Do nonesterified fatty acids regulate skeletal muscle protein turnover in humans?

The impact of forearm immobilization and acipimox administration on muscle amino acid metabolism and insulin sensitivity in healthy, young volunteers

Although the mechanisms underpinning short-term muscle disuse atrophy and associated insulin resistance remain to be elucidated, perturbed lipid metabolism might be involved. Our aim was to determine the impact of acipimox administration [i.e., pharmacologically lowering circulating nonesterified fatty acid (NEFA) availability] on muscle amino acid metabolism and insulin sensitivity during short-term disuse. Eighteen healthy individuals (age: 22 ± 1 years; body mass index: 24.0 ± 0.6 kg·m-2) underwent 2 days forearm immobilization with placebo (PLA; n = 9) or acipimox (ACI; 250 mg Olbetam; n = 9) ingestion four times daily. Before and after immobilization, whole body glucose disposal rate (GDR), forearm glucose uptake (FGU; i.e., muscle insulin sensitivity), and amino acid kinetics were measured under fasting and hyperinsulinemic-hyperaminoacidemic-euglycemic clamp conditions using forearm balance and l-[ring-2H5]-phenylalanine infusions. Immobilization did not affect GDR but decreased insulin-stimulated FGU in both groups, more so in ACI (from 53 ± 8 to 12 ± 5 µmol·min-1) than PLA (from 52 ± 8 to 38 ± 13 µmol·min-1; P < 0.05). In ACI only, and in contrast to our hypothesis, fasting arterialized NEFA concentrations were elevated to 1.3 ± 0.1 mmol·L-1 postimmobilization (P < 0.05), and fasting forearm NEFA balance increased approximately fourfold (P = 0.10). Forearm phenylalanine net balance decreased following immobilization (P < 0.10), driven by an increased rate of appearance [from 32 ± 5 (fasting) and 21 ± 4 (clamp) preimmobilization to 53 ± 8 and 31 ± 4 postimmobilization; P < 0.05] while the rate of disappearance was unaffected by disuse or acipimox. Disuse-induced insulin resistance is accompanied by early signs of negative net muscle amino acid balance, which is driven by accelerated muscle amino acid efflux. Acutely elevated NEFA availability worsened muscle insulin resistance without affecting amino acid kinetics, suggesting increased muscle NEFA uptake may contribute to inactivity-induced insulin resistance but does not cause anabolic resistance.NEW & NOTEWORTHY We demonstrate that 2 days of forearm cast immobilization in healthy young volunteers leads to the rapid development of insulin resistance, which is accompanied by accelerated muscle amino acid efflux in the absence of impaired muscle amino acid uptake. Acutely elevated fasting nonesterified fatty acid (NEFA) availability as a result of acipimox supplementation worsened muscle insulin resistance without affecting amino acid kinetics, suggesting increased muscle NEFA uptake may contribute to inactivity-induced insulin resistance but does not cause anabolic resistance.

Effect of acute and short-term dietary fat ingestion on postprandial skeletal muscle protein synthesis rates in middle-aged, overweight, and obese men

Muscle anabolic resistance to dietary protein is associated with obesity and insulin resistance. However, the contribution of excess consumption of fat to anabolic resistance is not well studied. The aim of these studies was to test the hypothesis that acute and short-term dietary fat overload will impair the skeletal muscle protein synthetic response to dietary protein ingestion. Eight overweight/obese men [46.4 ± 1.4 yr, body mass index (BMI) 32.3 ± 5.4 kg/m²] participated in the acute feeding study, which consisted of two randomized crossover trials. On each occasion, subjects ingested an oral meal (with and without fat emulsion), 4 h before the coingestion of milk protein, intrinsically labeled with [1-¹³C]phenylalanine, and dextrose. Nine overweight/obese men (44.0 ± 1.7 yr, BMI 30.1 ± 1.1 kg/m²) participated in the chronic study, which consisted of a baseline, 1-wk isocaloric diet, followed by a 2-wk high-fat diet (+25% energy excess). Acutely, incorporation of dietary amino acids into the skeletal muscle was twofold higher (P < 0.05) in the lipid trial compared with control. There was no effect of prior lipid ingestion on indices of insulin sensitivity (muscle glucose uptake, pyruvate dehydrogenase complex activity, and Akt phosphorylation) in response to the protein/dextrose drink. Fat overfeeding had no effect on muscle protein synthesis or glucose disposal in response to whey protein ingestion, despite increased muscle diacylglycerol C16:0 (P = 0.06) and ceramide C16:0 (P < 0.01) levels. Neither acute nor short-term dietary fat overload has a detrimental effect on the skeletal muscle protein synthetic response to dietary protein ingestion in overweight/obese men, suggesting that dietary-induced accumulation of intramuscular lipids per se is not associated with anabolic resistance.

