Persistence of inert macromolecules (Imposil) in the rat mesangium and glomerular functional disturbance.

Imposil iron-dextran is an inert tracer that has been used to study mesangial uptake and clearance of macromolecular material from the glomerular circulation. Such a tracer may be a useful marker of altered mesangial function in animals with some forms of glomerulonephritis. We have studied mesangial handling of intravenously injected Imposil (50 mg/100 g body weight) in normal rats by light, immunofluorescence and electron microscopy for up to 3 months. Mesangial cell uptake was maximal at 48-54 h. Extrusion and drainage of tracer to the vascular pole and distal tubule was evident at 3 days but iron was still present in mesangial cells at 3 months. Possible functional renal impairment resulting from persistent mesangially sequestered tracer was examined by measuring daily urine protein and iron excretion. A possible relationship between failure of mesangial cells to eliminate inert tracer and increasing glomerular permeability is demonstrated, suggesting that Imposil and similar inert macromolecules cannot be used for long-term studies of mesangial function.