1. In order to explain the vulnerability of medullary thick ascending limb of Heme's loop (mTAL) during hypoxia, adenosine 5′-triphosphate (ATP) content was measured in isolated rat mTAL cells during control conditions and chemically induced hypoxia and compared with those in medullary collecting duct (MCD) cells.

2. Basal ATP levels in mTAL and MCD were estimated as 3.6 and 2.1 mmol/l, respectively. Antimycin A (5 μmol/l) decreased the ATP content by 41% of the control value in the mTAL cells, but failed to reduce that of the MCD cells. Administration of sodium cyanide (5 mmol/l) drastically depleted ATP in the mTAL cells within 5 min (2–3% of control). On the other hand, ATP levels in MCD cells were sustained for at least 60 min after cyanide administration (64% of control).

3. When tubules were made permeable to sodium by the addition of nystatin, the effects of chemical hypoxia on the cell ATP levels were intensified in both segments, and this was partially blocked by pretreatment with ouabain, or by lowering the sodium concentration of the medium.

4. Higher doses of nystatin in mTAL caused a reduction in ATP levels even under control conditions, but its effect was prevented in low sodium medium.

5. The present study suggests that cell ATP levels can be altered by sodium, potassium-dependent adenosine triphosphatase activity, and that due to their high sodium-transporting activity, mTAL cells are more sensitive to reductions in ATP levels during hypoxia than are MCD cells.

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