In vivo imaging of intracellular Ca2+ after muscle contractions and direct Ca2+ injection in rat skeletal muscle in diabetes

Impacts of altered exercise volume, intensity, and duration on the activation of AMPK and CaMKII and increases in PGC-1α mRNA

The purpose of this review is to explore and discuss the impacts of augmented training volume, intensity, and duration on the phosphorylation/activation of key signaling protein - AMPK, CaMKII and PGC-1α - involved in the initiation of mitochondrial biogenesis. Specifically, we explore the impacts of augmented exercise protocols on AMP/ADP and Ca²⁺ signaling and changes in post exercise PGC - 1α gene expression. Although AMP/ADP concentrations appear to increase with increasing intensity and during extended durations of higher intensity exercise AMPK activation results are varied with some results supporting and intensity/duration effect and others not. Similarly, CaMKII activation and signaling results following exercise of different intensities and durations are inconsistent. The PGC-1α literature is equally inconsistent with only some studies demonstrating an effect of intensity on post exercise mRNA expression. We present a novel meta-analysis that suggests that the inconsistency in the PGC-1α literature may be due to sample size and statistical power limitations owing to the effect of intensity on PGC-1α expression being small. There is little data available regarding the impact of exercise duration on PGC-1α expression. We highlight the need for future well designed, adequately statistically powered, studies to clarify our understanding of the effects of volume, intensity, and duration on the induction of mitochondrial biogenesis by exercise.

Impaired fatigue resistance, sarcoplasmic reticulum function, and mitochondrial activity in soleus muscle of db/db mice

Type 2 diabetes mellitus (T2DM) is characterized by reduced exercise tolerance due to increased fatigability in skeletal muscle. In this study, we investigated muscle fatigue resistance of soleus (SOL) muscle in obese type 2 diabetic model mice (db/db). No differences in muscle volume, absolute force, or specific force in SOL muscle were observed between db/db mice and control mice (db/+), while fatigue resistance evaluated by repeated tetanic contractions was significantly lower in db/db mice (30th tetani, db/+: 63.7 ± 4.7%, db/db: 51.3 ± 4.8%). The protein abundance related to Ca2+ release from the sarcoplasmic reticulum (SR) in SOL muscle was not different between db/db mice and db/+ mice, while SR Ca2+ -ATPase (Ca2+ reuptake to SR) protein was decreased in db/db mice compared to db/+ mice (db/+: 1.00 ± 0.17, db/db: 0.60 ± 0.04, relative units). In addition, mitochondrial oxidative enzyme activity (succinate dehydrogenase) was decreased in the SOL muscle of db/db mice (p < 0.05). These data suggest that fatigue resistance in slow-twitch dominant muscle is impaired in mice with T2DM. Decreased mitochondrial oxidative enzyme activity and impairment of Ca2+ uptake to SR, or both might be involved in the mechanisms.

Ryanodine receptors mediate high intracellular Ca2+ and some myocyte damage following eccentric contractions in rat fast-twitch skeletal muscle

Eccentric contractions (ECC) facilitate cytosolic calcium ion (Ca2+) release from the sarcoplasmic reticulum (SR) and Ca2+ influx from the extracellular space. Ca2+ is a vital signaling messenger that regulates multiple cellular processes via its spatial and temporal concentration ([Ca2+]i) dynamics. We hypothesized that 1) a specific pattern of spatial/temporal intramyocyte Ca2+ dynamics portends muscle damage following ECC and 2) these dynamics would be regulated by the ryanodine receptor (RyR). [Ca2+]i in the tibialis anterior muscles of anesthetized adult Wistar rats was measured by ratiometric (i.e., ratio, R, 340/380 nm excitation) in vivo bioimaging with Fura-2 pre-ECC and at 5 and 24 h post-ECC (5 × 40 contractions). Separate groups of rats received RyR inhibitor dantrolene (DAN; 10 mg/kg ip) immediately post-ECC (+DAN). Muscle damage was evaluated by histological analysis on hematoxylin-eosin stained muscle sections. Compared with control (CONT, no ECC), [Ca2+]i distribution was heterogeneous with increased percent total area of high [Ca2+]i sites (operationally defined as R ≥ 1.39, i.e., ≥1 SD of mean control) 5 h post-ECC (CONT, 14.0 ± 8.0; ECC5h: 52.0 ± 7.4%, P < 0.01). DAN substantially reduced the high [Ca2+]i area 5 h post-ECC (ECC5h + DAN: 6.4 ± 3.1%, P < 0.01) and myocyte damage (ECC24h, 63.2 ± 1.0%; ECC24h + DAN: 29.1 ± 2.2%, P < 0.01). Temporal and spatially amplified [Ca2+]i fluctuations occurred regardless of DAN (ECC vs. ECC + DAN, P > 0.05). These results suggest that the RyR-mediated local high [Ca2+]i itself is related to the magnitude of muscle damage, whereas the [Ca2+]i fluctuation is an RyR-independent phenomenon.

Influence of Obesity and Type 2 Diabetes on Calcium Handling by Skeletal Muscle: Spotlight on the Sarcoplasmic Reticulum and Mitochondria

Obesity and diabetes have been shown to interfere with energy metabolism and cause peripheral insulin resistance in skeletal muscle. However, recent studies have focused on the effect metabolic insult has on the loss of muscle size, strength, and physical function. Contractile dysfunction has been linked to impaired intracellular Ca²⁺ concentration ([Ca²⁺]_i) regulation. In skeletal muscle, [Ca²⁺]_i homeostasis is highly regulated by Ca²⁺ transport across the sarcolemma/plasma membrane, the golgi apparatus, sarcoplasmic reticulum (SR), and mitochondria. Particularly, the SR and or mitochondria play an important role in the fine-tuning of this metabolic process. Recent studies showed that obesity and insulin resistance are associated with interactions between the SR and mitochondrial networks (the dynamic tubular reticulum formed by mitochondria), suggesting that metabolic disorders alter Ca²⁺ handling by these organelles. These interactions are facilitated by specific membrane proteins, including ion channels. This review considers the impact of metabolic disorders, such as obesity and type 2 diabetes, on the regulation of [Ca²⁺]_i in skeletal muscle. It also discusses the mechanisms by which this occurs, focusing chiefly on the SR and mitochondria networks. A deeper understanding of the effect of metabolic disorders on calcium handling might be useful for therapeutic strategies.

