Skeletal muscle sarcoplasmic reticulum glycogen status influences Ca2+ uptake supported by endogenously synthesized ATP

Subcellular localization- and fibre type-dependent utilization of muscle glycogen during heavy resistance exercise in elite power and Olympic weightlifters

AIM

Glycogen particles are found in different subcellular localizations, which are utilized heterogeneously in different fibre types during endurance exercise. Although resistance exercise typically involves only a moderate use of mixed muscle glycogen, the hypothesis of the present study was that high-volume heavy-load resistance exercise would mediate a pattern of substantial glycogen depletion in specific subcellular localizations and fibre types.

METHODS

10 male elite weightlifters performed resistance exercise consisting of four sets of five (4 × 5) repetitions at 75% of 1RM back squats, 4 × 5 at 75% of 1RM deadlifts and 4 × 12 at 65% of 1RM rear foot elevated split squats. Muscle biopsies (vastus lateralis) were obtained before and after the exercise session. The volumetric content of intermyofibrillar (between myofibrils), intramyofibrillar (within myofibrils) and subsarcolemmal glycogen was assessed by transmission electron microscopy.

RESULTS

After exercise, biochemically determined muscle glycogen decreased by 38 (31:45)%. Location-specific glycogen analyses revealed in type 1 fibres a large decrement in intermyofibrillar glycogen, but no or only minor changes in intramyofibrillar or subsarcolemmal glycogen. In type 2 fibres, large decrements in glycogen were observed in all subcellular localizations. Notably, a substantial fraction of the type 2 fibres demonstrated near-depleted levels of intramyofibrillar glycogen after the exercise session.

CONCLUSION

Heavy resistance exercise mediates a substantial utilization of glycogen from all three subcellular localization in type 2 fibres, while mostly taxing intermyofibrillar glycogen stores in type 1 fibres. Thus, a better understanding of the impact of resistance training on myocellular metabolism and performance requires a focus on compartmentalized glycogen utilization.

Predominant cause of prolonged low-frequency force depression changes during recovery after in situ fatiguing stimulation of rat fast-twitch muscle

To investigate time-dependent changes in sarcoplasmic reticulum (SR) Ca2+ release and myofibrillar (my-) Ca2+ sensitivity during recovery from prolonged low-frequency force depression (PLFFD), rat gastrocnemius muscles were electrically stimulated in situ. After 0 h (R0), 0.5 h (R0.5), 2 h (R2), 6 h (R6), or 12 h of recovery, the superficial gastrocnemius muscles were excised and used for biochemical and skinned fiber analyses. At R0, R0.5, R2, and R6, the ratio of force at 1 Hz to that at 50 Hz was decreased in the skinned fibers. The ratio of depolarization-induced force to the maximum Ca2+-activated force (depol/Ca2+ force ratio) was utilized as an indicator of SR Ca2+ release. At R0, both the depol/Ca2+ force ratio and my-Ca2+ sensitivity were decreased. At R0.5 and R2, my-Ca2+ sensitivity was recovered, while the depol/Ca2+ force ratio remained depressed. At R6, my-Ca2+ sensitivity was decreased again, whereas the depol/Ca2+ force ratio was nearly restored. Western blot analyses demonstrated that decreased my-Ca2+ sensitivity at R6 and reduced depol/Ca2+ force ratio at R0, R0.5, and R2 were accompanied by depressions in S-glutathionylated troponin I and increases in dephosphorylated ryanodine receptor 1, respectively. These results indicate that, in the early stage of recovery, reduced SR Ca2+ release plays a primary role in the etiology of PLFFD, whereas decreased my-Ca2+ sensitivity is involved in the late stage, and suggest that S-glutathionylation of troponin I and dephosphorylation of ryanodine receptor 1 contribute, at least partly, to fatiguing contraction-induced alterations in my-Ca2+ sensitivity and SR Ca2+ release, respectively.

Muscle glycogen content modifies SR Ca2+ release rate in elite endurance athletes

PURPOSE

The aim of the present study was to investigate the influence of muscle glycogen content on sarcoplasmic reticulum (SR) function and peak power output (Wpeak) in elite endurance athletes.

METHODS

Fourteen highly trained male triathletes (VO2max = 66.5 ± 1.3 mL O2·kg·min), performed 4 h of glycogen-depleting cycling exercise (HRmean = 73% ± 1% of maximum). During the first 4 h of recovery, athletes received either water (H2O) or carbohydrate (CHO), separating alterations in muscle glycogen content from acute changes affecting SR function and performance. Thereafter, all subjects received CHO-enriched food for the remaining 20-h recovery period.

RESULTS

Immediately after exercise, muscle glycogen content and SR Ca release rate was reduced to 32% ± 4% (225 ± 28 mmol·kg dw) and 86% ± 2% of initial levels, respectively (P < 0.01). Glycogen markedly recovered after 4 h of recovery with CHO (61% ± 2% of preexercise) and SR Ca release rate returned to preexercise level. However, in the absence of CHO during the first 4 h of recovery, glycogen and SR Ca release rate remained depressed, with the normalization of both parameters at the end of the 24 h of recovery after receiving a CHO-enriched diet. Linear regression demonstrated a significant correlation between SR Ca release rate and muscle glycogen content (P < 0.01, r = 0.30). The 4 h of cycling exercise reduced Wpeak by 5.5%-8.9% at different cadences (P < 0.05), and Wpeak was normalized after 4 h of recovery with CHO, whereas Wpeak remained depressed (P < 0.05) after water provision. Wpeak was fully recovered after 24 h in both the H2O and the CHO group.

