Beta2-agonist impairs muscle insulin sensitivity in persons with insulin resistance.

CONTEXT: Given the promising effects of prolonged treatment with beta2-agonist on insulin sensitivity in animals and non-diabetic individuals, the beta2-adrenergic receptor has been proposed as a target to counter peripheral insulin resistance. On the other hand, rodent studies also reveal that beta2-agonists acutely impair insulin action, posing a potential caveat for their use in treating insulin resistance. OBJECTIVE: To assess the impact of beta2-agonist on muscle insulin action and glucose metabolism and identify the underlying mechanism(s) in 10 insulin-resistant subjects. METHODS AND PARTICIPANTS: In a cross-over design, we assessed the effect of beta2-agonist on insulin-stimulated muscle glucose uptake during a 3-h hyperinsulinemic isoglycemic clamp with and without intralipid infusion in 10 insulin-resistant overweight subjects. Two hours into the clamp, we infused beta2-agonist. We collected muscle biopsies before, two hours into and by the end of the clamp and analyzed them using metabolomic and lipidomic techniques. RESULTS: We establish that beta2-agonist, independently from and additively to intralipid, impairs insulin-stimulated muscle glucose uptake via different mechanisms. In combination, beta2-agonist and intralipid nearly eliminates insulin-dependent muscle glucose uptake. While both beta2-agonist and intralipid elevated muscle glucose-6-phosphate, only intralipid caused accumulation of downstream muscle glycolytic intermediates, whereas beta2-agonist attenuated incorporation of glucose into glycogen. CONCLUSIONS: Our findings suggest that beta2-agonist inhibits glycogenesis while intralipid inhibits glycolysis in skeletal muscle of insulin-resistant individuals. These results should be addressed in future treatment of insulin resistance with beta2-agonist.

AMPKγ3 Controls Muscle Glucose Uptake in Recovery From Exercise to Recapture Energy Stores.

Exercise increases muscle glucose uptake independently of insulin signaling and represents a cornerstone for the prevention of metabolic disorders. Pharmacological activation of the exercise-responsive AMPK in skeletal muscle has been proven successful as a therapeutic approach to treat metabolic disorders by improving glucose homeostasis through the regulation of muscle glucose uptake. However, conflicting observations cloud the proposed role of AMPK as a necessary regulator of muscle glucose uptake during exercise. We show that glucose uptake increases in human skeletal muscle in the absence of AMPK activation during exercise and that exercise-stimulated AMPKγ3 activity strongly correlates to muscle glucose uptake in the postexercise period. In AMPKγ3-deficient mice, muscle glucose uptake is normally regulated during exercise and contractions but impaired in the recovery period from these stimuli. Impaired glucose uptake in recovery from exercise and contractions is associated with a lower glucose extraction, which can be explained by a diminished permeability to glucose and abundance of GLUT4 at the muscle plasma membrane. As a result, AMPKγ3 deficiency impairs muscle glycogen resynthesis following exercise. These results identify a physiological function of the AMPKγ3 complex in human and rodent skeletal muscle that regulates glucose uptake in recovery from exercise to recapture muscle energy stores. ARTICLE HIGHLIGHTS: Exercise-induced activation of AMPK in skeletal muscle has been proposed to regulate muscle glucose uptake in recovery from exercise. This study investigated whether the muscle-specific AMPKγ3-associated heterotrimeric complex was involved in regulating muscle glucose metabolism in recovery from exercise. The findings support that exercise-induced activation of the AMPKγ3 complex in human and mouse skeletal muscle enhances glucose uptake in recovery from exercise via increased translocation of GLUT4 to the plasma membrane. This work uncovers the physiological role of the AMPKγ3 complex in regulating muscle glucose uptake that favors replenishment of the muscle cellular energy stores.

Insulin Sensitization Following a Single Exercise Bout Is Uncoupled to Glycogen in Human Skeletal Muscle: A Meta-analysis of 13 Single-Center Human Studies.

