Inhibition of Glomerular Visceral Epithelial Cell Endocytosis During Nephrosis Induced by Puromycin Aminonucleoside

Puromycin aminonucleoside (PA), injected intravenously into rats in a single dose, causes proteinuria and ultrastructural pathology reminiscent of human lipoid nephrosis (puromycin aminonucleoside nephrosis (PAN]. Glomerular epithelial cell (GEC) endocytosis was studied in this model and in rats with protein-overload proteinuria using ultrastructural morphometry. Disappearance curves were constructed for protamine-heparin aggregates (PHA), which localized in the subepithelial region of the glomerular basal lamina following intravenous injection of protamine followed by heparin. Five groups were studied: (a) preproteinuric PAN, 4 days after PA (mean 4.5 +/- 1.5 mg of urinary protein/24 hours); (b) proteinuric PAN, 10 days after PA (mean 128 +/- 9.6 mg/24 hours); (c) recovery from PAN (mean 12.5 +/- 1.5 mg/24 hours); (d) protein-overload proteinuria, induced by injecting albumin intraperitoneally (mean 211 +/- 15.9 mg/24 hours); and (e) saline-injected controls (mean 1.2 +/- 0.2 mg/24 hours). Only the proteinuric PAN animals (group 2) had altered GEC endocytosis with a PHA half-disappearance time different from the group 5 saline controls (143.2 versus 72.6 minutes, p less than 0.05). The half-disappearance times in groups 1, 3, and 4 were 74.6, 80.7 and 86.5 minutes, respectively. Altered GEC function was further characterized by comparing PHA disappearance with the abundance of albumin-filled vacuoles in the GEC. Prolongation of PHA disappearance in group 2 correlated with the virtual absence of vacuoles; they were abundant in nonproteinuric phases of PAN and in animals with overload proteinuria. We conclude (a) GEC endocytosis is reduced only during the proteinuric phase of PAN, (b) GEC endocytosis is active during the preproteinuric phase of PAN and is a factor that may account for the absence of protein in the urine despite abnormal GBM permeability, (c) decreased GEC endocytosis during proteinuric PAN reflects abnormal cell metabolism due to PA and is not simply a consequence of albuminuria, as overload proteinuria did not produce diminished PHA or albumin uptake.