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

Infection with certain strains of Escherichia coil and endotoxemia results in renal glomerular thrombotic microangiopathy (TMA) characterized by endot helial swelling and prominent glomerular microthrombus formation. Nitric ox ide (NO) is an endogenous biologic modulator with diverse physiologic funct ions including vasodilation and inhibition of platelet adhesion and aggrega tion. NO is synthesized from conversion of L-arginine to L-citrulline by a family of NO synthases (NOS), which include constitutive and inducible isof orms. Indirect evidence supports the hypothesis that TMA is associated with depressed intrarenal NO production. However, the effect of TMA on renal ti ssue 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. S ubgroups 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 (eNO S and iNOS) were examined. Additionally, we examined the effect of endotoxi n on glomerular NO production, and eNOS and iNOS protein expression in vitr o. 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 T MA characterized by endothelial swelling, platelet accumulation/adhesion, a nd 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 lar gely by glomerular circulating neutrophils and endothelial cells, peritubul ar vascular endothelium, and collecting ducts of cortex and medulla. In vit ro incubation of isolated glomeruli with endotoxin also resulted in a marke d reduction in eNOS expression and a significant rise in iNOS content. Admi nistration of E. coli endotoxin leads to a sustained fail in renal eNOS exp ression both in vivo and in vitro. The associated decline in intrarenal end othelial NO production/availability may result in renal vasoconstriction an d a hypercoagulative state, which may contribute to the pathogenesis of end otoxin-induced TMA.