Regulation of muscle protein turnover: possible implications for modifying the responses to trauma and nutrient intake

Non-cardiac nucleic acid composition and protein synthesis rates in hypertension: studies on the spontaneously hypertensive rat (SHR) model

Various studies have shown the involvement of extracardiac tissues in hypertension, including the hepato-intestinal tract, musculo-skeletal system, skin, and the kidney. It was our hypothesis that these perturbations in non-cardiac tissues would also include alterations in protein metabolism. Thus, the reported differences in soleus contractile protein composition may be related to changes in muscle protein synthesis or reduced protein synthetic efficiencies. The aim of the present study was to characterise tissue composition of nucleic acids and rates of protein synthesis in non-cardiac tissues, such as liver, skeletal muscle (i.e., the Type I fibre-predominant soleus and Type II fibre-predominant plantaris), kidney, bone (tibia), skin and the gastrointestinal tract in a genetic model of hypertension (i.e., spontaneously hypertensive rats (SHRs), 15 weeks old) compared to their genetic aged-matched counterparts, i.e., normotensive Wistar-Kyoto (WKY) controls. Rates of protein synthesis were measured in vivo after injection with a flooding dose of L-[4-(3)H]phenylalanine. The results showed changed tissue wet weights (g per organ) for plantaris (+10%, P<0.05), liver (+25%, P<0.01), brain (-9%, P<0.01), jejunum (+39%, P<0.001) and tibia (+17%, P<0.001) in SHRs compared to WKY controls. Protein content (g or mg per organ) was increased in the liver (+32%, P<0. 01) and tibia (+37%, P<0.05). RNA contents (mg per organ) were increased in plantaris (+17%, P<0.01), liver (+22%, P<0.01) and jejunum (+11%, P<0.05). DNA (mg per organ) was increased in plantaris (+16%, P<0.025) and jejunum (+12%, P<0.025). The protein synthetic capacities (i.e., C(s), mg RNA/g protein) were higher in soleus (+41%, P<0.01) and plantaris (+6%, P<0.05) muscles of SHRs compared to WKYs, whereas values were lower in liver (-11%, P<0.01) and kidney (-6%, P<0.01) of SHRs compared to WKYs. The fractional rate of protein synthesis (i.e., k(s), the percentage of the protein pool renewed each day) was not significantly different for any of the tissues, though the rate of protein synthesis per unit RNA (i.e., k(RNA), mg protein/day per mg RNA) was reduced in the soleus (-24%, P<0.05) and the synthesis rate per unit DNA, i.e., k(DNA) (mg protein/day per mg DNA) was increased in the tibia (+31%, P<0.025). This is the first report of significant differences between indices of protein metabolism in extracardiac tissues in hypertension, which may reflect endocrine factors and/or the systemic influence of hypertension per se.

Hormonal regulation of human muscle protein metabolism

A continuous turnover of protein (synthesis and breakdown) maintains the functional integrity and quality of skeletal muscle. Hormones are important regulators of this remodeling process. Anabolic hormones stimulate human muscle growth mainly by increasing protein synthesis (growth hormone, insulin-like growth factors, and testosterone) or by decreasing protein breakdown (insulin). Unlike in growing animals, insulin's main anabolic effect on muscle protein in adult humans is an inhibition of protein breakdown. Protein synthesis is stimulated only in the presence of a high amino acid supply. A combination of the stress hormones (glucagon, glucocorticoids, and catecholamines) cause muscle catabolism, but the effects of the individual hormones on human muscle and their mechanisms of action remain to be clearly defined. Although thyroid hormone is essential during growth, both an excess and a deficiency cause muscle wasting by yet unknown mechanisms. A greater understanding of the regulation of human muscle protein metabolism is essential to elucidate mechanisms of muscle wasting.

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

BACKGROUND

Fasting and other catabolic states are characterized by reductions in the rate of protein synthesis. Most studies have investigated tissues such as skeletal muscle or liver, but impairments in the biochemistry of cardiovascular tissues also contribute to enhanced morbidity. The objectives of the present study were (1) to determine the response of protein synthesis in the heart and lung of young rats to overnight fasting; and (2) to determine whether protein synthesis could be ameliorated or modulated by refeeding or provision of enteral or parenteral nutrition.

METHODS

Fractional rates of protein synthesis (ie, the percentage of tissue protein renewed each day, ks) were measured in vivo in the ventricular muscle and lungs of young male Wistar rats (body weight, 100 to 130 g) with a "flooding" dose of L-[4(3)H]phenylalanine. Rats were fed ad libitum or fasted overnight. Fasted rats were subjected to various treatments.

RESULTS

When nutrient supply in fasted rats recommenced by refeeding for 1 hour, there were small but significant increases in the rates of ventricular protein synthesis, although the infusion of amino acids and glucose for 1 hour had no significant effect. Increases in ventricular ks were also obtained when amino acids were infused for 6 hours. Infusion of glucose alone for 6 hours did not cause a significant increase in ventricular ks. The effect of infusing a mixture of glucose plus amino acids for 6 hours was similar to the effects of amino acids alone. In all instances, ventricular ks in rats infused with amino acid for 6 hours did not attain rates observed in fed rats. ks was reduced in the lung after overnight fasting but was unresponsive to refeeding or to acute or chronic provision of amino acids and glucose by either IV or oral routes. Measurements also suggested that changes in neither insulin nor glucagon per se were responsible for the amino acid-induced increases in heart protein synthesis. However, acute treatment of rats with anti-insulin serum reduced rates of ventricular ks below values observed in fed rats. Anti-insulin serum also increased lung ks.

CONCLUSIONS

It was concluded that rates of heart protein synthesis could be increased by the chronic provision (ie, 6 hours) of nutrients by oral or IV routes. In contrast, the lung was insensitive to these treatments. The observations have important implications for clinical situations, which are characterized by diminished cardiopulmonary protein synthesis.