Membrane Proteins Increase with the Repeated Bout Effect

Skeletal Muscle Heat Shock Protein Content and the Repeated Bout Effect

The "Repeated Bout Effect" (RBE) occurs when a skeletal muscle is preconditioned with a few lengthening contractions (LC) prior to exposing the muscle to a greater number of LC. The preconditioning (PC) results in significantly less damage and preservation of force. Since it takes only a few LC to increase muscle heat shock protein (HSP) content, it was of interest to examine the relationship between HSPs and the RBE. To do this, one tibialis anterior (TA) muscle from Sprague-Dawley rats (n = 5/group) was preconditioned with either 0, 5, or 15 lengthening contractions (LC) and exposed to a treatment of 60 LC 48 h later. Preconditioning TA muscles with 15 LC, but not 5 LC, significantly elevated muscle αB-crystallin (p < 0.05), HSP25 (p < 0.05), and HSP72 content (p < 0.001). These preconditioned TA muscles also showed a significantly (p < 0.05) reduced loss of active torque throughout the subsequent 60 LC. While there was a trend for all preconditioned muscles to maintain higher peak torque levels throughout the 60 LC, no significant differences were detected between the groups. Morphologically, preconditioned muscles appeared to show less discernible muscle fiber damage. In conclusion, an elevated skeletal muscle HSP content from preconditioning may contribute to the RBE.

Contralateral versus ipsilateral protective effect against muscle damage of the elbow flexors and knee extensors induced by maximal eccentric exercise

The present study compared the ipsilateral repeated bout effect (IL-RBE) and contralateral repeated bout effect (CL-RBE) of the elbow flexors (EF) and knee flexors (KF) for the same interval between bouts to shed light on their mechanisms. Fifty-two healthy sedentary young (20-28 years) men were randomly assigned to the IL-EF, IL-KF, CL-EF, and CL-KF groups (n = 13/group). Thirty maximal eccentric contractions of the EF were performed in IL-EF and CL-EF, and 60 maximal eccentric contractions of the KF were performed in IL-KF and CL-KF, with a 2-week interval between bouts. Changes in muscle damage markers such as maximal voluntary contraction (MVC) torque, muscle soreness, and plasma creatine kinase activity, and proprioception measures before to 5 days post-exercise were compared between groups. Changes in all variables were greater (p < 0.05) after the first than second bout for all groups, and the changes were greater (p < 0.05) for the EF than KF. The changes in all variables after the second bout were greater (p < 0.05) for the CL than IL condition for both EF and KF. The magnitude of the average protective effect was similar between CL-EF (33%) and CL-KF (32%), but slightly greater (p < 0.05) for IL-EF (67%) than IL-KF (61%). These demonstrate that the magnitude of CL-RBE relative to IL-RBE was similar between the EF and KF (approximately 50%), regardless of the greater muscle damage for the EF than KF. It appears that the CL-RBE is more associated with neural adaptations at cerebrum, cerebellum, interhemispheric inhibition, and coricospinal tract, but the IL-RBE is induced by additional adaptations at muscles.

RAD140 (Testolone) negatively impacts skeletal muscle adaptation, frailty status and mortality risk in female mice

RAD140 is a selective androgen receptor modulator that produces anabolic effects within skeletal muscle. Thus, RAD140 may be effective at treating sarcopenia. No long-term studies have investigated how RAD140 influences strength in ageing muscle. This study aimed to determine how 10 weeks of RAD140 supplementation impacts strength, recovery from exercise, and overall health in ageing mice. Young and adult females were assigned to receive RAD140 (5 mg/kg) or a control solution. Dorsiflexor muscles were exposed to repeated bouts of eccentric contractions, and torque was measured before and after each bout. Adaptive potential and strength gains were calculated to assess the efficacy of RAD140 in muscle, while frailty status and mortality risk were used to measure health span. Supplementation of RAD140 increased frailty status and mortality risk in the young and adult treated groups compared to the controls (p ≤ 0.042). RAD140 decreased adaptive potential in young (p = 0.040) but not adult mice (p = 0.688). Torque did not differ between groups after 2-3 weeks of recovery (p ≥ 0.135). In conclusion, long-term RAD140 supplementation reduced indices of overall health and failed to improve strength in female mice, suggesting that RAD140 (at a 5mg/kg dosage) may be more detrimental than beneficial in delaying or preventing sarcopenia.

Adaptability to eccentric exercise training is diminished with age in female mice

The ability of skeletal muscle to adapt to eccentric contractions has been suggested to be blunted in older muscle. If eccentric exercise is to be a safe and efficient training mode for older adults, preclinical studies need to establish if older muscle can effectively adapt and if not, determine the molecular signatures that are causing this impairment. The purpose of this study was to quantify the extent age impacts functional adaptations of muscle and identify genetic signatures associated with adaptation (or lack thereof). The anterior crural muscles of young (4 mo) and older (28 mo) female mice performed repeated bouts of eccentric contractions in vivo (50 contractions/wk for 5 wk) and isometric torque was measured across the initial and final bouts. Transcriptomics was completed by RNA-sequencing 1 wk following the fifth bout to identify common and differentially regulated genes. When torques post eccentric contractions were compared after the first and fifth bouts, young muscle exhibited a robust ability to adapt, increasing isometric torque 20%-36%, whereas isometric torque of older muscle decreased up to 18% (P ≤ 0.047). Using differential gene expression, young and older muscles shared some common transcriptional changes in response to eccentric exercise training, whereas other transcripts appeared to be age dependent. That is, the ability to express particular genes after repeated bouts of eccentric contractions was not the same between ages. These molecular signatures may reveal, in part, why older muscles do not appear to be as adaptive to exercise training as young muscles.NEW & NOTEWORTHY The ability to adapt to exercise training may help prevent and combat sarcopenia. Here, we demonstrate young mouse muscles get stronger whereas older mouse muscles become weaker after repeated bouts of eccentric contractions, and that numerous genes were differentially expressed between age groups following training. These results highlight that molecular and functional plasticity is not fixed in skeletal muscle with advancing age, and the ability to handle or cope with physical stress may be impaired.

Longitudinal assessment of strength and body composition in a mouse model of chronic alcohol-related myopathy

BACKGROUND

Excessive, chronic alcohol consumption can result in muscle atrophy and weakness (i.e., alcoholic myopathy) that impairs the quality of life. However, the precise mechanisms responsible for ethanol's detrimental impact on skeletal muscle have not been fully elucidated, in part due because the time course of disease development and progression are not well established. Therefore, we examined muscle strength and body composition longitudinally using an established preclinical mouse model of chronic alcoholic myopathy.

METHODS

To establish a time course of chronic alcoholic myopathy, we fed High Drinking in the Dark (HDID) female mice (n = 7) 20% ethanol for ~32 weeks (following a 2-week ethanol ramping period). We assessed in vivo isometric contractility of the left ankle dorsiflexor and lean mass via NMR every 4 weeks. Outcomes were compared with age-matched control HDID mice that did not consume ethanol (n = 8).

RESULTS

At study completion, mice who consumed ethanol were 12% weaker than control mice (p = 0.015). Compared to baseline, consuming ethanol resulted in an acute transient reduction in dorsiflexion torque at Week 4 (p = 0.032) that was followed by a second, more sustained reduction at Week 20 (p < 0.001). Changes in lean mass paralleled those of dorsiflexor torque, with ~40% of the variance in dorsiflexor torque being explained by the variance in lean mass of the ethanol group (p < 0.001). Dorsiflexor torque normalized to lean mass (mN·m/g lean mass) did not differ between the ethanol and control groups from Weeks 4 to 32 (p ≥ 0.498).

