Selective vulnerability of the medullary thick ascending limb to anoxia in the isolated perfused rat kidney.

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Abstract
A specific anatomical lesion sharply localized to the cells of the medullary thick ascending limbs (mTAL) and characterized by mitochondrial swelling progressing to nuclear pyknosis and cell death is elicited reproducibly in isolated rat kidneys perfused for 15 or 90 min with cell-free albumin-Ringer's medium gassed with 5% CO2, 95% O2 (O2 content, 1.5 vol/100 ml). The lesion, involving about half of mTALs, appears first in mTALs removed from vascular bundles and near the inner medulla, areas most likely to be anoxic. Hypoxic perfusion (O2 content 0.12 vol/100 ml) exaggerates the lesion, wiping out gradations of damage and extending it to all mTALs. O2-enriched perfusions using rat erythrocytes (O2 content 7.1 vol/100 ml) completely eliminates the lesion (unless gassed with carbon monoxide). Similarly, supplementation of the perfusion medium with a purified hemoglobin (O2 content 5.8 vol/100 ml) prevents mTAL injury. Perfusion with a fluorinated hydrocarbon blood substitute, Oxypherol (O2 content 4.3 vol/100 ml) also attenuates the lesion. These findings suggest that the mTAL is exquisitely susceptible to anoxic damage because of low O2 supply imposed by the medullary vascular system and the high rate of metabolism mandated by active reabsorption of sodium chloride. The vulnerability of the mTAL to anoxic injury could play a key role in the pathogenesis of ischemic renal injury.