Comp. Biochem. Physiol. Vol. 93A,No. 3, pp. 597-600, 1989 0300-9629/89$3.00+0.00 Printed in Great Britain © 1989MaxwellPergamonMacmillan plc AMMONIA METABOLISM DURING THE SUCKLING PERIOD IN THE RAT M. V. GARCiA,J. MARTIN-BARRIENTOS and J. M. MEDINA* Dpto. de Bioquimica y Biologia Molecular, Facultad de Farmacia, Aptdo 449. 37080, Salamanca, Spain (Received 12 December 1988) Abstract--1. The time-courses of blood glutamine, glutamate, alanine, ammonia, urea and allantoin concentrations during the first 15 days of extrauterine life were studied. 2. Glutamine and glutamate concentrations followed the same pattern and correlated positively, suggesting that both amino acids are utilized or released synchronously. 3. Alanine concentrations decreased during the first 3 days, reaching levelsclose to those of adults which persisted up to the end of the observation period. 4. A highly significantcorrelation was found between ammonia and urea concentrations, suggestingthat during the suckling period urea synthesis may be limited by blood ammonia availability. 5. The time-course of allantoin concentrations suggests that the synthesis of purines was enhanced during the first day, decreasing sharply during the 2nd to steeply increase to the end of the suckling period. INTRODUCTION The neonatal period is characterized by rapid growth which is achieved while the rat is entirely dependent on the intake of a milk diet to supply it with essential nutrients. In most animals, milk is relatively rich in fat compared to carbohydrate (Jennes, 1974). The protein content of rat milk is low compared with the normal solid food intake of adults, resulting in a relatively lower protein intake for the suckling ani- mals. The metabolism of dietary fat supplies most of the energy requirements of the suckling animals (Snell, 1982) through hepatic oxidation of fatty acids until weaning, when a change to a high-carbohydrate diet occurs. Once suckling is established, the oxi- dation of fatty acids results in an increase in the plasma concentrations of ketone bodies (Snell and Walker, 1973; Ferr6 et al., 1978; Girard and Ferrr, 1982) which are used by the brain as a source of energy and for carbon skeletons (Williamson and Buckley, 1973). Concurrent with the production of ketone bodies, suckling animals need a constant influx of glucose; this is partially covered by the milk content. Glucose requirements are completed by an active gluconeo- genesis during the suckling period, in which the main gluconeogenic substrates are lactate, glycerol and gluconeogenic amino acids (Girard and Ferrr, 1982; Patel et al., 1982). This would involve deamination of the amino acids and probably excretion of nitrogen in the form of urea. However, urea production may be attenuated during the suckling period, which implies that amino acid deamination and/or urea biosynthesis in the liver is also decreased in these circumstances in order to minimize the metabolism of amino acids by this route (Snell, 1982). Nevertheless, the amino acids supplied to the newborn in the mother's milk are mainly consumed for processes of synthesis, i.e. protein biosynthesis and the synthesis *Author to whom all correspondence should be addressed. of nucleotides, nucleic acids and other nitrogen products. In the present work we attempted to investigate tissue interrelationships in nitrogen metabolism and disposal in the newborn. Accordingly, the time- courses of major blood metabolites for nitrogen metabolism in newborn rats during the suckling period, i.e. alanine, glutamine, glutamate, ammonia, urea and allantoin, were studied. MATERIALS AND METHODS Animals Albino Wistar rats fed on stock laboratory diet (carbo- hydrate 58.7%, protein 17.0%, fat 3.0% and added salts and vitamins) and of known gestational age were used for the experiments. Females with a mean weight of 250 g were caged with males overnight and conception was considered to occur at 1 p.m.; this was verified the following morning by the presence of spermatozoa in the vaginal smears. Pregnant rats of 21.5 days of gestation (21.7 days for full gestation) were killed by cervical dislocation and the foetuses were delivered by rapid hysterectomy (0 time), killed by decapitation and 100/~1 of blood was collected in disposable pipettes and deproteinized. Naturally delivered newborns were maintained with their mothers up to the time of the experiments. The neonates were then killed by decapitation and their blood collected. Age-matched virgin rats were used as adult controls. Reagents Substrates, coenzymesand enzymes were purchased from Boehringer (Mannheim, FRG) or Sigma Chemical Co. (St Louis, MO, USA). Standard analytical grade laboratory reagents were obtained from Merck (Darmstad, FRG) or Sigma. Analytical methods The concentrations of metabolites were measured by fluorimetric techniques, essentially based on the spectro- photometric methods developed for ammonia (Kun and Keamey, 1974), urea (Gutmann and Bergmeyer, 1974), alanine (Williamson, 1974), glutamine and glutamate (Lund, 1985) and allantoin (Sumi et al., 1976). The 597