Exercise profoundly influences glycemic control by enhancing muscle insulin sensitivity, thus promoting glucometabolic health. While prior glycogen breakdown so far has been deemed integral for muscle insulin sensitivity to be potentiated by exercise, the mechanisms underlying this phenomenon remain enigmatic. We have combined original data from 13 of our studies that investigated insulin action in skeletal muscle either under rested conditions or following a bout of one-legged knee extensor exercise in healthy young male individuals (n = 106). Insulin-stimulated glucose uptake was potentiated and occurred substantially faster in the prior contracted muscles. In this otherwise homogenous group of individuals, a remarkable biological diversity in the glucometabolic responses to insulin is apparent both in skeletal muscle and at the whole-body level. In contrast to the prevailing concept, our analyses reveal that insulin-stimulated muscle glucose uptake and the potentiation thereof by exercise are not associated with muscle glycogen synthase activity, muscle glycogen content, or degree of glycogen utilization during the preceding exercise bout. Our data further suggest that the phenomenon of improved insulin sensitivity in prior contracted muscle is not regulated in a homeostatic feedback manner from glycogen. Instead, we put forward the idea that this phenomenon is regulated by cellular allostatic mechanisms that elevate the muscle glycogen storage set point and enhance insulin sensitivity to promote the uptake of glucose toward faster glycogen resynthesis without development of glucose overload/toxicity or feedback inhibition.

Muscle hypertrophic effect of inhaled beta2 -agonist is associated with augmented insulin-stimulated whole-body glucose disposal in young men.

Rodent studies highlight enhancement of glucose tolerance and insulin sensitivity as potential clinically relevant effects of chronic beta2 -agonist treatment. However, the doses administered to rodents are not comparable with the therapeutic doses used for humans. Thus, we investigated the physiological effects of prolonged beta2 -agonist treatment at inhaled doses resembling those used in respiratory diseases on insulin-stimulated whole-body glucose disposal and putative mechanisms in skeletal muscle and adipose tissue of healthy men. Utilizing a randomized placebo-controlled parallel-group design, we assigned 21 healthy men to 4 weeks daily inhalation of terbutaline (TER; 4 mg × day-1 , n = 13) or placebo (PLA, n = 8). Before and after treatments, we assessed subjects' whole-body insulin-stimulated glucose disposal and body composition, and collected vastus lateralis muscle and abdominal adipose tissue biopsies. Glucose infusion rate increased by 27% (95% CI: 80 to 238 mg × min-1 , P = 0.001) in TER, whereas no significant changes occurred in PLA (95% CI: -37 to 195 mg × min-1 , P = 0.154). GLUT4 content in muscle or adipose tissue did not change, nor did hexokinase II content or markers of mitochondrial volume in muscle. Change in lean mass was associated with change in glucose infusion rate in TER (r = 0.59, P = 0.03). Beta2 -agonist treatment in close-to-therapeutic doses may augment whole-body insulin-stimulated glucose disposal in healthy young men and part of the change is likely to be explained by muscle hypertrophy. These findings highlight the therapeutic potential of beta2 -agonists for improving insulin sensitivity. KEY POINTS: While studies in rodents have highlighted beta2 -agonists as a means to augment insulin sensitivity, these studies utilized beta2 -agonists at doses inapplicable to humans. Herein we show that a 4-week treatment period with daily therapeutic inhalation of beta2 -agonist increases insulin-stimulated whole-body glucose disposal in young healthy lean men. This effect was associated with an increase of lean mass but not with changes in GLUT4 and hexokinase II or basal glycogen content in skeletal muscle nor GLUT4 content in abdominal adipose tissue. These findings suggest that the enhanced insulin-stimulated whole-body glucose disposal induced by a period of beta2 -agonist treatment in humans, at least in part, is attributed to muscle hypertrophy. Our observations extend findings in rodents and highlight the therapeutic potential of beta2 -agonists to enhance the capacity for glucose disposal and whole-body insulin sensitivity, providing important knowledge with potential application in insulin resistance.

Personalized phosphoproteomics identifies functional signaling.

Protein phosphorylation dynamically integrates environmental and cellular information to control biological processes. Identifying functional phosphorylation amongst the thousands of phosphosites regulated by a perturbation at a global scale is a major challenge. Here we introduce 'personalized phosphoproteomics', a combination of experimental and computational analyses to link signaling with biological function by utilizing human phenotypic variance. We measure individual subject phosphoproteome responses to interventions with corresponding phenotypes measured in parallel. Applying this approach to investigate how exercise potentiates insulin signaling in human skeletal muscle, we identify both known and previously unidentified phosphosites on proteins involved in glucose metabolism. This includes a cooperative relationship between mTOR and AMPK whereby the former directly phosphorylates the latter on S377, for which we find a role in metabolic regulation. These results establish personalized phosphoproteomics as a general approach for investigating the signal transduction underlying complex biology.

The beta2 -adrenergic receptor - a re-emerging target to combat obesity and induce leanness?

