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1. At high internal K concentrations the efflux of Na from red cells increases with internal Na concentration following an S-shaped curve. As internal K is reduced the S-shaped region and the value of internal Na for which the Na efflux is half-maximal are both shifted progressively towards zero.
2. The effects of internal Na on the shape of the Na efflux curves can be quantitatively accounted for if it is assumed that the rate of Na efflux is linearly related to the number of pump units having three identical and non-interacting sites occupied by Na.
3. The effects of internal K on the shape of the Na efflux curves are fully explained if it is assumed that the inner sites for Na of the Na pump also behave as identical and non-interacting sites for internal K, being the K-carrier complexes unable to promote Na translocation. The apparent affinity of the Na pump for internal K is about 50 times less than for internal Na.
4. Internal K not only alters the apparent affinity of the Na pump for Na, but also affects its turnover rate. The turnover rate of Na: K exchange increases with internal K following a curve which saturates at about 30 mM internal K. The turnover rate for Na:Na exchange increases linearly with internal K.
5. The linear dependence of the rate of Na:Na exchange on internal K explains why, when internal Na is increased at the expense of internal K, the rate of Na:Na exchange progressively decreases after passing through a maximum.
6. The effects of external Na on the rate of Na:Na exchange can be satisfactorily explained assuming that they are due to the occupation by external Na of three identical and non-interacting sites on each pump unit. The apparent affinity of the Na pump for external Na is about 160 times less than the apparent affinity for internal Na.
7. Under all the experimental conditions tested, it was found that the relation between flux and cation concentration at one of the surfaces of the cell membrane is altered only by a constant factor by changes in the cation composition at the opposite surface of the cell membrane. This fact strongly suggests that there are no interactions between the inner and outer sites of the Na pump.
8. The effects of inner and outer cations on both the Na:K and the Na:Na exchanges catalysed by the Na pump suggest that cation fluxes are proportional to the number of pump units having its inner and outer sites simultaneously occupied by the relevant cations. It seems therefore that sequential models for ion transort do not provide an adequate description of the molecular mechanism of active transport in red cells.
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