Distribution of endogenous albumin in the glomerular wall of streptozotocin-induced diabetic rats as revealed by high-resolution immunocytochemistry

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Abstract
Endogenous albumin was revealed with high resolution in the glomerular wall of renal tissue from normoglycaemic and long-term streptozotocin-induced hyperglycaemic rats applying the protein A-gold immunocytochemical approach. In tissues from normal animals, albumin antigenic sites were detected at the level of the endothelial cell basal plasma membrane and in the subendothelial side of the lamina densa of the glomerular basal laminae. The epithelial side of the laminae was weakly labelled, while the urinary space was devoid of labelling. In the podocytes, labelling for albumin was confined to few lysosomal structures. In diabetic animals, concomitant with hyperglycaemy, low insulin levels, significant glycosuria, proteinuria and albuminuria, the glomerular basal laminae displayed the characteristic increase in thickness found in diabetic microangiopathy (404 ± 45 nm versus 190 ± 10 nm). Major basal laminae deposits were also found in the mesangial regions. Albumin antigenic sites were detected throughout the entire thickness of the glomerular basal laminae without any preferential accumulation at any particular site. Labelling was also found over flocculent material present in the urinary space. Numerous densely labelled lysosomal structures were present in the podocytes. The basal laminae deposits in the mesangial regions were labelled for albumin. Morphometrical evaluations made on the distribution of the labelling confirmed the qualitative observations. Two sites for albumin retention were revealed in the glomerular wall of the normal animal: the endothelial cell basal membrane (< 10 nm) and the subendothelial side of the lamina densa (50 nm). Besides the endothelial cell basal membrane ( < 10 nm) for the diabetic animals, no other site of retention was detected; the distribution of albumin was uniform throughout the basal laminae. These results are in agreement with the physiological demonstration of restricted passage of albumin through the glomerular wall in the normal condition and its loss in diabetes.