CONCLUSION

In conclusion, the present results suggest that low muscle glycogen depresses muscle SR Ca release rate, which may contribute to fatigue and delayed recovery of Wpeak 4 h postexercise.

Weakness, SR function and stress in gastrocnemius muscles of aged male rats

Aging is associated with a decline in muscle force that exceeds loss of muscle mass, suggesting that factors other than sarcopenia affect age-related muscle weakness. Here, we investigate in situ muscle force and sarcoplasmic reticulum (SR) properties in gastrocnemius muscles of adult (6-8 months) and aged (24 months) rats. Despite minimal loss of muscle mass, peak tetanic force was significantly reduced (-28%) in aged muscles. Adjusting for differences in muscle cross-sectional area mitigated the age difference (-23%), but it remained significant. The SR calcium release function was also impaired (-17%) with aging, although calcium uptake was not, and SR-associated glycogen increased (+30%) with aging. Western blotting revealed age related increases in Grp78, serinepalmitoyltransferase and neutral sphingomyelinase, suggesting that age increased the stress response and ceramide metabolism in the SR. In contrast Parkin, a protein associated with autophagic signaling, was reduced in the aged SR. These findings are consistent with a hypothesis that age-related impairments of the SR, possibly due to impaired autophagy and/or altered membrane metabolism, contribute to age-related muscle weakness, independent of changes in muscle mass.

Physiological aspects of the subcellular localization of glycogen in skeletal muscle

Glucose is stored in skeletal muscle fibers as glycogen, a branched-chain polymer observed in electron microscopy images as roughly spherical particles (known as β-particles of 10–45 nm in diameter), which are distributed in distinct localizations within the myofibers and are physically associated with metabolic and scaffolding proteins. Although the subcellular localization of glycogen has been recognized for more than 40 years, the physiological role of the distinct localizations has received sparse attention. Recently, however, studies involving stereological, unbiased, quantitative methods have investigated the role and regulation of these distinct deposits of glycogen. In this report, we review the available literature regarding the subcellular localization of glycogen in skeletal muscle as investigated by electron microscopy studies and put this into perspective in terms of the architectural, topological, and dynamic organization of skeletal muscle fibers. In summary, the distribution of glycogen within skeletal muscle fibers has been shown to depend on the fiber phenotype, individual training status, short-term immobilization, and exercise and to influence both muscle contractility and fatigability. Based on all these data, the available literature strongly indicates that the subcellular localization of glycogen has to be taken into consideration to fully understand and appreciate the role and regulation of glycogen metabolism and signaling in skeletal muscle. A full understanding of these phenomena may prove vital in elucidating the mechanisms that integrate basic cellular events with changing glycogen content.

Increased intramyocellular lipids but unaltered in vivo mitochondrial oxidative phosphorylation in skeletal muscle of adipose triglyceride lipase-deficient mice

Adipose triglyceride lipase (ATGL) is a lipolytic enzyme that is highly specific for triglyceride hydrolysis. The ATGL-knockout mouse (ATGL(-/-)) accumulates lipid droplets in various tissues, including skeletal muscle, and has poor maximal running velocity and endurance capacity. In this study, we tested whether abnormal lipid accumulation in skeletal muscle impairs mitochondrial oxidative phosphorylation, and hence, explains the poor muscle performance of ATGL(-/-) mice. In vivo ¹H magnetic resonance spectroscopy of the tibialis anterior of ATGL(-/-) mice revealed that its intramyocellular lipid pool is approximately sixfold higher than in WT controls (P = 0.0007). In skeletal muscle of ATGL(-/-) mice, glycogen content was decreased by 30% (P < 0.05). In vivo ³¹P magnetic resonance spectra of resting muscles showed that WT and ATGL(-/-) mice have a similar energy status: [PCr], [P(i)], PCr/ATP ratio, PCr/P(i) ratio, and intracellular pH. Electrostimulated muscles from WT and ATGL(-/-) mice showed the same PCr depletion and pH reduction. Moreover, the monoexponential fitting of the PCr recovery curve yielded similar PCr recovery times (τPCr; 54.1 ± 6.1 s for the ATGL(-/-) and 58.1 ± 5.8 s for the WT), which means that overall muscular mitochondrial oxidative capacity was comparable between the genotypes. Despite similar in vivo mitochondrial oxidative capacities, the electrostimulated muscles from ATGL(-/-) mice displayed significantly lower force production and increased muscle relaxation time than the WT. These findings suggest that mechanisms other than mitochondrial dysfunction cause the impaired muscle performance of ATGL(-/-) mice.

Role of glycogen availability in sarcoplasmic reticulum Ca2+ kinetics in human skeletal muscle

Glucose is stored as glycogen in skeletal muscle. The importance of glycogen as a fuel during exercise has been recognized since the 1960s; however, little is known about the precise mechanism that relates skeletal muscle glycogen to muscle fatigue. We show that low muscle glycogen is associated with an impairment of muscle ability to release Ca(2+), which is an important signal in the muscle activation. Thus, depletion of glycogen during prolonged, exhausting exercise may contribute to muscle fatigue by causing decreased Ca(2+) release inside the muscle. These data provide indications of a signal that links energy utilization, i.e. muscle contraction, with the energy content in the muscle, thereby inhibiting a detrimental depletion of the muscle energy store.

