title: Measurement of Na+-K+ coupling ratio of Na+-K+-ATPase in rabbit proximal tubules.
abstract: A combination of 23Na nuclear magnetic resonance (NMR) spectroscopy and a K+-selective electrode was used to make simultaneous measurements of net Na+ and K+ fluxes across plasma membranes of rabbit renal proximal tubules after an abrupt stimulation of Na+-K+-ATPase. After a step in extracellular K+ concentration ([K+]o) from low to higher concentration (0.1-0.3 mM to 0.5-5.2 mM) at 25 degrees C, net extrusion of Na+ and uptake of K+ were observed. These fluxes were completely inhibited by ouabain (10(-3) M). Because initial rates of K+ uptake in presence or absence of Ba2+ (a known inhibitor of plasma membrane K+ conductance) were indistinguishable, net K+ flux was virtually unidirectional. Because suspension buffers contained neither glucose nor amino acids and the ratio of net Na+ and K+ fluxes (JNa and JK, respectively) was constant over a wide range of transmembrane Na+ gradients and absolute values of the JNa and JK, it is likely that changes in electrogenic or passive net fluxes across plasma membranes were insignificant in the first 30-45 s after the [K+]o step. Thus the ratio of these initial net Na+ and K+ fluxes corresponds closely to the Na+-K+ coupling ratio of the Na+-K+-ATPase. In 12 experiments, the measured Na+-K+-ATPase coupling ratio was 1.54 +/- 0.07 (SE). The coupling ratio was constant over a wide range of intracellular Na+ content, intracellular sodium concentration, [K+]o and transmembrane Na+ gradient. The coupling ratio also remained constant over an eightfold range of Na+-K+-ATPase rates.
mesh_terms: D000818:Animals; D001464:Barium / Q000494:pharmacology; D005260:Female; D007425:Intracellular Membranes / Q000378:metabolism; D007473:Ion Channels / Q000502:physiology; D007687:Kidney Tubules, Proximal / Q000201:enzymology / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism*; D011817:Rabbits; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
chemical_list: D007473:Ion Channels; D001464:Barium; D010042:Ouabain; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium