Glomerular podocytic injury in protein overload proteinuria.

Injection of rats with large doses of bovine serum albumin causes proteinuria which may persist long after the period of overload has ended. In order to assess in this model of proteinuria the relative importance of podocytic epithelial changes versus alterations in anionic groups in the glomerular capillary wall a morphological study has been made of animals in which the kidneys were fixed by vascular perfusion or by in situ drip fixation. By transmission electron microscopy, podocytes showed protein droplets, cytoplasmic vacuoles, spreading of epithelial cytoplasm with loss of foot processes, and focal separation of epithelium from the glomerular basement membrane, occasionally with cytoplasmic disruption. Staining with colloidal iron showed no reduction in the density of anionic groups per unit area on epithelial cell surfaces or elsewhere in glomeruli. However, the reduced surface area of epithelial cells caused by the changes to their structure accounts adequately for the less intense glomerular colloidal iron staining evident by light microscopy. Changes in podocyte structure, particularly those leading to focal cytoplasmic defects on the outer surface of the glomerular basement membrane, appear to be more important than loss of glomerular anionic groups for the development of proteinuria in protein overload nephropathy.