Thirst and salt appetite responses in young and old Brown Norway rats

Male Brown Norway rats aged 4 mo (young) and 20 mo (old) received a series of experimental challenges to body fluid homeostasis over ∼3 mo. Water was available for drinking in some tests, and both water and 0.3 M NaCl were available in others. The series included three episodes of extracellular fluid depletion (i.e., furosemide + 20 h of sodium restriction), two tests involving intracellular fluid depletion (i.e., hypertonic saline: 1 or 2 M NaCl at 2 ml/kg body wt sc), one test involving overnight food and fluid restriction, and testing with captopril adulteration of the drinking water (0.1 mg/ml) for several days. Old rats were significantly heavier than young rats throughout testing. Old rats drank less water and 0.3 M NaCl after sodium deprivation than young rats, in terms of absolute and body weight-adjusted intakes. Old rats drank only half as much water as young rats in response to subcutaneous hypertonic NaCl when intakes were adjusted for body weight. Old rats drank less 0.3 M NaCl than young rats after overnight food and fluid restriction when intakes were adjusted for body weight. In response to captopril adulteration of the drinking water, young rats significantly increased daily ingestion of 0.3 M NaCl when it was available as an alternative to water and significantly increased daily water intakes when only water was available, in terms of absolute and body weight-adjusted intakes. Old rats had no response to captopril treatment. These results add important new information to previous reports that aging rats have diminished thirst and near-absent salt appetite responses to regulatory challenges.