Treatment of obesity with repurposed or novel drugs is an expanding research field. One approach is to target beta2 -adrenergic receptors because they regulate the metabolism and phenotype of adipose and skeletal muscle tissue. Several observations support a role for the beta2 -adrenergic receptor in obesity. Specific human beta2 -adrenergic receptor polymorphisms are associated with body composition and obesity, for which the Gln27Glu polymorphism is associated with obesity, while the Arg16Gly polymorphism is associated with lean mass in men and the development of obesity in specific populations. Individuals with obesity also have lower abundance of beta2 -adrenergic receptors in adipose tissue and are less sensitive to catecholamines. In addition, studies in livestock and rodents demonstrate that selective beta2 -agonists induce a so-called 'repartitioning effect' characterized by muscle accretion and reduced fat deposition. In humans, beta2 -agonists dose-dependently increase resting metabolic rate by 10-50%. And like that observed in other mammals, only a few weeks of treatment with beta2 -agonists increases muscle mass and reduces fat mass in young healthy individuals. Beta2 -agonists also exert beneficial effects on body composition when used concomitantly with training and act additively to increase muscle strength and mass during periods with resistance training. Thus, the beta2 -adrenergic receptor seems like an attractive target in the development of anti-obesity drugs. However, future studies need to verify the long-term efficacy and safety of beta2 -agonists in individuals with obesity, particularly in those with comorbidities.

Salbutamol Increases Leg Glucose Uptake and Metabolic Rate but not Muscle Glycogen Resynthesis in Recovery From Exercise.

CONTEXT: Exercise blunts the effect of beta2-agonists on peripheral glucose uptake and energy expenditure. Whether such attenuation extends into recovery is unknown. OBJECTIVE: To examine the effect of a beta2-agonist on leg glucose uptake and metabolic rate in recovery from exercise. METHODS: Using leg arteriovenous balance technique and analyses of thigh muscle biopsies, we investigated the effect of a beta2-agonist (24 mg of oral salbutamol) vs placebo on leg glucose, lactate, and oxygen exchange before and during quadriceps exercise, and 0.5 to 5 hours in recovery from quadriceps exercise, as well as on muscle glycogen resynthesis and activity in recovery. Twelve healthy, lean, young men participated. RESULTS: Before exercise, leg glucose uptake was 0.42 ±â€…0.12 and 0.20 ±â€…0.02 mmol × min-1 (mean ±â€…SD) for salbutamol and placebo (P = .06), respectively, while leg oxygen consumption was around 2-fold higher (P < .01) for salbutamol than for placebo (25 ±â€…3 vs 14 ±â€…1 mL × min-1). No treatment differences were observed in leg glucose uptake, lactate release, and oxygen consumption during exercise. But in recovery, cumulated leg glucose uptake, lactate release, and oxygen consumption was 21 mmol (95% CI 18-24, P = .018), 19 mmol (95% CI 16-23, P < .01), and 1.8 L (95% CI 1.6-2.0, P < .01) higher for salbutamol than for placebo, respectively. Muscle glycogen content was around 30% lower (P < .01) for salbutamol than for placebo in recovery, whereas no treatment differences were observed in muscle glycogen resynthesis or glycogen synthase activity. CONCLUSION: Exercise blunts the effect of beta2-agonist salbutamol on leg glucose uptake, but this attenuation diminishes in recovery. Salbutamol increases leg lactate release in recovery, which may relate to glycolytic trafficking due to excessive myocellular glucose uptake.

ß2-Adrenergic agonist salbutamol augments hypertrophy in MHCIIa fibers and sprint mean power output but not muscle force during 11 weeks of resistance training in young men.

