Modification of anionic sites in myocardial capillary basal laminae of diabetic rats.

Capillary basal laminar thickening is a distinctive feature of diabetic microangiopathy; however, the mechanism responsible for this abnormality remains to be clarified. Recent reports have described a reduction in the distribution of anionic sites in diabetic glomerular basement membranes, with the suggestion that this reduction may generate a compensatory synthesis of basal laminar constituents, causing laminar thickening. In order to provide additional information, the character and distribution of the basal laminar anionic profile were examined in the myocardium of diabetic rats. Diabetes mellitus was induced in 14 rats by injection with streptozotocin, ip; 6 rats served as controls. Myocardial tissue was subjected to Charonis' procedure for the demonstration of anionic sites with the cationic electron-dense dye, ruthenium red, following the sacrifice of the animals at intervals up to 11 months after the induction of the diabetes. The tissues were then processed routinely for electron microscopic examination. A total of 20 electron micrographs, at magnifications of 13,000x and 33,000x, were obtained from each rat for the quantitation of anionic sites. A length measuring 6 micron along each basal lamina was utilized for determining the number of anionic loci. Results of this study show that (1) the number and size of anionic sites in myocardial basal laminae is reduced in diabetic rats, (2) this decrease becomes more pronounced with prolongation of the diabetes, (3) it is detectable prior to the demonstration of basal laminar thickening by electron microscopy, and (4) enzyme digestion treatments indicated that heparan sulfate proteoglycan is the essential stainable component of the anionic sites. These findings provide evidence that the laminar anionic profile is altered in the diabetic myocardium and support the view that this abnormality constitutes a significant initial event in the pathogenesis of basal laminar thickening.