Sirtuin 3 overexpression preserves maximal sarco(endo)plasmic reticulum calcium ATPase activity in the skeletal muscle of mice subjected to high fat-high sucrose-feeding

Sarco(endo)plasmic reticulum calcium (Ca²⁺) ATPase (SERCA) transports Ca²⁺ in muscle. Impaired SERCA activity contributes to diabetic myopathy. Sirtuin (SIRT) 3 regulates muscle metabolism and function. However, it is unknown if SIRT3 regulates muscle SERCA activity. We determined if SIRT3 overexpression enhances SERCA activity in mouse gastrocnemius muscle and if SIRT3 overexpression preserves gastrocnemius SERCA activity in a model of type 2 diabetes, induced by high fat-high sucrose (HFHS)-feeding. We also determined if the acetylation status of SERCA proteins in mouse gastrocnemius is altered by SIRT3 overexpression or HFHS-feeding. Wild-type (WT) mice and SIRT3 transgenic (SIRT3_TG) mice, overexpressing SIRT3 in skeletal muscle, were fed a standard- or HFHS-diet for 4-months. SIRT3_TG and WT mice developed obesity and glucose intolerance after 4-months of HFHS-feeding. SERCA V_max was higher in gastrocnemius of SIRT3TG mice, compared to WT mice. HFHS-fed mice had lower SERCA1a protein levels and lower SERCA V_max in their gastrocnemius than control-fed mice. The decrease in SERCA V_max in gastrocnemius muscle due to HFHS-feeding was attenuated by SIRT3 overexpression in HFHS-fed SIRT3_TG mice. SERCA1a and SERCA2a acetylation in mouse gastrocnemius was not altered by genotype or diet. These findings suggest SIRT3 overexpression improves SERCA function in diabetic mouse skeletal muscle.

Type I diabetes suppresses intracellular calcium ion increase normally evoked by heat stress in rat skeletal muscle

Heat stress, via its effects on muscle intracellular Ca2+ concentrations ([Ca2+]i), has been invoked as a putative therapeutic countermeasure to type 1 diabetes-induced muscle atrophy. Using a circulation- and neurally intact in vivo muscle preparation, we tested the hypothesis that impaired muscle Ca2+ homeostasis in type 1 diabetic rats is due to attenuated heat stress tolerance mediated via transient receptor potential vanilloid 1 (TRPV1). Male Wistar rats were randomly assigned to one of the following four groups: 1) healthy control 30°C (CONT 30°C); 2) CONT 40°C; 3) diabetes 30°C (DIA 30°C); and 4) DIA 40°C. The temperature of 40°C was selected because it exceeds the TRPV1 activation threshold. Spinotrapezius muscles of Wistar rats were exteriorized in vivo and loaded with the fluorescent Ca2+ probe Fura-2 AM. [Ca2+]i was estimated over 20 min using fluorescence microscopy (340/380 nm ratio) in quiescent muscle held at the required temperature, using a calibrated heat source applied to the ventral muscle surface. Western blotting was performed to determine the protein expression levels of TRPV1 in spinotrapezius muscle. After 20 min of heat stress, the CONT 40°C condition induced a 12.3 ± 5% [Ca2+]i (P < 0.05) elevation that was markedly absent in the DIA 40°C or other conditions. Thus, no significant differences were found among DIA 40°C, DIA 30°C, and CONT 30°C. TRPV1 protein expression was decreased by 42.0 ± 9% in DIA compared with CONT (P < 0.05) and, unlike CONT, heat stress did not increase TRPV1 phosphorylation. In conclusion, diabetes suppresses TRPV1 protein expression and function and inhibits the elevated myocyte [Ca2+]i evoked normally by heat stress. These results suggest that capsaicin or other therapeutic strategies to increase Ca2+ accumulation via TRPV1 might be more effective than hyperthermic therapy for type 1 diabetic patients.

Maintenance of contractile force and increased fatigue resistance in slow-twitch skeletal muscle of mice fed a high-fat diet

Consumption of a high-fat diet (HFD) significantly increases exercise endurance performance during treadmill running. However, whether HFD consumption increases endurance capacity via enhanced muscle fatigue resistance has not been clarified. In this study, we investigated the effects of HFDs on contractile force and fatigue resistance of slow-twitch dominant muscles. The soleus (SOL) muscle of male C57BL/6J mice fed an HFD (60% kcal from fat) or a low-fat diet (LFD) for 12 wk was analyzed. Muscle contractile force was measured under resting conditions and during fatigue induced by repeated tetanic contractions (100 Hz, 50 contractions, and 2-s intervals). Differences in muscle twitch or tetanic force were not evident between HFD and LFD groups, whereas fatigue resistance was higher in the HFD groups. The SOL muscle of HFD-fed mice showed increased levels of markers related to oxidative capacity such as succinate dehydrogenase (SDH) and citrate synthase (CS) activity. In addition, electron microscopy analyses indicated that the total number of mitochondria and mitochondrial volume density increased in the SOL muscle of the HFD groups. These findings suggest that HFD consumption induces increased muscle fatigue resistance in slow-twitch dominant muscle fibers. This effect of HFD may be related to elevated oxidative enzyme activity, high mitochondrial content, or both.NEW & NOTEWORTHY In this study, we examined the effects of HFDs on muscle contractile force and fatigue resistance of slow-twitch dominant muscles ex vivo. We found that contractile function was comparable between the HFD groups and the LFD group, whereas fatigue resistance was higher in the HFD groups. This effect of HFD may be related to elevated oxidative enzyme activity, high mitochondrial content, or both.