In this study, we examined the effect of ß2-agonist salbutamol at oral doses during a period of resistance training on sprint performance, quadriceps contractile function, skeletal muscle hypertrophy, fiber type composition, maximal activity of enzymes of importance for anaerobic energy turnover, and sarcoplasmic reticulum Ca2+ handling in young men. Twenty-six men (23 ± 2 yr; means ± SD) were randomized to daily intake of oral salbutamol (16 mg/day; RES+SAL) or placebo (RES) during 11 wk of full-body resistance training 3 times/wk. Mean power output during 10-s maximal cycling increased more (P = 0.027) in RES+SAL (+12%) than in RES (+7%), whereas peak power output increased similarly (RES+SAL: +8%; RES: +7%; P = 0.400). Quadriceps dynamic peak torque and maximal voluntary isometric torque increased by 13 and 14% (P ≤ 0.001) in RES+SAL and 13 and 13% (P ≤ 0.001) in RES, respectively. Myosin heavy-chain (MHC) isoform distribution transitioned from MHCI and MHCIIx toward MHCIIa in RES+SAL (P = 0.002), but not in RES (P = 0.323). MHCIIa cross-sectional-area increased more (P = 0.040) in RES+SAL (+35%) than RES (+21%). Sarcoplasmic reticulum Ca2+ release rate increased in both groups (RES+SAL: +9%, P = 0.048; RES: +13%, P = 0.008), whereas Ca2+-uptake rate increased only in RES (+12%, P = 0.022) but was not different from the nonsignificant change in RES+SAL (+2%, P = 0.484). Maximal activity of lactate dehydrogenase increased only in RES+SAL (+13%, P = 0.008). Muscle content of the dihydropyridine receptor, ryanodine receptor 1, and sarcoplasmic reticulum Ca2+-ATPase isoform 1 and 2 did not change with the intervention in either group (P ≥ 0.100). These observations indicate that the enhancement of sprint mean power output induced by salbutamol is at least partly attributed to greater hypertrophy of MHCIIa fibers and transition toward the MHCIIa isoform.NEW & NOTEWORTHY Here, we show that daily oral treatment with selective ß2-agonist salbutamol induces muscle fiber isoform transition from myosin-heavy-chain (MHC)-I toward MHCIIa and augments hypertrophy of MHCIIa fibers during a period of resistance training. Compared with placebo, salbutamol enhanced sprint mean power output, whereas peak power output and measures of muscle strength increased similarly during the resistance training period despite augmented hypertrophy with salbutamol. Thus, salbutamol is a muscle anabolic drug that can enhance sprint ability adaptations to resistance training.

Effect of beta2 -adrenergic agonist and resistance training on maximal oxygen uptake and muscle oxidative enzymes in men.

While beta2 -adrenoceptor stimulation has been shown to increase lean mass and to alter metabolic properties of skeletal muscle, adaptations in muscle oxidative enzymes and maximal oxygen uptake ( V ˙ O2max ) in response to beta2 -adrenergic agonist treatment are inadequately explored in humans, particularly in association with resistance training. Herein, we investigated beta2 -adrenergic-induced changes in V ˙ O2max , leg and arm composition, and muscle content of oxidative enzymes in response to treatment with the selective beta2 -adrenergic agonist terbutaline with and without concurrent resistance training in young men. Forty-six subjects were randomized to 4 weeks of lifestyle maintenance (n = 23) or resistance training (n = 23). Within the lifestyle maintenance and resistance training group, subjects received daily terbutaline (8 × 0.5 mg) (n = 13) or placebo (n = 10) treatment. No apparent treatment by training interactions was observed during the study period. Terbutaline increased leg and arm lean mass with the intervention, whereas no treatment differences were observed in absolute V ˙ O2max and incremental peak power output (iPPO). Treatment main effects were observed for V ˙ O2 -reserve (P < .05), V ˙ O2max relative to body mass (P < .05), V ˙ O2max relative to leg lean mass (P < .01), and iPPO relative to leg lean mass, in which terbutaline had a negative effect compared with placebo. Furthermore, content of electron transport chain complex I-V decreased by 11% (P < .05) for terbutaline compared with placebo. Accordingly, chronic treatment with the selective beta2 -adrenergic agonist terbutaline may negatively affect V ˙ O2max and iPPO in relative terms, but not in absolute.

In-season adaptations to intense intermittent training and sprint interval training in sub-elite football players.

This study investigated the in-season effect of intensified training comparing the efficacy of duration-matched intense intermittent exercise training with sprint interval training in increasing intermittent running performance, sprint ability, and muscle content of proteins related to ion handling and metabolism in football players. After the first two weeks in the season, 22 sub-elite football players completed either 10 weeks of intense intermittent training using the 10-20-30 training concept (10-20-30, n = 12) or sprint interval training (SIT, n = 10; work/rest ratio: 6-s/54-s) three times weekly, with a ~20% reduction in weekly training time. Before and after the intervention, players performed a Yo-Yo intermittent recovery test level 1 (Yo-Yo IR1) and a 30-m sprint test. Furthermore, players had a muscle biopsy taken from the vastus lateralis. Yo-Yo IR1 performance increased by 330 m (95%CI: 178-482, P ≤ 0.01) in 10-20-30, whereas no change was observed in SIT. Sprint time did not change in 10-20-30 but decreased by 0.04 second (95%CI: 0.00-0.09, P ≤ 0.05) in SIT. Muscle content of HADHA (24%, P ≤ 0.01), PDH-E1α (40%, P ≤ 0.01), complex I-V of the electron transport chain (ETC) (51%, P ≤ 0.01) and Na+ , K+ -ATPase subunits α2 (33%, P ≤ 0.05) and ß1 (27%, P ≤ 0.05) increased in 10-20-30, whereas content of DHPR (27%, P ≤ 0.01) and complex I-V of the ETC (31%, P ≤ 0.05) increased in SIT. Intense intermittent training, combining short sprints and a high aerobic load, is superior to regular sprint interval training in increasing intense intermittent running performance during a Yo-Yo IR1 test and muscle content of PDH-E1α and HADHA in sub-elite football players.

