, after an intravenous infusion, was more rapid in diabetics without insulin antibody binding than in controls. In respect of their influence on insulin pharmacokinetics, moderate concentrations of insulin antibodies may be of positive advantage to all diabetics without endogenous insulin secretion and are not responsible for the insulin resistance of type 1 diabetes.