Cortisol rapidly stimulates calcium waves in the developing trunk muscle of zebrafish

Glucocorticoids (GCs) play a role in stress coping by activating the glucocorticoid receptor (GR), a ligand-bound transcription factor. GCs also exert rapid effects that are nongenomic by modulating second messenger signaling, including Ca²⁺. However, the mechanism of action of GCs in modulating cytoplasmic free calcium level ([Ca²⁺]i) is unclear. We hypothesized that cortisol increases ([Ca²⁺]i) in zebrafish (Danio rerio) muscle, and this is independent of GR activation. Indeed, cortisol rapidly stimulated ([Ca²⁺]i) rise in the developing trunk muscle (DTM), and this response was not abolished in the GR knockout zebrafish. The rapid cortisol-induced ([Ca²⁺]i) rise was reduced with EGTA, and completely abolished by the pharmacological inhibition of the calcium release-activated calcium channel (CRACC). Also, cortisol stimulation rapidly increased the expression of Orai1, the pore forming protein subunit of CRACC, in the DTM. Altogether, rapid nongenomic action of cortisol on muscle function may involve Ca²⁺ signaling by CRACC gating in zebrafish.

Skeletal Muscle Function, Structure, and Metabolism in Patients With Heart Failure With Reduced Ejection Fraction and Heart Failure With Preserved Ejection Fraction

BACKGROUND

Reduced exercise capacity in patients with heart failure (HF) could be partially explained by skeletal muscle dysfunction. We compared skeletal muscle function, structure, and metabolism among clinically stable outpatients with HF with preserved ejection fraction, HF with reduced ejection fraction, and healthy controls (HC). Furthermore, the molecular, metabolic, and clinical profile of patients with reduced muscle endurance was described.

METHODS

Fifty-five participants were recruited prospectively at the University Hospital Jena (17 HF with preserved ejection fraction, 18 HF with reduced ejection fraction, and 20 HC). All participants underwent echocardiography, cardiopulmonary exercise testing, 6-minute walking test, isokinetic muscle function, and skeletal muscle biopsies. Expression levels of fatty acid oxidation, glucose metabolism, atrophy genes, and proteins as well as inflammatory biomarkers were assessed. Mitochondria were evaluated using electron microscopy.

RESULTS

Patients with HF with preserved ejection fraction showed compared with HF with reduced ejection fraction and HC reduced muscle strength (eccentric extension: 13.3±5.0 versus 18.0±5.9 versus 17.9±5.1 Nm/kg, P=0.04), elevated levels of MSTN-2 (myostatin-2), FBXO-32 (F-box only protein 32 [Atrogin1]) gene and protein, and smaller mitochondrial size (P<0.05). Mitochondrial function and fatty acid and glucose metabolism were impaired in HF-patients compared with HC (P<0.05). In a multiple regression analysis, GDF-15 (growth and differentiation factor 15), CPT1B (carnitine palmitoyltransferase IB)-protein and oral anticoagulation were independent factors for predicting reduced muscle endurance after adjusting for age (log10 GDF-15 [pg/mL] [B, -54.3 (95% CI, -106 to -2.00), P=0.043], log10 CPT1B per fold increase [B, 49.3 (95% CI, 1.90-96.77), P=0.042]; oral anticoagulation present [B, 44.8 (95% CI, 27.90-61.78), P<0.001]).

CONCLUSIONS

Patients with HF with preserved ejection fraction have worse muscle function and predominant muscle atrophy compared with those with HF with reduced ejection fraction and HC. Inflammatory biomarkers, fatty acid oxidation, and oral anticoagulation were independent factors for predicting reduced muscle endurance.

Sex differences in mitochondrial Ca2+ handling in mouse fast-twitch skeletal muscle in vivo

We investigated sex differences in mitochondrial Ca²⁺ handling properties in mouse fast-twitch skeletal muscle. Changes in cytoplasmic Ca²⁺ concentration ([Ca²⁺]_cyto) were measured in vivo using tibialis anterior muscles from male and female mice. The muscles were exposed to increasing concentrations of cyclopiazonic acid [CPA; sarcoplasmic reticulum (SR) Ca²⁺-ATPase inhibitor] (from 10 to 30 to 50 μM at 10 min intervals). Thirty minutes after treatment, [Ca²⁺]_cyto was increased by 31.6 ± 2.0% and 13.5 ± 4.5% of initial [Ca²⁺]_cyto in male and female muscles, respectively, and there was a significant difference between sexes. However, muscle preincubation for 5 min with 10 μM carbonyl cyanide-4-(trifluoromethoxy) phenylhydrazone (an inhibitor of mitochondria Ca²⁺ uptake) eradicated this difference between sexes with respect to the CPA-induced [Ca²⁺]_cyto increase. Both intermyofibrillar mitochondrial number and volume, assessed in longitudinal fiber sections, were higher in females compared with males (mitochondria number: 13.1 ± 1.0 in males vs. 19.9 ± 2.3 in females; mitochondrial volume: 0.034 ± 0.004 μm³/μm³ fiber volume in males vs. 0.066 ± 0.008 μm³/μm³ fiber volume in females, both P < 0.05). There were no sex differences in the content of SR Ca²⁺-ATPase, mitochondrial Ca²⁺ uniporter, mitofusin (Mfn) 1, or Mfn2. These results suggest that 1) mitochondrial Ca²⁺ uptake ability is greater in female than male myocytes, and 2) this superior Ca²⁺ uptake ability of female myocytes is due, partly, to the higher intermyofibrillar mitochondrial content but not to the expression of mitochondrial proteins related to mitochondrial Ca²⁺ uptake.NEW & NOTEWORTHY This investigation presents evidence that female versus male fast-twitch muscle exhibits a greater mitochondrial calcium ion uptake capability that is partly conferred by the higher intermyofibrillar mitochondrial volume density.

