REGULATION OF RENAL NA-ATPASE IN RAT THICK ASCENDING LIMB DURING K+ DEPLETION - EVIDENCE FOR MODULATION OF NA+ AFFINITY(,K+)

1. NaCl reabsorption along the loop of Henle is reduced in K+-depleted rats. Because Na+,K+-ATPase energizes this transport and because K+ d epletion is known to induce an upregulation of Na+,K+-ATPase in most t issues, the regulation of this enzyme was investigated at the level of single thick ascending limbs of the loop of Henle freshly microdissec ted from rats fed either a normal (control rats) or a low-K+ diet (LK rats). 2. Within 2 weeks of K+ depletion, Na+,K+-ATPase activity and [ H-3]ouabain binding were increased by 30-50% in the medullary portion of the thick ascending limb (MTAL). 3. Despite this increase in the nu mber of Na+,K+-ATPase units, the transport capacity of the Na+,K+ pump , determined by ouabain-sensitive Rb+ uptake in the presence of an ext racellular concentration of Rb+ mimicking the kalaemia determined in c ontrol (4.0 mar Rb+) and LK rats (2.3 mM Rb+), was reduced in MTAL fro m LK rats. 4. Inhibition of the Na+,K+ pump was not accounted for by c hanges in either extracellular K+ or intracellular Na+ concentrations, but by a decrease in the pump affinity for Na+. 5. Because this chang e in the apparent affinity of the Na+,K+ pump for Na+ was detectable i n intact but not in permeabilized MTAL cells, it is probably induced h y a rapidly; reversible cytosolic factor.