1. In order to define a physiological role for circulating inhibitors of sodium, potassium-dependent adenosine triphosphatase (Na+,K+-ATPase), plasma was obtained from control, water deplete, water repleted, sodium deplete and sodium loaded rats. The effect of this plasma on Na+,K+-ATPase activity, and its transport equivalent 86Rb uptake, was measured in separated guinea pig renal cortical tubules.

2. Plasma from water deplete rats had a raised plasma osmolality and sodium concentration and a significant inhibitory effect on Na+,K+-ATPase (14%) and 86Rb uptake (24%) compared with control or water repleted rats.

3. Inhibition of Na+,K+-ATPase and 86Rb transport was not seen with plasma from rats after dietary sodium loading (urine sodium 5.2 ± 0.9 mmol/day) compared with low sodium diet controls (urine sodium 0.41 ± 0.08 mmol/day).

4. Des-amino arginine vasopressin in vivo produced no inhibition of Na+,K+-ATPase or Rb transport.

5. These studies suggest, that in terms of common homoeostatic insults, circulating inhibitors of Na+,K+-ATPase are more responsive to water depletion than to oral sodium loading. The inhibitors may fulfil a physiological role in increasing sodium excretion to maintain osmolality after dehydration.

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