In-vitro stimulation on the rat epitrochlearis muscle. II. Effects of catecholamines and nutrients on protein degradation and amino acid metabolism

The influence of catecholamines and branched-chain amino acids (BCAA) plus insulin on protein degradation and amino acid metabolism was investigated in isolated and electrically stimulated rat epitrochlearis muscles. 10(-7) M adrenaline significantly increased the total amount of muscle tyrosine during 40 min of stimulation with 50 Hz (I s min 1) pulse trains. On the other hand, BCAA + insulin at normal and five times normal plasma concentrations had no effect on muscle tyrosine. Muscle 3- methylhistidine was not influenced by any of the treatments. Muscle release and content of aspartate, alanine, glutamate and glutamine showed individual response characteristics to catecholamines and BCAA + insulin. The data indicate that adrenaline can induce an increased total protein degradation in rat fast muscle during acute contractions in vitro and, furthermore, that BCAA + insulin does not retard protein breakdown during acute muscle contraction.     

Effect of branched-chain amino acid and carbohydrate supplementation on the exercise-induced change in plasma and muscle concentration of amino acids in human subjects

Five male endurance-trained subjects performed exhaustive exercise on a cycle ergometer at a work rate corresponding to 75% of their Vo2max after reduction of their muscle glycogen stores. During exercise the subjects were given in random order a 6% carbohydrate solution contining 7 g L^-1 of branched-chain amino acids (BCAA), a 6% CHO solution and flavoured water. The physical performance was lowered in four of the five subjects when they were given flavoured water during exercise as compared with the two conditions when CHO was supplied. No difference in performance was found when the subjects were given CHO+BCAA or only CHO during exercise. When CHO+BCAA was supplied the plasma and muscle (vastus lateralis) concentrations of BCAA increased during exercise by 120 and 35%, respectively. In the other conditions there was no change or a slight decrease in the plama concentrations of BCAA, but the muscle concentrations of BCAA were decreased after exercise. The plasma concentration of glutamine over the whole exercise period and 5 min after exercise was higher when CHO+BCAA were supplied during exercise compared with a supply of CHO alone or water. However, exercise caused no change in the muscle concentration of glutamine, whereas that of glutamate decreased in all three conditions. A supply of CHO+BCAA or CHO alone did not affect the exercise-induced increase in the plasma and muscle concentration of aromatic amino acids, indicating that neither BCAA nor CHO influenced the net protein degradation during exercise.     

Effects of adrenaline on lactate, glucose, lipid and protein metabolism in the placebo controlled bilaterally perfused human leg

Aim: Adrenaline has widespread metabolic actions, including stimulation of lipolysis and induction of insulin resistance and hyperlactatemia. Systemic adrenaline administration, however, generates a very complex hormonal and metabolic scenario. No studies employing regional, placebo controlled and adrenaline infusion exist. Our study was designed to test the hypothesis that local placebo controlled leg perfusion with adrenaline directly increases local lactate release, stimulates lipolysis, induces insulin resistance and leaves protein metabolism unaffected. Methods: We studied seven healthy volunteers with bilateral femoral vein and artery catheters during 3‐h basal and 3‐h hyperinsulinemic (0.6 mU kg^?1 min^?1) euglycemic clamp conditions. One femoral artery was perfused with saline and the other with adrenaline (0.4 μg min m^?2). Lipid metabolism was quantified with [9,10‐³H] palmitate and amino acid metabolism with ¹⁵N‐phenylalanine and lactate and glucose by raw arterio‐venous differences. Results: Femoral vein plasma adrenaline increased ≈eightfold in the perfused leg with unaltered blood flows. Adrenaline perfusion significantly increased local leg lactate release from 0.01 to 0.25 mmol min^?1 per leg, palmitate release in the basal state 11.5–16.9 μmol min^?1 per leg and during the clamp 2.62–8.44 μmol min^?1 per leg. Glucose uptake decreased during the clamp from ≈180 to 30 μmol min^?1 per leg. Phenylalanine kinetics was not affected by adrenaline. Conclusion: Adrenaline directly increases lactate release and lipolysis and inhibits insulin‐stimulated glucose uptake in the perfused human leg. Adrenaline has no direct effects on peripheral amino acid metabolism. Adrenaline‐induced lactate release from striated muscle may be an important mechanism underlying hyperlactatemia in the critically ill.     

