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1. The Na content of rat red cells was found to be 4·40 m-mole/l. cells. When incubated in K(Rb)-free Na this value was doubled in 1 hr, whereas in K(Rb)-free choline it was reduced to about 35% in the same period of time.
2. In cells with elevated Na (13·70 m-mole/l.) the activation curve of Rb influx by external Rb reached the same Vmax in sodium as in choline. The shape of the curve was sigmoid in the first case (Km about 1·05 mM) and hyperbolic in the second (Km about 0·20 mM).
3. The activation curve of rubidium influx by internal sodium was linear at least up to 12 m-mole/l. cells with a slope of 0·84. From this concentration it could increase more steeply, though the data is insufficient to assure it.
4. In normal cells the efflux of Na in K(Rb)-free Na Ringer was 5·64 m-mole/l. cells. hr, and it was reduced to 4·32 m-mole by 10-4 M ouabain. This was accompanied by a reduction of Na influx by 4·14 m-mole, representing then a Na-Na ouabain-sensitive exchange mechanism.
5. At a concentration of 5 mM, external Rb increased Na efflux in 2·32 m-mole/l. cells. hr above the K(Rb)-free levels, and reduced Na influx by 2·13 m-mole.
6. It is proposed that the Na pump is able to operate even in the absence of external K(Rb), though at reduced rate and on a Na-Na exchange basis (Na is the only monovalent cation in the bathing solution). External K(Rb) would have two actions: to increase the rate of shuttling of the carrier (catalytic effect) and to switch the Na-Na to a Na-K(Rb) exchange.
7. These results raise a question of the real significance of the Na/K(Rb) `coupling' ratio and the K-free effect on the Na pump mechanism.
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