Abstract

The renal action of ouabain on reabsorption of sodium and on reabsorption and secretion of potassium was evaluated by stop-flow technique in pentobarbitalized dogs during osmotic diuresis. Ouabain (50 µg/kg) was injected directly into the renal artery to minimize side-effects.

In 11 dogs tested, ouabain impaired distal Na reabsorption as reflected in inability to reduce sodium concentration to its normal low value at the minimum inflection of the Na stop-flow curve.

A ‘slow-flow’ technique demonstrated that concentrations at the Na minimum truly reflect the actual concentrations developed in the distal sites during stop-flow. When the diuretic urine flow is slowed by urethral clamping to 0.5 ml/min, there is time for low-Na distal fluid to form continually and fill the entire distal system including the polyethylene catheter. ‘Slow-flow’ concentrations of Na are identical to those at the stop-flow minimum, whether low as in normal dogs or high as after ouabain. The urine sample cut at the Na minimum must contain very little contaminating high-Na fluid originating from tubular segments above and below the true low-Na distal sites.

The action of ouabain was maximal within 10 min. It decayed exponentially at a rate of 60% per hour and is thus reversible in a manner that would allow competitive action against aldosterone which augments reabsorption at this site after adrenalectomy.

The ‘additional proximal reabsorbate’ of stop-flow yields no positive indication of proximal impairment.

The distal summit which represents potassium secretion occurs so infrequently on the stop-flow curve that we have insufficient data to test ouabain action. Ouabain obliterated 2 of the 3 summits developed in the 9 dogs tested. The distal minimum inflection of the potassium curve which we have taken to represent potassium reabsorption occurred in 6 of 9 dogs. Ouabain elevated drastically 5 of these minima suggesting inhibition of reabsorption at this site.