Chronic periodic fluid redistribution effect on muscle calcium in healthy subjects during prolonged hypokinesia

Studies have shown that chronic periodic fluid shifting upwards is not sensed as excessive fluid volume and excretion mechanisms are not activated. To determine if chronic periodic fluid and volume shifting upwards can affect muscle calcium (Ca²⁺) during hypokinesia (HK) we measured muscle Ca²⁺ content, plasma Ca²⁺ concentration, and Ca²⁺ losses in urine and feces. Studies were conducted on 40 healthy male volunteers. They were divided into four equal groups: active control subjects (ACS), hypokinetic subjects (HKS), periodic fluid redistribution control subjects (PFRCS), and periodic fluid redistribution hypokinetic subjects (PFRHS). Plasma Ca²⁺ level decreased (p < 0.05) in Ca²⁺ repleted muscle, muscle Ca²⁺ level increased (p < 0.05), and Ca²⁺ losses in urine and feces decreased (p < 0.05) in the PFRHS group compared with the HKS group. Plasma Ca²⁺ level increased (p < 0.05) in Ca²⁺ deficient muscle, muscle Ca²⁺ level decreased (p < 0.05), and Ca²⁺ losses in urine and feces increased (p < 0.05) in the HKS group compared with their pre-experimental levels and the values in their respective control groups (ACS and PFRCS). This study shows that the muscle Ca²⁺ content increases and Ca²⁺ excretion decreases, suggesting the clinical potential of chronic periodic fluid and volume redistribution in treatment of muscle Ca²⁺ deficiency.     

Substrate use and biochemical response to a 3,211-km bicycle tour in trained cyclists

The purpose of this study was to assess the physiological adaptations in physically fit individuals to a period of intensified training. Ten trained males cycled outdoors ~170 km day⁻¹ on 19 out of 21 days. Expired gas was collected on days 1 and 21 during maximal graded exercise and used for the determination of gross efficiency and whole body substrate use. Muscle biopsies were obtained before and after exercise on days 2 and 22 for the determination of mtDNA/gDNA ratio, gene expression, metabolic enzyme activity and glycogen use. Muscle glycogen before and after exercise, fat oxidation, and gross efficiency increased, carbohydrate oxidation decreased (p < 0.05), and VO_2max did not change over the 21 days of training. Citrate synthase (CS), β-hydroxyacyl CoA dehydrogenase (β-HAD) and cytochrome c oxidase (COX) enzyme activity did not change with training. CS and β-HAD mRNA did not change with acute exercise or training. COX (subunit IV) mRNA increased with acute exercise (p < 0.05) but did not change over the 21 days. PGC-1α mRNA increased with acute exercise, but did not increase to the same degree on day 22 as it did on day 2 (p < 0.05). UCP3 mRNA decreased with training (p < 0.05). Acute exercise caused an increase in mitofusin2 (MFN2) mRNA (p < 0.05) and a trend for an increase in mtDNA/gDNA ratio (p = 0.057). However, training did not affect MFN2 mRNA or mtDNA/gDNA ratio. In response to 3,211 km of cycling, changes in substrate use and gross efficiency appear to be more profound than mitochondrial adaptations in trained individuals.     

Exercise-induced decreases in sarcoplasmic reticulum Ca2+–ATPase activity attenuated by high-resistance training

Muscle biopsies were performed on the vastus lateralis muscle prior to and during a high-resistance training (HRT) programme in order to examine the effects of hypertrophy on sarcoplasmic reticulum Ca²⁺ ATPase activity at rest and during exercise. In six male untrained volunteers (peak aerobic power, V˙O ₂ peak = 3.39 ± 0.13 L min^?1, mean ± SE), the resting Ca²⁺ ATPase activity (μmol min^?1 g wet wt^?1) at 0 (4.89 ± 0.20), 4 (5.62 ± 0.56), 7 (5.15 ± 0.41) and 12 (4.82 ± 0.11) weeks was unchanged by HRT. During cycle ergometer exercise, prior to training, Ca²⁺-ATPase was reduced (P < 0.05) by 14% during the initial 30 min at 58% VO ₂ peak and (P < 0.05) a further 19% during 30 min at 72% VO ₂ peak. Following 7 and 12 weeks of training, the decreases in SR Ca²⁺-ATPase were less pronounced (P < 0.05). These results indicate that muscle hypertrophy, although incapable of altering Ca²⁺-ATPase pump activity at rest, can attenuate the decrease observed in exercise by mechanism(s) as yet unknown.     

