1. We investigated whether the previously reported circulating frusemide-like factor in rats with malignant renal hypertension was specific to this syndrome, or was also present in rats with chronic uraemia. 86Rb uptake (K+ transport) of monolayers of vascular smooth muscle cells was measured in the plasma of rats with malignant one-kidney, one-clip hypertension and of uraemic rats (60–70% nephrectomy).

2. Compared with control plasma, ouabain-insensitive 86Rb uptake of cells was reduced and ouabain-sensitive 86Rb uptake was unchanged in the plasma of rats with malignant hypertension. The reverse was found in the plasma of uraemic rats.

3. Inhibition of ouabain-sensitive 86Rb uptake of cells occurred in the plasma of rats with malignant hypertension when an angiotensin II antagonist was added to the reaction mixture.

4. The findings confirm the presence of a frusemide-like plasma factor in malignant hypertension and a ouabain-like plasma factor in uraemia. The presence of a ouabain-like plasma factor in malignant hypertension is camouflaged by an elevated circulating angiotensin II level, which stimulates K+ transport.

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