Ins(1,4,5)P(3) facilitates ATP accumulation via phosphocreatine/creatine kinase in the endoplasmic reticulum extracted from MDCK cells

So far, the content and accumulation of ATP in isolated endoplasmic reticulum (ER) are little understood. First, we confirmed using electron microscopic and Western blotting techniques that the samples extracted from MDCK cells are endoplasmic reticulum (ER). The amounts of ATP in the extracted ER were measured from the filtrate after a spinning down of ultrafiltration spin column packed with ER. When the ER sample (5mug) after 3days freezing was suspended in intracellular medium (ICM), 0.1% Triton X and ultrapure water (UPW), ATP amounts from the ER with UPW were the highest and over 10 times compared with that from the control with ICM, indicating that UPW is the most effective tool in destroying the ER membrane. After a 10-min-incubation with ICM containing phosphocreatine (PCr)/creatine kinase (CK) of the fresh ER. ATP amounts in the filtrate obtained by spinning down were not changed from that in the control (no PCr/CK). However, ATP amounts in the filtrate from the second spinning down of the ER (treated with PCr/CK) suspended in UPW became over 10-fold compared with the control. When 1muM inositol(1,4,5)trisphosphate (Ins(1,4,5)P(3)) was added in the incubation medium (ICM with PCr/CK), ATP amounts from the filtrate after the second spinning down were further enhanced around three times. This enhancement was almost canceled by Ca(2+)-removal from ICM and by adding thapsigargin, a Ca(2+)-ATPase inhibitor, but not by 2-APB and heparin, Ins(1,4,5)P(3) receptor antagonists. Administration of 500muM adenosine to the incubation medium (with PCr/CK) failed to enhance the accumulation of ATP in the ER. These findings suggest that the ER originally contains ATP and ATP accumulation in the ER is promoted by PCr/CK and Ins(1,4,5)P(3).

Effect of previous exhaustive exercise on metabolism and fatigue development during intense exercise in humans

The present study examined how metabolic response and work capacity are affected by previous exhaustive exercise. Seven subjects performed an exhaustive cycle exercise ( approximately 130%-max; EX2) after warm-up (CON) and 2 min after an exhaustive bout at a very high (VH; approximately 30 s), high (HI; approximately 3 min) or low (LO; approximately 2 h) intensity. Compared with CON, performance during EX2 was reduced (P<0.05) more in HI and LO than in VH (61+/-4% and 68+/-3% vs 35+/-4%). The muscle glycogen before EX2 was lower (P<0.05) in LO than in HI and VH, but the muscle glycogen utilization rates during EX2 were not different. Muscle glycogen concentration before EX2 was related (P<0.05) to the mean rate of muscle glycogen utilization during EX2 in HI and VH, and the mean rate of muscle lactate accumulation in LO. In HI, muscle pH before EX2 was lower (P<0.05) compared with VH and LO, but the same in HI and VH at the end of EX2. In HI, muscle pH before and after EX2 was inversely related (P<0.05) to the decrease in EX2 performance. Thus, muscle glycogen availability and low muscle pH do not per se control but appear to affect the rate of glycogenolysis/glycolysis and fatigue development during a repeated high-intensity exercise lasting 1/2-2 min.

No relationship between enzyme activity and structure of nucleotide binding site in sarcoplasmic reticulum Ca(2+)-ATPase from short-term stimulated rat muscle

AIM

We examined whether structural alterations to the adenine nucleotide binding site (ANBS) within sarcoplasmic (endo) reticulum Ca(2+)-ATPase (SERCA) would account for contraction-induced changes in the catalytic activity of the enzyme as assessed in vitro.

METHODS

Repetitive contractions were induced in rat gastrocnemius by electrical nerve stimulation. Measurements of sarcoplasmic reticulum properties were performed on control and stimulated muscles immediately after or at 30 min after the cessation of 5-min stimulation. In order to examine the properties at the ANBS, the binding capacity of SERCA to fluorescence isothiocyanate (FITC), a competitive inhibitor at the ANBS, was analysed in microsomes.

RESULTS

Short-term electrical stimulation evoked a 23.9% and 32.6% decrease (P < 0.05) in SERCA activity and in the FITC binding capacity, respectively, in the superficial region of the muscle. Whereas SERCA activity reverted to normal levels during 30-min recovery, a restoration of the FITC binding capacity did not occur.

CONCLUSION

The discordant changes between the enzyme activity and the FITC binding suggest that, at least during recovery after exercise, changes in SERCA activity may not correlate closely with structural alterations to the ANBS within the enzyme.

Lymphocytes in the development of lung inflammation: a role for regulatory CD4+ T cells in indirect pulmonary lung injury

Although roles for myelocytes have been suggested in the pathophysiology of indirect acute lung injury (ALI not due to a direct insult to the lung), the contribution of various regulatory lymphoid subsets is unknown. We hypothesized a role for lymphocytes in this process. Using a sequential model of indirect ALI induced in mice by hemorrhagic shock followed 24 h later by polymicrobial sepsis; we observed a specific and nonredundant role for each lymphocyte subpopulation in indirect ALI pathophysiology. In particular, we showed that CD4(+) T cells are specifically recruited to the lung in a dendritic cell-independent but IL-16-dependent process and diminish neutrophil recruitment through increased IL-10 production. Most importantly, this appears to be mediated by the specific subpopulation of CD4(+)CD25(+)Foxp3(+) regulatory T cells. Although indirect ALI has constantly been described as a proinflammatory pathology mediated by cells of the innate immune system, we now demonstrate that cells of the adaptive immune response play a major role in its pathophysiology as well. Most importantly, we also describe for the first time the nature of the regulatory mechanisms activated in the lung during indirect ALI, with CD4(+) regulatory T cells being central to the control of neutrophil recruitment via increased IL-10 production.

