Benedict J. Canny

Early signaling responses to divergent exercise stimuli in skeletal muscle from well-trained humans

Skeletal muscle from strength‐ and endurance‐trained individuals represents diverse adaptive states. In this regard, AMPK‐PGC‐1α signaling mediates several adaptations to endurance training, while up‐regulation of the Akt‐TSC2‐mTOR pathway may underlie increased protein synthesis after resistance exercise. We determined the effect of prior training history on signaling responses in seven strength‐trained and six endurance‐trained males who undertook 1 h cycling at 70% VO2peak or eight sets of five maximal repetitions of isokinetic leg extensions. Muscle biopsies were taken at rest, immediately and 3 h postexercise. AMPK phosphorylation increased after cycling in strength‐trained (54%; P<0.05) but not endurance‐trained subjects. Conversely, AMPK was elevated after resistance exercise in endurance‐ (114%; P<0.05), but not strength‐trained subjects. Akt phosphorylation increased in endurance‐ (50%; P<0.05), but not strength‐trained subjects after cycling but was unchanged in either group after resistance exercise. TSC2 phosphorylation was decreased (47%; P<0.05) in endurance‐trained subjects following resistance exercise, but cycling had little effect on the phosphorylation state of this protein in either group. p70S6K phosphorylation increased in endurance‐ (118%; P<0.05), but not strength‐trained subjects after resistance exercise, but was similar to rest in both groups after cycling. Similarly, phosphorylation of S6 protein, a substrate for p70 S6K, was increased immediately following resistance exercise in endurance‐ (129%; P<0.05), but not strength‐trained subjects. In conclusion, a degree of “response plasticity” is conserved at opposite ends of the endurance‐hypertrophic adaptation continuum. Moreover, prior training attenuates the exercise specific signaling responses involved in single mode adaptations to training.

Central administration of leptin to ovariectomized ewes inhibits food intake without affecting the secretion of hormones from the pituitary gland: evidence for a dissociation of effects on appetite and neuroendocrine function.

We have studied the effect of leptin on food intake and neuroendocrine function in ovariectomized ewes. Groups (n = 5) received intracerebroventricular infusions of either vehicle or leptin (20 microg/h) for 3 days and were blood sampled over 6 h on days -1, 2, and for 3 h on day 3 relative to the onset of the infusion. The animals were then killed to measure hypothalamic neuropeptide Y expression by in situ hybridization. Plasma samples were assayed for metabolic parameters and pituitary hormones. Food intake was reduced by leptin, but did not change in controls. Leptin treatment elevated plasma lactate and nonesterified fatty acids, but did not affect glucose or insulin levels, indicating a state of negative energy balance that was met by the mobilization of body stores. Pulse analysis showed that the secretion of LH and GH was not affected by leptin treatment, nor were the mean plasma concentrations of FSH, PRL, or cortisol. Expression of messenger RNA for neuropeptide Y in the arcuate nucleus was reduced by the infusion of leptin, primarily due to reduced expression per cell rather than a reduction in the number of cells observed. Thus, the action of leptin to inhibit food intake is dissociated from neuroendocrine function. These results suggest that the metabolic effects of leptin are mediated via neuronal systems that possess leptin receptors rather than via endocrine effects.

Short-term exercise training in humans reduces AMPK signalling during prolonged exercise independent of muscle glycogen

We examined the effect of short‐term exercise training on skeletal muscle AMP‐activated protein kinase (AMPK) signalling and muscle metabolism during prolonged exercise in humans. Eight sedentary males completed 120 min of cycling at 66 ± 1%, then exercise trained for 10 days, before repeating the exercise bout at the same absolute workload. Participants rested for 72 h before each trial while ingesting a high carbohydrate diet (HCHO). Exercise training significantly (P < 0.05) attenuated exercise‐induced increases in skeletal muscle free AMP: ATP ratio and glucose disposal and increased fat oxidation. Exercise training abolished the 9‐fold increase in AMPK α2 activity observed during pretraining exercise. Since training increased muscle glycogen content by 93 ± 12% (P < 0.01), we conducted a second experiment in seven sedentary male participants where muscle glycogen content was essentially matched pre‐ and post‐training by exercise and a low CHO diet (LCHO; post‐training muscle glycogen 52 ± 7% less than in HCHO, P < 0.001). Despite the difference in muscle glycogen levels in the two studies we obtained very similar results. In both studies the increase in ACCβ Ser221 phosphorylation was reduced during exercise after training. In conclusion, there is little activation of AMPK signalling during prolonged exercise following short‐term exercise training suggesting that other factors are important in the regulation of glucose disposal and fat oxidation under these circumstances. It appears that muscle glycogen is not an important regulator of AMPK activation during exercise in humans when exercise is begun with normal or high muscle glycogen levels.