Effect of infusing branched-chain amino acid during incremental exercise with reduced muscle glycogen content

Summary The aim of this study was to investigate whether, when muscle glycogen is reduced, a pre-exercise infusion of branched-chain amino acids (BCAA) modifies exercise performance or the metabolic and respiratory responses to incremental exercise. Six moderately trained volunteers took part in the following protocol on two occasions. On day 1, at 9 a.m. in the postabsorptive state, they performed a graded incremental exercise (increases of 35 W every 4 min) to exhaustion (Ex-1). A meal of 1,000 kcal (4,200 kJ; 60% protein, 40% fat) was consumed at 12 p.m. No food was then allowed until the end of the experiment (20–21 h later). A 90-min period of exercise at alternating high and moderate intensities, designed to deplete muscle glycogen, was performed between 6 p.m. and 7.30 p.m. The morning after (day 2), the subjects randomly received either a mixed solution of BCAA (260 mg × kg^–1 × h^–1 for 70 min), or saline. They then repeated the graded incremental exercise to exhaustion (Ex-2). Metabolic and respiratory measurements suggested a muscle glycogen-depleted state had been achieved. No significant differences were observed in total work performed, maximal oxygen uptake or plasma ammonia, alanine, and blood pyruvate concentrations in the two treatments. After BCAA infusion, higher blood lactate concentrations were observed at maximal power output in comparison with those during saline [BCAA 4.97 (SEM 0.41) mmol × l^–1, Saline 3.88 (SEM 0.47) mmol × l^–1,P < 0.05]. In summary, in conditions of reduced muscle glycogen content, after a short period of fasting, BCAA infusion had no significant effect on the total work that could be performed during a graded incremental exercise.     

Alterations in amino acid concentrations in the plasma and muscle in human subjects during 24 h of simulated adventure racing

This investigation was designed to evaluate changes in plasma and muscle levels of free amino acids during an ultra-endurance exercise and following recovery. Nine male ultra-endurance trained athletes participated in a 24-h standardized endurance trial with controlled energy intake. The participants performed 12 sessions of running, kayaking and cycling (4 × each discipline). Blood samples were collected before, during and after exercise, as well as after 28 h of recovery. Muscle biopsies were taken before the test and after exercise, as well as after 28 h of recovery. During the 24-h exercise, plasma levels of branched-chain (BCAA), essential amino acids (EAA) and glutamine fell 13, 14 and 19% (P < 0.05), respectively, whereas their concentrations in muscle were unaltered. Simultaneously, tyrosine and phenylalanine levels rose 38 and 50% (P < 0.05) in the plasma and 66 and 46% (P < 0.05) in muscle, respectively. After the 24-h exercise, plasma levels of BCAA were positively correlated with muscle levels of glycogen (r ² = 0.73, P < 0.05), as was the combined concentrations of muscle tyrosine and phenylalanine with plasma creatine kinase (R ² = 0.55, P < 0.05). Following 28-h of recovery, plasma and muscle levels of amino acids had either returned to their initial levels or were elevated. In conclusion, ultra-endurance exercise caused significant changes elevations in plasma and muscle levels of tyrosine and phenylalanine, which suggest an increase in net muscle protein breakdown during exercise. There was a reduction in plasma concentrations of EAA and glutamine during exercise, whereas no changes were detected in their muscle concentration after exercise.     