Distinct effects of subcellular glycogen localization on tetanic relaxation time and endurance in mechanically skinned rat skeletal muscle fibres

In vitro experiments indicate a non-metabolic role of muscle glycogen in contracting skeletal muscles. Since the sequence of events in excitation\#8211;contraction (E\#8211;C) coupling is known to be located close to glycogen granules, at specific sites on the fibre, we hypothesized that the distinct compartments of glycogen have specific effects on muscle fibre contractility and fatigability. Single skeletal muscle fibres (n = 19) from fed and fasted rats were mechanically skinned and divided into two segments. In one segment glycogen localization and volume fraction were estimated by transmission electron microscopy. The other segment was mechanically skinned and, in the presence of high and constant myoplasmic ATP and PCr, electrically stimulated (10 Hz, 0.8 s every 3 s) eliciting repeated tetanic contractions until the force response was decreased by 50% (mean +/- S.E.M., 81 +/- 16, range 22-252 contractions). Initially the total myofibrillar glycogen volume percentage was 0.46 +/- 0.07%, with 72 +/- 3% in the intermyofibrillar space and 28 +/- 3% in the intramyofibrillar space. The intramyofibrillar glycogen content was positively correlated with the fatigue resistance capacity (r(2) = 0.32, P = 0.02). Intermyofibrillar glycogen was inversely correlated with the half-relaxation time in the unfatigued tetanus (r(2) = 0.25, P = 0.03). These results demonstrate for the first time that two distinct subcellular populations of glycogen have different roles in contracting single muscle fibres under conditions of high myoplasmic ATP.

Sarcoplasmic Reticulum Ca2+-ATPase Activity and Glycogen Content in Various Fiber Types after Intensive Exercise in Thoroughbred Horses

To find a new parameter indicating muscle fitness in Thoroughbred horses, we examined time-dependent recovery of glycogen content and sarcoplasmic reticulum (SR) Ca²⁺-ATPase activity of skeletal muscle after intensive treadmill running. Two repeated 50-sec running sessions (13 m/sec) were performed on a flat treadmill (approximately 90%VO₂max). Muscle samples of the middle gluteal muscle were taken before exercise (pre) and 1 min, 20 min, 60 min, and 24 hr after exercise. Muscle fiber type composition was determined in the pre muscle samples by immunohistochemical staining with monoclonal antibody to myosin heavy chain. SR Ca²⁺-ATPase activity of the muscle and glycogen content of each muscle fiber type were determined with biochemical analysis and quantitative histochemical staining, respectively. As compared to the pre value, the glycogen content of each muscle fiber type was reduced by 15–27% at 1 min, 20 min, and 60 min after the exercise and recovered to the pre value at 24 hr after exercise test. These results indicate that 24 hr is enough time to recover glycogen content after short-term intensive exercise. The mean value of the SR Ca²⁺-ATPase activity showed a slight decrease (not significant) immediately after exercise, and complete recovery at 60 min after exercise. There were no significant relationship between the changes in glycogen content of each muscle fiber type and SR Ca²⁺-ATPase. Although further studies are needed, SR Ca²⁺-ATPase is not a useful parameter to detect muscle fitness, at least in Thoroughbred horses.

Muscle metabolic, SR Ca2+-cycling responses to prolonged cycling, with and without glucose supplementation

This study investigated the effects of prolonged exercise, with and without glucose supplementation, on metabolism and sarcoplasmic reticulum (SR) Ca2+-handling properties in working vastus lateralis muscle. Fifteen untrained volunteers [peak O2consumption (V̇o2peak) = 3.45 ± 0.17 l/min; mean ± SE] cycled at ∼60% V̇o2peakon two occasions, during which they were provided with either an artificially sweetened placebo beverage (NG) or a 6% glucose (G) beverage (∼1.00 g carbohydrate/kg body mass). Beverage supplementation started at 30 min of exercise and continued every 15 min thereafter. SR Ca2+handling, metabolic, and substrate responses were assessed in tissue extracted from the vastus lateralis at rest, after 30 min and 90 min of exercise, and at fatigue in both conditions. Plasma glucose during G was 15–23% higher ( P < 0.05) than those observed during NG following 60 min of exercise until fatigue. Cycle time to fatigue was increased ( P < 0.05) by ∼19% during G (137 ± 7 min) compared with NG (115 ± 6 min). Prolonged exercise reduced ( P < 0.05) maximal Ca2+-ATPase activity (−18.4%), SR Ca2+uptake (−27%), and both Phase 1 (−22.2%) and Phase 2 (−34.2%) Ca2+-release rates during NG. The exercise-induced reductions in SR Ca2+-cycling properties were not altered during G. The metabolic responses to exercise were all unaltered by glucose supplementation, since no differences in respiratory exchange ratios, carbohydrate and lipid oxidation rates, and muscle metabolite and glycogen contents were observed between NG and G. These results indicate that the maintenance of blood glucose homeostasis by glucose supplementation is without effect in modifying the muscle metabolic, endogenous glycogen, or SR Ca2+-handling responses.

Muscle energetics in exercising horses

An optimally functional musculoskeletal system is crucial for athletic performance and even minor perturbations can limit athletic ability. The introduction of the muscle biopsy technique in the 1970s created a window of opportunity to examine the form and function of equine skeletal muscle. Muscle histochemical and biochemical analyses have allowed characterization of the properties of equine muscle fibres and their influence on, and adaptation to, physical exertion. Analyses of exercise responses during standardized treadmill exercise and field studies have illustrated the role of cellular energetics in determining athletic suitability for specific disciplines, mechanisms of fatigue, adaptations to training and the affect of diet on metabolic responses. This article provides a review of the tools available to study muscle energetics in the horse, discusses the muscular metabolic pathways and summarizes the energetics of exercise.