Effects of local pressure and vibration on muscle pain from eccentric exercise and hypertonic saline.

&NA; In human subjects the triceps surae of one leg was exercised eccentrically by asking subjects to walk backwards on an inclined treadmill. Before the exercise controlled local pressure, applied to the muscle with an electromagnet, produced mild soreness, which was reduced when the pressure was combined with vibration. When delayed‐onset muscle soreness (DOMS) had set in, 24–48 h after the exercise, vibration increased pain from local pressure. Vibrating at different frequencies suggested 80 Hz as the optimal frequency. During 2‐h testing post‐exercise, evidence of a change in character of the effects of vibration was first detected at 6 h. It persisted up to 72 h post‐exercise. When muscle pain was generated in an unexercised triceps by injection of hypertonic (5%) saline, controlled local pressure applied to the sore area increased pain levels by 32% while pressure plus vibration reduced this to 11%. In a subject with DOMS, local pressure again increased pain from saline by 32% but combining it with vibration increased pain further by an additional 20%. The effect of vibration on DOMS could be abolished with a large nerve fibre block applied to the sciatic nerve. It is concluded that the vibration effects are the result of stimulation of large‐diameter mechanoreceptive afferents in the muscle which, it is speculated, play a role in generating DOMS.

Effect of sodium bicarbonate on muscle metabolism during intense endurance cycling

INTRODUCTION Sodium bicarbonate (NaHCO3) ingestion has been shown to increase both muscle glycogenolysis and glycolysis during brief submaximal exercise. These changes may be detrimental to performance during more prolonged, exhaustive exercise. This study examined the effect of NaHCO3 ingestion on muscle metabolism and performance during intense endurance exercise of approximately 60 min in seven endurance-trained men. METHODS Subjects ingested 0.3 g.kg-1 body mass of either NaHCO3 or CaCO3 (CON) 2 h before performing 30 min of cycling exercise at 77 +/- 1% .VO(2peak) followed by completion of 469 +/- 21 kJ as quickly as possible (approximately 30 min, approximately 80% .VO(2peak)). RESULTS Immediately before, and throughout exercise, arterialized-venous plasma HCO3- concentrations were higher (P < 0.05) whereas plasma and muscle H+ concentrations were lower (P < 0.05) in NaHCO3 compared with CON. Blood lactate concentrations were higher (P < 0.05) during exercise in NaHCO3, but there was no difference between trials in muscle glycogen utilization or muscle lactate content during exercise. Reductions in PCr and ATP and increases in muscle Cr during exercise were also unaffected by NaHCO3 ingestion. Accordingly, exercise performance time was not different between treatments. CONCLUSION NaHCO3 ingestion resulted in a small muscle alkalosis but had no effect on muscle metabolism or intense endurance exercise performance in well-trained men.

Divergent cell signaling after short-term intensified endurance training in human skeletal muscle

Endurance training represents one extreme in the continuum of skeletal muscle plasticity. The molecular signals elicited in response to acute and chronic exercise and the integration of multiple intracellular pathways are incompletely understood. We determined the effect of 10 days of intensified cycle training on signal transduction in nine inactive males in response to a 1-h acute bout of cycling at the same absolute workload (164 +/- 9 W). Muscle biopsies were taken at rest and immediately and 3 h after the acute exercise. The metabolic signaling pathways, including AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR), demonstrated divergent regulation by exercise after training. AMPK phosphorylation increased in response to exercise ( approximately 16-fold; P < 0.05), which was abrogated posttraining (P < 0.01). In contrast, mTOR phosphorylation increased in response to exercise ( approximately 2-fold; P < 0.01), which was augmented posttraining (P < 0.01) in the presence of increased mTOR expression (P < 0.05). Exercise elicited divergent effects on mitogen-activated protein kinase (MAPK) pathways after training, with exercise-induced extracellular signal-regulated kinase (ERK) 1/2 phosphorylation being abolished (P < 0.01) and p38 MAPK maintained. Finally, calmodulin kinase II (CaMKII) exercise-induced phosphorylation and activity were maintained (P < 0.01), despite increased expression ( approximately 2-fold; P < 0.05). In conclusion, 10 days of intensified endurance training attenuated AMPK, ERK1/2, and mTOR, but not CaMKII and p38 MAPK signaling, highlighting molecular pathways important for rapid functional adaptations and maintenance in response to intensified endurance exercise and training.

