Effects of dehydration on adult and newborn rats

Abstract

There is some indication that decreases of the body water load fail to induce a substantial rise of the urinary osmotic pressure in very young children: Young & McCance (1942) investigating infants aged 5-12 weeks who suffered from gastroenteritis, were able to show that the osmotic pressure of the urine rarely rose to that of urine samples obtained from adults at comparable minute volumes. This apparent inability to concentrate the urine under stress was also seen in a normal full-term infant of less than 48 hr., who secreted a hypotonic urine even when its fluid intake was restricted by the omission of two feeds (McCance & Young, 1941). It was shown by Heller (1944) that the osmotic pressure of urine samples collected from normal infants during the first 2 days after birth, i.e. when fluid intake is low, did not reach the figures obtained in urines of normal adults. However, in the absence of any quantitative criterion for the degree of dehydration in these infants, it might be asked whether the conditions were stringent enough (the watery nature of the food should here be considered) to exclude a rise of urinary concentration after further deprivation of water. Since the limitations of clinical tests prevent deliberate withdrawal of fluid from normal newborn infants for more than a short period, it was decided to compare the effects of dehydration in newborn rats deprived of fluid for 24 hr. with those in adult rats suffering from approximately the same relative extrarenal water loss. METHODS E:xperimental animals. Male albino rats, 12-48 hr. old, and male adult rats (weighing 180-310g.) were used. The adults, which were of the same strain as the newborn animals, were kept on a standard diet containing 18% casein. The newborn rats were removed from their cage after they had suckled at least once and were then kept at 33/340 C. for 24 hr. by placing them in a thermoregulated oven. As milk supplies both food and water to the newborn animals, the adult rats were deprived of both water and food during the period of dehydration.Determination of extrarenal uter low. The extrarenal water loss of newborn rats was determined by weighing the animals on an analytical balance. Animals which passed faeces or urine were

Ontogenetic Development of the Pro-Enkephalin B (= Pro-Dynorphin) Opioid Peptide System in the Rat Pituitary

“…In agreement with earlier reports [8,9,24], oxytocin was found to be more than 10 times less con centrated than vasopressin in the neurointermediate pitu itary of newborns, while in the neurointermediate pituitary of adult rats, both hormones occurred in roughly equimolar concentrations. Thus, the increase in the concentration of vasopressin was 10 times less than that of oxytocin between birth and adulthood.…”

The fate of the antidiuretic activity of Pitressin in rats

“…In man, Lauson (1951) found that after infusion of 50 mU Pitressin/hr, the urine flow fell to a minimum of about 0 70 ml./min; during dehydration, however, the urine excretion in man can be as low as 0-35-0 40 ml./min (McCance & Young, 1944). Likewise, in rats, after 48 hr of dehydration, the urine excretion was 000033 ml./100 g/min, while that following infusions of Pitressin of 20,uU/100 g/min (or more) was never lower than 0 0015 ml./100 g/min, even when the water load was only 2-0 ml./100 g. In neither man nor rat did either the period of dehydration or the degree of hydration (2.0 ml./100 g in rat and man) produce a change in glomerular filtration (McCance & Young, 1944;Heller, 1949) which if present could have contributed to the enhanced antidiuretic effect. It may be then that even a moderate water load is enough to decrease the sensitivity of man or animal to the action of Pitressin.…”

Maturation of the hypothalamo‐neurohypophysial system

“…Our finding, that the pituitaries of infant rats deprived of milk for 24 hr contained 20-30 % less vasopressor-antidiuretic activity than those of litter-mate controls, could be so interpreted. It would be remarkable if this applied, for it would mean that antidiuretic hormone is released at an age when the kidneys are unable to respond to it (Heller, 1949;Adolph, 1957). The alternative explanation could be that the neuro-hormonal reflex is not activated, but that withdrawal of food and fluid at so early an age inhibits hormone synthesis or transport.…”