Reversibility and partial reactions of the Na(+)-K+ pump of rat erythrocytes.
An assay was developed to characterize the kinetic parameters of the Na(+)-K+ pump of rat erythrocytes under conditions as physiological as possible. Changes in the red cell Na+ and Rb+ content were determined in Na+ media (containing 2.5 mM inorganic phosphate (PO4) as a function of cell Na+ (2-8 mmol/l) and extracellular Rb+ (0.2-5 mM). Evaluation of the data revealed that under these conditions the Na(+)-K+ pump mediates, in addition to forward running 3 Nai+: 2 Rbo+ exchange, 1 Ki+:Rbo+ exchange and pump reversal (3 Nao+:2 Ki+ exchange). The two latter modes of Na(+)-K+ pump operation are accelerated by PO4 and lowering of cell Na+. At physiological cation and PO4 concentrations, 1Ki+:Rbo+ exchange contributes by 30-60% to total ouabain-sensitive Rb+ uptake. Thereby, the stoichiometry of ouabain-sensitive Na+ net-extrusion to Rb+ uptake is reduced to values between 1.0 and 0.5. Only at cell Na+ contents above 20 mmol/l the Na+:Rb+ stoichiometry approaches the value of 3:2 = 1.5. At certain constellations of Nai+ and Rbo+ the Na(+)-K+ pump cannot perform any net-transport of Na+ and K+ (Rb+). These equilibrium points are not far from those expected from thermodynamic considerations. The results demonstrate that in normal rat erythrocytes the reversible reaction cycle of the Na(+)-K+ pump runs in several modes of operation. The "abnormal" modes complicate the interpretation of unidirectional fluxes mediated by the Na(+)-K+ pump.
Duhm
J
J
Institute of Physiology, University of Munich, F.R.G.
Zicha
J
J
D000818:Animals; D001693:Biological Transport, Active; D004912:Erythrocytes / Q000378:metabolism*; D007474:Ion Exchange; D007700:Kinetics; D010042:Ouabain / Q000494:pharmacology; D010710:Phosphates / Q000494:pharmacology; D011188:Potassium / Q000097:blood / Q000378:metabolism*; D051381:Rats; D012413:Rubidium / Q000378:metabolism; D012964:Sodium / Q000097:blood / Q000378:metabolism*
D010710:Phosphates; D010042:Ouabain; D012964:Sodium; D012413:Rubidium; D011188:Potassium