Muscle protein synthesis and balance responsiveness to essential amino acids ingestion in the presence of elevated plasma free fatty acid concentrations

CONTEXT

Elevated plasma free fatty acid (FFA) concentrations are observed under various clinical circumstances and are associated with impaired glucose disposal in skeletal muscle.

OBJECTIVE

The aim of the study was to determine the effects of elevated plasma FFA concentrations on the response of protein synthesis and balance in muscle after essential amino acids (EAAs) ingestion.

DESIGN

Leg protein kinetics were determined in young healthy individuals before and after the ingestion of EAAs at 10 h after the initiation of either lipid (Liposyn/heparin+EAA) or saline (saline+EAA) infusions.

RESULTS

Plasma insulin responses where higher (P <0.05) in the Liposyn/heparin+EAA group than the saline+EAA group both before (14 +/- 4 vs. 6 +/- 1 microIU . ml(-1)) and after (1038 +/- 257 vs. 280 +/- 87 microIU . ml(-1) . 210 min(-1)) the EAA ingestion. After the EAA ingestion, the rates of both leg phenylalanine disappearance (Rd; nmol . min(-1) . kg lean leg mass(-1)) and muscle proteins fractional synthesis (FSR; % . h(-1)) increased (P <0.05) in both the Liposyn/heparin+EAA and saline+EAA groups, but these changes were not different between the two groups (Rd, 102 +/- 32 vs. 118 +/- 34; FSR, 0.014 +/- 0.005 vs. 0.018 +/- 0.007; P > 0.05). Although the leg phenylalanine rate of appearance (Ra; nmol . min(-1) . kg lean leg mass(-1)) was lower (381 +/- 47 vs. 518 +/- 40) and the balance was greater (-109 +/- 20 vs. -172 +/- 17) in the Liposyn/heparin+EAA group compared to the saline+EAA group before the EAA ingestion (P <0.05), the changes in both of these parameters were not different between groups after the EAA ingestion (P > 0.05).

CONCLUSIONS

Elevated plasma FFA concentrations do not interfere with the response of muscle protein synthesis and balance to a bolus ingestion of EAAs.

Dose-response effects of free fatty acids on amino acid metabolism and ureagenesis

AIM

Free fatty acids (FFAs) are important fuels and have vital protein-sparing effects, particularly during conditions of metabolic stress and fasting. However, it is uncertain whether these beneficial effects are evident throughout the physiological range or only occur at very high FFA concentrations. It is also unclear whether secondary alterations in hormone levels and ketogenesis play a role. We therefore aimed at describing dose-response relationships between amino acid metabolism and circulating FFA concentrations at clamped hormone levels.

METHODS

Eight healthy men were studied on four occasions (6 h basal, 2 h glucose clamp). Endogenous lipolysis was blocked with acipimox and Intralipid was infused at varying rates (0, 3, 6 or 12 microL kg(-1) min(-1)) to obtain four different levels of circulating FFAs. Endogenous growth hormone, insulin and glucagon secretion was blocked by somatostatin (300 microg h(-1)) and replaced exogenously. 15N-phenylalanine, 2H4-tyrosine and 13C-urea were infused continuously to assess protein turnover and ureagenesis.