Influence of supplementation with branched-chain amino acids in combination with resistance exercise on p70S6 kinase phosphorylation in resting and exercising human skeletal muscle

Aim: Skeletal muscle growth is thought to be regulated by the mammalian target of rapamycin (mTOR) pathway, which can be activated by resistance exercise and branched-chain amino acids (BCAA). The major aim of the present study was to distinguish between the influence of resistance exercise and BCAA on key enzymes considered to be involved in the regulation of protein synthesis, including p70^S6 kinase (p70^S6k). Methods: Nine healthy subjects (four men and five women) performed unilateral resistance exercise on two occasions separated by 1 month. Subjects were randomly supplied either a mixture of BCAA or flavoured water. Muscle biopsies were taken from both resting and exercising muscle before, after and 1 h after exercise. Results: Phosphorylation of Akt was unaltered by either resistance exercise and/or BCAA supplementation whereas mTOR phosphorylation was enhanced (P < 0.05) to a similar extent in both exercising and resting muscle following exercise in the absence (70–90%) and presence of BCAA supplementation (80–130%). Phosphorylation of p70^S6k was unaffected by resistance exercise alone; however, BCAA intake increased (P < 0.05) this phosphorylation in both legs following exercise. In resting muscle, a 5- and 16-fold increase in p70^S6k was observed immediately after and 1 h after exercise, respectively, as compared to 11- and 30-fold increases in the exercising muscle. Phosphorylation of eukaryotic elongation factor 2 was attenuated 1 h after exercise (P < 0.05) in both resting (10–40%) and exercising muscle (30–50%) under both conditions. Conclusion: The present findings indicate that resistance exercise and BCAA exert both separate and combined effects on the p70^S6k phosphorylation in an Akt-independent manner.     

Effects of protein supplementation during prolonged exercise at moderate altitude on performance and plasma amino acid pattern

Summary The effects of two levels of protein intake on muscle performance and energy metabolism were studied in humans submitted to repeated daily sessions of prolonged exercise at moderate altitude. For this purpose, 29 healthy males, were exposed to seven successive stages of ski-mountaineering at altitudes between 2500 and 3 800 m, and to an isocaloric diet (4000 kcal·day^–1, 16760 kJ·day^–1) with either 1.5g·kg^–1·day^–1 (C group,n =14), or 2.5 g·kg^–1·day^–1 (PR group,n =15) protein intake. Measurements made after the ski-mountaineering programme did not show any change in body mass. The peak torque during maximal isometric voluntary contraction (MVC) of the quadriceps muscle was unaffected by the repeated exercises, whereas the endurance time at 50% MVC was decreased in PR subjects (–26.8%,P<0.001). Increased levels of both free fatty acids (+147%,P<0.001) and glycerol (+170%,P<0.001) observed in C subjects would suggest that lipolysis was enhanced after the repeated exercise. The plasma amino acid pattern was altered after completion of the ski-mountaineering programme; the plasma concentration of the three branched-chain amino acids (BCAA) was significantly decreased in C subjects, whereas the higher level of protein intake (PR group) greatly minimized the exercise-induced decrease in serum BCAA.     

Effect of branched-chain amino acid supplementation during unloading on regulatory components of protein synthesis in atrophied soleus muscles

Maintenance of skeletal muscle mass depends on the equilibrium between protein synthesis and protein breakdown; diminished functional demand during unloading breaks this balance and leads to muscle atrophy. The current study analyzed time-course alterations in regulatory genes and proteins in the unloaded soleus muscle and the effects of branched-chain amino acid (BCAA) supplementation on muscle atrophy and abundance of molecules that regulate protein turnover. Short-term (6 days) hindlimb suspension of rats resulted in significant losses of myofibrillar proteins, total RNA, and rRNAs and pronounced atrophy of the soleus muscle. Muscle disuse induced upregulation and increases in the abundance of the eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1), increases in gene and protein amounts of two ubiquitin ligases (muscle RING-finger protein 1 and muscle atrophy F-box protein), and decreases in the expression of cyclin D1, the ribosomal protein S6 kinase 1, the mammalian target of rapamycin (mTOR), and ERK1/2. BCAA addition to the diet did not prevent muscle atrophy and had no apparent effect on regulators of proteasomal protein degradation. However, BCAA supplementation reduced the loss of myofibrillar proteins and RNA, attenuated the increases in 4E-BP1, and partially preserved cyclin D1, mTOR and ERK1 proteins. These results indicate that BCAA supplementation alone does not oppose protein degradation but partly preserves specific signal transduction proteins that act as regulators of protein synthesis and cell growth in the non-weight-bearing soleus muscle.     