Measurement of sarcoplasmic reticulum Ca2+ ATPase activity using high-performance liquid chromatography

The goal of this investigation was to develop an assay whereby we could measure changes in ATP, ADP, and phosphocreatine (PCr) during stimulation of the sarcoplasmic reticulum (SR) Ca2+ ATPase. After stopping the enzyme reaction, compounds were extracted by perchloric acid and separated by reversed-phase high-performance liquid chromatography (HPLC). Absorbance of ATP and ADP was monitored at 260 nm, and detection of PCr was done at 205 nm. Chromatograms show that peaks associated with each compound are clearly separated and easily detected. The SR Ca2+ ATPase assay was run for various time periods and using varying free [Ca2+]. The changes in ATP and ADP contents were linear with increasing time and varied as expected with increasing free [Ca2+]. The ATPase activities determined using changes in ATP and ADP were nearly identical to those determined using previously established assays. When PCr was added to the assay, we were able to confirm that the Ca2+ ATPase uses ATP that is synthesized locally from PCr via creatine kinase (CK). The results indicate that this is a valid and reliable method for examining SR Ca2+ ATPase activity and for investigating its interaction with CK.

Mechanical and Metabolic Responses with Exercise and Dietary Carbohydrate Manipulation

PURPOSE

To investigate the effects of altered muscle glycogen content on the mechanical and metabolic responses to prolonged exercise of moderate intensity.

METHODS

Eight volunteers (.VO2peak = 49.3 +/- 1.2 mL.kg(-1).min(-1)) cycled to fatigue on two occasions: after a 3-d low-carbohydrate diet (Lo CHO), which had been preceded by glycogen-depleting exercise, and then after a 3-d high-carbohydrate diet (Hi CHO). Metabolic and mechanical properties were assessed at both Lo CHO and Hi CHO before exercise (Pre), at 30 min of exercise (30 min), at fatigue in Lo CHO (Post 1), and again at fatigue after a brief rest (Post 2).

RESULTS

For the Lo CHO cycle, time to fatigue averaged 66.7 +/- 4.5 and 9.5 +/- 1.7 min for Post 1 and Post 2, respectively. For Hi CHO, Post 2 time to fatigue was 64.9 +/- 6.3 min. Muscle glycogen was elevated (P < 0.05) by approximately 40% in Hi CHO compared with Lo CHO. Phosphocreatine, although higher (P < 0.05) by approximately 25% during exercise in Hi CHO, was not different at Pre. Similar but reciprocal effects (P < 0.05) were observed for inorganic phosphate and creatine. Force at low frequencies of stimulation was maximally reduced (P < 0.05) by approximately 26-38% by 30 min of exercise, regardless of condition.

CONCLUSION

A 7-d exercise-dietary protocol leads to both an elevation in muscle glycogen and improved energy homeostasis during exercise. Although these adaptations may explain the improved cycle performance, they are not related to the progression of muscle fatigue as assessed statically at low frequencies of stimulation.

Expression of Integrin β3 Is Correlated to the Properties of Quiescent Hemopoietic Stem Cells Possessing the Side Population Phenotype

With significant attention paid to the field of tissue-specific stem cells, the identification of stem cell-specific markers is of considerable importance. Previously, the side population (SP) phenotype, with the capacity to efflux the DNA-binding dye Hoechst 33342, has been recognized as a common feature of adult tissue-specific stem cells. In this study, we show that high expression of integrin beta(3) (CD61) is an attribute of SP cells isolated from mouse bone marrow. Additionally, we confirmed that the expression of integrin beta(3) is correlated with properties of quiescent hemopoietic stem cells (HSCs) including the strength of the SP phenotype, cell cycle arrest, expression of HSC markers, and long-term hemopoiesis. Importantly, Lineage(-) (Lin(-))/integrin beta(3)(high) (beta(3)(high)) SP cells have as strong a capacity for long-term hemopoiesis as c-Kit(+)/Sca-1(+)/Lin(-) SP cells, which are regarded as one of the most highly enriched HSC populations. Finally, the integrin beta(3) subunit that is present in SP cells having the properties of HSCs, is associated with integrin alpha(v) (CD51). Therefore, our results demonstrate that high expression of integrin beta(3) is correlated to the properties of quiescent HSCs and suggest that the integrin beta(3) subunit is available as a common surface marker of tissue-specific stem cells.

Comparative effects of a low-carbohydrate diet and exercise plus a low-carbohydrate diet on muscle sarcoplasmic reticulum responses in males

