The kidney in potassium depletion. I. Na+-K+-ATPase activity and [3H]ouabain binding in MCT

The effect of potassium depletion on renal Na+-K+-ATPase was studied in rats. K depletion produced a striking, time-dependent increase in Na+-K+-ATPase activity of the outer medullary collecting tubules (inner stripe; MCTis). After 3 wk on the K-free diet, when the urine was almost potassium-free, Na+-K+-ATPase activity in MCTis was over fourfold higher than in control animals (2,964 +/- 185 vs. 645 +/- 108 pmol X mm-1 X h-1). Repletion of potassium restored enzyme activity to base line within 7 days (t1/2 = 3.8 days), which corresponds to the catabolic rate of the renal enzyme, suggesting the cessation of enhanced synthesis that took place during K deprivation. Changes in Na+-K+-ATPase activity and aldosterone levels during both K depletion and repletion occurred in opposite directions and were therefore independent of each other. [3H]Ouabain binding to intact MCTis, reflecting the number of pump sites on the basolateral membrane, was similar in K-depleted and control animals; in contrast, tubule permeabilization that exposes additional pump units to the ligand, unmasked a nearly fourfold increase in [3H]ouabain binding (50.0 +/- 6.8 vs. 13.2 +/- 1.7 fmols X mm-1) in K-depleted rats, comparable to the increment in Na+-K+-ATPase activity. These results show that K depletion leads to a marked increase in Na+-K+-ATPase activity of MCTis, and suggest that the new enzyme units are located at a ouabain-inaccessible site in the intact tubule, i.e., either in an intracellular compartment or at the luminal membrane, where they may be involved in potassium reabsorption.