Handling of a sodium load by rats on a low sodium intake and frusemide

1. Groups of rats (n = 9-10 per group) were given a medium sodium (Na) diet or a low Na diet or a low Na diet plus low or high dose frusemide in order to have their body Na in a state of surplus or deficit or neither. 2. Body Na was measured by a 22Na whole body counting method involving Na-free chow and the drinking fluid as the only source of Na (22Na-labelled NaCl). Intraperitoneal NaCl (same specific activity) loads were given and their excretion was measured by repeated measurements of body Na over the next 48 h. 3. Rats in surplus excreted more than the load; those in neither surplus nor deficit excreted more or less exactly the load (allowing for growth); those with a small deficit retained enough Na to make up most of the deficit; those with a deficit that was larger than the load retained approximately the whole load. 4. The results support the Strauss-Hollenberg concept that there is a basal body Na above which Na is excreted and below which any available Na is retained.