The effect of recreational soccer training and running on postural balance in untrained men

The aim of this study was to examine the effect of intense intermittent exercise performed as soccer training or interval running in comparison with continuous endurance running exercise on postural balance in young healthy untrained males. Young sedentary men were randomized to soccer training (SOC, n = 10), continuous running (RUN; n = 9), high-intensity interval running (INT; n = 7) or no training (CON; n = 9). Postural balance was evaluated pre and post 12 weeks of training using a 30-s single-leg stance test on a force plate (AMTI) to yield center of pressure (CoP) sway path and 1-min beam standing (Flamingo test). CoP sway length decreased by 18.2% (p < 0.01), 14.6% (p < 0.05) and 12.8% (p < 0.05) in SOC, INT and RUN, respectively. CoP sway area decreased in SOC (−30.2%; p < 0.01) and INT (−23.4%; p < 0.01) but remained unaffected in RUN. Acceleration parameters (Mean CoP acc, SD accX, SD accY) decreased in SOC only (17–19%, p < 0.05). All training groups demonstrated fewer falls (37–41%, p < 0.01) in the Flamingo test. No changes were observed in CON. Relationships (r > 0.40) were observed between pre-training values in CoP sway area versus muscle fiber area, explosive muscle strength and countermovement jump velocity. Postural control was improved in response to 12 weeks of soccer training and high-intensity interval running, respectively, while less-marked changes were observed following continuous running. Notably, the reduced variability in CoP acceleration after soccer training indicates that this training regimen may produce superior improvements in postural sensory-motor function.     

Effects of high-intensity training and acute exercise on in vitro function of rat sarcoplasmic reticulum

To evaluate the effects of high-intensity training and/or a single bout of exercise on in vitro function of the sarcoplasmic reticulum (SR), the rats were subjected to 8 weeks of interval running program (final training: 2.5-min running × 4 sets per day, 50 m/min at 10% incline). Following training, SR function, i.e., Ca²⁺-ATPase activity and Ca²⁺-uptake and release rates, was examined in homogenates of the superficial region of the vastus lateralis muscle from rats subjected to a single bout of treadmill running (50 m/min at 10% incline) for 2.5 min or to exhaustion. Training brought about a 12.4% increase (P < 0.05) in SR Ca²⁺-uptake rate in rested muscles. This change was not accompanied by alterations in Ca²⁺-ATPase activity, Ca²⁺-release rate, Ca²⁺ dependence of enzyme and protein contents of Ca²⁺-ATPase and ryanodine receptor. A single bout of high-intensity exercise to exhaustion evoked significant reductions (P < 0.05) in SR function, irrespective of whether or not the animals were trained. For 2.5-min run and exhausted rats, no differences existed between SR functions of untrained and trained muscles. These data suggest that high-intensity training may be capable of enhancing SR Ca²⁺-sequestering ability, and may not protect against decreasing SR function with high-intensity exercise.     

Training in the fasted state facilitates re-activation of eEF2 activity during recovery from endurance exercise