We employed a glycogen-depleting session of exercise followed by a low-carbohydrate (CHO) diet to investigate modifications that occur in muscle sarcoplasmic reticulum (SR) Ca2+-cycling properties compared with low-CHO diet alone. SR properties were assessed in nine untrained males [peak aerobic power (V̇o2 peak) = 43.6 ± 2.6 (SE) ml·kg−1·min−1] during prolonged cycle exercise to fatigue performed at ∼58% V̇o2 peakafter 4 days of low-CHO diet (Lo CHO) and after glycogen-depleting exercise plus 4 days of low-CHO (Ex+Lo CHO). Compared with Lo CHO, Ex+Lo CHO resulted in 12% lower ( P < 0.05) resting maximal Ca2+-ATPase activity ( Vmax= 174 ± 12 vs. 153 ± 10 μmol·g protein−1·min−1) and smaller reduction in Vmaxinduced during exercise. A similar effect was observed for Ca2+uptake. The Hill coefficient, defined as slope of the relationship between cytosolic free Ca2+concentration and Ca2+-ATPase activity, was higher ( P < 0.05) at rest (2.07 ± 0.15 vs. 1.90 ± 0.10) with Ex+Lo CHO, an effect that persisted throughout the exercise. The coupling ratio, defined as the ratio of Ca2+uptake to Vmax, was 23–30% elevated ( P < 0.05) at rest and during the first 60 min of exercise with Ex+Lo CHO. The ∼27 and 34% reductions ( P < 0.05) in phase 1 and phase 2 Ca2+release, respectively, observed during exercise with Lo CHO were not altered by Ex+Lo CHO. These results indicate that when prolonged exercise precedes a short-term Lo CHO diet, Ca2+sequestration properties and efficiency are improved compared with those during Lo CHO alone.

Reinnervation-induced alterations in rat skeletal muscle

Denervation-induced myofiber atrophy can be reversed by reinnervation. Growing reinnervated myofibers upregulate numerous molecules, many of which determine the muscle fiber type. In the present study we aimed at identifying factors that might contribute specifically to myofiber growth after reinnervation. The common peroneal nerve of 15 male Wistar rats was cut and resutured without delay (9 animals) or with a delay of 4 weeks (6 animals). We studied the transcriptional repertoire of intact reinnervated tibialis anterior muscle by microarray gene analysis. We assessed SC activation by immunolabeling using anti-MyoD and -myogenin antibodies. The percentage of SC expressing MyoD reached up to 50% of M-cadherin+ cells whereas the percentage of SC expressing myogenin was normal (<10%) in all muscles examined. The values of ipsi- and contralateral muscles did not differ significantly from one another between right and left leg (p<0.05). Thirteen known genes were differentially regulated after reinnervation compared with contralateral muscles. Five of them determine the slow-twitch fiber type (four and a half LIM domains 3, cardiac beta-myosin heavy chain, calsequestrin 2, troponin C (slow), and heart myosin light chain), and three of them are neurally regulated (thrombospondin 4, transferrin receptor, cardiac ankyrin repeat protein). The results strengthen the notion that reinnervaton affects the molecular repertoire of the myofibers directly, leading to fiber type transformation and partial reversal of the denervation phenotype. By contrast, SC do not appear to be affected by reinnervation directly. They can be activated both in reinnervated and contralateral muscles, and they do not fully differentiate. This makes them unlikely to contribute to myofiber growth.

Effects of reduced glycogen on structure and in vitro function of rat sarcoplasmic reticulum Ca2+-ATPase

The aim of this study was to examine the effects of reduced glycogen concentration on sarcoplasmic reticulum (SR) Ca(2+)-ATPase activity in rat fast-twitch muscles. In the first experiment, the gastrocnemius (GAS) muscle from one leg was removed, followed by starvation for 24-72 h, after which the remaining GAS was removed. Intra-animal comparisons revealed that starvation caused a 25% reduction (P<0.05) in the glycogen concentration but no change in SR Ca(2+)-ATPase activity in the GAS. In the second experiment, the SR was purified from a mixture of the GAS and vastus lateralis muscles. In half of the samples obtained from each animal, glycogen was extracted from the SR by treatment with glucoamylase. Treatment resulted in a 94.1 and 70.2% decrease (P<0.01) in glycogen and glycogen phosphorylase, respectively, and a 41.5% increase (P<0.05) in a fluorescein isothiocyanate (FITC) binding to SR Ca(2+)-ATPase. On the other hand, SR Ca(2+)-ATPase activity and the affinity of the enzyme for ATP were unaltered. These results do not implicate depletion of muscle glycogen as a contributor to impaired SR Ca(2+)-ATPase activity as measured in vitro. Therefore, it is concluded that muscle glycogen does not influence exercise tolerance and work productivity in working muscles by modulating the structure of protein involved in Ca(2+) sequestering. Furthermore, it is suggested that the FITC binding assay may be inappropriate as a method for examining the mechanisms for the altered activity of SR Ca(2+)-ATPase.

Metabolic and sarcoplasmic reticulum Ca2+cycling responses in human muscle 4 days following prolonged exercise

