title: Active ion transport in the renal proximal tubule. II. Ionic dependence of the Na pump.
abstract: The dependence of Na pump activity on intracellular and extracellular Na+ and K+ was investigated using a suspension of rabbit cortical tubules that contained mostly (86%) proximal tubules. The ouabain-sensitive rate of respiration (QO2) was used to measure the Na pump activity of intact tubules, and the Na,K-ATPase hydrolytic activity was measured using lysed proximal tubule membranes. The dependence (K0.5) of the Na pump on intracellular Na+ was affected by the relative intracellular concentration of K+, ranging from approximately 10 to 15 mM at low K+ and increasing to approximately 30 mM as the intracellular K+ was increased. The Na pump had a K0.5 for extracellular K+ of 1.3 mM in the presence of saturating concentrations of intracellular Na+. Measurements of the Na,K-ATPase activity under comparable conditions rendered similar values for the K0.5 of Na+ and K+. The Na pump activity in the intact tubules saturated as a function of extracellular Na at approximately 80 mM Na, with a K0.5 of 30 mM. Since Na pump activity under these conditions could be further stimulated by increasing Na+ entry with the cationophore nystatin, these values pertain to the Na+ entry step and not to the Na+ dependence of the intracellular Na+ site. When tubules were exposed to different extracellular K+ concentrations and the intracellular Na+ concentration was subsaturating, the Na pump had an apparent K0.5 of 0.4 mM for extracellular K. Under normal physiological conditions, the Na pump is unsaturated with respect to intracellular Na+, and indirect analysis suggests that the proximal cell may have an intracellular Na+ concentration of approximately 35 mM.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D002414:Cations, Monovalent / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism*; D005260:Female; D006706:Homeostasis; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D007473:Ion Channels / Q000502:physiology*; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D009994:Osmolar Concentration; D010101:Oxygen Consumption; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D011817:Rabbits; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism
chemical_list: D002414:Cations, Monovalent; D007473:Ion Channels; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium