Aging and fluid homeostasis in rats

Area of Science:

• Physiology
• Gerontology
• Neuroscience

Background:

• Aging can affect the body's ability to maintain fluid and electrolyte balance.
• Understanding age-related changes in thirst and salt appetite is crucial for health in older populations.

Purpose of the Study:

• To assess the capacity of aging rats to defend body fluid homeostasis.
• To investigate age-related changes in water and salt intake in response to various stimuli.
• To explore the role of brain angiotensin II receptors in age-related fluid balance.

Main Methods:

• Longitudinal and cross-sectional studies on Fischer 344 (FR) and Sprague-Dawley (SD) rats at different ages (5, 10, 15, 20 months).
• Administration of dipsogenic (water-seeking) and natriorexigenic (salt-seeking) stimuli, including hypertonic NaCl, angiotensin (ANG) I, II, III, isoproterenol, furosemide, and a kininase II inhibitor (ramipril).
• Measurement of water and NaCl intake, urinary excretion, and brain angiotensin II receptor density via autoradiography.

Main Results:

• No consistent age-related declines in water intake were observed in response to most stimuli.
• Older rats excreted water and hypotonic NaCl loads more slowly.
• A significant age-related decline in salt appetite was observed, particularly in response to ramipril, with older male rats showing no appetite.
• Brain angiotensin II receptor density declined significantly in the paraventricular nucleus and slightly in the organum vasculosum laminae terminalis of older rats.

Conclusions:

• Aging rats exhibit major deficits in salt appetite, which is a key component of fluid homeostasis.
• The decline in salt appetite is associated with reduced brain angiotensin II receptor density.
• While water intake is largely preserved, impaired salt appetite may contribute to fluid balance issues in aging.