Glomerular mesangium. Analysis of the increased activity observed in experimental acute aminonucleoside nephrosis in the rat.

Kinetic studies have revealed increased mesangial macromolecular uptake in acute aminonucleoside of puromycin (PAN) nephrosis in the rat. The mechanisms leading to this increased activity, however, are poorly understood. Therefore, we studied mesangial function and ultrastructure in rats 8 days after intravenous injection of 6 mg. of PAN per 100 gm. of body weight. One hour after intravenous injection of 20 mg. of colloidal carbon per 100 gm., no differences in mesangial carbon could be detected between PAN rats and controls. After 24 hours, the amount of mesangial carbon was significantly higher in PAN rats; rates of disappearance did not differ. Ultrastructural examination revealed no differences in localization of mesangial carbon at the 1-hour interval. After 24 hours, mesangial cells of PAN rats showed an increased number of lysosomes filled with carbon; the number of carbon particles in the mesangial matrix was not increased. Using an ultrastructural immunoperoxidase technique, we found increased amounts of endogenous IgG in the extracellular space of segmental mesangial lobules of PAN rats. IgG was mainly present in matrix substance of electron-lucent aspect, and quantitative morphometric analysis revealed an increase in volume of this loose permeable matrix component (mesangial channels). No increased staining of IgG was found in the lacis area indicating that there was no increased mesangial egress of macromolecules in PAN nephrosis. The increased volume of the permeable mesangial channels in segmental mesangial lobules of PAN rats may lead to a pooling of tracer material with consequent increased phagocytosis by mesangial cells.