Phenylbutyrate exerts adverse effects on liver regeneration and amino acid concentrations in partially hepatectomized rats

Phenylbutyrate is recommended in urea cycle disorders and liver injury to enhance nitrogen disposal by the urine. However, hypothetically there may be adverse responses to the use of phenylbutyrate in the treatment of liver disease because of its role as a histone deacetylase inhibitor and its stimulatory effect on branched‐chain alpha‐keto acid dehydrogenase, the rate‐limiting enzyme in the catabolism of branched‐chain amino acids (BCAA; valine, leucine and isoleucine). We report the effects of phenylbutyrate on liver regeneration and amino acid levels in plasma of partially hepatectomized (PH) rats. Phenylbutyrate or saline was administered at 12‐h intervals to PH or laparotomized rats. Phenylbutyrate delayed the onset of liver regeneration compared to the saline‐treated controls, as indicated by lower hepatic DNA specific activities 18 and 24 h post‐PH, decreased hepatic fractional protein synthesis rates 24 h post‐PH and lowered the increases in liver weights and hepatic protein and DNA contents 48 h after PH. Hepatic DNA fragmentation (a hallmark of apoptosis) was higher in the phenylbutyrate‐treated animals than in controls. Phenylbutyrate decreased the glutamine and BCAA concentrations and the ratio of the BCAA to aromatic amino acids (phenylalanine and tyrosine) in the blood plasma in both hepatectomized and laparotomized animals. In conclusion, the delayed onset of liver regeneration and the decrease in BCAA/AAA ratio in blood suggest that phenylbutyrate administration may be disastrous in subjects with acute hepatic injury and BCAA supplementation is needed when phenylbutyrate is used therapeutically.     

Forearm and leg amino acid metabolism in the basal state and during combined insulin and amino acid stimulation after a 3‐day fast

Aim: Fasting is characterized by a progressive loss of protein, but data on protein kinetics are unclear and few have studied the effects of re‐feeding. The present study was designed to test the hypothesis that a combined infusion of insulin and amino acids after fasting would induce compensatory increases in protein synthesis and reductions in protein breakdown at the whole body level and in muscle. Methods: We included 10 healthy male volunteers and studied them twice: (1) in the post‐absorptive state and (2) after 72 h of fasting. Amino acid kinetics was measured using labelled phenylalanine and tyrosine, whole body energy expenditure was assessed and urea nitrogen synthesis rates were calculated. Results: After fasting we observed an increase in arterial blood concentration of branched chain amino acids and a decrease in gluconeogenic amino acids (P < 0.05). Isotopically determined whole body, forearm and leg phenylalanine fluxes were unaltered apart from a 30% decrease in phenylalanine‐to‐tyrosine conversion (2.0 vs. 1.4 μmol kg^?1 h^?1, P < 0.01). During infusion of insulin and amino acids, amino acid concentrations increased. Conclusion: Our data indicate that after a 72‐h fast basal and insulin/amino acid‐stimulated regional phenylalanine fluxes in leg and forearm muscle are unaltered. During fasting concentrations of gluconeogenic amino acids decrease and hepatic and/or renal phenylalanine‐to‐tyrosine conversion decreases. Thus, as opposed to glucose and lipid metabolism, fasting does not induce insulin resistance as regards amino acid metabolism.     

Carbohydrate metabolism and uraemia — Mechanisms for glycogenolysis and gluconeogenesis

Summary Disturbances of carbohydrate metabolism during acute uraemia are characterized by the degradation of liver and muscle glycogen with a simultaneous activation of hepatic gluconeogenesis. After binephrectomy, the substitution of essential amino acids and keto analogues stimulate liver, but not skeletal muscle glycogen synthesis. Serine proves to be an optimal substrate for liver gluconeogenesis and muscle glycogen generation under acute uraemic conditions. Propranolol does not influence glycogenolysis of skeletal muscle in acutely uraemic rats. During starvation, acute uraemia leads to an increase of total carbohydrate content as well as of glycogen and glucose concentrations in heart muscle Alterations in carbohydrate contents are not observed in the kidney after ureter ligation.Enhanced glycogenolysis of skeletal muscle and liver during acute uraemia may be due to activation of phosphorylase kinase caused by the increased serum concentrations of various hormones (glucagon, catecholamines, parathormone) as well as free proteolytic activity, an increase of intracellular Ca²⁺-concentration and finally by alterations in the structure of contractile proteins.This work was supported by the Deutsche Forschungsgemeinschaft (Ho 781/1)     

