1. The sodium content, the ouabain-sensitive sodium efflux and efflux rate constant and the ouabain-binding capacity were measured in the erythrocytes of 53 patients with hypokalaemia and in 37 healthy controls. The sodium content alone was measured in a further 57 patients with hypokalaemia.

2. In the patients with hypokalaemia there was an increase in the average sodium content of the erythrocytes, which was entirely due to a reduction in the ouabain-sensitive efflux rate constant.

3. The ratio of the ouabain-sensitive efflux rate constant to the number of sodium pumps was decreased in the patients with hypokalaemia, and was directly related to the plasma potassium.

4. Many patients with moderate hypokalaemia had normal erythrocyte sodium and potassium contents and normal ouabain-sensitive efflux rate constant. These patients had an increased number of sodium pumps, which compensated for the inhibitory effect of hypokalaemia on each sodium pump.

5. This increase in the number of sodium pumps was common even in patients who had probably had hypokalaemia for less than 2 weeks. This finding suggests that there are latent sodium pumps within the circulating erythrocytes.

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