ß2-Agonist Induces Net Leg Glucose Uptake and Free Fatty Acid Release at Rest but Not During Exercise in Young Men.

Objective: The role of selective ß2-adrenergic stimulation in regulation of leg glucose uptake and free fatty acid (FFA) balance is inadequately explored in humans. The objective of this study was to investigate ß2-adrenergic effects on net leg glucose uptake and clearance, as well as FFA balance at rest and during exercise. Design: The study was a randomized, placebo-controlled crossover trial where 10 healthy men received either infusion of ß2-agonist terbutaline (0.2 to 0.4 mg) or placebo. Net leg glucose uptake and clearance and FFA balance were determined at rest and during 8 minutes of knee extensor exercise using Fick's principle. Vastus lateralis muscle biopsies were collected at rest and at cessation of exercise. The primary outcome measure was net leg glucose uptake. Results: At rest, net leg glucose uptake and clearance were 0.35 (±0.16) mmol/min and 41 (±17) mL/min (mean ± 95% CI) higher (P < 0.001) for terbutaline than placebo, corresponding to increases of 84% and 70%. During exercise, no treatment differences were observed in net leg glucose uptake, whereas clearance was 101 (±86) mL/min lower (P < 0.05) for terbutaline than placebo. At rest, terbutaline induced a net leg FFA release of 21 (±14) µmol/min, being different from placebo (P = 0.04). During exercise, net leg FFA uptake was not different between the treatments. Conclusions: These observations indicate that ß2-agonist alters net leg glucose uptake and clearance, as well as FFA balance in humans, which is associated with myocellular ß2-adrenergic and insulin-dependent signaling. Furthermore, the study shows that exercise confounds the ß2-adrenergic effect on net leg glucose uptake and FFA balance.

Chronic ß2 -adrenoceptor agonist treatment alters muscle proteome and functional adaptations induced by high intensity training in young men.

KEY POINTS: While several studies have investigated the effects of exercise training in human skeletal muscle and the chronic effect of ß2 -agonist treatment in rodent muscle, their effects on muscle proteome signature with related functional measures in humans are still incompletely understood. Herein we show that daily ß2 -agonist treatment attenuates training-induced enhancements in exercise performance and maximal oxygen consumption, and alters muscle proteome signature and phenotype in trained young men. Daily ß2 -agonist treatment abolished several of the training-induced enhancements in muscle oxidative capacity and caused a repression of muscle metabolic pathways; furthermore, ß2 -agonist treatment induced a slow-to-fast twitch muscle phenotype transition. The present study indicates that chronic ß2 -agonist treatment confounds the positive effect of high intensity training on exercise performance and oxidative capacity, which is of interest for the large proportion of persons using inhaled ß2 -agonists on a daily basis, including athletes. ABSTRACT: Although the effects of training have been studied for decades, data on muscle proteome signature remodelling induced by high intensity training in relation to functional changes in humans remains incomplete. Likewise, ß2 -agonists are frequently used to counteract exercise-induced bronchoconstriction, but the effects ß2 -agonist treatment on muscle remodelling and adaptations to training are unknown. In a placebo-controlled parallel study, we randomly assigned 21 trained men to 4 weeks of high intensity training with (HIT+ß2 A) or without (HIT) daily inhalation of ß2 -agonist (terbutaline, 4 mg dose-1 ). Of 486 proteins identified by mass-spectrometry proteomics of muscle biopsies sampled before and after the intervention, 32 and 85 were changing (false discovery rate (FDR) ≤5%) with the intervention in HIT and HIT+ß2 A, respectively. Proteome signature changes were different in HIT and HIT+ß2 A (P = 0.005), wherein ß2 -agonist caused a repression of 25 proteins in HIT+ß2 A compared to HIT, and an upregulation of 7 proteins compared to HIT. ß2 -Agonist repressed or even downregulated training-induced enrichment of pathways related to oxidative phosphorylation and glycogen metabolism, but upregulated pathways related to histone trimethylation and the nucleosome. Muscle contractile phenotype changed differently in HIT and HIT+ß2 A (P ≤ 0.001), with a fast-to-slow twitch transition in HIT and a slow-to-fast twitch transition in HIT+ß2 A. ß2 -Agonist attenuated training-induced enhancements in maximal oxygen consumption (P ≤ 0.01) and exercise performance (6.1 vs. 11.6%, P ≤ 0.05) in HIT+ß2 A compared to HIT. These findings indicate that daily ß2 -agonist treatment attenuates the beneficial effects of high intensity training on exercise performance and oxidative capacity, and causes remodelling of muscle proteome signature towards a fast-twitch phenotype.

