Regulation of protein metabolism during postnatal development.

The foetal and neonatal periods of mammalian development are associated with a profound anabolic state leading to an unprecedented rate of growth. Although both energy and protein intake are important in determining the postnatal growth rate of the rat, studies using artificial diets have shown that it is protein intake which is the most significant factor (Miller, 1970). The weight gain during postnatal development is reflected in body protein deposition, which increases more than 10-fold from birth to 30 days (Winick & Noble, 1965). However, the rate of protein accumulation is not constant throughout this period, but occurs in a biphasic manner, with spurts at 4 and 13 days coincident with increased rates of total body protein synthesis (Miller, 1969). The mechanisms underlying the biphasic pattern are unclear, although the second phase occurs at the time of weaning, when there is an increase in dietary protein and energy intake (Snell & Walker, 1972). Skeletal muscle and skin together account for 58-87% of total body protein synthesis (Miller, 1969). On the other hand, the growth rate and protein content of the liver in the early postnatal period is decreased (Winick & Noble, 1965). This is partly due to the diversion of hepatic protein synthesis towards increased synthesis of secretory proteins (Conde & Scornik, 1977) which are required for the rapidly expanding plasma volume. In addition, amino acids may be spared from intracellular liver protein synthesis in order to divert them to the more rapidly growing body tissues. In view of these demands and the finding that carcass nitrogen retention is greater than 70% in the suckling rat (Miller, 1970), it would seem that amino acid catabolism by the liver and subsequent nitrogen excretion must be attenuated in the early postnatal period.