Global Gene Expression in Skeletal Muscle from Well-Trained Strength and Endurance Athletes

PURPOSE We used gene microarray analysis to compare the global expression profile of genes involved in adaptation to training in skeletal muscle from chronically strength-trained (ST), endurance-trained (ET), and untrained control subjects (Con). METHODS Resting skeletal muscle samples were obtained from the vastus lateralis of 20 subjects (Con n = 7, ET n = 7, ST n = 6; trained [TR] groups >8 yr specific training). Total RNA was extracted from tissue for two color microarray analysis and quantative (Q)-PCR. Trained subjects were characterized by performance measures of peak oxygen uptake (V x O 2peak) on a cycle ergometer and maximal concentric and eccentric leg strength on an isokinetic dynamometer. RESULTS Two hundred and sixty-three genes were differentially expressed in trained subjects (ET + ST) compared with Con (P < 0.05), whereas 21 genes were different between ST and ET (P < 0.05). These results were validated by reverse transcriptase polymerase chain reaction for six differentially regulated genes (EIFSJ, LDHB, LMO4, MDH1, SLC16A7, and UTRN. Manual cluster analyses revealed significant regulation of genes involved in muscle structure and development in TR subjects compared with Con (P <or= 0.05) and expression correlated with measures of performance (P < 0.05). ET had increased whereas ST had decreased expression of gene clusters related to mitochondrial/oxidative capacity (P <or= 0.05). These mitochondrial gene clusters correlated with V x O 2peak (P < 0.05). V x O 2peak also correlated with expression of gene clusters that regulate fat and carbohydrate oxidation (P < 0.05). CONCLUSION We demonstrate that chronic training subtly coregulates numerous genes from important functional groups that may be part of the long-term adaptive process to adapt to repeated training stimuli.

Force matching at the elbow joint is disturbed by muscle soreness.

These experiments are concerned with the ability of human subjects to match isometric torque in their elbow flexor muscles when biceps of one arm is made sore. Pain was induced by injection of hypertonic saline. Subjects were asked to generate a level of torque, 30% of maximum, with one arm, the reference arm. To achieve the required torque, subjects were given visual feedback. Subjects were then asked to match this torque with their other arm, the indicator arm. In control measurements, subjects were consistent in their matching ability and often were quite accurate. However, when biceps of one arm was made sore, subjects consistently and significantly underestimated the level of torque being generated by the sore arm. Painful heat applied to the skin over biceps produced a similar pattern of errors. Heating skin remote from elbow flexors had no significant effect. One interpretation of these findings is that the nociceptive input from the sore region of skin or muscle leads to reduced excitability of the motor cortex. That, in turn, disturbs the relationship between the centrally generated effort and motor output, leading to matching errors.

AMPK activation is fiber type specific in human skeletal muscle: effects of exercise and short-term exercise training

AMP-activated protein kinase (AMPK) has been extensively studied in whole muscle biopsy samples of humans, yet the fiber type-specific expression and/or activation of AMPK is unknown. We examined basal and exercise AMPK-alpha Thr(172) phosphorylation and AMPK subunit expression (alpha(1), alpha(2), and gamma(3)) in type I, IIa, and IIx fibers of human skeletal muscle before and after 10 days of exercise training. Before training basal AMPK phosphorylation was greatest in type IIa fibers (P < 0.05 vs. type I and IIx), while an acute bout of exercise increased AMPK phosphorylation in all fibers (P < 0.05), with the greatest increase occurring in type IIx fibers. Exercise training significantly increased basal AMPK phosphorylation in all fibers, and the exercise-induced increases were uniformly suppressed compared with pretraining exercise. Expression of AMPK-alpha(1) and -alpha(2) was similar between fibers and was not altered by exercise training. However, AMPK-gamma(3) was differentially expressed in skeletal muscle fibers (type IIx > type IIa > type I), irrespective of training status. Thus skeletal muscle AMPK phosphorylation and AMPK expression are fiber type specific in humans in the basal state, as well as during exercise. Our findings reveal fiber type-specific differences that have been masked in previous studies examining mixed muscle samples.

Effects of ovariectomy and 17β-oestradiol replacement on [Ca2+]i in female rat cardiac myocytes

1. The present study investigated the effects of ovariectomy (OVX) and 17β‐oestradiol replacement on [Ca2+]i in rat freshly isolated cardiac myocytes.