1. We have reported that the bufadienolide, bufalin (purified from toad skin), was more potent than ouabain in inhibiting the sodium/potassium-dependent adenosine triphosphatase from canine kidney (Sigma) [Brownlee, A. A., Lee, G. & Mills, I. H. J. Physiol. (London) 1987; 390, 94P].

2. The activities of bufalin and cinobufotalin were compared with ouabain in the [3H]ouabain binding assay and on 86Rb uptake in human erythrocytes.

3. When the percentage binding of ouabain-sensitive [3H]ouabain was plotted against the log of the concentration of drug in mol/l, it was shown that the bufalin curve was shifted to the left of that of ouabain and that of cinobufotalin was to the right

4. Linear regression lines were fitted to the data transformed as the log of (p/1–p) plotted against the log of the drug concentration, where p is the proportion of maximal ouabain-sensitive activity at the drug concentration being considered. The IC50 (the concentration of drug producing a 50% change in the maximal ouabain-sensitive response) was 1.4 × 10−9 mol/l for bufalin, 9.7 × 10−9 mol/l for ouabain and 1.70 × 10−7 mol/l for cinobufotalin.

5. The introduction of bufalin 1 h before ouabain reduced the binding of [3H]ouabain to 23.4 ± 1.5% (P < 0.001). Bufalin added in the second hour reduced the ouabain-sensitive binding from 100 ± 1.9% to 87.4 ± 2.9% (P < 0.01).

6. The oubain-sensitive 86Rb uptake curves showed that that of bufalin was to the left of ouabain and that of cinobufotalin was to the right.

7. Regression lines were fitted to the 86Rb uptake data as described for the ouabain-binding data. The IC50 was 1.05 × 10−8 mol/l for bufalin, 4.4 × 10−8 mol/l for ouabain and 4.3 × 10−7 mol/l for cinobufotalin.

8. The differences in potencies are not all explained by what is known of the effects of structure on the cardiac toxicity of cardenolides and bufadienolides.

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