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The effects.of a high salt diet (8% NaCI) on blood pressure and intra-erythrocytic Na+ content were studied in Wistar rats. The ability of the plasma to inhibit the renal Na+,K+-ATPase activity and to cross-react with digoxin antibodies was also investigated.After 1 week, neither systolic blood pressure nor intra-erythrocytic Na+ content were modified, but plasma extracts slightly inhibited renal Na+,K+-ATPase (70.9 ± 1.7 versus 76.3 ± 2.1 ixmol Pi/mg per h, P=0.05).After 2 weeks, the plasma inhibitory activity, systolic blood pressure and intra-erythrocytic Na+ content were higher than corresponding values in control animals (65.5 ± 1.6 versus 79.1 + 2.8 mol Pi/mg per h, P<0.001; 132 + 2 versus 114 + 4 mmHg, P<0.001, and 4.95 ± 0.32 versus 3.81 ± 0.36 mmol/l cells, P<0.05, respectively).After 3 months, the plasma digoxin-like immunoreactivity and its ability to inhibit the Na+ pump were elevated (68.7 ± 7.9 versus 48.2 ± 5.4 pg/ml, P<0.02; 57.8 ± 1.8 versus 72.9 ± 1.8 ixmol Pi/mg per h, P<0.001, respectively) whereas intra-erythrocytic Na+ content had returned to control levels. The results demonstrated that this high salt intake led to simultaneous increases in systolic blood pressure and in the activity of a digitalis-like compound present in plasma. The inhibition of Na+,K+-ATPase was correlated with systolic blood pressure and digoxin-like immunoreactivity (r=0.569, n=76, P<0.001 and r=0.414, n=34, P<0.02, respectively). The associated changes in plasma and plasma extract content of total cholesterol, triglycerides and unesterified fatty acids were measured. High salt diet was accompanied by a significant increase in non-esterified fatty acid concentration in the plasma extracts inhibiting the Na+,K+-ATPase activity and these may have contributed to the inhibitory activity.