1. The effects of a 2 litre intravenous infusion of saline (0.9% NaCl solution) over 3 h on erythrocyte transmembrane sodium transport were studied in 12 normal human subjects.

2. After saline infusion a significant (P < 0.01) reduction of both outward Na+, K+ pump- and Na+, K+ cotransport-mediated Na+ effluxes was observed. The Na+, Li+ countertransport rate and the passive Na+ permeability did not change.

3. The incubation of the subjects' erythrocytes, obtained on a separate occasion, with their own plasma taken after the saline infusion, induced an inhibition of both Na+, K+ pump and Na+, K+ cotransport outward sodium fluxes. The percentage decrease after incubation was closely correlated with the percentage reduction induced by the saline infusion in vivo (r = 0.93 for the pump and r = 0.96 for cotransport; P < 0.01).

4. These data suggest that extracellular fluid volume expansion affects the release of circulating factors modulating sodium transport by the Na+, K+ pump and by Na+, K+ cotransport.

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