Early Postnatal Metabolism of Amino Acids in Rat

Abstract

SummaryThe production of 14C0, from glutamate and alanine studied in the newborn rat after injection of labeled precursors was intense and revealed an important catabolism from 130 nmollhr in a 0-hr neonate to 2000 nmollhr in 3-week-old rat. The degradative metabolism of leucine was low at birth (10 nmollhr), but increased up to 500 nmol/hr in 3-week-old rat. Phenylalanine was practically not metabolized into CO,, but its incorporation into protein was higher than leucine, alanine, and glutamate incorporation. Glucose is the major fuel utilized by newborn rat for its energy supply (300-560 nmollhr). However, the relative part of amino acids in energy production is important in the early neonate (0 and 1 hr after birth). SpeculationIn the early newborn rat, the important oxidative metabolism of amino acids (before phosphoenolpyruvate carboxykinase (PEPCK) biosynthesis and gluconeogenesis activation) indicates that they may be directly utilized for energy requirements. This amino acid oxidation could be considered as a transient energetic pathway in species showing a delayed development of PEPCK activity (e.g., the human neonate).In the fetus, the energy supply is provided by the mother through the placental barrier. Amino acids and glucose are the two major nutrients which cross the rat placenta: freely for glucose (18) or by an active transport mechanism for amino acids (2). Maternal glucose appears to be the major oxidative fuel for the fetus in utero (I), but it is also utilized during the last days of gestation for the constitution of endogenous stores: hepatic glycogen and triglycerides in brown adipose tissue. Amino acids are essentially used for macromolecule synthesis.However, metabolic measurements in fetal lamb indicate that amino acids are also utilized for energy supply. The fetal urea production (a reflection of amino acid catabolism) and its excretion to the maternal circulation support this view (9).In the first hours following birth, as during starvation, a transient hypoglycemia is observed; carbohydrate stores are then utilized for energy requirements whereas endogenous triglycerides assure thermogenesis in brown adipose tissue. An active gluconeogenesis from amino acids appears after some hours and maintains constant the plasma glucose concentration.In the present study, the respective participations of amino acids in oxidative processes (directly or after conversion into carbohydrates) and in protein biosynthesis were investigated in vivo during the early postnatal period. MATERIALS AND METHODSNewborn rats (Sprague-Dawley strain) were obtained by cesarian section on the 21.5th day of gestation under light ether anesthesia. Metabolic measurements have been performed previously on control animals delivered without anesthesia; no significant differences in the 14C02 production were noted and the values were pooled. Labeled precursor (0.5 or 1 pCi) was injected intraperitoneally and the respiratory 14C0, was monitored for 1 hr starting immediately after injection. The relative concentrations of pr...

Fetal and Neonatal Responses to Maternal Canine Starvation: Circulating Fuels and Neonatal Glucose Production

“…When maternal ovine starvation occurs, fetal urea production increases acutely so that amino acid oxidation may account for the majority of fetal oxygen consumption (36). During early neonatal fasting in rats before gluconeogenesis is active, amino acids are capable of entering oxidative pathways for energy production (7). Once hepatic gluconeogenesis is active, 25% of the amino acids derived from the diet may be oxidized elsewhere after conversion to glucose (39).…”