Acute effects of phenylbutyrate on glutamine,branched‐chain amino acid and protein metabolism in skeletal muscles of rats

Phenylbutyrate (PB) acts as chemical chaperone and histone deacetylase inhibitor, which is used to decrease ammonia in urea cycle disorders and has been investigated for use in the treatment of a number of lethal illnesses. We performed in vivo and in vitro experiments to examine the effects of PB on glutamine (GLN), branched‐chain amino acid (BCAA; valine, leucine and isoleucine) and protein metabolism in rats. In the first study, animals were sacrificed one hour after three injections of PB (300mg/kg b.w.) or saline. In the second study, soleus (SOL, slow twitch) and extensor digitorum longus (EDL, fast twitch) muscles were incubated in a medium with or without PB (5 mM). L‐[1‐¹⁴C] leucine was used to estimate protein synthesis and leucine oxidation, and 3‐methylhistidine release was used to evaluate myofibrillar protein breakdown. PB treatment decreased GLN, BCAA and branched‐chain keto acids (BCKAs) in blood plasma, decreased BCAA and increased GLN concentrations in muscles, and increased GLN synthetase activities in muscles. Addition of PB to incubation medium increased leucine oxidation (55% in EDL, 29% in SOL), decreased BCKA and increased GLN in medium of both muscles, increased GLN in muscles, decreased protein synthesis in SOL and increased proteolysis in EDL. It is concluded that PB decreases BCAA, BCKA and GLN in blood plasma, activates BCAA catabolism and GLN synthesis in muscle and exerts adverse effects on protein metabolism. The results indicate that BCAA and GLN supplementation is needed when PB is used therapeutically and that PB may be a useful prospective agent which could be effective in management of maple syrup urine disease.     

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 μL kg^?1 min^?1) to obtain four different levels of circulating FFAs. Endogenous growth hormone, insulin and glucagon secretion was blocked by somatostatin (300 μg 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 μmol 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).     

The effects of high intensity exercise on muscle and plasma levels of alpha-ketoisocaproic acid

Summary Alpha-ketoisocaproic acid (KIC) is the product of the transamination of the indispensable amino acid leucine, which is the first step in the complete degradation of leucine. To determine the effects of intense exercise on muscle and blood levels of KIC, 7 male volunteers performed cycle exercise to exhaustion. After pedaling at an intensity of 90 W for 3 min, the load was increased by 60 W every 3 min until volitional fatigue. Muscle biopsies were obtained prior to and immediately after exercise and rapidly frozen for later determination of KIC. During exercise, blood lactate levels increased as expected, while plasma KIC levels did not change. Following exercise, plasma KIC levels rose significantly with peak values occurring 15 min after exercise and did not return to pre-exercise values until 60 min after exercise. In contrast, muscle KIC levels increased during exercise from a pre-exercise mean of 49.4±4.1 Μmol · kg⁻¹ wet wt to 78.1±6.5 Μmol · kg⁻¹ after exercise, an average increase of 48% (P<0.05). These data indicate that during intense exercise, leucine transamination in muscle may continue at a faster rate than the decarboxylation of KIC. In addition, plasma levels of KIC did not reflect the intracellular accumulation of KIC during exercise, suggesting a delay in the diffusion of KIC from muscle.     