Regional differences in Ca2+ entry along the proximal-middle-distal muscle axis during eccentric contractions in rat skeletal muscle

Eccentric (ECC) contraction-induced muscle damage is associated with calcium ion (Ca²⁺) influx from the extracellular milieu through stretch-activated channels. It remains unknown whether Ca²⁺ influx consequent to repetitive ECC contractions is nonuniform across different muscle regions. We tested the hypothesis that there are regional differences in Ca²⁺ entry along the proximal-middle-distal muscle axis. Tibialis anterior (TA) muscles of adult male Wistar rats were exposed by reflecting the overlying skin and fasciae and ECC contractions evoked by peroneal nerve stimulation paired with simultaneous ankle extension (50 times/set, 2 protocols: 1 set and 10 sets). During ECC in the proximal, middle, and distal TA, we determined 1) muscle fiber extension by high-speed camera (200 frames/s) and 2) Ca²⁺ accumulation by in vivo bioimaging (Ca²⁺-sensitive probe Fura-2-acetoxymethyl ester). Muscle fiber extension from resting was significantly different among regions (i.e., proximal, 4.0%: < middle, 11.2%: < distal, 17.0%; ECC phase length at 500th contraction). Intracellular Ca²⁺ accumulation after 1 set of ECC was higher in the distal (1.46 ± 0.04, P < 0.05) than the proximal (1.27 ± 0.04) or middle (1.26 ± 0.05) regions. However, this regional Ca²⁺ accumulation difference disappeared by 32.5 min after the 1 set protocol when the muscle was quiescent and by contraction set 5 for the 10-set protocol. The initial preferential ECC-induced Ca²⁺ accumulation observed distally was associated spatially with the greater muscle extension compared with that of the proximal and middle regions. Disappearance of the regional Ca²⁺ accumulation disparity in quiescent and ECC-contracting muscle might be explained, in part, by axial Ca²⁺ propagation and account for the uniformity of muscle damage across regions evident 3 days post-ECC.NEW & NOTEWORTHY After 1 set of 50 eccentric (ECC) contractions in the anterior tibialis muscle, intracellular Ca²⁺ ([Ca²⁺]i) accumulation evinces substantial regional heterogeneity that is spatially coherent with muscle length changes (i.e., distal [Ca²⁺]i > middle, proximal). However, irrespective of whether 50 or 500 ECC contractions are performed, this heterogeneity is subsequently abolished, at least in part, by axial intracellular Ca²⁺ propagation. This Ca²⁺ homogenization across regions is consistent with the absence of any interregional difference in muscle damage 3 days post-ECC.

Accumulation of intramyocyte TRPV1-mediated calcium during heat stress is inhibited by concomitant muscle contractions

Heat stress promotes intramyocyte calcium concentration ([Ca²⁺]_i) accumulation via transient receptor potential vanilloid 1 (TRPV1) channels. We tested the hypothesis that muscle contractile activity concomitant with heat stress would accelerate the increase in [Ca²⁺]_i via TRPV1, further impairing [Ca²⁺]_i homeostasis. Spinotrapezius muscles of adult Wistar rats were exteriorized in vivo and loaded with the fluorescent Ca²⁺ probe fura 2-AM. Heat stress (muscle surface temperature 40°C) was used as TRPV1 activator. An isometric contraction (100 Hz, 5-10 V, 30 s) was induced electrically concomitant with heat stress. [Ca²⁺]_i was determined for 20 min using in vivo fluorescence microscopy, and the phosphorylation response of TRPV1 was determined by Western blotting. Heat stress induced a significant [Ca²⁺]_i increase of 18.5 ± 8.1% at 20 min and TRPV1 phosphorylation (+231%), which was inhibited by addition of the TRPV1 inhibitor (capsazepine). However, contrary to expectations, the heat stress and isometric contraction condition almost completely inhibited TRPV1 phosphorylation and the consequent [Ca²⁺]_i elevation (<2.8% accumulation during heat stress, P > 0.05). In conclusion, this in vivo physiological model demonstrated that isometric muscle contraction(s) can suppress the phosphorylation response of TRPV1 and maintain [Ca²⁺]_i homeostasis during heat stress. NEW & NOTEWORTHY This investigation is the first document the dynamics of intramyocyte calcium concentration ([Ca²⁺]_i) increase in the myoplasm of skeletal muscle fibers in response to heat stress where the muscle blood flow is preserved. Heat stress at 40°C drives a myoplasmic [Ca²⁺]_i accumulation in concert with transient receptor potential vanilloid 1 (TRPV1) phosphorylation. However, muscle contraction caused TRPV1 channel deactivation by dephosphorylation of TRPV1. TRPV1 inactivation via isometric contraction(s) permits maintenance of [Ca²⁺]_i homeostasis even under high imposed muscle temperature.

