Cyclosporine A (CyA) nephorotoxicity is associated with impaired renal hemodynamic funtion and increased production of the vasoconstrictor eicosanoid thromboxane A2 (TxA2). In CyA toxic rats, renal dysfunction cna be partially reversed by inhibitors of thromoboxane sysnthase. However, interpretation of these results is complicated since inhibitance of thromboxane synthase may cause accumulation of prostaglandin endoperoxides that can act as partial agonists at the TxA2 receptor and may blunt the efficacy of treatment. Furthermore, these endoperoxides may be used as substrate for production of vasodilator prostaglandins causing beneficial effects on hemodynamics which are independent of thromboxane inhibition. To more specially examine the role of TxA2 in CyA toxicity, we investigated the effects of the thromboxane receptor antagonist GR32191 on renal hemodynamics in a rat model of CyA nephrotoxicity. In this model, administration of CyA resulted in a significant decrease in glomerular filtration rate (GFR) 2.85±0.26 [CyA] vs 6.82±0.96 ml/min/kg [vehicle]; p<0.0005) and renal blood flow (RBF) (21.6±2.31 [CyA] vs 31.8±3.60 ml/min/kg [vehicle]; p<0.025). Renal vascular resistance (RVR) was significantly higher in rats given CyA compared to animals treated with CyA vehicle (5.32±0.55 [cyCyA] vs 3.54±0.24 mm Hg/min/ml/kg [vehicle]; p<0.05). These hemodynamic alterations were associated with a significant increase in urinary excretion of unmetabolized, "native" thromboxane B2 (TxB2 (103±18 [CyA] vs 60±16 pg/hour [vehicle]; p<0.05). Only minimal histomorphologic changes were apparent by light microscopic examination of kidneys from both CyA and vehicle treated animals. However, with immunoperoxidase staining, a significantly greater number of cells experssing the rat common leukocyte antigen was found in the renal interstitium of rats given CyA*. There was no detectable increase in monocytes/macrophages in the kidneys of CyA toxic animals. In rats treated with CyA, intraarterial infusion of GR32191 at maximally tolerated doses significanlty increased GFR and RBD, and decreased RVR. Although both RBF and RVR were restored to levels not different from controls, GFR remained significantly reduced following administration of GR32191. These data suggest that the potent vasoconstrictor TxA2 plays an important role in mediating renal dysfunction in CyA nephrotoxicity. However, other factors may be important in producing nephrotoxicity associated with CyA.
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