Nutrition is an important co-factor in exercise-induced training adaptations in muscle. We compared the effect of 6 weeks endurance training (3 days/week, 1–2 h at 75% VO_2peak) in either the fasted state (F; n = 10) or in the high carbohydrate state (CHO, n = 10), on Ca²⁺-dependent intramyocellular signalling in young male volunteers. Subjects in CHO received a carbohydrate-rich breakfast before each training session, as well as ingested carbohydrates during exercise. Before (pretest) and after (posttest) the training period, subjects performed a 2 h constant-load exercise bout (~70% of pretest VO_2peak) while ingesting carbohydrates (1 g/kg h⁻¹). A muscle biopsy was taken from m. vastus lateralis immediately before and after the test, and after 4 h of recovery. Compared with pretest, in the posttest basal eukaryotic elongation factor 2 (eEF2) phosphorylation was elevated in CHO (P < 0.05), but not in F. In the pretest, exercise increased the degree of eEF2 phosphorylation about twofold (P < 0.05), and values returned to baseline within the 4 h recovery period in each group. However, in the posttest dephosphorylation of eEF2 was negated after recovery in CHO, but not in F. Independent of the dietary condition training enhanced the basal phosphorylation status of Phospholamban at Thr¹⁷, 5′-AMP-activated protein kinase α (AMPKα), and Acetyl CoA carboxylase β (ACCβ), and abolished the exercise-induced increase of AMPKα and ACCβ (P < 0.05). In conclusion, training in the fasted state, compared with identical training with ample carbohydrate intake, facilitates post-exercise dephosphorylation of eEF2. This may contribute to rapid re-activation of muscle protein translation following endurance exercise.     

Epac activation, altered calcium homeostasis and ventricular arrhythmogenesis in the murine heart

The recently described exchange protein directly activated by cAMP (Epac) has been implicated in distinct protein kinase A-independent cellular signalling pathways. We investigated the role of Epac activation in adrenergically mediated ventricular arrhythmogenesis. In contrast to observations in control conditions (n = 20), monophasic action potentials recorded in 2 of 10 intrinsically beating and 5 of 20 extrinsically paced Langendorff-perfused wild-type murine hearts perfused with the Epac activator 8-pCPT-2′-O-Me-cAMP (8-CPT, 1 μM) showed spontaneous triggered activity. Three of 20 such extrinsically paced hearts showed spontaneous ventricular tachycardia (VT). Programmed electrical stimulation provoked VT in 10 of 20 similarly treated hearts (P < 0.001; n = 20). However, there were no statistically significant accompanying changes (P > 0.05) in left ventricular epicardial (40.7 ± 1.2 versus 44.0 ± 1.7 ms; n = 10) or endocardial action potential durations (APD₉₀; 51.8 ± 2.3 versus 51.9 ± 2.2 ms; n = 10), transmural (ΔAPD₉₀) (11.1 ± 2.6 versus 7.9 ± 2.8 ms; n = 10) or apico-basal repolarisation gradients, ventricular effective refractory periods (29.1 ± 1.7 versus 31.2 ± 2.4 ms in control and 8-CPT-treated hearts, respectively; n = 10) and APD₉₀ restitution characteristics. Nevertheless, fluorescence imaging of cytosolic Ca²⁺ levels demonstrated abnormal Ca²⁺ homeostasis in paced and resting isolated ventricular myocytes. Epac activation using isoproterenol in the presence of H-89 was also arrhythmogenic and similarly altered cellular Ca²⁺ homeostasis. Epac-dependent effects were reduced by Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibition with 1 μM KN-93. These findings associate VT in an intact cardiac preparation with altered cellular Ca²⁺ homeostasis and Epac activation for the first time, in the absence of altered repolarisation gradients previously implicated in reentrant arrhythmias through a mechanism dependent on CaMKII activity.     

Effect of acute sprint interval exercise on central and peripheral artery distensibility in young healthy males

Peripheral arterial distensibility is improved with sprint interval exercise training in young healthy participants (Rakobowchuk et al. in Am J Physiol Regul Integr Comp Physiol 295:R236–R242, 2008). To fully understand the mechanisms contributing to these training effects it is useful to examine the acute responses to sprint interval exercise. Following supine rest, nine healthy males completed either a single sprint interval (Wingate test) or a multiple sprint interval exercise session (4 Wingate tests each separated by 4.5 min). Following exercise, participants recovered for 60 min while central and peripheral arterial distensibility measurements were conducted at discrete time points, using applanation tonometry and ultrasound imaging and continuously, using central and peripheral pulsewave velocity (PWV). Single and multiple sprint interval exercise sessions caused similar changes in all variables. Heart rate was increased throughout recovery (p < 0.05), while central artery PWV was increased until 20 min of recovery (p < 0.05) and lower extremity PWV was decreased until ~45 min (p < 0.05). Distensibility of the superficial femoral artery showed a trend for a reduction at 2 min post-exercise (p = 0.06). These results indicate that extremely high intensity exercise transiently increases central artery stiffness, while metabolite induced vasodilation reduces peripheral stiffness in exercised limbs well into recovery.     

