Vasopressin in aged rats: Longitudinal studies of vasopressin excretion in Sprague-Dawley and Fischer 344 strains

Abstract

In order to provide physiological baseline values for future experimental procedures, indices of vasopressin secretion were assessed in male Sprague-Dawley (SD) and Fischer 344 (F344) rats at 3 and 20 months of age. Daily water intake, urine volume, urine osmolality, and urinary vasopressin excretion were monitored in SD rats for 30 days, and in F344 rats for 60 days. In the SD strain, daily water and urine volumes, expressed as ml/24 hr/100 g b.wt., were consistently lower in aged animals, as was a calculation of water balance (water intake-urine output volumes/24 hr). Although mean VP concentration in urine appeared higher in aged rats (33.9±20.4 pg/ml) than in young (16.3±7.7 pg/ml), total daily VP excretion was comparable for both ages when expressed as a function of body weight [80.6±37.3 pg for 3 months old (m.o.) and 81.9±47.2 pg/24 hr/100 g b.wt. for 3 and 20 m.o. respectively]. Young and old F344 males showed comparable daily drinking and urine volumes, and water balance, during two months of monitoring, but VP excretion was lower ( p<0.025) in aged rats (83.8±19.0 pg/24 hr/100 g b.wt.) than in 3 m.o. rats (213.0±48.1 pg/24 hr/100 g b.wt.). Urine VP concentration was comparable (69.6±20.6 for 3 m.o.; 59.8±25.6 pg/ml for 20 m.o.). Mean urine osmolality was not significantly different among groups. Urine osmolality and daily urine volumes showed a significant correlation with daily VP excretion among young, but not aged, rats of both strains. Thus, water balance and urine osmolality remained unchanged with age in F344 male rats, despite decreased VP excretion. Water balance was lower in aged SD males, despite maintenance of urinary VP excretion and urine osmolality. The rapid weight gain and decline in water balance in SD males dictate against their use as a model for studies of VP secretion in aging. The data should serve, however, as a basis for further studies on VP secretion and water balance in aged rats of either strain.