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A highly sensitive cytochemical method for the assay of the ability of plasma and extracts of human urine to stimulate renal glucose-6-phosphate dehydrogenase (G6PD) activity in vitro is described. In the proximal convoluted tubules there was a linear increase of G6PD activity with the logarithm of concentration of a highly purified natriuretic extract from normal human urine (0·384–384 ng active material/l) which was used as a standard. The stimulation of G6PD obtained with dilutions of normal human plasma was parallel to that produced by the standard. The sensitivity of the assay permitted the measurement of as little as 0·384 ng active material/l of the natriuretic extract (0·001 units/ml) and dilutions of 1/10 000 could be detected using normal human plasma. The mean ± S.E.M. index of precision was 0·068± 0·003 (n = 9).

It is known that inhibition of sodium potassium-dependent adenosine triphosphatase (Na⁺-K⁺-ATPase) is associated with a rise in G6PD activity. We have confirmed this observation by demonstrating that ouabain, a potent inhibitor of Na⁺ -K⁺-ATPase, stimulates renal G6PD activity in our assay and that natriuretic extract, human plasma and ouabain stimulated renal G6PD activity in vitro and simultaneously inhibited renal Na⁺-K⁺-ATPase activity in vitro.

The plasma from 12 normal subjects (five of whom were previously shown to inhibit renal Na⁺-K⁺-ATPase activity in vitro in a manner related to sodium intake) stimulated renal G6PD activity in vitro, and this activity was also directly related to sodium intake.

It is suggested that the change in the capacity of plasma to stimulate renal G6PD activity in vitro is a marker of the concentration of a circulating sodium transport inhibitor.

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