Potassium transport by the isolated perfused kidney.

Rat kidneys perfused outside of the body with an artificial medium are able to increase their fractional excretion of potassium in response to a rising concentration of potassium in the medium but never show net secretion of potassium. By contrast, isolated perfused kidneys from chronically potassium-loaded rats regularly secrete potassium in excess of the amount filtered. Ouabain completely blocks the secretion of potassium by these isolated kidneys, suggesting that Na-K-ATPase mediates potassium secretion by potassium-adapted rats. Neither sodium deprivation, pretreatment with deoxycorticosterone, nor pretreatment with methylprednisolone prepared the kidney to secrete potassium, despite stimulation of Na-K-ATPase activity in cortex or outer medulla. Potassium loading was the only maneuver tested that increased the activity of Na-Katpase in the inner medulla (white papilla) and also produced potassium secretion by the isolated kidney. Surgical ablation of the papilla abolished the net secretion of potassium normally seen in perfused kidneys of potassium-adapted rats, thus underlining the importance of the papilla in the process of potassium adaptation.