Electron microscopic determination of the sequence of acute tubular and vascular injury induced in the rat kidney by a carcinogenic dose of dimethylnitrosamine.

The early sequence of ultrastructural changes induced in the rat kidney by a single, carcinogenic dose (60 mg/kg of body weight) of dimethylnitrosamine (DMN) was traced in 6-week-old, protein-deprived female Wistar rats. The first alterations, moderate lipid droplet formation and proliferation of smooth endoplasmic reticulum, were observed between 2 and 24 hours, primarily in the P2 segment of the proximal tubule. There was no accompanying evidence of cytoplasmic degeneration within tubule epithelium at this stage. By 18 to 24 hours, cytotoxicity was observed in zone 1 resident cortical fibrocytes, followed by focal necrosis of vascular endothelium of the zone 1 peritubular capillary network at 2 to 4 days. With disruption of the capillary lining, red cells leaked into the interstitial space. At the same time, overt cytotoxicity occurred specifically in the P2 segment of the nephron, with marked lipid accumulation and focal cytoplasmic degradation, leading to scattered single-cell necrosis spatially related to the focally damaged capillaries by 5 days. From day 4, infiltration of mononuclear phagocytes into the interstitial space of zone 1 was prominent. Macrophages were observed to be involved in erythrophagocytosis during the course of this inflammatory reaction. Capillary repair appeared to be complete by 7 days, and proximal tubule regeneration was ongoing through the 11th day. By 14 days, the inflammatory process was decreasing. The following hypothesis is presented as an explanatory basis for this pattern of injury and acknowledges the pharmacokinetics of DMN in the protein-deprived rat. The results suggest the possibility that DMN might be metabolically activated to a toxic species by the tubular microsomal enzyme system in the P2 segment of the proximal nephron with subsequent diffusion of a reactive intermediate(s), or toxic cell product, to produce cytotoxic injury in the susceptible mesenchymal cell populations, the resident cortical fibrocyte , and the adjacent capillary endothelium. The observed time sequence also suggests that transient circulatory disturbance presumed to result from the focal endothelial necrosis, in turn, causes ischemic injury to the associated P2 segment of the proximal tubule as a relatively delayed event. Resolution of the renal lesion by 3 weeks is associated with endothelial and tubule regeneration and clearance of the extravasated red cells from the interstitial space by infiltrated macrophages. Additionally, the cell populations involved as targets in the acute toxic response to DMN implicate the proximal convoluted tubule and either the resident cortical fibrocyte or peritubular capillary endothelium as origins