The effect of dehydration level on the NaCl concentration chosen by rats

The relationship between the level of dehydration (3%, 6%, and 8% body weight) and the preference for water and saline in the rehydration were assessed in rats for 15 h. At the 6% and 8% dehydration levels, rats provided with tap water and 0.9% NaCl solution simultaneously consumed more water than NaCl solution (calculated Na concentration was about 57 mEq/l) within 3 h and then consumed more NaCl solution than water (about 116 mEq/l). The shift of the Na concentration of fluid occurred when about 107% of lost cations and 82% of lost fluid were regained. Urine output and urinary Na output were about 14% of the ingested fluid during the initial 4 h; after that, the output was in amounts almost equal to the ingestion. However, the 3% dehydrated rats consumed almost equal amount of tap water and NaCl solution (about 68 mEq/l) throughout the 15 h. The result indicates that at more than 3% dehydration level, rats initially choose hypotonic NaCl solution, and the volume loss of body fluid is then regained by the consumption of almost isotonic Na solution.