Electrocardiogram Signals to Assess Zebrafih Heart Regeneration: Implication of Long QT Intervals

Zebrafish is an emerging model system for cardiac conduction and regeneration. Zebrafish heart regenerates after 20% ventricular resection within 60 days. Whether cardiac conduction phenotype correlated with cardiomyocyte regeneration remained undefined. Longitudinal monitoring of the adult zebrafish heart (n = 12) was performed in terms of atrial contraction (PR intervals), ventricular depolarization (QRS complex) and repolarization (heart rated corrected QTc interval). Baseline electrocardiogram (ECG) signals were recorded one day prior to resection and twice per week over 59 days. Immunostaining for gap junctions with anti-Connexin-43 antibody was compared between the sham (n = 5) and ventricular resection at 60 days post-resection (dpr) (n = 7). Heart rate variability, QTc prolongation and J-point depression developed in the resected group but not in the sham. Despite a trend toward heart rate variability in response to ventricular resection, the differences between the resected and sham fish were, by and large, statistically insignificant. At 10 dpr, J-point depression was statistically significant (sham: −0.179 ± 0.061 mV vs. ventricular resection: −0.353 ± 0.105 mV, p < 0.01, n = 7). At 60 days, histology revealed either cardiomyocyte regeneration (n = 4) or scar tissues (n = 3). J-point depression was no longer statistically significant at 59 dpr (sham: −0.114 ± 0.085 mV; scar tissue: −0.268 ± 0.178 mV, p > 0.05, n = 3; regeneration: −0.209 ± 0.119 mV, p > 0.05, n = 4). Despite positive Connexin-43 staining in the regeneration group, QTc intervals remained prolonged (sham: 325 ± 42 ms, n = 5; scar tissues: 534 ± 51 ms, p < 0.01, n = 3; regeneration: 496 ± 31 ms, p < 0.01, n = 4). Thus, we observed delayed electric repolarization in either the regenerated hearts or scar tissues. Moreover, early regenerated cardiomyocytes lacked the conduction phenotypes of the sham fish.     

Contraction-induced changes in skeletal muscle Na+,K+ pump mRNA expression – importance of exercise intensity and Ca2+-mediated signalling

Aim: To investigate if exercise intensity and Ca²⁺ signalling regulate Na⁺,K⁺ pump mRNA expression in skeletal muscle. Methods: The importance of exercise intensity was evaluated by having trained and untrained humans perform intense intermittent and prolonged exercise. The importance of Ca²⁺ signalling was investigated by electrical stimulation of rat soleus and extensor digitorum longus (EDL) muscles in combination with studies of cell cultures. Results: Intermittent cycling exercise at ∼85% of VO_2peak increased (P < 0.05) α1 and β1 mRNA expression ∼2-fold in untrained and trained subjects. In trained subjects, intermittent exercise at ∼70% of VO_2peak resulted in a less (P < 0.05) pronounced increase (∼1.4-fold; P < 0.05) for α1 and no change in β1 mRNA. Prolonged low intensity exercise increased (P < 0.05) mRNA expression of α1 ∼3.0-fold and α2 ∼1.8-fold in untrained but not in trained subjects. Electrical stimulation of rat soleus, but not EDL, muscle increased (P < 0.05) α1 mRNA expression, but not when combined with KN62 and cyclosporin A incubation. Ionomycin incubation of cultured primary rat skeletal muscle cells increased (P < 0.05) α1 and reduced (P < 0.001) α2 mRNA expression and these responses were abolished (P < 0.05) by co-incubation with cyclosporin A or KN62. Conclusion: (1) Exercise-induced increases in Na⁺,K⁺ pump α1 and β1 mRNA expression in trained subjects are more pronounced after high- than after moderate- and low-intensity exercise. (2) Both prolonged low and short-duration high-intensity exercise increase α1 mRNA expression in untrained subjects. (3) Ca²⁺_i regulates α1 mRNA expression in oxidative muscles via Ca²⁺/calmodulin-dependent protein kinase (CaMK) and calcineurin signalling pathways.     

