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

Abstract

Thunhorst, Robert L., and Alan Kim Johnson. Thirst and salt appetite responses in young and old Brown Norway rats. Am J Physiol Regul Integr Comp Physiol 284: R317-R327, 2003. First published October 10, 2002 10.1152/ ajpregu.00368.2002 Norway rats aged 4 mo (young) and 20 mo (old) received a series of experimental challenges to body fluid homeostasis over ϳ3 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 ϩ 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 weightadjusted 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 nearabsent salt appetite responses to regulatory challenges. aging; drinking; diuresis; natriuresis; dehydration; hypovolemia ELDERLY HUMANS CONSUME ADEQUATE fluids on a daily basis (7). Yet dehydration is a major health risk for the elderly (5), and the elderly are susceptible to dehydration because of increased fluid losses (41). Aged kidneys have a decreased ability to concentrate urine (17, 27) and to conserve sodium (8), along with a relative resistance to vasopressin (25), and release less renin when challenged (6). These changes with age predispose the elderly to hypovolemia and dehydration because of the reduced ability to conserve water and sodium during times of relative water and sodium loss. The elderly have decreased thirst sensation (24, 26, 32). As a result, they are slower than young people to restore fluid balance after exercise-induced dehydration (19) and water deprivation (26). It thus appears that the elderly are more vulnerable to the consequences of physiological challenges to body fluid homeostasis and that diminished thirst in the elderly contributes to their problems of fluid balance (15).The capacities of the aging kidney have been extensively investigated in rat models of aging. As in elderly humans, kidneys of aging r...

Hypotension- and osmotically induced thirst in old Brown Norway rats

“…Consistent with our previous work (40), young rats drank about twice as much as middle-aged adult and old rats in response to hypertonic loads of NaCl. Middle-aged adult and old rats never differed in their drinking responses.…”

“…Most drinking occurred in the first 30 min of testing for all ages and, at each hypertonic concentration of NaCl, drinking in the first 30 min was proportional to the load. Together with previous work (40), the present results indicate that age-related reductions in osmotic drinking begin at least by 12 mo of age in the Brown Norway strain and are apparent using a dose (2 ml/kg body wt of 0.5 M) of NaCl solution calculated to produce a 1% increase in plasma osmolality. Notably, however, in the first 30 min of testing, all ages drank significantly more water in response to all hypertonic loads compared with basal intakes observed after isotonic saline.…”

“…The variability in responding by older rats, and presumably the stress involved in testing, can be reduced by extensive preexperimental handling (36). Similarly, we have observed highly variable daily water intakes in old rats compared with young rats upon initial receipt from the supplier, which we attributed to shipping stress or a response to the novel laboratory environment (40). For this reason, we allow 3-4 wk for adaptation to laboratory conditions before beginning aging studies rather than the 1-2 wk that we typically use in studies unrelated to aging.…”

“…We (40) and others (25) find that aged rats drink less than young rats to peripherally administered hypertonic NaCl solution, but this finding is also not consistent for all laboratories (25,34). Our previous work (40) indicated that 20-mo-old Brown Norway rats are refractory to osmotic loads producing 2-4% increases in estimated plasma osmolality, but we have not used smaller loads or older rats. To characterize declines in osmotic thirst with age more thoroughly, a further goal of the present work was to assess osmotically induced drinking over additional doses of NaCl and across a wider range of ages than in our previous tests.…”

SGK1 as a determinant of kidney function and salt intake in response to mineralocorticoid excess

“…Interestingly, the effect of mineralocorticoids or salt depletion on salt appetite may be modified by glucocorticoids (18,34,50) and amphetamine (5). Moreover, salt appetite has been shown to be dependent on the age of the animal (54). It would be interesting to explore whether SGK1 participates in the modulation of salt appetite by other stimuli and by age.…”