AGING AND FLUID HOMEOSTASIS IN RATS

The capacity of aging rats to defend body fluid homeostasis in respons e to a variety of dipsogenic and natriorexigenic stimuli was assessed. Male and female rats of both the Fischer 344 (FR) and Sprague-Dawley (SD) strains were used and tested at target ages of similar to 5, 10, 15, and 20 mo in both longitudinal and cross-sectional studies. There were no consistent age-related declines in water intake in response to water deprivation or acute administration of hypertonic NaCl; angiote nsin (ANG) I, II, III; or isoproterenol. Likewise, there were no major impairments in either urinary excretion of the hypertonic NaCl load o r excretion of water or hypotonic NaCl loads, although the latter were excreted more slowly in the older cohorts. The preference/aversion fu nctions for NaCl solutions differed between SD and FR rats, but did no t change with age except in male FR rats that lost their aversion to d ilute NaCl at 20 mo of age. Intake of hypotonic NaCl solution after ac ute sodium depletion (furosemide treatment) showed a partial decline w ith age, and the older rats sustained larger estimated sodium deficits after a 6-h repletion period. A more complete age-related decline was observed in the intake of hypertonic NaCl stimulated by chronic dieta ry administration of a kininase II inhibitor (ramipril). Male rats of 15-20 mo of age showed no ramipril-induced sodium appetite. Brain ANG II receptor density, determined by autoradiography, declined by almost 50% in the paraventricular nucleus at 20 mo of age and declined sligh tly in the organum vasculosum laminae terminalis but did not decline i n either the supraoptic nucleus or subfornical organ. Thus the major d eficits in fluid intake in aging rats are related to salt appetite; th e mechanism was not identified definitively.