Dysfunction of muscle contraction with impaired intracellular Ca2+ handling in skeletal muscle and the effect of exercise training in male db/db mice

Type 2 diabetes is characterized by reduced contractile force production and increased fatigability of skeletal muscle. While the maintenance of Ca²⁺ homeostasis during muscle contraction is a requisite for optimal contractile function, the mechanisms underlying muscle contractile dysfunction in type 2 diabetes are unclear. Here, we investigated skeletal muscle contractile force and Ca²⁺ flux during contraction and pharmacological stimulation in type 2 diabetic model mice ( db/db mice). Furthermore, we investigated the effect of treadmill exercise training on muscle contractile function. In male db/db mice, muscle contractile force and peak Ca²⁺ levels were both lower during tetanic stimulation of the fast-twitch muscles, while Ca²⁺ accumulation was higher after stimulation compared with control mice. While 6 wk of exercise training did not improve glucose tolerance, exercise did improve muscle contractile dysfunction, peak Ca²⁺ levels, and Ca²⁺ accumulation following stimulation in male db/db mice. These data suggest that dysfunctional Ca²⁺ flux may contribute to skeletal muscle contractile dysfunction in type 2 diabetes and that exercise training may be a promising therapeutic approach for dysfunctional skeletal muscle contraction. NEW & NOTEWORTHY The purpose of this study was to examine muscle contractile function and Ca²⁺ regulation as well as the effect of exercise training in skeletal muscle in obese diabetic mice ( db/db). We observed impairment of muscle contractile force and Ca²⁺ regulation in a male type 2 diabetic animal model. These dysfunctions in muscle were improved by 6 wk of exercise training.

Involvement of O-GlcNAcylation in the Skeletal Muscle Physiology and Physiopathology: Focus on Muscle Metabolism

Skeletal muscle represents around 40% of whole body mass. The principal function of skeletal muscle is the conversion of chemical energy toward mechanic energy to ensure the development of force, provide movement and locomotion, and maintain posture. This crucial energy dependence is maintained by the faculty of the skeletal muscle for being a central place as a "reservoir" of amino acids and carbohydrates in the whole body. A fundamental post-translational modification, named O-GlcNAcylation, depends, inter alia, on these nutrients; it consists to the transfer or the removal of a unique monosaccharide (N-acetyl-D-glucosamine) to a serine or threonine hydroxyl group of nucleocytoplasmic and mitochondrial proteins in a dynamic process by the O-GlcNAc Transferase (OGT) and the O-GlcNAcase (OGA), respectively. O-GlcNAcylation has been shown to be strongly involved in crucial intracellular mechanisms through the modulation of signaling pathways, gene expression, or cytoskeletal functions in various organs and tissues, such as the brain, liver, kidney or pancreas, and linked to the etiology of associated diseases. In recent years, several studies were also focused on the role of O-GlcNAcylation in the physiology and the physiopathology of skeletal muscle. These studies were mostly interested in O-GlcNAcylation during muscle exercise or muscle-wasting conditions. Major findings pointed out a different "O-GlcNAc signature" depending on muscle type metabolism at resting, wasting and exercise conditions, as well as depending on acute or long-term exhausting exercise protocol. First insights showed some differential OGT/OGA expression and/or activity associated with some differential stress cellular responses through Reactive Oxygen Species and/or Heat-Shock Proteins. Robust data displayed that these O-GlcNAc changes could lead to (i) a differential modulation of the carbohydrates metabolism, since the majority of enzymes are known to be O-GlcNAcylated, and to (ii) a differential modulation of the protein synthesis/degradation balance since O-GlcNAcylation regulates some key signaling pathways such as Akt/GSK3β, Akt/mTOR, Myogenin/Atrogin-1, Myogenin/Mef2D, Mrf4 and PGC-1α in the skeletal muscle. Finally, such involvement of O-GlcNAcylation in some metabolic processes of the skeletal muscle might be linked to some associated diseases such as type 2 diabetes or neuromuscular diseases showing a critical increase of the global O-GlcNAcylation level.

Long-term, but not short-term high-fat diet induces fiber composition changes and impaired contractile force in mouse fast-twitch skeletal muscle

In this study, we investigated the effects of a short-term and long-term high-fat diet (HFD) on morphological and functional features of fast-twitch skeletal muscle. Male C57BL/6J mice were fed a HFD (60% fat) for 4 weeks (4-week HFD) or 12 weeks (12-week HFD). Subsequently, the fast-twitch extensor digitorum longus muscle was isolated, and the composition of muscle fiber type, expression levels of proteins involved in muscle contraction, and force production on electrical stimulation were analyzed. The 12-week HFD, but not the 4-week HFD, resulted in a decreased muscle tetanic force on 100 Hz stimulation compared with control (5.1 ± 1.4 N/g in the 12-week HFD vs. 7.5 ± 1.7 N/g in the control group; P < 0.05), whereas muscle weight and cross-sectional area were not altered after both HFD protocols. Morphological analysis indicated that the percentage of type IIx myosin heavy chain fibers, mitochondrial oxidative enzyme activity, and intramyocellular lipid levels increased in the 12-week HFD group, but not in the 4-week HFD group, compared with controls (P < 0.05). No changes in the expression levels of calcium handling-related proteins and myofibrillar proteins (myosin heavy chain and actin) were detected in the HFD models, whereas fast-troponin T-protein expression was decreased in the 12-week HFD group, but not in the 4-week HFD group (P < 0.05). These findings indicate that a long-term HFD, but not a short-term HFD, impairs contractile force in fast-twitch muscle fibers. Given that skeletal muscle strength largely depends on muscle fiber type, the impaired muscle contractile force by a HFD might result from morphological changes of fiber type composition.