Beta2-adrenergic stimulation increases energy expenditure at rest, but not during submaximal exercise in active overweight men.

PURPOSE: ß2-Agonists have been proposed as weight-loss treatment, because they elevate energy expenditure. However, it is unknown what effect ß2-agonists have on energy expenditure in overweight individuals. Furthermore, the influence of ß2-agonist R- and S-enantiomer ratio for the increased energy expenditure is insufficiently explored. METHODS: Nineteen males were included in the study of which 14 completed. Subjects were 31.6 (±3.5) years [mean (±95% CI)] and had a fat percentage of 22.7 (±2.1)%. On separate days, subjects received either placebo or inhaled racemic (rac-) formoterol (2 × 27 µg). After an overnight fast, energy expenditure and substrate oxidation were estimated by indirect calorimetry at rest and during submaximal exercise. Plasma (R,R)- and (S,S)-formoterol enantiomer levels were measured by ultra-performance liquid chromatograph-mass spectrometry. RESULTS: At rest, energy expenditure and fat oxidation were 12% (P ≤ 0.001) and 38% (P = 0.006) higher for rac-formoterol than placebo. Systemic (R,R):(S,S) formoterol ratio was correlated with change in energy expenditure at rest in response to rac-formoterol (r = 0.63, P = 0.028), whereas no association was observed between fat percentage and rac-formoterol-induced change in energy expenditure. During exercise, energy expenditure was not different between treatments, although carbohydrate oxidation was 15% higher (P = 0.021) for rac-formoterol than placebo. Rac-formoterol-induced shift in substrate choice from rest to exercise was related to plasma ln-rac-formoterol concentrations (r = 0.75, P = 0.005). CONCLUSION: Selective ß2-adrenoceptor agonism effectively increases metabolic rate and fat oxidation in overweight individuals. The potential for weight loss induced by ß2-agonists may be greater for R-enantiopure formulations.

Mechanisms underlying enhancements in muscle force and power output during maximal cycle ergometer exercise induced by chronic ß2-adrenergic stimulation in men.

The study was a randomized placebo-controlled trial investigating mechanisms by which chronic ß2-adrenergic stimulation enhances muscle force and power output during maximal cycle ergometer exercise in young men. Eighteen trained men were assigned to an experimental group [oral terbutaline 5 mg/30 kg body weight (bw) twice daily (TER); n = 9] or a control group [placebo (PLA); n = 9] for a 4-wk intervention. No changes were observed with the intervention in PLA. Isometric muscle force of the quadriceps increased (P ≤ 0.01) by 97 ± 29 N (means ± SE) with the intervention in TER compared with PLA. Peak and mean power output during 30 s of maximal cycling increased (P ≤ 0.01) by 32 ± 8 and 25 ± 9 W, respectively, with the intervention in TER compared with PLA. Maximal oxygen consumption (V̇o2max) and time to fatigue during incremental cycling did not change with the intervention. Lean body mass increased by 1.95 ± 0.8 kg (P ≤ 0.05) with the intervention in TER compared with PLA. Change in single fiber cross-sectional area of myosin heavy chain (MHC) I (1,205 ± 558 µm(2); P ≤ 0.01) and MHC II fibers (1,277 ± 595 µm(2); P ≤ 0.05) of the vastus lateralis muscle was higher for TER than PLA with the intervention, whereas no changes were observed in MHC isoform distribution. Expression of muscle proteins involved in growth, ion handling, lactate production, and clearance increased (P ≤ 0.05) with the intervention in TER compared with PLA, with no change in oxidative enzymes. Our observations suggest that muscle hypertrophy is the primary mechanism underlying enhancements in muscle force and peak power during maximal cycling induced by chronic ß2-adrenergic stimulation in humans.