Short-term exercise provides left ventricular myocardial protection against intermittent hypoxia-induced apoptosis in rats

We investigated whether exercise provides beneficial effects to attenuate intermittent hypoxia (IH)-induced myocardial apoptosis. Male Sprague-Dawley rats were randomly assigned to four groups: control (CON), IH, exercise (EXE) or IH interspersed with EXE (IHEXE). IH rats were exposed to repetitive hypoxia–reoxygenation cycles (30 s of 5% O₂; 45 s of 21% O₂, 6 h day⁻¹) during the light phase (1000–1600 h) for 12 consecutive days. EXE rats were habituated to treadmill running for 5 days, permitted 2 days of rest, followed by 5 exercise bouts (30 m min⁻¹ for 60 min on a 2% grade) on consecutive days during the dark phase (2000–2200 h). IHEXE rats were exposed to IH during the light phase interspersed with exercise programs during the dark phase on the same day. Apoptosis levels, cytochrome c (Cyt-c), cleaved caspase-3, oxidative stress and antioxidant capacity were determined in the left ventricular (LV) myocardium. IH rats showed higher myocardial levels of the apoptotic index, mitochondria-released Cyt-c, cleaved caspase-3 and oxidative stress and lower catalase activity levels than CON rats (p < 0.05, for all). These changes were not observed in EXE rats (p > 0.05, for all) except that catalase activity increased (p < 0.05). IHEXE rats showed lower myocardial levels of apoptotic index, mitochondria-released Cyt-c, cleaved caspase-3 and oxidative stress and higher catalase activity levels (p < 0.05, for all) than IH rats. We conclude that short-term exercise provides potent cardioprotective effects by attenuating IH-induced myocardial apoptosis.     

Magnesium sulfate enhances exercise performance and manipulates dynamic changes in peripheral glucose utilization

The effect of magnesium supplementation on exercise performance remains controversial. In the present study, the effects of magnesium sulfate on exercise performance and blood glucose metabolism were examined. In order to provide a non-invasive measure of continuous exercise, we developed an auto-blood sampling system was coupled to a microdialysis analyzer to detect the dynamic changes in glucose metabolism in conscious and freely moving gerbils subjected to forced swimming. Gerbils were pretreated with saline or magnesium sulfate (90 mg kg⁻¹, ip) 30 min before exercise. The duration times were significantly increased by 71% in the magnesium sulfate-treated groups (p < 0.01) when compared with those in the control. Another group of gerbils were subjected to blood sampling assay. A catheter was implanted in the jugular vein of each gerbil for collecting blood samples by the computer-aided blood sampler. The basal levels of plasma glucose, lactate, and magnesium were 6,245 ± 662, 1,067 ± 309, and 590 ± 50 μM, respectively, with no significant difference between groups. Plasma glucose, lactate, and magnesium levels increased to 134 and 204%, 369 and 220%, and 155 and 422% of basal levels during swimming in both the control and magnesium sulfate-treated groups, respectively (p < 0.05). Pretreatment with magnesium sulfate elevated glucose and magnesium levels to 175 and 302% of the basal levels (p < 0.05), respectively, whereas pretreatment with magnesium sulfate reduced the lactate levels 150% of the basal level (p < 0.05) during swimming. Furthermore, the magnesium levels increased to about 152–422% of basal levels during forced swimming and the recovery period (p < 0.05). The present study demonstrates that magnesium sulfate improved the duration time of forced swimming exercise. In addition, magnesium raised glucose levels and attenuated lactate levels during forced swimming. These results indicate that positive effects of magnesium supplementation may contribute to the enhancement of exercise performance in athletes.     