This study investigated the effects of prolonged exercise on muscle sarcoplasmic reticulum (SR) Ca2+cycling properties and the metabolic responses with and without a session of exercise designed to reduce muscle glycogen reserves while on a normal carbohydrate (CHO) diet. Eight untrained males (VO2peak = 3.81 ± 0.12 L/min, mean ± SE) performed a standardized cycle-to-fatigue at 55% VO2peakwhile on a normal CHO diet (Norm CHO) and 4 days following prolonged exercise while on a normal CHO diet (Ex+Norm CHO). Compared to rest, exercise in Norm CHO to fatigue resulted in significant reductions (p < 0.05) in Ca2+uptake (3.17 ± 0.21 vs. 2.47 ± 0.12 µmol·(g protein)–1·min–1), maximal Ca2+ATPase activity (Vmax, 152 ± 12 vs. 119 ± 9 µmol·(g protein)–1·min–1) and both phase 1 (15.1 ± 0.98 vs. 13.1 ± 0.28 µmol·(g protein)–1·min–1) and phase 2 (6.56 ± 0.33 vs. 4.91 ± 0.28 µmol·(g protein)–1·min–1) Ca2+release in vastus lateralis muscle. No differences were observed between Norm CHO and Ex-Norm CHO in the response of these properties to exercise. Compared with Norm CHO, Ex+Norm CHO resulted in higher (p < 0.05) resting Ca2+uptake (3.17 ± 0.21 vs. 3.49 ± 0.24 µmol·(g protein)·min–1and higher ionophore ratio, defined as the ratio of Vmaxmeasured with and without the Ca2+-ionophore A23187, (2.3 ± 0.3 vs. 4.4 ± 0.3 µmol·(g protein)·min–1) at fatigue. No differences were observed between conditions in the concentration of muscle glycogen, the high-energy phosphates (ATP and PCr), or metabolites (Pi, Cr, and lactate). Ex+Norm CHO also failed to modify the exercise-induced changes in CHO and fat oxidation. We conclude that prolonged exercise to fatigue performed 4 days following glycogen-depleting exercise while on a normal CHO diet elevates resting Ca2+uptake and prevents increases in SR membrane permeability to Ca2+as measured by the ionophore ratio. Key words: Ca2+cycling, glycogen depletion, contractile activity, recovery.

N-acetylcysteine fails to modulate the in vitro function of sarcoplasmic reticulum of diaphragm in the final phase of fatigue

AIM

In the present study, we tested the hypothesis whether N-acetylcysteine (NAC), a non-specific antioxidant, might influence fatigue by modulating Ca2+-handling capacity by the sarcoplasmic reticulum (SR).

METHODS

In the presence (10 mm) or absence of NAC, bundles of rat diaphragm were stimulated with tetanic trains (350 ms, 30-40 Hz) at 1 train every 2 s for 300 s. SR functions, as assessed by SR Ca2+-uptake and release rates and SR Ca2+-ATPase activity, were measured in vitro on muscle homogenates.

RESULTS

Following the 300-s stimulation, the force developed by NAC-treated muscles is approximately 1.8-fold higher (P < 0.05) than that of muscles without NAC treatment. Stimulation elicited an 18-30% depression in SR function (P < 0.05). Despite the differing degrees of fatigue between NAC-treated and non-treated muscles, SR functions in these muscles were reduced to similar extents.

CONCLUSIONS

These results suggest that modulation of SR function measured in vitro may not be a major contributor to inhibition of diaphragmic fatigue with antioxidant, at least, in the final phase of fatigue where force output is remarkably reduced.

Regulation of the cell surface expression of human BCRP/ABCG2 by the phosphorylation state of Akt in polarized cells

Human breast cancer resistance protein (BCRP/ABCG2) is believed to act as an efflux pump to protect the body from drugs and toxins. BCRP is known to accept many kinds of endogenous and exogenous compounds as substrates and to be localized on the apical membrane of various tissues. Expression of BCRP is also reported on the side population cells, and a recent report suggested involvement of Akt in the modulation of the side population phenotype. In the present study, we have characterized the effect of Akt on the polarized expression of BCRP using LLC-PK1 cells. After treatment with phosphatidylinositol 3-kinase (PI3K) inhibitors, internalization of stably transfected BCRP from the apical surface was observed after immunohistochemical staining, and the relative expression level of BCRP on the cell surface decreased to 49 +/- 14 and 51 +/- 8% of the control for LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride] and wortmannin treatment, respectively. This phenomenon was supported by the observation of internalized BCRP in presence of dominant negative-Akt. When the cells were treated with epidermal growth factor, the cell surface expression of BCRP was increased to 228 +/- 43% of the control accompanied by Akt stimulation. These results suggest that the relative expression of BCRP on the cell surface is regulated by the PI3K-Akt signaling pathway with a positive correlation in polarized cells. Alteration in Akt activities may influence the cellular extrusion of BCRP substrates by modifying epithelial BCRP localization.

Effects of acute creatine kinase inhibition on metabolism and tension development in isolated single myocytes

This study investigated the effects of acute creatine kinase (CK) inhibition (CKi) on contractile performance, cytosolic Ca2+concentration ([Ca2+]c), and intracellular Po2(Pi[Formula: see text]) in Xenopus laevis isolated myocytes during a 2-min bout of isometric tetanic contractions (0.33-Hz frequency). Peak tension was similar between trials during the first contraction but was significantly ( P < 0.05) attenuated for all subsequent contractions in CKivs. control (Con). The fall in Pi[Formula: see text](ΔPi[Formula: see text]) from resting values was significantly greater in Con (26.0 ± 2.2 Torr) compared with CKi(17.8 ± 1.8 Torr). However, the ratios of Con to CKiend-peak tension (1.53 ± 0.11) and ΔPi[Formula: see text](1.49 ± 0.11) were similar, suggesting an unaltered aerobic cost of contractions. Additionally, the mean response time (MRT) of ΔPi[Formula: see text]was significantly faster in CKivs. Con during both the onset (31.8 ± 5.5 vs. 49.3 ± 5.7 s; P < 0.05) and cessation (21.2 ± 4.1 vs. 68.0 ± 3.2 s; P < 0.001) of contractions. These data demonstrate that initial phosphocreatine hydrolysis in single skeletal muscle fibers is crucial for maintenance of sarcoplasmic reticulum Ca2+release and peak tension during a bout of repetitive tetanic contractions. Furthermore, as Pi[Formula: see text]fell more rapidly at contraction onset in CKicompared with Con, these data suggest that CK activity temporally buffers the initial ATP-to-ADP concentration ratio at the transition to an augmented energetic demand, thereby slowing the initial mitochondrial activation by mitigating the energetic control signal (i.e., ADP concentration, phosphorylation potential, etc.) between sites of ATP supply and demand.