Improved skeletal muscle Ca2+ regulation in vivo following contractions in mice overexpressing PGC-1α

In skeletal muscle, resting intracellular Ca²⁺ concentration ([Ca²⁺]_i) homeostasis is exquisitely regulated by Ca²⁺ transport across the sarcolemmal, mitochondrial, and sarcoplasmic reticulum (SR) membranes. Of these three systems, the relative importance of the mitochondria in [Ca²⁺]_i regulation remains poorly understood in in vivo skeletal muscle. We tested the hypothesis that the capacity for Ca²⁺ uptake by mitochondria is a primary factor in determining [Ca²⁺]_i regulation in muscle at rest and following contractions. Tibialis anterior muscle of anesthetized peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α)-overexpressing (OE, increased mitochondria model) and wild-type (WT) littermate mice was exteriorized in vivo and loaded with the fluorescent probe fura 2-AM, and Rhod 2-AM Ca²⁺ buffering and mitochondrial [Ca²⁺] were evaluated at rest and during recovery from fatiguing tetanic contractions induced by electrical stimulation (120 s, 100 Hz). In addition, the effects of pharmacological inhibition of SR (thapsigargin) and mitochondrial [carbonyl cyanide-4-(trifluoromethoxy) phenylhydrazone (FCCP)] function were examined at rest. [Ca²⁺]_i in WT remained elevated for the entire postcontraction recovery period (+6 ± 1% at 450 s), but in PGC-1α OE [Ca²⁺]_i returned to resting baseline within 150 s. Thapsigargin immediately and substantially increased resting [Ca²⁺]_i in WT, whereas in PGC-1α OE this effect was delayed and markedly diminished (WT, +12 ± 3; PGC-1α OE, +1 ± 2% at 600 s after thapsigargin treatment, P < 0.05). FCCP abolished this improvement of [Ca²⁺]_i regulation in PGC-1α OE. Mitochondrial [Ca²⁺] accumulation was observed in PGC-1α OE following contractions and thapsigargin treatment. In the SR, PGC-1α OE downregulated SR Ca²⁺-ATPase 1 (Ca²⁺ uptake) and parvalbumin (Ca²⁺ buffering) protein levels, whereas mitochondrial Ca²⁺ uptake-related proteins (Mfn1, Mfn2, and mitochondrial Ca²⁺ uniporter) were upregulated. These data demonstrate a heretofore unappreciated role for skeletal muscle mitochondria in [Ca²⁺]_i regulation in vivo following fatiguing tetanic contractions and at rest.

In vivo Ca2+ dynamics induced by Ca2+ injection in individual rat skeletal muscle fibers

In contrast to cardiomyocytes, store overload‐induced calcium ion (Ca²⁺) release (SOICR) is not considered to constitute a primary Ca²⁺ releasing system from the sarcoplasmic reticulum (SR) in skeletal muscle myocytes. In the latter, voltage‐induced Ca²⁺ release (VICR) is regarded as the dominant mechanism facilitating contractions. Any role of the SOICR in the regulation of cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) and its dynamics in skeletal muscle in vivo remains poorly understood. By means of in vivo single fiber Ca²⁺ microinjections combined with bioimaging techniques, we tested the hypothesis that the [Ca²⁺]_i dynamics following Ca²⁺ injection would be amplified and fiber contraction facilitated by SOICR. The circulation‐intact spinotrapezius muscle of adult male Wistar rats (n = 34) was exteriorized and loaded with Fura‐2 AM to monitor [Ca²⁺]_i dynamics. Groups of rats underwent the following treatments: (1) 0.02, 0.2, and 2.0 mmol/L Ca²⁺ injections, (2) 2.0 mmol/L Ca²⁺ with inhibition of ryanodine receptors (RyR) by dantrolene sodium (DAN), and (3) 2.0 mmol/L Ca²⁺ with inhibition of SR Ca²⁺ ATPase (SERCA) by cyclopiazonic acid (CPA). A quantity of 0.02 mmol/L Ca²⁺ injection yielded no detectable response, whereas peak evoked [Ca²⁺]_i increased 9.9 ± 1.8% above baseline for 0.2 mmol/L and 23.8 ± 4.3% (P < 0.05) for 2.0 mmol/L Ca²⁺ injections. The peak [Ca²⁺]_i in response to 2.0 mmol/L Ca²⁺ injection was largely abolished by DAN and CPA (−85.8%, −71.0%, respectively, both P < 0.05 vs. unblocked) supporting dependence of the [Ca²⁺]_i dynamics on Ca²⁺ released by SOICR rather than injected Ca²⁺ itself. Thus, this investigation demonstrates the presence of a robust SR‐evoked SOICR operant in skeletal muscle in vivo.

