Sodium Pump-Catalyzed ATP-ADP Exchange in Red Blood Cells: The Effects of Intracellular and Extracellular Na and K Ions

Abstract

This chapter discusses sodium pump-catalyzed ATP–ADP exchange in red blood cells together with the effects of intracellular and extracellular Na and K ions. When human red blood cells are incubated in K-free media containing Na ions, an ouabain-sensitive exchange of intracellular and extracellular Na ions is observed. Subsequent studies on this transport mode of Na pump have established that the Na–Na exchange rate is increased by elevated levels of ADP, is independent of ATP in the range 300 μM–1500 μM, and will not take place in resealed ghosts containing ADP but not ATP. Although ATP is required for the Na pump to support Na–Na exchange, there is no net hydrolysis of ATP associated with the transport. These observations support the suggestion that the biochemical events catalyzed by the Na pump protein while mediating Na–Na exchange are the reversible phosphorylation of the protein by ATP and its dephosphorylation by ADP. This chapter characterize the sidedness of the Na-activating (and inhibiting) effects and discusses the validity of the hypothesis that ATP–ADP exchange and Na–Na exchange are coupled simultaneous biochemical and transporting reactions.