Concurrent EMG feedback acutely improves strength and muscle activation

The purpose of this study was to investigate the acute effects of electromyographic (EMG) feedback on muscle activation and strength during maximal voluntary concentric and eccentric muscle actions. 15 females performed two sets of three lengthening and three shortening maximal voluntary isokinetic knee extensions at 20° s⁻¹ over 60° range of motion. After the first set, subjects were randomized to either a control group (n = 8) or a feedback group (n = 7). In the second set, the control group performed tasks identical to those in the first set, whereas the feedback group additionally received concurrent visual feedback of the EMGrms from Vastus Medialis (VM). Knee extensor strength and EMG activation of VM, Vastus lateralis (VL) and hamstrings (HAM) were measured during the MVCs. Analyses were performed separately in a 1 s preactivation phase, a 1 s initial movement phase and a 1 s late movement phase. EMG feedback was associated with significantly higher knee extensor strength in all phases (20.5% p < 0.05, 18.2% p < 0.001 and 19% p < 0.001, respectively) for the eccentric MVCs and in the preactivation phase (16.3%, p < 0.001) and initial movement phases (7.2%, p < 0.05) for concentric MVCs. EMG feedback from VM further improved activation in VM and HAM but not VL. These findings suggested that concurrent visual EMG feedback from VM could acutely enhance muscle strength and activation. Before recommending implementation of EMG feedback in resistance training paradigms, the feedback parameters needs to be optimized and its long-term effects needs to be scrutinized.     

The endothelial microparticle response to a high fat meal is not attenuated by prior exercise

Triglyceride-rich postprandial lipoproteins are known to activate endothelial cells in vitro, contributing to atherosclerosis. Endothelial microparticles (EMP) are membranous vesicles released into the circulation from vascular endothelial cells that permit cell activation to be monitored in vivo. The objective of the study was to examine changes in EMP following a high fat meal, consumed with and without prior exercise. Eight recreationally active young men underwent two oral fat tolerance tests following either 100 min exercise at 70% VO₂peak (EX trial) or no exercise (CON trial) on the previous evening. Postprandial triglycerides were reduced (1.97 ± 0.31 vs. 1.17 ± 0.13 mmol L⁻¹, p < 0.05) and HDL-cholesterol (HDL-C) increased (1.20 ± 0.07 vs. 1.30 ± 0.08 mmol L⁻¹, p < 0.05) in the EX compared to CON trial. EMP (CD31+/42b−) increased postprandially (p < 0.05). However, counts were not different between trials (postprandial CON and EX trial counts × 10³ μL⁻¹, 3.10 ± 0.14 vs. 3.26 ± 0.37). There were no changes in sICAM-1 or sVCAM-1 postprandially and no differences between trials. Interleukin-6 (IL-6) and leukocytes increased postprandially (p < 0.05). IL-6 values were not different between trials. Leukocytes were higher at 0 h in the EX trial with CON and EX trial values similar at 6 h. EMP, but not sICAM-1 or sVCAM-1, increase in response to a high fat meal. However, EMP are not attenuated by acute exercise, despite a considerable reduction in postprandial lipemia and an increase in HDL-C. M. Harrison and R. P. Murphy contributed equally to this work.     

A possible role of the cholinergic and purinergic receptor interaction in the regulation of the rat urinary bladder function

The contractile activation of the upper (dome) and lower (base) parts of the urinary bladder show some differences. Cellular mechanisms that might be responsible for cholinergic effects blocking non-adrenergic non-cholinergic contractions in the base of the rat urinary bladder were investigated. Smooth muscle cells were thus freshly isolated or cultured both from the dome and the base of the rat urinary bladder and the contribution from cholinergic and purinergic pathways to their Ca²⁺ homeostasis was examined. The expression of nicotinic acetylcholine (nAChR) and P2X2 purinergic receptors on the cultured cells and on tissue sections was investigated. The ATP-evoked Ca²⁺ transients in rat smooth muscle cells did not show any desensitization. However, when ATP was administered together with carbamylcholine (CCh), the latter essentially prevented ATP from evoking Ca²⁺ transients in smooth muscle cells from the base (suppression to 12 ± 2.5% of control, n = 57; p < 0.01), but not from the dome (99 ± 5% of control, n = 52; p > 0.05) of the rat urinary bladder. While atropine was unable to modify (6 ± 3% of control, n = 14; p < 0.05), α-bungarotoxin (118 ± 12% of control, n = 20; p > 0.05) blocked the inhibitory effects of CCh. Additionally, α7 subunits of nAChR and P2X2 purinergic receptors were identified using immunocytochemistry, immunohistochemistry, and Western blot in cultured urinary bladder smooth muscle cells, in urinary bladder sections, and in urinary bladder muscle strips, respectively, suggesting that the activation of nAChR modifies the action of ATP.     