Protein turnover,amino acid requirements and recommendations for athletes and active populations

Skeletal muscle is the major deposit of protein molecules. As for any cell or tissue, total muscle protein reflects a dynamic turnover between net protein synthesis and degradation. Noninvasive and invasive techniques have been applied to determine amino acid catabolism and muscle protein building at rest, during exercise and during the recovery period after a single experiment or training sessions. Stable isotopic tracers (¹³C-lysine, ¹⁵N-glycine, ²H₅-phenylalanine) and arteriovenous differences have been used in studies of skeletal muscle and collagen tissues under resting and exercise conditions. There are different fractional synthesis rates in skeletal muscle and tendon tissues, but there is no major difference between collagen and myofibrillar protein synthesis. Strenuous exercise provokes increased proteolysis and decreased protein synthesis, the opposite occurring during the recovery period. Individuals who exercise respond differently when resistance and endurance types of contractions are compared. Endurance exercise induces a greater oxidative capacity (enzymes) compared to resistance exercise, which induces fiber hypertrophy (myofibrils). Nitrogen balance (difference between protein intake and protein degradation) for athletes is usually balanced when the intake of protein reaches 1.2 g·kg⁻¹·day⁻¹ compared to 0.8 g·kg⁻¹·day⁻¹ in resting individuals. Muscular activities promote a cascade of signals leading to the stimulation of eukaryotic initiation of myofibrillar protein synthesis. As suggested in several publications, a bolus of 15-20 g protein (from skimmed milk or whey proteins) and carbohydrate (± 30 g maltodextrine) drinks is needed immediately after stopping exercise to stimulate muscle protein and tendon collagen turnover within 1 h.     

The Effect of 6-Aminonicotinamide Blockade of the Pentose Phosphate Pathway on Catecholamines in the Rat Adrenal Medulla,Superior Cervical Ganglion,Hypothalamus and Synaptosome Fractions

The effect on tissue catecholamines of blockade of the pentose phosphate pathway with 6-aminonicotinamide (6-AN) was studied in the rat. 6-AN at 35–50 mg kg^-1 persistently lowered the adrenaline content in the adrenal gland to less than 10% of control values and caused a 50% loss of noradrenaline, which recovered. When the amine turnover rate was increased by a preceding period of drum stress, 6-AN also consistently depressed noradrenaline in the gland. 6-AN was without significant effect on the noradrenaline concentration in heart tissue, hypothalamus and superior cervical ganglion and did not affect the uptake or release of catecholamines in vitro. The possibility is discussed that 6-AN interferes with the biosynthesis of catecholamines, when it blocks the pentose phosphate pathway, by decreasing the supply of reducing equivalents in the form of NADPH which are necessary for the tetrahydropteridine cofactors of tyrosine hydroxylase.     

Improved insulin‐stimulated glucose uptake and glycogen synthase activation in rat skeletal muscles after adrenaline infusion: role of glycogen content and PKB phosphorylation

Aim: Effects of in vivo adrenaline infusion on subsequent insulin‐stimulated glucose uptake and glycogen synthase activation was investigated in slow‐twitch (soleus) and fast‐twitch (epitrochlearis) muscles. Furthermore, role of glycogen content and Protein kinase B (PKB) phosphorylation for modulation insulin sensitivity was investigated. Methods: Male Wistar rats received adrenaline from osmotic mini pumps (≈150 μg kg^?1 h^?1) for 1 or 12 days before muscles were removed for in vitro studies. Results: Glucose uptake at physiological insulin concentration was elevated in both muscles after 1 and 12 days of adrenaline infusion. Insulin‐stimulated glycogen synthase activation was also improved in both muscles. This elevated insulin sensitivity occurred despite the muscles were exposed to hyperglycaemia in vivo. After 1 day of adrenaline infusion, glycogen content was reduced in both muscles; insulin‐stimulated PKB ser⁴⁷³ phosphorylation was increased in both muscles only at the highest insulin concentration. After 12 days of adrenaline infusion, glycogen remained low in epitrochlearis, but returned to normal level in soleus; insulin‐stimulated PKB phosphorylation was normal in both muscles. Conclusion: Insulin‐stimulated glucose uptake and glycogen synthase activation were increased after adrenaline infusion. Increased insulin‐stimulated glucose uptake and glycogen synthase activation after adrenaline infusion cannot be explained by a reduction in glycogen content or an increase in PKB phosphorylation. The mechanisms for the improved insulin sensitivity after adrenaline treatment deserve particular attention as they occur in conjunction with hyperglycaemia.     

