Nair, K. Sreekumaran

During fasting, a lack of GH increases protein loss by close to 50%, but the underlying mechanisms remain uncertain. The present study tests the hypothesis that the anabolic actions of GH depend on mobilization of lipids. Seven normal subjects were examined on four occasions during a 37-h fast with infusion of somatostatin, insulin, and glucagon for the final 15 h: 1) with GH replacement, 2) with GH replacement and antilipolysis with acipimox, 3) without GH and with antilipolysis, and 4) with GH replacement, antilipolysis, and infusion of intralipid. Urinary urea excretion, serum urea concentrations, and muscle protein breakdown (assessed by labeled phenylalanine) increased by almost 50% during fasting with suppression of lipolysis. Addition of GH during fasting with antilipolysis did not influence indexes of protein degradation, whereas restoration of high FFA levels regenerated proportionally low concentrations of urea and decreased whole body protein degradation (phenylalanine to tyrosine conversion) by 10–15%, but failed to affect muscle protein metabolism. Thus, the present data provide strong evidence that FFA are important protein-sparing agents during fasting. The finding that inhibition of lipolysis eliminates the ability of GH to restrict fasting protein loss indicates that stimulation of lipolysis is the principal protein-conserving mechanism of GH.

Mitochondria are the primary site of skeletal muscle fuel metabolism and ATP production. Although insulin is a major regulator of fuel metabolism, its effect on mitochondrial ATP production is not known. Here we report increases in vastus lateralis muscle mitochondrial ATP production capacity (32–42%) in healthy humans ( P < 0.01) i.v. infused with insulin (1.5 milliunits/kg of fat-free mass per min) while clamping glucose, amino acids, glucagon, and growth hormone. Increased ATP production occurred in association with increased mRNA levels from both mitochondrial (NADH dehydrogenase subunit IV) and nuclear [cytochrome c oxidase (COX) subunit IV] genes (164–180%) encoding mitochondrial proteins ( P < 0.05). In addition, muscle mitochondrial protein synthesis, and COX and citrate synthase enzyme activities were increased by insulin ( P < 0.05). Further studies demonstrated no effect of low to high insulin levels on muscle mitochondrial ATP production for people with type 2 diabetes mellitus, whereas matched nondiabetic controls increased 16–26% ( P < 0.02) when four different substrate combinations were used. In conclusion, insulin stimulates mitochondrial oxidative phosphorylation in skeletal muscle along with synthesis of gene transcripts and mitochondrial protein in human subjects. Skeletal muscle of type 2 diabetic patients has a reduced capacity to increase ATP production with high insulin levels.

▪ Abstract 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.

Synthesis of Tyr in the human body occurs by hydroxylation of the indispensable amino acid Phe. Until now, it was believed that in humans, this process was restricted to the liver, but we provide compelling evidence of production of Tyr from Phe in the kidney. To determine whether the human kidney produces Tyr, we measured Tyr balance, the Tyr appearance rate, and the Phe-to-Tyr conversion in 12 healthy human subjects by using [ 15 N]Phe and [ 2 H 4 ]Tyr as tracers. Renal plasma flow was measured by using paraaminohippurate, and sampling from the femoral artery and renal veins was performed. The results were compared with those obtained in 12 control subjects undergoing hepatic vein catheterization and infusion of identical tracers. In all 12 subjects, there was a net uptake of Phe by the kidney (2.2 ± 1.2 μmol/min), whereas Tyr was released (5.3 ± 1.5 μmol/min). In contrast, there was a net uptake of both Phe (9.5 ± 1.2 μmol/min) and Tyr (14.3 ± 1.3 μmol/min) by the splanchnic bed. Phe conversion to Tyr occurred at a rate of 5.2 ± 1.2 μmol/min in kidney and 3.0 ± 0.7 μmol/min in the splanchnic bed. The kidney contributed a substantial amount of Tyr to the systemic circulation where the splanchnic bed was a net remover of Tyr. Our results demonstrate that the kidney is the major donor of Tyr to the systemic circulation by its conversion of Phe to Tyr. This observation may have important clinical implications for patients with both renal and hepatic disease, who may be at risk of Phe overloading and Tyr deficiency, and it should be considered when parenteral or enteral nutrients are administered rich in Phe and low in Tyr.

A progressive decline in muscle performance in the rapidly expanding aging population is causing a dramatic increase in disability and health care costs. A decrease in muscle endurance capacity due to mitochondrial decay likely contributes to this decline in muscle performance. We developed a novel stable isotope technique to measure in vivo rates of mitochondrial protein synthesis in human skeletal muscle using needle biopsy samples and applied this technique to elucidate a potential mechanism for the age-related decline in the mitochondrial content and function of skeletal muscle. The fractional rate of muscle mitochondrial protein synthesis in young humans (24 ± 1 year) was 0.081 ± 0.004%·h −1 , and this rate declined to 0.047 ± 0.005%·h −1 by middle age (54 ± 1 year; P < 0.01). No further decline in the rate of mitochondrial protein synthesis (0.051 ± 0.004%·h −1 ) occurred with advancing age (73 ± 2 years). The mitochondrial synthesis rate was about 95% higher than that of mixed protein in the young, whereas it was approximately 35% higher in the middle-aged and elderly subjects. In addition, decreasing activities of mitochondrial enzymes were observed in muscle homogenates (cytochrome c oxidase and citrate synthase) and in isolated mitochondria (citrate synthase) with increasing age, indicating declines in muscle oxidative capacity and mitochondrial function, respectively. The decrease in the rates of mitochondrial protein synthesis is likely to be responsible for this decline in muscle oxidative capacity and mitochondrial function. These changes in muscle mitochondrial protein metabolism may contribute to the age-related decline in aerobic capacity and muscle performance.