Impaired glucose tolerance plus hyperlipidaemia induced by diet promotes retina microaneurysms in New Zealand rabbits

Type 2 diabetes mellitus is a complex disorder with a worldwide prevalence estimated to be in the range of 1–5%, with greatest impact in developing nations (King et al. 1998). According to the latest report from the National Health and Nutrition Examination Survey (NHANES), type 2 diabetes has a 12.9% prevalence among individuals aged 20 years or more, and over 40% of the adult population may present any hyperglycemic disorder (prediabetes plus diabetes) (Cowie et al. 2009). Micro- and macrovascular complications account for the disease-related morbidity and mortality of type 2 diabetes (Stirban & Tschoepe 2008), and diabetic retinopathy has been reported as the most common cause of blindness in adults (King et al. 1998). In experimental animals, diabetes occurs either spontaneously or by chemical, surgical, genetic or combined techniques, and these models share many clinical characteristics of the human disease (Srinivasan & Ramarao 2007; Chatzigeorgiou et al. 2009). However, there are limitations to the use of such models in research, including animal availability, costs, long periods of induction, heterogeneity in the degree of glucose disturbances and high mortality rates. Chemically induced diabetes obtained using alloxan or streptozotocin promotes irreversible toxic effects on beta cells, with insulin deficiency. However, the increment in blood glucose may not be uniform (Rees & Alcolado 2005; Lenzen 2008; Szkudelski 2001). Non-rodent animal models (swine and non-human primates) have been used to depict spontaneous type 2 diabetes, but they are expensive and require sophisticated animal facilities (Wagner et al. 2006). Models of diet-induced impaired glucose tolerance or type 2 diabetes in rodents are scarce in the literature. Yin et al. (2002) induced hyperglycaemia in rabbits by adding sucrose 37% and lard 10% to the standard laboratory chow and showed mild hyperglycaemia, hyperinsulinaemia and dyslipidaemia, increased lipid peroxidation, decreased NO/NOS levels and aortic atherosclerosis. However, involvement of other target organs was not assessed. Most of what is known about diabetes complications in rabbits derives from models of alloxan-induced diabetes. Oxidative stress and non-enzymatic glycosylation are considered major factors contributing to the extent of chronic complications of diabetes in the kidneys, liver and pancreas (Szkudelski 2001; Griesmacher et al. 1995; Bulut et al. 2001; Sochor et al. 1979; Mir & Darzi 2009). Retinopathy seems to be a consequence of glucose toxicity and increased oxidative stress. Early cellular and molecular changes in the retinal vasculature have been related to glucose-associated mechanisms of toxicity, alterations in vascular permeability and capillary obliteration (Kvanta 2006). The United Kingdom Prospective Diabetes Study (UKPDS) has shown that LDL-cholesterol, followed by HDL-cholesterol, is the main risk factor for cardiovascular events (Turner et al. 1998). To assess initial complications of hyperglycaemia, we modified the protocol of Yin et al. (2002) by adding cholesterol to the diet given chronically to rabbits. To obtain a more comprehensive model, we evaluated these animals 24 weeks after diet-induced hyperglycaemia/hypercholesterolaemia for early functional and/or morphologic alterations in the kidneys, pancreas, liver, aortas and, particularly, to examine retinopathy.