Down-regulation of renal endothelial nitric oxide synthase expression in experimental glomerular thrombotic microangiopathy

Infection with certain strains of Escherichia coli and endotoxemia results in renal glomerular thrombotic microangiopathy (TMA) characterized by endothelial swelling and prominent glomerular microthrombus formation. Nitric oxide (NO) is an endogenous biologic modulator with diverse physiologic functions including vasodilation and inhibition of platelet adhesion and aggregation. NO is synthesized from conversion of L-arginine to L-citrulline by a family of NO synthases (NOS), which include constitutive and inducible isoforms. Indirect evidence supports the hypothesis that TMA is associated with depressed intrarenal NO production. However, the effect of TMA on renal tissue NOS expression has not been fully elucidated. We studied rats with TMA induced by iv bolus injection of high dose (20 mg/kg) E. coli endotoxin. Subgroups of six animals each were sacrificed before or at 30, 90, 180, 360, and 720 minutes after the administration of endotoxin. Renal histology and tissue expression of endothelial and inducible nitric oxide synthases (eNOS and iNOS) were examined. Additionally, we examined the effect of endotoxin on glomerular NO production, and eNOS and iNOS protein expression in vitro. Glomerular capillary thrombosis developed by 180 minutes after endotoxin administration in approximately half of the animals. The glomeruli without thrombotic lesions apparent by light microscopy disclosed early signs of TMA characterized by endothelial swelling, platelet accumulation/adhesion, and patchy fibrinogen deposition. These morphologic changes were associated with a marked reduction of renal tissue eNOS expression beyond 180 minutes after the endotoxin administration. The fall in eNOS expression was coupled with a significant rise in iNOS protein abundance, which was expressed largely by glomerular circulating neutrophils and endothelial cells, peritubular vascular endothelium, and collecting ducts of cortex and medulla. In vitro incubation of isolated glomeruli with endotoxin also resulted in a marked reduction in eNOS expression and a significant rise in iNOS content. Administration of E. coli endotoxin leads to a sustained fail in renal eNOS expression both in vivo and in vitro. The associated decline in intrarenal endothelial NO production/availability may result in renal vasoconstriction and a hypercoagulative state, which may contribute to the pathogenesis of endotoxin-induced TMA.