Cytoplasmic Confinement of Breast Cancer Resistance Protein (BCRP/ABCG2) as a Novel Mechanism of Adaptation to Short-Term Folate Deprivation

The unique capability of breast cancer resistance protein (BCRP/ABCG2) to export mono-, di-, and triglutamates of folates should limit cellular proliferation under conditions of folate deprivation, particularly upon BCRP overexpression. Here, we explored the mode of adaptation of BCRP-overexpressing cells to short-term folate deprivation. MCF-7/MR cells grown in high folate medium (2.3 muM folic acid) containing mitoxantrone had 62% of their overexpressed BCRP in the plasma membrane and only 38% in the cytoplasm. In contrast, cells grown for 2 weeks in folic acid-free medium followed by an adaptation week in low folate medium (1 nM folic acid) had 86% of BCRP in the cytoplasm and only 14% in the plasma membrane. Unlike BCRP, various transmembrane proteins retained their normal plasma membrane localization in folate-deprived cells. Folate deprivation was also associated with a 3-fold decrease in BCRP and multidrug resistance protein 1 (MRP1/ABCC1) levels. Confocal microscopy with folate-deprived cells revealed that cytoplasmic BCRP colocalized with calnexin, an established endoplasmic reticulum resident. The loss of BCRP from the plasma membrane in folate-deprived cells consistently resulted in a 4.5-fold increase in [(3)H]folic acid accumulation relative to MCF-7/MR cells. Hence, cellular adaptation to shortterm folate deprivation results in a selective confinement of BCRP to the cytoplasm along with a moderate decrease in BCRP and MRP1 levels aimed at preserving the poor intracellular folate pools. These results constitute a novel mechanism of cellular adaptation to short-term folate deprivation and provide further support to the possible role of BCRP in the maintenance of cellular folate homeostasis.

Effects of fatigue on depolarization- and caffeine-induced contractures of skinned fibres

AIMS

Fatigue has been shown to cause intrinsic alterations in sarcoplasmic reticulum (SR) Ca2+ release.

METHODS

In this investigation, frog semitendinosus muscles were stimulated to fatigue, in vitro (80 Hz, 100 ms, 1 train s-1, 5 min). Immediately after stimulation, single fibres were removed and skinned using either chemically or mechanically skinning. Contralateral muscle were treated similarly but were not stimulated.

RESULTS

In fatigued, saponin skinned fibres, contracture responses to low [caffeine] (4-8 mm) were depressed compared with control. However, responses to high concentrations (10-15 mm) were not different between conditions. In the fatigued, mechanically skinned fibres, responses to chloride depolarization were depressed at all [chloride] (20-100 mm) compared with control.

CONCLUSIONS

These results suggest that fatigue causes intrinsic alterations in both the SR Ca2+ release channel as well as communication between the transverse-tubule and the SR.

Adenosine receptor, protein kinase G, and p38 mitogen-activated protein kinase-dependent up-regulation of serotonin transporters involves both transporter trafficking and activation

Serotonin (5-hydroxytryptamine; 5-HT) transporters (SERTs) are critical determinants of synaptic 5-HT inactivation and the targets for multiple drugs used to treat psychiatric disorders. In support of prior studies, we found that short-term (5-30 min) application of the adenosine receptor (AR) agonist 5'-N-ethylcarboxamidoadenosine (NECA) induces an increase in 5-HT uptake Vmax in rat basophilic leukemia 2H3 cells that is enhanced by pretreatment with the cGMP phosphodiesterase inhibitor sildenafil. NECA stimulation is blocked by the A3 AR antagonist 3-ethyl-5-benzyl-2-methyl-phenylethynyl-6-phenyl-1,4(+/-)dihydropyridine-3,5-dicarboxylate (MRS1191), by the phospholipase C inhibitor 1-(6-[[17beta-3-methoxyestra-1,3,5(10)-trien-17-yl] amino]hexyl)-1H-pyrrole-2,5-dione (U73122), by the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester, and by the guanyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. Hydroxylamine, a nitric-oxide donor, and 8-bromo-cGMP, a membrane-permeant analog of cGMP, mimic the effects of NECA on 5-HT uptake, whereas the protein kinase G (PKG) inhibitor N-[2-(methylamino)ethy]-5-isoquinoline-sulfonamide (H8) blocks NECA, hydroxylamine, and 8-bromo-cGMP effects. NECA stimulation activates p38 mitogen-activated protein kinase (MAPK), whereas p38 MAPK inhibitors block NECA stimulation of SERT activity, as does the protein phosphatase 2A (PP2A) inhibitor calyculin A. 5-HT-displaceable [125I]3beta-(4-iodophenyl)-tropane-2beta-carboxylic acid methylester tartrate (RTI-55) whole-cell binding is increased by NECA or sildenafil, and both surface binding and cell surface SERT protein are elevated after NECA or sildenafil stimulation of AR/SERT-cotransfected Chinese hamster ovary cells. Whereas p38 MAPK inhibition blocks NECA stimulation of 5-HT activity, it fails to blunt stimulation of SERT surface density. Moreover, inactivation of existing surface SERTs fails to eliminate NECA stimulation of SERT. Together, these results reveal two PKG-dependent pathways supporting rapid SERT regulation by A3 ARs, one leading to enhanced SERT surface trafficking, and a separate, p38 MAPK-dependent process augmenting SERT intrinsic activity.