In-season strength maintenance training increases well-trained cyclists’ performance

We investigated the effects of strength maintenance training on thigh muscle cross-sectional area (CSA), leg strength, determinants of cycling performance, and cycling performance. Well-trained cyclists completed either (1) usual endurance training supplemented with heavy strength training twice a week during a 12-week preparatory period followed by strength maintenance training once a week during the first 13 weeks of a competition period (E + S; n = 6 [♂ = 6]), or (2) usual endurance training during the whole intervention period (E; n = 6 [♂ = 5, ♀ = 1]). Following the preparatory period, E + S increased thigh muscle CSA and 1RM (p < 0.05), while no changes were observed in E. Both groups increased maximal oxygen consumption and mean power output in the 40-min all-out trial (p < 0.05). At 13 weeks into the competition period, E + S had preserved the increase in CSA and strength from the preparatory period. From the beginning of the preparatory period to 13 weeks into the competition period, E + S increased peak power output in the Wingate test, power output at 2 mmol l⁻¹ [la⁻], maximal aerobic power output (W _max), and mean power output in the 40-min all-out trial (p < 0.05). The relative improvements in the last two measurements were larger than in E (p < 0.05). For E, W _max and power output at 2 mmol l⁻¹ [la⁻] remained unchanged. In conclusion, in well-trained cyclists, strength maintenance training in a competition period preserved increases in thigh muscle CSA and leg strength attained in a preceding preparatory period and further improved cycling performance determinants and performance.     

Cardiac β-adrenergic responsiveness with exercise

Left ventricular performance is enhanced with chronic exercise training. Alterations in cardiomyocyte β-adrenergic responsiveness (BAR) may, in part, mediate this response. In this study, cardiac BAR and the expression of some key cardiac hypertrophic signaling molecules following 3 months of treadmill training were examined. Four-month old, female, Wistar Kyoto (WKY) rats were randomly assigned into either a sedentary (WKY-SED, n = 15) or an exercise-trained (WKY-TRD, n = 11) group. All rats were maintained on a 12-h light/dark cycle, and fed ad libitum. Exercise training consisted of motorized treadmill training at 25 m/min, 0% grade, 60 continuous minutes, 5 days/week for a period of 12 weeks. RT-PCR was used to establish basal cardiac calcineurin A, ANP, and AKT mRNA expression. In vitro cardiac BAR responsiveness was determined in Langendorff, isolated hearts. Following baseline, isoproterenol (ISO) was incrementally infused at concentrations ranging from 1 × 10⁻¹⁰ to 1 × 10⁻⁷ mol/L. There were no group differences for heart weight, heart to body weight ratio, calcineurin A, ANP, or AKT mRNA levels between WKY-SED and WKY-TRD. WKY-TRD showed enhanced cardiac BAR relative to WKY-SED (at ISO 1 × 10⁻⁷ mol/L; P < 0.05). Moderate intensity treadmill exercise improved cardiac BAR responsiveness to a high concentration of isoproterenol. This adaptation was independent of training-induced alterations in cardiac hypertrophy or hypertrophic marker expression.     

The effects of mild one-legged isometric or dynamic training

Summary Four men isometrically trained their stronger leg for 19 weeks (attempted knee extension against a restraining strap incrementally increasing to 30 brief maximal contractions x 6 wk⁻¹). Five others similarly trained dynamically (repeated knee extension against a 63 N resistance force, incrementally increasing to 300 extensions x 6 wk⁻¹). Before, at regular intervals during training and after de-training (between 7–11 weeks) measurements were made using trained and control legs of: Maximum Voluntary Isometric Contraction (M.V.C.), Endurance at 60% M.V.C., Knee Extension Performance Test (K.E.P.T.) and One-legged Work Test. Isometric training produced a 30% (p<0.01) increase in M.V.C. with a 15% (p<0.05) increase in the control leg. These changes persisted with some deterioration after the de-training period. Endurance at 60% M.V.C. remained unchanged, even though M.V.C. was increasing in both trained and control legs. There was some evidence that isometric training improved the cardio-vascular response to one-legged exercise. Dynamic training did not result in changes in M.V.C, Endurance at 60% M.V.C. or the One-legged work Test, but K.E.P.T. (time taken for 50 knee extensions at a comfortable pace against 63 N resistance) improved by 33% (p<0.01) and 28% (p<0.01) in the trained and control legs respectively. Isometric training resulted in similar improvements in performance of K.E.P.T. (28%, p<0.05, trained leg; 18%, p<0.05 control leg). For similar time spent in training, isometric work appeared more effective than dynamic work in improving the parameters of muscle function, these improvements appeared to be both centrally (C.N.S.) and locally mediated.     