RESULTS

We obtained four distinct levels of FFA concentrations ranging from 0.03 to 2.1 mmol L(-1) and 3-hydroxybutyrate concentrations from 10 to 360 micromol L(-1). Whole-body phenylalanine turnover and phenylalanine-to-tyrosine degradation decreased with increasing FFA levels as did insulin-stimulated forearm fluxes of phenylalanine. Phenylalanine, tyrosine and urea concentrations also decreased progressively, whereas urea turnover was unperturbed.

CONCLUSION

Circulating FFAs decrease amino acid concentrations and inhibit whole-body phenylalanine fluxes and phenylalanine-to-tyrosine conversion. Our data cover FFA concentrations from 0 to 2 mmol L(-1) and indicate that FFAs exert their protein conserving effects in the upper physiological range (>1.5 mmol L(-1)).

Effect of short-term and long-term fasting on transcriptional regulation of metabolic genes in rat tissues

Ninety-eight genes/ESTs with differential expressions in epididymal adipose tissue of fed and 3-day fasting (F3) rats were identified by microarray analysis. Genes for lipogenesis, glycolysis, and glucose aerobic oxidation were decreased in response to starvation. Further study was performed to investigate the expression patterns of these genes in rat tissues after short- and long-term starvations. The results of the increased expression of the pyruvate dehydrogenase kinase 4 (PDK4) gene and decreased pyruvate dehydrogenase (PDH) in rat muscle together with decreased fatty acid synthase (FAS) in rat adipose tissue after 1 day of fasting (F1) suggested from transcriptional level that glucose aerobic oxidation was down-regulated in rat muscle and synthesis of saturated fatty acids was inhibited in rat adipose tissue after short-term fasting. It was noted that the transcriptions of genes involved in the fatty acid oxidation, such as very-long-chain Acyl-CoA dehydrogenase (LCAH), Acyl-CoA oxidase (ACO), carnitine palmitoyltransferase-I (CPT-I), and carnitine-acylcarnitine translocase (CAT)L, were greatly increased in F1 rat liver, then began to decrease in F3 and 5-day fasting (F5) rat liver, combined with significantly increased serum non-esterified fatty acids (NEFA) in F1 rats and increased urea in F5 rats, suggesting that inhibition of the oxidation of lipid and not the decreased availability of these fuels may play an important role in the phase II-phase III of fasting transition in the long-term fasting rats.

Blockade of fatty acid oxidation mimics phase II-phase III transition in a fasting bird, the king penguin

This study tests the hypothesis that the metabolic and endocrine shift characterizing the phase II-phase III transition during prolonged fasting is related to a decrease in fatty acid (FA) oxidation. Changes in plasma concentrations of various metabolites and hormones and in lipolytic fluxes, as determined by continuous infusion of [2-(3)H]glycerol and [1-(14)C]palmitate, were examined in vivo in spontaneously fasting king penguins in the phase II status (large fat stores, protein sparing) before, during, and after treatment with mercaptoacetate (MA), an inhibitor of FA oxidation. MA induced a 7-fold decrease in plasma beta-hydroxybutyrate and a 2- to 2.5-fold increase in plasma nonesterified fatty acids (NEFA), glycerol, and triacylglycerols. MA also stimulated lipolytic fluxes, increasing the rate of appearance of NEFA and glycerol by 60-90%. This stimulation might be partly mediated by a doubling of circulating glucagon, with plasma insulin remaining unchanged. Plasma glucose level was unaffected by MA treatment. Plasma uric acid increased 4-fold, indicating a marked acceleration of body protein breakdown, possibly mediated by a 2.5-fold increase in circulating corticosterone. Strong similarities between these changes and those observed at the phase II-phase III transition in fasting penguins support the view that entrance into phase III, and especially the end of protein sparing, is related to decreased FA oxidation, rather than reduced NEFA availability. MA could be therefore a useful tool for understanding mechanisms underlying the phase II-phase III transition in spontaneously fasting birds and the associated stimulation of feeding behavior.