Chronic aerobic exercise enhances components of the classical and novel insulin signalling cascades in Sprague–Dawley rat skeletal muscle

Aim: The aim of this study was to provide a more extensive evaluation of the effects of chronic aerobic exercise on various components of the insulin signalling cascade in normal rodent skeletal muscle because of the limited body of literature that exists in this area of investigation. Methods: Male Sprague–Dawley rats were assigned to either control (n = 7) or chronic aerobic exercise (n = 7) groups. Aerobic exercise animals were run 3 day week^?1 for 45 min on a motor‐driven treadmill (32 m min^?1, 15% grade) for a 12 week period. Following the training period, all animals were subjected to hind limb perfusion in the presence of 500 μU mL^?1 insulin to determine what effect chronic aerobic training had on various components of the insulin signalling cascade, c‐Cbl protein concentration and c‐Cbl phosphorylation. Results: Twelve weeks of aerobic training did not alter skeletal muscle Akt 1/2 protein concentration, Akt Ser 473 phosphorylation, Akt Thr 308 phosphorylation, Akt 1 activity, aPKC‐ζ protein concentration, aPKC‐λ protein concentration or c‐Cbl protein concentration. In contrast, chronic aerobic exercise increased insulin‐stimulated phosphatidylinositol 3‐kinase, Akt 2 kinase and aPKC‐ζ/λ kinase activities, as well as c‐Cbl tyrosine phosphorylation, in a fibre type specific response to aerobic training. In addition, chronic aerobic exercise enhanced insulin‐stimulated plasma membrane glucose transporter 4 (GLUT4) protein concentration. Conclusion: Collectively, these findings suggest that chronic aerobic exercise enhances components of both the classical and novel insulin signalling cascades in normal rodent skeletal muscle, which may contribute to an increased insulin‐stimulated plasma membrane GLUT4 protein concentration.     

Stress hormones initiate prolonged changes in the muscle amino acid pattern.

Eight healthy volunteers were given an infusion containing cortisol, glucagon and adrenaline during 6 h. Muscle biopsies were taken before and at 6, 12 and 24 h. During the infusion serum cortisol, glucagon, glucose and insulin were increased. The stress hormone infusion induced characteristic changes in the muscle and plasma amino acid patterns similar to those seen early in protein catabolism. Muscle glutamine decreased at 12 and 24 h by -18.2 +/- 3.8 and -28.8 +/- 4.8%, respectively. The branched chain amino acids decreased at 6 h by -54.6 +/- 4.2% while increased levels (by 54.7 +/- 13.1%) were seen at 24 h. Plasma amino acids decreased during the infusion period and returned to basal during the postinfusion period. Despite a short-term infusion during 6 h the muscle amino acid pattern was still affected at 12 and 24 h and some of the changes were more accentuated at those timepoints as compared with the changes seen at 6 h.     

Adrenaline inhibition of insulin release: role of the repolarization of the B cell membrane

Activation of ₂-adrenergic receptors affects several signalling pathways in pancreatic B cells. However, since adrenaline can inhibit insulin release by interfering with a late step of the secretory process, the functional significance of the earlier effects is unclear. In this study, normal mouse islets were used to determine whether the repolarization of the B cell membrane caused by adrenaline contributes to the inhibition of insulin release. The decrease in ⁸⁶Rb efflux and the repolarization of the B cell membrane produced by adrenaline were attenuated by tolbutamide, which depolarizes by blocking ATP-sensitive K⁺ channels, and by arginine, which depolarizes because of its transport in a charged form. It is also known that adrenaline does not affect the membrane potential and ⁸⁶Rb efflux in B cells depolarized by high K⁺. These three depolarizing conditions similarly shifted to the right the concentration dependence of adrenaline inhibition of insulin release: the effect of 1 nM and 10 nM adrenaline was reduced, but high concentrations of adrenaline still inhibited insulin release nearly completely under all conditions. In contrast, increasing insulin release by cytochalasin B did not alter the inhibitory potency of adrenaline. It is concluded that the repolarization of the B cell membrane and the ensuing decrease in Ca²⁺ influx play a significant role in the inhibition of insulin release by low concentrations of adrenaline. When high concentrations are used, a more distal effect becomes predominant.