THE sodium pump can catalyse several modes of cation exchange (for example Na:K, Na:Na, and K:K), which have been characterised mainly in human red blood cells1-3, squid giant axon4-6, and frog skeletal muscle7-9. In all three tissues it seems that intracellular ADP levels determine the extent to which the pump can engage in Na:Na exchange 2,6,9,10. Yet, despite its obvious kinetic implications, the basic question of how the 'normal' Na:K exchange is affected by the appearance of ADP-induced Na:Na exchange, has not been answered. Here we present evidence that, in frog skeletal muscle, elevated intracellular ADP levels have little effect on pre-existing electrogenic Na:K exchange, but cause Na:Na exchange to appear in addition to ongoing Na:K exchange.
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