1. A circulating ouabain-like factor which inhibits the Na+, K+-pump has been implicated in volume-expanded states. To assess the role of this putative factor in normovolaemic rats, we measured erythrocyte and renal Na+, K+-adenosine triphosphatase activity after the infusion of a mixture of high-affinity digoxin-binding Fab fragments (Digibind) capable of removing digoxin from pump sites.

2. Compared with either saline (vehicle) or sheep immunoglobin G, infusion of the antidigoxin antibody caused a moderate increase of Na+, K+-adenosine triphosphatase activity in the erythrocyte (saline 348 ± 12; immunoglobulin G 339 ± 16; antidigoxin antibody 432 ± 22 nmol h−1 mg−1; P < 0.005 by analysis of variance) and a larger increase in the renal cortex (saline 9.7 ± 0.9; immunoglobulin G 9 ± 1.4; antidigoxin antibody 24.3 ± 1.8 μmol h−1 mg−1; P < 0.0005 by analysis of variance) without a change in blood pressure

3. These results are consistent with the presence of a digoxin-like inhibitor of the Na+, K+ -pump in normal rats.

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