Differential effects of aging on fluid intake in response to hypovolemia, hypertonicity, and hormonal stimuli in Munich Wistar rats

Abstract

A significant proportion of aged humans may have impaired thirst and inadequate fluid intake after a period of fluid deprivation. We have studied the water drinking responses, relative to body weight, of Munich Wistar (MW) rats in response to osmotic, hypovolemic, dehydrational, and angiotensin (Ang)-related stimuli as they aged from 3 to 24 months. Young 3-months-old (m.o.) rats had the largest daily fluid intakes and drinking responses to hypertonic and dehydrational stimuli, suggesting that they have accentuated thirst in comparison with older age groups. There were no differences in daily fluid intake from 6 -24 m.o.; however, drinking responses to i.p. injection of hypertonic 0.4 mol͞liter NaCl gradually declined over this period so that in 24-m.o. rats the response was only half that of 6-m.o. rats. Water intake after 24-h water deprivation also declined gradually over 24 months. Drinking responses to hypovolemia induced by s.c. injection of colloid (polyethylene glycol) were unchanged in 6-to 15-m.o. rats, then declined precipitously in 18-to 24-m.o. rats. Drinking responses to s.c. Ang II or s.c. isoproterenol were not reduced in 24-m.o. rats, nor was the drinking associated with feeding. Therefore, there are specific impairments of water intake in response to hypertonicity and hypovolemia in aged MW rats, but Ang-related drinking is not reduced. Like aged humans, aged MW rats exhibit high plasma atrial natriuretic peptide levels and impaired cardiovascular reflexes that could contribute to the impairment of thirst with age.thirst ͉ osmoregulation ͉ angiotensin ͉ age ͉ dehydration A dequate water intake is essential for life in most mammals. Thirst provides a specific motivational stimulus to ingest fluids when bodily fluid water content falls or its tonicity rises. Thus, factors that impair the thirst mechanism can have severe deleterious effects, leading to dehydration and even death. A significant proportion of aged human subjects has an impairment of thirst, particularly after a period of fluid deprivation (1, 2). Thus, they may be at risk for dehydration and in conditions of hot weather become severely dehydrated and at risk for heat stroke and circulatory collapse (3).Whether thirst in general is reduced in aged humans, or there is a specific deficit in thirst in response to a particular dipsogenic stimulus such as hypertonicity, hypovolemia, or some other factor is unresolved. Fluid deprivation, resulting in dehydration that entails both hypertonicity and hypovolemia of extracellular fluid, usually has resulted in lower thirst ratings in elderly subjects (1, 2, 4-7). However, diminished thirst and reduced water drinking in response to i.v. infusion of hypertonic saline in elderly compared with younger subjects has been reported by some (8, 9) but not all investigators (10, 11). The effect of aging on human thirst in response to hypovolemia per se appears not to have been investigated. Some strains of rats have been surveyed as suitable models for studying the effect of aging on thirst. Aged rats o...

Hypotension- and osmotically induced thirst in old Brown Norway rats

“…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.…”

“…There are conflicting reports about whether old rats drink less in response to hypotension. For example, old rats have been found to drink less after subcutaneous injections of isoproterenol compared with young rats (24,34,37), although not always (25,34). These previous studies did not record arterial pressure or control for differences in arterial pressure that may occur with aging (24,25,34,37).…”

“…For example, old rats have been found to drink less after subcutaneous injections of isoproterenol compared with young rats (24, 34, 37), although not always (25,34). These previous studies did not record arterial pressure or control for differences in arterial pressure that may occur with aging (24,25,34,37). This information is important because old rats may have impaired reception or processing of baroreceptor-related information, as suggested by the diminished reflex tachycardia that older rats typically have in response to hypotension (1, 2, 7, 9, 19).…”

Effects of β-adrenergic receptor agonists on drinking and arterial blood pressure in young and old rats

“…Stimulation of vascular ␤ 2 -adrenergic receptors causes vasodilation and reductions in arterial pressure, which reflexively increase HR and renin secretion through baroreceptor mechanisms (21, 26). The water drinking response is a result of isoproterenol-induced hypotension and renin secretion (21,26,31).Several studies have used isoproterenol to examine agerelated declines in drinking by rats (24,25,34,35). However, the relative contributions of the ␤ 1 -and ␤ 2 -adrenergic receptor subtypes to age-related drinking have not been examined.…”

“…Several studies have used isoproterenol to examine agerelated declines in drinking by rats (24,25,34,35). However, the relative contributions of the ␤ 1 -and ␤ 2 -adrenergic receptor subtypes to age-related drinking have not been examined.…”