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 mol/liter 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.

Fig. 1.

Body weight and daily food and water intake over 24 months in MW rats (n = 6–8 rats per group). There were no significant differences in food intake or water intake between 6-m.o. rats and any of the older age groups; 3-m.o. MW rats had significantly larger (P < 0.01, indicated by ∗∗) food and water intakes compared with the other MW rats.

Fig. 2.

Water intake in response to i.p. injection of hypertonic 0.4 mol/liter NaCl (2 ml/100 g) in MW rats aged 3 (n = 7), 6 (n = 8), 10 (n = 5), 15 (n = 5), 18 (n = 6), and 24 (n = 6) m.o. Significant difference from 6-m.o. rats is indicated by # (P < 0.01); significant difference from 3-m.o. rats is indicated by ∗ (P < 0.05) and ∗∗ (P < 0.01).

Fig. 3.

The volume of water drunk in 2 h after water deprivation for 24 h (Top), the loss of body weight (Middle), and food intake (Bottom) during the period of water deprivation in MW rats aged 3 (n = 6), 6 (n = 7), 10 (n = 5), 15 (n = 5), 18 (n = 8), and 24 (n = 7) m.o. Statistical analysis of water intake by ANOVA and Student–Neuman–Keuls test shows a significant difference from 3-m.o. rats, P < 0.01 (a); a significant difference from 6-m.o. rats, P < 0.05 (b); and a significant difference from 15-m.o. rats, P < 0.05 (c). There were no significant differences in weight loss between the groups (n.m., not measured), whereas the only significant difference in food intake was that of 3-m.o. rats, which was significantly greater (P < 0.01 indicated by a; P < 0.05 indicated by d) than all other groups.

Fig. 4.

Water intake in response to s.c. injection of Ang II (100 mg/kg) in MW rats aged 3 (n = 5), 6 (n = 5), 10 (n = 5), 16 (n = 5), and 24 (n = 7) m.o. There were no significant differences between any of the age groups.

Fig. 5.

Water intake in response to s.c. injection of colloid (PEG, 1.5 ml/100 g) in MW rats aged 3 (n = 7), 6 (n = 5), 10 (n = 4), 15 (n = 4), and 18–24 (n = 9) m.o. A significant difference from 6-m.o. rats is indicated by # (P < 0.01). Significant difference from 3-m.o. rats is indicated by ∗ (P < 0.05) and ∗∗ (P < 0.01).