The effect of lactation on the fractional synthetic rate of protein in the liver and muscle of rats

A model of whole-body protein turnover based on leucine kinetics in rodents

The measurement of fractional synthesis rate is based on the following assumptions: amino acids for protein synthesis are supplied by an intracellular pool; amino acids from protein degradation are not recycled preferentially to protein synthesis; and proteins turn over at a homogeneous rate. To test these assumptions, a mechanistic, theoretical model of protein turnover for a nongrowing 26-g mouse was developed on the basis of data from the literature. The model consisted of three protein pools turning over at fast (102 micromol Leu, t1/2= 11.5 h), medium (212 micromol Leu, t1/2 = 16.6 h) or slow (536 micromol Leu, t1/2 = 71.5 h) rates and extracellular (1.69 micromol Leu), leucyl-tRNA (0.0226 micromol Leu) and intracellular (5.72 micromol Leu) amino acid pools that exchanged amino acids. The flow of amino acids from the protein pools to the leucyl-tRNA pool determined the amount of recycling. The flow of amino acids from the extracellular pool to aminoacyl tRNA determined the amount of channeling. Two flooding dose data sets were used to evaluate specific radioactivity changes predicted by the model. Predictions of specific radioactivities using flooding dose, pulse dose or continuous infusion methods indicated that the model can be a useful tool in estimating the rates of channeling and recycling. However, it was found that use of data from flooding dose experiments might cause inaccurate predictions of certain fluxes.

Maternal protein reserves and their influence on lactational performance in rats. 2. Effects of dietary protein restriction during gestation and lactation on tissue protein metabolism and Na+, K(+)-ATPase (EC 3.6.1.3) activity

Changes in tissue protein synthesis and an associated membrane transport system in rats were investigated during lactation and under conditions of dietary protein restriction. Following mating, female Sprague-Dawley rats (second parity) were caged individually and offered a high-protein diet (H; 215 g crude protein (N x 6.25; CP)/kg dry matter (DM)) ad lib. until day 12 of gestation. Subsequently half continued to receive diet H, whilst the remainder were offered a low-protein diet (L; 65 g CP/kg DM) until parturition. On day 1 of lactation females were then allocated to either diet H or another low-protein diet (L2; 90 g CP/kg DM) which were offered ad lib. until day 13 of lactation, giving four lactation groups HH, LH, HL2 and LL2. On days 1 and 13 of lactation groups of females were used in the estimation of tissue protein synthesis (flooding dose of [3H] phenylalanine) and Na+, K(+)-ATPase (EC 3.6.1.3) activity (polarographically) in skeletal muscle, mammary gland, liver and duodenal mucosa. By day 1 of lactation diet L had reduced fractional and absolute synthesis rates (FSR and ASR) of muscle protein (P < 0.05) and the O2 consumption associated with Na+, K(+)-ATPase, although not significantly (P < 0.10). Rates of protein synthesis in the other tissues studied were not affected on day 1 of lactation by the gestation dietary treatment. By day 13 of lactation the feeding of diet L2 had reduced muscle FSR and ASR of group HL2 to rates that were lower than those on day 1 (P < 0.05), comparable to those of group LL2 and lower than those of groups HH and LH (P < 0.05). Diet H had allowed group LH to increase their muscle protein synthesis compared with that on day 1 (P < 0.05). Muscle Na+, K(+)-ATPase activity on day 13 of lactation was also lower in groups offered diet L2 (P < 0.05). Mammary protein synthesis was increased during lactation with the feeding of diet H (P < 0.05), which was prevented by diet L2 such that rates of groups HL2 and LL2 were lower than those of the two high-protein groups on day 13 (P < 0.01). Mammary respiration and in particular Na+, K(+)-ATPase activity was increased during lactation by the feeding of diet H (P < 0.05). Rates of protein synthesis and respiration in liver and duodenal mucosa were not significantly affected by the gestational or lactational dietary treatments.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolic fuels and reproduction in female mammals

A complete reproductive cycle of ovulation, conception, pregnancy, and lactation is one of the most energetically expensive activities that a female mammal can undertake. A reproductive attempt at a time when calories are not sufficiently available can result in a reduced return on the maternal energetic investment or even in the death of the mother and her offspring. Numerous physiological and behavioral mechanisms link reproduction and energy metabolism. Reproductive attempts may be interrupted or deferred when food is scarce or when other physiological processes, such as thermoregulation or fattening, make extraordinary energetic demands. Food deprivation suppresses both ovulation and estrous behavior. The neural mechanisms controlling pulsatile release of gonadotropin-releasing hormone (GnRH) and, consequently, luteinizing hormone secretion and ovarian function appear to respond to minute-to-minute changes in the availability of metabolic fuels. It is not clear whether GnRH-secreting neurons are able to detect the availability of metabolic fuels directly or whether this information is relayed from detectors elsewhere in the brain. Although pregnancy is less affected by fuel availability, both lactational performance and maternal behaviors are highly responsive to the energy supply. When a reproductive attempt is made, changes in hormone secretion have dramatic effects on the partitioning and utilization of metabolic fuels. During ovulatory cycles and pregnancy, the ovarian steroids, estradiol and progesterone, induce coordinated changes in the procurement, ingestion, metabolism, storage, and expenditure of metabolic fuels. Estradiol can act in the brain to alter regulatory behaviors, such as food intake and voluntary exercise, as well as adenohypophyseal and autonomic outputs. At the same time, ovarian hormones act on peripheral tissues such as adipose tissue, muscle, and liver to influence the metabolism, partitioning and storage of metabolic fuels. During lactation, the peptide hormones, prolactin and growth hormone, rather than estradiol and progesterone, are the principal hormones controlling partitioning and utilization of metabolic fuels. The interactions between metabolic fuels and reproduction are reciprocal, redundant, and ubiquitous; both behaviors and physiological processes play vital roles. Although there are species differences in the particular physiological and behavioral mechanisms mediating nutrition-reproduction interactions, two findings are consistent across species: 1) Reproductive physiology and behaviors are sensitive to the availability of oxidizable metabolic fuels. 2) When reproductive attempts are made, ovarian hormones play a major role in the changes in ingestion, partitioning, and utilization of metabolic fuels.

Protein metabolism in the mouse during pregnancy and lactation

Protein synthesis was measured in vivo in the whole body and in a number of individual tissues in mice at various stages of pregnancy and lactation. The absolute rate of protein synthesis in the whole body increased from 640 mg/day in virgin mice to 1590 mg/day by day 18 of pregnancy, and to 2100 mg/day by day 15 of lactation. Large proportions of these increments were contributed by the rapidly growing foetuses and placentae in the pregnant animals and by protein synthesis in the mammary glands during lactation. In addition, a substantial stimulation of growth and protein synthesis was also observed in the liver and the gastrointestinal tract. Gastrocnemius muscle showed no changes in protein metabolism, indicating that in the well-fed mouse this tissue is not required to play a role as a protein reserve during pregnancy and lactation.