|ISSN No. 1606-7754 Vol.14 No.1 April 2006|
The association between glycoxidation and chronic or acute hyperglycaemia in type 2 diabetic patients with retinopathy
Marzena Dworacka, Hanna Winiarska, Sebastian Kuczyński
Department of Pharmacology, Poznań University of Medical Sciences, 60-806 Poznań, Rokietnicka 5a, Polan
Retinopathy is the main cause of blindness in persons with diabetes and the most frequent type of diabetic microvascular complication. There are several factors considered to be retinopathy risk factors. Among them, the role of chronic hypergly-caemia and its consequence glycoxidation, has already been well established but the influence of acute hyperglycaemia on glycoxidation has not been fully elucidated. We have analyzed the association between daily glycaemic fluctuations (acute hyperglycaemia), estimated from plasma 1,5-Anhydroglucitol concentrations, and glycoxidation expressed as serum N-carboxy(methyl)lysine concentrations, in type 2 diabetic patients with and without retinopathy. 1,5-anhydroglucitol concen-trations in plasma were determined by an enzymatic method and N-carboxy(methyl)lysine concentrations in serum were estimated by ELISA. After allowing for the effects of hyperlipidaemia and residual β-cell function we found no relation between 1,5-anhydroglucitol concentrations in plasma and N-carboxy(methyl)lysine concentrations in serum and conclude that glycoxidation in patients with type 2 diabetes with retinopathy is not directly, if at all, related to acute hyperglycaemic epi-sodes. (Int J Diabetes Metab 14: 27-31, 2006)
Key words: hyperglycaemia, diabetes type 2, glycoxidation, microvascular complications, retinopathy
Survival among diabetic patients has been significantly prolonged by modern therapeutic regimens. Nowadays, the clinical impact of diabetes is dominated by its long-term effects on the circulatory, peripheral and other systems and organs. These late complications are the main causes of the increased morbidity and mortality in diabetic persons.
Retinopathy, the main cause of blindness in persons with diabetes, is the most frequent microvascular complication.1,2 but many questions concerning the pathogenesis of chronic diabetic complications have yet to be fully answered.
Several factors associated with progression to retinopathy are regarded as crucial in the pathogenesis of microangiopathy, including degree of metabolic control, duration of diabetes, body mass index, blood pressure, genetic determinants, immu-nologic mechanisms, hormones, and growth factors.3-6 The role of hyperglycaemia, and perhaps its associated nonenzymatic glycation and oxidation, appears to be of central importance in the development of microangiopathy.7,8
The consequences of chronic hyperglycaemia include increased activation of the uronic acid pathway and structural and func-tional changes, stimulation of the polyol pathway and changes in activity of protein kinase C,9 stimulation of nonenzymatic glycation, and oxygen free radical production.10,11
Nonenzymatic glycation results in formation of covalent connections between the alde-hyde group of glucose and free amino groups of proteins. The ultimate products of these reaction are advanced glycation end products (AGEs) – structurally differentiated chemi-cal compounds. Changes in structure of collagen and intercellular matrix due to AGE formation influence the properties of connective tissue. AGEs also react with macrophage receptors, releasing cytokines and growth factors and, in this way, AGEs stimulate de-structive and mitogenic processes. Moreover, the synthesis of AGE is a source of free radicals.10,11 The toxic oxygen derivatives inhibit nitric oxide synthase and this reaction is the source of the next pool of free radicals. Reduced availability of a vasodilator such as nitric oxide could contribute to the reduced blood flow throughout small blood ves-sels.10,12
Chronic hyperglycaemia in diabetes is a major source of free radicals, which may be generated via at least three different pathways: glucose auto-oxidation, non-enzymatic glycation and activation of the sorbitol pathway.12 Non-enzymatic glyca-tion and oxidative reactions are closely related and termed glycoxidation.
N-carboxy(methyl)lysine (CML) has been detected and measured in tissues and serum.12-14 CML is a non-fluorescent, non-cross-linked structure formed as a product of both glycoxidation and lipoxidation14 and was recommended as biochemical marker of two correlated processes: nonenzymatic glycation and glycoxidation.12
The role of glycoxidation in the development and progression of diabetic retinopathy has been well documented15,16 and a correlation between glycoxidation and the incidence of retinopathy has been found in hamsters.15-17 Microvascular complica-tions, including retinopathy, are associated with hyperglycaemia measured as fasting glucose concentrations and HbA1c,18 but these parameters do not indicate acute postprandial hyperglycaemia.19 Postprandial hyperglycaemia, defined as short-term daily glucose fluctuations, was recently confirmed as the most significant macroangiopathic (cardiovascular) risk factor.20,21 The only retrospective plasma marker of such glycaemic spikes is the 1,5-anhydroglucitol (1,5-AG) concentration. The 1,5-AG concentration in plasma promptly decreases with each hyperglycaemic episode,22,23 even short-lasting ones. 1,5-AG is eliminated by the kidneys and 1,5-AG reabsorption is competitively inhibited by glucosuria. Hyperglycaemic episodes stimu-lating glucosuria result in rapid 1,5-AG elimination in the urine.22 The reference values of 1,5-AG levels in the plasma of healthy humans range between 14.4 – 30.2 mg/l, while in patients with diabetes the plasma 1,5-AG levels are markedly re-duced, i.e. in patients with type 2 diabetes 1,5-AG concentrations in plasma vary between 0.9 – 26.6 mg/l, depending on the frequency and duration of hyperglycaemic episodes.23 HbA1c, the routine retrospective parameter of chronic hyperglycaemia, is not designed to capture acute, short-lasting, hyperglycaemic spikes.24
We investigated whether serum concentrations of CML are increased and linked to acute hyperglycaemia in patients with type 2 diabetes and retinopathy. To exclude the potential influence of residual β-cell function on CML concentrations25,26 we randomized patients according to their C-peptide level.