Effect of variations in dietary sodium intake on sodium excretion in mature rats

Sprague-Dawley rats weighing 400 g or more were studied to determine whether their continued weight gain affects renal sodium handling. Rats maintained on a wide range of sodium intakes gained 3.9 +/- 0.4 g/day. The intercept of a linear regression of intake against urinary excretion provided an estimate of the minimum daily requirement for sodium intake of 247 +/- 33 microEq/day. When more than this required amount was ingested, the animals excreted the excess quantitatively in the urine. When less was ingested they continued to gain weight at a slower rate, 1.6 +/- 0.6 g/day, and remained in positive sodium balance. Nonetheless, they developed a sodium deficit manifested as retention of a sodium challenge. Thus, on an adequate dietary intake the normal physiological state of Sprague-Dawley rats of this size is one of chronic sodium retention rather than neutral sodium balance. In contrast, when inadequate sodium is ingested a deficit develops in the absence of external losses. These observations have important implications for the interpretation of studies of renal sodium handling in these animals.