Energy barriers to sodium extrusion from sodium-rich kidney cortex slices.

1. Rat and guinea-pig kidney cortex slices were made Na-rich by leaching in cold NaCl and then allowed to recover in Ringer solution at 30 or 25 degrees C. Net Na extrusion was systematically decreased by the use of re-immersion media with constant Na and diminishing K, or with constant K and increasing Na, and the energy requirements for Na extrusion under these conditions were calculated.2. The critical energy barrier at which no net Na loss can be achieved was somewhat lower in these tissues than found by other authors for frog sartorius muscle.3. Inclusion of pyruvate or insulin and lactate in the recovery fluid had no effect.4. Methylsulphate apparently enters renal cells during incubation and is therefore unsuitable for use with this tissue as a source of impermeant anion.