Ventricular muscle and lung protein synthesis in vivo in response to fasting, refeeding, and nutrient supply by oral and parenteral routes

Influence of starvation, surgery, and sepsis on cardiac protein synthesis in rats: effects of parenteral nutrition, glutamine, and growth hormone

The effect of sepsis on the rate of protein synthesis in the heart is poorly described. We have investigated changes in protein synthesis in the ventricles of the heart over time after cecal ligation and puncture (CLP) in rats in comparison with sham-operated and unoperated animals (ad libitum). All operated animals were starved from the time of surgery to the time of sacrifice. When operated animals were compared with ad libitum animals, ventricular weight and ventricular protein, and DNA and RNA contents were unchanged at 24 h, but were invariably reduced at 72 and 96 h. Fractional rate of protein synthesis (FSR), RNA activity, and cellular efficiency were reduced at 24 h and further reduced at 72 and 96 h. There were no differences, however, between septic and sham-operated animals. Eighteen hours after CLP, additional groups of rats were infused intravenously with 0.9% sodium chloride, parenteral nutrition (PN), or PN with glutamine, and were given a single dose of 400 microg recombinant human growth hormone (rhGH) or an equal volume of 0.9% sodium chloride. FSR was higher in animals given PN when compared with those given 0.9% sodium chloride only, and did not differ from FSR measured in unoperated animals. There was no additional benefit from the acute administration of either glutamine-enriched PN or rhGH. These results indicate that ventricular protein synthesis is markedly reduced by surgery and starvation, but that superimposed sepsis does not further influence these changes. PN can prevent the fall in cardiac protein synthesis associated with starvation, surgery, and sepsis, but neither glutamine nor rhGH produced any additional benefit.

Day length has a major effect on the response of protein synthesis rates to feeding in growing Japanese quail

We investigated the effect of day length on mixed protein fractional synthesis rates (K(S)) in 14- and 21-d-old Japanese quail (Coturnix c. japonica) habituated to either a long day length, 18 h light/6 h dark (LDL), or short day length, 6 h light/18 h dark (SDL), with free access to food during the light period. Rates of protein synthesis were measured by a flooding dose of L-[1-(13)C]leucine. In both groups, we measured K(S) of pectoral muscle, liver and heart after an overnight period of food deprivation and after 2-h food access at dawn. Rates of protein synthesis were also measured in LDL quail starved for 18 h and refed for 2 h. SDL chicks were smaller and had lower tissue weights at 2 wk of age than did LDL chicks (P<0.05). Starvation led to a lower rate of protein synthesis in those animals starved for 18 h. Food availability after starvation for 18 h induced a significant rise in tissue protein synthesis in both SDL and LDL quail (P<0.05). This increase was absent in LDL quail after a 6-h starvation period. There was an increase in K(S) to ad hoc changes in food supply. By determining the daily period in which feeding can occur, day length has a major effect on protein synthesis rates. This effect will determine the overall growth chicks are able to achieve that have been subjected to different day lengths.

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.

Protein synthesis in the heart in vivo, its measurement and patho-physiological alterations

Changes in cardiac protein composition occur in a variety of patho-physiological situations and are usually accompanied by modifications in protein synthesis. Although adjustments in protein synthesis during starvation may be adaptive, the alterations in protein synthesis seen in response to ethanol ingestion may be pathological and an important step in the genesis of alcoholic heart muscle disease. The alterations in heart muscle in hypertension are initially adaptive but in the long term they are deleterious, and involve both transcription and translation. While adequate methods exist for quantifying the amount of mRNA for contractile and non-contractile proteins, such studies of gene-expression provide no dynamic information on the rate at which tissue proteins are lost or accrued. This can only be determined by measuring the rate of protein turnover, i.e. either protein synthesis or protein breakdown. Techniques for directly determining the rates of protein breakdown are limited or involve surgical procedures. Methods for measuring the rate of protein synthesis are described, and are illustrated by their application to the investigation of starvation and ethanol toxicity. In particular, attention is focused on the fact that reliable rates of protein synthesis are obtained only if the specific radioactivity of the precursor at the site of protein synthesis (aminoacyl-tRNA) is assessed.