Contraction-induced changes in TNFα and Akt-mediated signalling are associated with increased myofibrillar protein in rat skeletal muscle

Resistance training results in skeletal muscle hypertrophy, but the molecular signalling mechanisms responsible for this altered phenotype are incompletely understood. We used a resistance training (RT) protocol consisting of three sessions [day 1 (d1), day 3 (d3), day 5 (d5)] separated by 48 h recovery (squat exercise, 4 sets × 10 repetitions, 3 min recovery) to determine early signalling responses to RT in rodent skeletal muscle. Six animals per group were killed 3 h after each resistance training session and 24 and 48 h after the last training session (d5). There was a robust increase in TNFα protein expression, and IKK^Ser180/181 and p38MAPK^{Thr180/Tyr182} phosphorylation on d1 (P < 0.05), which abated with subsequent RT, returning to control levels by d5 for TNFα and IKK^Ser180/181. There was a trend for a decrease in MuRF-1 protein expression, 48 h following d5 of training (P = 0.08). Notably, muscle myofibrillar protein concentration was elevated compared to control 24 and 48 h following RT (P < 0.05). Akt^Ser473 and mTOR^Ser2448 phosphorylation were unchanged throughout RT. Phosphorylation of p70S6k^Thr389 increased 3 h post-exercise on d1, d3 and d5 (P < 0.05), whilst phosphorylation of S6^Ser235/236 increased on d1 and d3 (P < 0.05). Our results show a rapid attenuation of inflammatory signalling with repeated bouts of resistance exercise, concomitant with summation in translation initiation signalling in skeletal muscle. Indeed, the cumulative effect of these signalling events was associated with myofibrillar protein accretion, which likely contributes to the early adaptations in response to resistance training overload in the skeletal muscle.     

CaMKIIβ-mediated LIM-kinase activation plays a crucial role in BDNF-induced neuritogenesis

LIM-kinase 1 (LIMK1) regulates actin cytoskeletal reorganization by phosphorylating and inactivating actin-depolymerizing factor and cofilin. We examined the role of LIMK1 in brain-derived neurotrophic factor (BDNF)-induced neuritogenesis in primary-cultured rat cortical neurons. Knockdown of LIMK1 or expression of a kinase-dead LIMK1 mutant suppressed BDNF-induced enhancement of primary neurite formation. By contrast, expression of an active form of LIMK1 promoted primary neuritogenesis in the absence of BDNF. BDNF-induced neuritogenesis was inhibited by KN-93, an inhibitor of Ca²⁺/calmodulin-dependent protein kinases (CaMKs), but not by STO-609, an inhibitor of CaMK-kinase (CaMKK). CaMKK activity is required for the activation of CaMKI and CaMKIV, but not CaMKII, which suggests that CaMKII is principally involved in BDNF-induced enhancement of neuritogenesis. Knockdown of CaMKIIβ, but not CaMKIIα, suppressed BDNF-induced neuritogenesis. Active CaMKIIβ promoted neuritogenesis, and this promotion was inhibited by knockdown of LIMK1, indicating that CaMKIIβ is involved in BDNF-induced neuritogenesis via activation of LIMK1. Furthermore, in vitro kinase assays revealed that CaMKIIβ phosphorylates LIMK1 at Thr-508 in the kinase domain and activates the cofilin-phosphorylating activity of LIMK1. In summary, these results suggest that CaMKIIβ-mediated activation of LIMK1 plays a crucial role in BDNF-induced enhancement of primary neurite formation.