Cation activation of the pig kidney sodium pump: transmembrane allosteric effects of sodium.

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Abstract
We have studied activation by Na or Rb ions of different transport modes of the Na-K pump, using phospholipid vesicles reconstituted with pig kidney Na-K-ATPase. The shape of the activation curves, sigmoid or quasi-hyperbolic, depends on the nature of the cation at the opposite surface and not on the specific mode of transport. ATP-dependent Na uptake into K-containing vesicles (Na-K exchange) is activated by cytoplasmic Na along a highly sigmoid curve in the absence of extracellular Na (Hill number, nH = 1.9). Activation displays progressively less-sigmoid curves as extracellular Na is raised to 150 mM (nH = 1.2). The maximal rate of the Na-K exchange is not affected. Na is not transported from the extracellular face by the pump in the presence of excess extracellular K, and the transmembrane effects of the extracellular Na are therefore 'allosteric' in nature. ATP-dependent Na-Na exchange (Lee & Blostein, 1980) and classical ATP-plus-ADP-dependent Na-Na exchange are activated by cytoplasmic Na along hyperbolic curves. ATP-dependent Na uptake into Tris-containing vesicles is activated by cytoplasmic Na along a somewhat sigmoidal curve. (ATP + Pi)-dependent Rb-Rb exchange is activated by cytoplasmic and extracellular Rb along strictly hyperbolic curves. The same applies for Rb-Rb exchange in the presence or absence of ATP or Pi alone. The presence of a high concentration of extracellular Na together with extracellular Rb induces a sigmoidal activation by cytoplasmic Rb of (ATP + Pi)-dependent Rb-Rb exchange (nH = 1.45) but does not affect the maximal rate of exchange. Slow passive Rb fluxes through the pump observed in the absence of other pump ligands (see Karlish & Stein, 1982 alpha) are activated by cytoplasmic Rb along a strictly hyperbolic curve with extracellular Rb, nH = 1.0 (Rb-Rb exchange), along a strongly sigmoid curve with extracellular Na, nH = 1.5 (Rb-Na exchange), and along less-sigmoid curves with extracellular Tris, nH = 1.24 (net Rb flux) or extracellular Li, nH = 1.2 (Rb-Li exchange). Activation of the passive Rb fluxes by extracellular Rb is hyperbolic in the presence of cytoplasmic Rb, Li or Tris but is sigmoid in the presence of cytoplasmic Na (nH = 1.36). Inhibition by cytoplasmic Na of passive Rb fluxes from the cytoplasmic to the extracellular face of the pump depends on the nature of the cation at the extracellular surface.(ABSTRACT TRUNCATED AT 400 WORDS)