pH buffering of single rat skeletal muscle fibers in the in vivo environment

Homeostasis of intracellular pH (pHi) has a crucial role for the maintenance of cellular function. Several membrane transporters such as lactate/H(+) cotransporter (MCT), Na(+)/H(+) exchange transporter (NHE), and Na(+)/HCO3 (-) cotransporter (NBC) are thought to contribute to pHi regulation. However, the relative importance of each of these membrane transporters to the in vivo recovery from the low pHi condition is unknown. Using an in vivo bioimaging model, we pharmacologically inhibited each transporter separately and all transporters together and then evaluated the pHi recovery profiles following imposition of a discrete H(+) challenge loaded into single muscle fibers by microinjection. The intact spinotrapezius muscle of adult male Wistar rats (n = 72) was exteriorized and loaded with the fluorescent probe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-acetoxymethyl ester (10 μM). A single muscle fiber was then loaded with low-pH solution [piperazine-N,N'-bis(2-ethanesulfonic acid) buffer, pH 6.5, ∼2.33 × 10(-3) μl] by microinjection over 3 s. The rats were divided into groups for the following treatments: 1) no inhibitor (CONT), 2) MCT inhibition (by α-Cyano-4-hydroxyciannamic acid; 4 mM), 3) NHE inhibition (by ethylisopropyl amiloride; 0.5 mM), 4) NBC inhibition (by DIDS; 1 mM), and 5) MCT, NHE, and NBC inhibition (All blockade). The fluorescence ratio (F500 nm/F445 nm) was determined from images captured during 1 min (60 images/min) and at 5, 10, 15, and 20 min after injection. The pHi at 1-2 s after injection significantly decreased from resting pHi (ΔpHi = -0.73 ± 0.03) in CONT. The recovery response profile was biphasic, with an initial rapid and close-to-exponential pHi increase (time constant, τ: 60.0 ± 7.9 s). This initial rapid profile was not affected by any pharmacological blockade but was significantly delayed by carbonic anhydrase inhibition. In contrast, the secondary, more gradual, return toward baseline that restored CONT pHi to 84.2% of baseline was unimpeded by MCT, NHE, and NBC blockade separately but abolished by All blockade (ΔpHi = -0.60 ± 0.07, 72.8% initial pHi, P < 0.05 vs. CONT). After injection of H(+) into, or superfusion onto, an adjacent fiber pHi of the surrounding fibers decreased progressively for the 20-min observation period (∼7.0, P < 0.05 vs. preinjection/superfusion). In conclusion, these results support that, after an imposed H(+) load, the MCT, NHE, and NBC transporters are not involved in the initial rapid phase of pHi recovery. In contrast, the gradual recovery phase was abolished by inhibiting all three membrane transporter systems simultaneously. The alteration of pHi in surrounding fibers suggest that H(+) uptake by neighboring fibers can help alleviate the pH consequences of myocyte H(+) exudation.

Overload-induced skeletal muscle hypertrophy is not impaired in STZ-diabetic rats

The aim of this study was to evaluate the effect of overload-induced hypertrophy on extensor digitorum longus (EDL) and soleus muscles of streptozotocin-induced diabetic rats. The overload-induced hypertrophy and absolute tetanic and twitch forces increases in EDL and soleus muscles were not different between diabetic and control rats. Phospho-Akt and rpS6 contents were increased in EDL muscle after 7 days of overload and returned to the pre-overload values after 30 days. In the soleus muscle, the contents of total and phospho-Akt and total rpS6 were increased in both groups after 7 days. The contents of total Akt in controls and total rpS6 and phospho-Akt in the diabetic rats remained increased after 30 days. mRNA expression after 7 days of overload in the EDL muscle of control and diabetic animals showed an increase in MGF and follistatin and a decrease in myostatin and Axin2. The expression of FAK was increased and of MuRF-1 and atrogin-1 decreased only in the control group, whereas Ankrd2 expression was enhanced only in diabetic rats. In the soleus muscle caused similar changes in both groups: increase in FAK and MGF and decrease in Wnt7a, MuRF-1, atrogin-1, and myostatin. Differences between groups were observed only in the increased expression of follistatin in diabetic animals and decreased Ankrd2 expression in the control group. So, insulin deficiency does not impair the overload-induced hypertrophic response in soleus and EDL muscles. However, different mechanisms seem to be involved in the comparable hypertrophic responses of skeletal muscle in control and diabetic animals.

Increased intrinsic mitochondrial respiratory capacity in skeletal muscle from rats with streptozotocin-induced hyperglycemia

Type I diabetes mellitus (T1DM) is a chronic disorder, characterized by an almost or complete insulin deficiency. Widespread tissue dysfunction and deleterious diabetes-complications are associated with long-term elevations of blood glucose. The aim of this study was to investigate the effects of type I diabetes, as induced by streptozotocin, on the mitochondria in skeletal muscles that predominantly consist of either slow or fast twitch fibers. Soleus (primarily slow twitch fiber type) and the plantaris muscle (mainly fast twitch fiber type) were removed in order to measure mitochondrial protein expression and integrated mitochondrial respiratory function. Mitochondrial capacity for oxidative phosphorylation (OXPHOS) was found to be higher in the slow (more oxidative) soleus muscle from STZ rats when evaluating lipid and complex I linked OXPHOS capacity, whereas no difference was detected between the groups when evaluating the more physiological complex I and II linked OXPHOS capacity. These findings indicate that chronic hyperglycemia results in an elevated intrinsic mitochondrial respiratory capacity in both soleus and, at varying degree, plantaris muscle, findings that are consistent with human T1DM patients.

In vivo Ca2+ buffering capacity and microvascular oxygen pressures following muscle contractions in diabetic rat skeletal muscles: fiber-type specific effects

In Type 1 diabetes, skeletal muscle resting intracellular Ca2+ concentration ([Ca2+]i) homeostasis is impaired following muscle contractions. It is unclear to what degree this behavior is contingent upon fiber type and muscle oxygenation conditions. We tested the hypotheses that: 1) the rise in resting [Ca2+]i evident in diabetic rat slow-twitch (type I) muscle would be exacerbated in fast-twitch (type II) muscle following contraction; and 2) these elevated [Ca2+]i levels would relate to derangement of microvascular partial pressure of oxygen (PmvO2) rather than sarcoplasmic reticulum dysfunction per se. Adult male Wistar rats were divided randomly into diabetic (DIA: streptozotocin ip) and healthy (CONT) groups. Four weeks later extensor digitorum longus (EDL, predominately type II fibers) and soleus (SOL, predominately type I fibers) muscle contractions were elicited by continuous electrical stimulation (120 s, 100 Hz). Ca2+ imaging was achieved using fura 2-AM in vivo (i.e., circulation intact). DIA increased fatigability in EDL ( P < 0.05) but not SOL. In recovery, SOL [Ca2+]i either returned to its resting baseline within 150 s (CONT 1.00 ± 0.02 at 600 s) or was not elevated in recovery at all (DIA 1.03 ± 0.02 at 600 s, P > 0.05). In recovery, EDL CONT [Ca2+]i also decreased to values not different from baseline (1.06 ± 0.01, P > 0.05) at 600 s. In marked contrast, EDL DIA [Ca2+]i remained elevated for the entire recovery period (i.e., 1.23 ± 0.03 at 600 s, P < 0.05). The inability of [Ca2+]i to return to baseline in EDL DIA was not associated with any reduction of SR Ca2+-ATPase (SERCA) 1 or SERCA2 protein levels (both increased 30–40%, P < 0.05). However, PmvO2 recovery kinetics were markedly slowed in EDL such that mean PmvO2 was substantially depressed (CONT 27.9 ± 2.0 vs. DIA 18.4 ± 2.0 Torr, P < 0.05), and this behavior was associated with the elevated [Ca2+]i. In contrast, this was not the case for SOL ( P > 0.05) in that neither [Ca2+]i nor PmvO2 were deranged in recovery with DIA. In conclusion, recovery of [Ca2+]i homeostasis is impaired in diabetic rat fast-twitch but not slow-twitch muscle in concert with reduced PmvO2 pressures.

