31 July 2000
Endotoxin-induced hypotension and altered renal microcirculation could lead to tubular injury, particularly at the physiologically hypoxic outer medulla. We explored this hypothesis in isolated perfused kidneys and in vivo in rats subjected to endotoxemia. Rat kidneys were removed 15 min after endotoxin injection in vivo (from Escherichia coli 0127:B8, 1 mg/kg i.p.) and perfused with oxygenated medium supplemented with 20 amino acids and endotoxin. Glomerular filtration rate and filtration fraction markedly declined (0.4 ± 0.1 ml/min and 1.1 ± 0.1, respectively) as compared with control kidneys (0.7 ± 0.1 ml/min and 1.8 ± 0.1, n = 8–12 per group; p < 0.05). Hypoxic injury to medullary thick ascending limbs in the innermost outer medulla increased (47 ± 9% of tubules vs. 16 ± 8% in controls, p < 0.05). When rats were preconditioned with an additional endotoxin injection 16 h earlier (a manipulation that markedly reduces cortical and medullary blood flow), glomerular filtration rate and filtration fraction further declined to 0.1 ± 0.0 ml/min and 0.4 ± 0.1, respectively (p < 0.01), and tubular sodium reabsorption fell to 81 ± 12 vs 98 ± 0% in controls (p < 0.05). Tubular damage, however, did not increase (20 ± 7%), probably reflecting a decline in reabsorptive workload and oxygen requirement. In rats subjected to a single or two repeated daily doses of endotoxin (1 mg/kg i.p.) plasma creatinine comparably rose 41% on the average over 24 h, creatinine clearance fell by 27% (p < 0.0001), but tubular damage was absent. By contrast, in rats preconditioned with indomethacin and the nitric oxide synthase inhibitor N<sup>G</sup>-nitro- L-arginine methyl ester (10 mg/kg), the addition of endotoxin markedly augmented outer medullary hypoxic tubular damage both in S<sub>3</sub> segments (27 ± 10 vs 1 ± 1%) and in medullary thick ascending limbs (38 ± 11 vs. 10 ± 5%, n = 7–8; p < 0.05). It is concluded that under special conditions, such as altered medullary oxygen balance or defective nitric oxide or prostaglandin synthesis, endotoxin may predispose to hypoxic outer medullary tubular damage.