-AMPactivated protein kinase (AMPK) is a major mediator of the exercise response and a molecular target to improve insulin sensitivity. To determine if the anaerobic component of the exercise response, which is exaggerated when sprint is performed in severe acute hypoxia, influences sprint exercise-elicited Thr 172 -AMPK␣ phosphorylation, 10 volunteers performed a single 30-s sprint (Wingate test) in normoxia and in severe acute hypoxia (inspired PO2: 75 mmHg). Vastus lateralis muscle biopsies were obtained before and immediately after 30 and 120 min postsprint. Mean power output and O2 consumption were 6% and 37%, respectively, lower in hypoxia than in normoxia. O2 deficit and muscle lactate accumulation were greater in hypoxia than in normoxia. Carbonylated skeletal muscle and plasma proteins were increased after the sprint in hypoxia. Thr 172 -AMPK␣ phosphorylation was increased by 3.1-fold 30 min after the sprint in normoxia. This effect was prevented by hypoxia. The NAD ϩ -to-NADH.H ϩ ratio was reduced (by 24-fold) after the sprints, with a greater reduction in hypoxia than in normoxia (P Ͻ 0.05), concomitant with 53% lower sirtuin 1 (SIRT1) protein levels after the sprint in hypoxia (P Ͻ 0.05). This could have led to lower liver kinase B1 (LKB1) activation by SIRT1 and, hence, blunted Thr 172 -AMPK␣ phosphorylation. Ser 485 -AMPK␣1/Ser 491 -AMPK␣2 phosphorylation, a known negative regulating mechanism of Thr 172 -AMPK␣ phosphorylation, was increased by 60% immediately after the sprint in hypoxia, coincident with increased Thr 308 -Akt phosphorylation. Collectively, our results indicate that the signaling response to sprint exercise in human skeletal muscle is altered in severe acute hypoxia, which abrogated Thr 172 -AMPK␣ phosphorylation, likely due to lower LKB1 activation by SIRT1.sprint; AMP-activated protein kinase; signaling; muscle; metabolism AMP-ACTIVATED PROTEIN KINASE (AMPK) is a metabolic energy sensor activated by Thr 172 phosphorylation of the ␣-subunit, mainly in response to an increase of the AMP-to-ATP ratio (25). AMPK is involved in the regulation of feeding and body weight (42), lipid metabolism (26), glucose homeostasis (62), and mitochondrial biogenesis (69) and is a key player in the adaptation to exercise training (48). AMPK␣ phosphorylation of Thr 172 increases markedly in response to sprint exercise (22), most likely due to the elevation of the AMP-to-ATP ratio (11). Whether free radicals may also play a role in contractionmediated Thr 172 -AMPK␣ phosphorylation in skeletal muscle remains controversial (41,52). In cell cultures, hypoxia and anoxia increase Thr 172 -AMPK␣ phosphorylation more through the release of free radicals than through an increase in the AMP-to-ATP ratio (15). In contrast, chronic hypoxia (5 and 12 days of exposure to 5,500 m above sea level) did not increase skeletal muscle Thr 172 -AMPK␣ phosphorylation in rats (10). The influence of the inspired O 2 fraction (FI O 2 ) on exerciseinduced Thr 172 -AMPK␣ phosphorylation has been scarcely studied in humans (63)....

This investigation examined the influence of the number of repetitions per set on power output and muscle metabolism during leg press exercise. Six trained men (age 34±6 yr) randomly performed either 5 sets of 10 repetitions (10REP), or 10 sets of 5 repetitions (5REP) of bilateral leg press exercise, with the same initial load and rest intervals between sets. Muscle biopsies (vastus lateralis) were taken before the first set, and after the first and the final sets. Compared with 5REP, 10REP resulted in a markedly greater decrease (P<0.05) of the power output, muscle PCr and ATP content, and markedly higher (P<0.05) levels of muscle lactate and IMP. Significant correlations (P<0.01) were observed between changes in muscle PCr and muscle lactate (R2 = 0.46), between changes in muscle PCr and IMP (R2 = 0.44) as well as between changes in power output and changes in muscle ATP (R2 = 0.59) and lactate (R2 = 0.64) levels. Reducing the number of repetitions per set by 50% causes a lower disruption to the energy balance in the muscle. The correlations suggest that the changes in PCr and muscle lactate mainly occur simultaneously during exercise, whereas IMP only accumulates when PCr levels are low. The decrease in ATP stores may contribute to fatigue.

We have explored the role of mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase in regulating ketogenesis. We had previously cloned the cDNA for mitochondrial HMG-CoA synthase and have now studied the regulation in vivo of the expression of this gene in rat liver. The amount of processed mitochondrial HMG-CoA synthase mRNA is rapidly changed in response to cyclic AMP, insulin, dexamethasone and refeeding, and is greatly increased by starvation, fat feeding and diabetes. We conclude that one point of ketogenic control is exercised at the level of genetic expression of mitochondrial HMG-CoA synthase.

To examine whether blood lactate and ammonia concentrations can be used to estimate the functional state of the muscle contractile machinery with regard to muscle lactate and adenosine triphosphate (ATP) levels during leg press exercise. Thirteen men (age, 34 ± 5 years; 1 repetition maximum leg press strength 199 ± 33 kg) performed either 5 sets of 10 repetitions to failure (5×10RF), or 10 sets of 5 repetitions not to failure (10×5RNF) with the same initial load (10RM) and interset rests (2 minutes) on 2 separate sessions in random order. Capillary blood samples were obtained before and during exercise and recovery. Six subjects underwent vastus lateralis muscle biopsies at rest, before the first set and after the final exercise set. The 5×10RF resulted in a significant and marked decrease in power output (37%), muscle ATP content (24%), and high levels of muscle lactate (25.0 ± 8.1 mmol·kg wet weight), blood lactate (10.3 ± 2.6 mmol·L), and blood ammonia (91.6 ± 40.5 μmol·L). During 10×5RNF no or minimal changes were observed. Significant correlations were found between: (a) blood ammonia and muscle ATP (r = -0.75), (b) changes in peak power output and blood ammonia (r = -0.87) and blood lactate (r = -0.84), and (c) blood and muscle lactate (r = 0.90). Blood lactate and ammonia concentrations can be used as extracellular markers for muscle lactate and ATP contents, respectively. The decline in mechanical power output can be used to indirectly estimate blood ammonia and lactate during leg press exercise.