Mitochondrial succinate dehydrogenase is involved in stimulus-secretion coupling and endogenous ROS formation in murine beta cells

AIMS/HYPOTHESIS

Generation of reduction equivalents is a prerequisite for nutrient-stimulated insulin secretion. Mitochondrial succinate dehydrogenase (SDH) fulfils a dual function with respect to mitochondrial energy supply: (1) the enzyme is part of mitochondrial respiratory chains; and (2) it catalyses oxidation of succinate to fumarate in the Krebs cycle. The aim of our study was to elucidate the significance of SDH for beta cell stimulus-secretion coupling (SSC).

METHODS

Mitochondrial variables, reactive oxygen species (ROS) and cytosolic Ca(2+) concentration ([Ca(2+)]c) were measured by fluorescence techniques and insulin release by radioimmunoassay in islets or islet cells of C57Bl/6N mice.

RESULTS

Inhibition of SDH with 3-nitropropionic acid (3-NPA) or monoethyl fumarate (MEF) reduced glucose-stimulated insulin secretion. Inhibition of the ATP-sensitive K(+) channel (KATP channel) partly prevented this effect, whereas potentiation of antioxidant defence by superoxide dismutase mimetics (TEMPOL and mito-TEMPO) or by nuclear factor erythroid 2-related factor 2 (Nrf-2)-mediated upregulation of antioxidant enzymes (oltipraz, tert-butylhydroxyquinone) did not diminish the inhibitory influence of 3-NPA. Blocking SDH decreased glucose-stimulated increase in intracellular FADH2 concentration without alterations in NAD(P)H. In addition, 3-NPA and MEF drastically reduced glucose-induced hyperpolarisation of mitochondrial membrane potential, indicative of decreased ATP production. As a consequence, the glucose-stimulated rise in [Ca(2+)]c was significantly delayed and reduced. Acute application of 3-NPA interrupted glucose-driven oscillations of [Ca(2+)]c. 3-NPA per se did not elevate intracellular ROS, but instead prevented glucose-induced ROS accumulation.

CONCLUSIONS/INTERPRETATION

SDH is an important regulator of insulin secretion and ROS production. Inhibition of SDH interrupts membrane-potential-dependent SSC, pointing to a pivotal role of mitochondrial FAD/FADH2 homeostasis for the maintenance of glycaemic control.

O-GlcNAcylation, contractile protein modifications and calcium affinity in skeletal muscle

O-GlcNAcylation, a generally undermined atypical protein glycosylation process, is involved in a dynamic and highly regulated interplay with phosphorylation. Akin to phosphorylation, O-GlcNAcylation is also involved in the physiopathology of several acquired diseases, such as muscle insulin resistance or muscle atrophy. Recent data underline that the interplay between phosphorylation and O-GlcNAcylation acts as a modulator of skeletal muscle contractile activity. In particular, the O-GlcNAcylation level of the phosphoprotein myosin light chain 2 seems to be crucial in the modulation of the calcium activation properties, and should be responsible for changes in calcium properties observed in functional atrophy. Moreover, since several key structural proteins are O-GlcNAc-modified, and because of the localization of the enzymes involved in the O-GlcNAcylation/de-O-GlcNAcylation process to the nodal Z disk, a role of O-GlcNAcylation in the modulation of the sarcomeric structure should be considered.

In vivo calcium regulation in diabetic skeletal muscle

In skeletal muscle, dysfunctional contractile activity has been linked to impaired intracellular Ca(2+) concentration ([Ca(2+)]i) regulation. Muscle force production is impaired and fatigability and muscle fragility deteriorate with diabetes. Use of a novel in vivo model permits investigation of [Ca(2+)]i homeostasis in diabetic skeletal muscle. Within this in vivo environment we have shown that diabetes perturbs the Ca(2+) regulatory system such that resting [Ca(2+)]i homeostasis following muscle contractions is compromised and elevations of [Ca(2+)]i are exacerbated. This review considers the impact of diabetes on the capacity of skeletal muscle to regulate [Ca(2+)]i, following muscle contractions and, in particular, the relationship between muscle fatigue and elevated [Ca(2+)]i in a highly ecologically relevant circulation-intact environment. Importantly, the role of mitochondria in calcium sequestration and the possibility that diabetes impacts this process is explored. Given the profound microcirculatory dysfunction in diabetes this preparation offers the unique opportunity to study the interrelationships among microvascular function, blood-myocyte oxygen flux and [Ca(2+)]i as they relate to enhanced muscle fatigability and exercise intolerance.