CONCLUSIONS

These results indicate that reductions in muscle mass and strength due to chronic, excessive ethanol intake are dynamic, not necessarily linear, processes. Moreover, the findings confirm that ethanol-induced weakness is primarily driven by muscle atrophy (i.e., loss of muscle quantity). Future studies should consider how chronic alcoholic myopathy develops and progresses rather than identifying changes after it has been diagnosed.

Excessive Ethanol Intake in Mice Does Not Impair Recovery of Torque after Repeated Bouts of Eccentric Contractions

PURPOSE

Alcoholics develop muscle atrophy and weakness from excessive ethanol (EtOH) intake. To date, most research has examined outcomes of alcohol-induced atrophy and weakness under basal or unstressed conditions despite physical stress being a normal occurrence in a physiological setting. Therefore, this study set out to determine if recovery of torque is impaired after repetitive bouts of physical stress in skeletal muscle during excessive short-term (experiment 1) and long-term (experiment 2) EtOH consumption.

METHODS

Twenty male and female mice were assigned to receive either 20% EtOH in their drinking water or 100% water. Short- and long-term consumption was predetermined to be EtOH intake starting at 4 and 26 wk, respectively. Anterior crural muscles performed repeated bouts of physical stress using in vivo eccentric contractions, with tetanic isometric torque being measured immediately pre- and postinjury. A total of 10 bouts were completed with 14 d between each bout within bouts 1-5 (experiment 1) and bouts 6-10 (experiment 2), and 12 wk between bouts 5 and 6.

RESULTS

Mice consuming EtOH had blood alcohol concentrations up to 270 mg·dL -1 . In experiment 1, five bouts of eccentric contractions did not reduce recovery of torque, regardless of sex or EtOH treatment ( P ≥ 0.173). Similarly, in experiment 2, preinjury torques did not differ from day 14 values regardless of sex or treatment ( P ≥ 0.322). However, there was a group effect in female mice for bouts 6 and 10 during experiment 2, with female EtOH mice being weaker than controls ( P ≤ 0.002).

CONCLUSIONS

Excessive short- or long-term EtOH misuse in a mouse model did not affect the muscle's ability to regain strength after repeated bouts of eccentric contractions, suggesting that EtOH may not be as detrimental to recovery as once predicted.

Changes in Insulin Sensitivity and Lipid Profile Markers Following Initial and Secondary Bouts of Multiple Eccentric Exercises

An acute bout of eccentric exercise affects insulin sensitivity and lipid profile, but how the magnitude of muscle damage affects them is not clear. We compared changes in blood insulin sensitivity and lipid markers after the first (EC1) and second (EC2) eccentric exercise bouts. Fifteen sedentary young men performed arm, leg and trunk muscle eccentric exercises, and repeated them 2 weeks later. Fasting blood samples were taken before, 2 h and 1–5 days after each exercise bout to analyze plasma creatine kinase (CK) activity, serum glucose (GLU), insulin, homeostasis model assessment (HOMA), triacylglycerols (TG), total (TC) and low- (LDLC) and high-density lipoprotein cholesterol (HDLC) concentrations as well as TC/HDLC ratio. Changes in these measures were compared between bouts and relationships to peak plasma CK activity were analyzed. Plasma CK activity increased (p < 0.05) after EC1 (peak: 101,668 ± 58,955 IU/L) but not after EC2. The magnitude of changes in GLU (peak after EC1: 26 ± 10% vs. EC2: 7 ± 6%), insulin (46 ± 27% vs. 15 ± 8%), HOMA (86 ± 48% vs. 24 ± 15%), TC (−20 ± 5% vs. −6 ± 4%), TG (−32 ± 11% vs. −6 ± 3%), LDHC (−47 ± 15% vs. −12 ± 9%), HDLC (35 ± 26% vs. 7 ± 4%), and TC/HDLC ratio (−139 ± 13% vs. −11 ± 7%) were significantly greater after EC1 than EC2. Peak plasma CK activity was significantly (p < 0.05) correlated with the peak changes in blood insulin sensitivity and lipid markers for the combined data of EC1 and EC2. These results suggest that the greater the magnitude of muscle damage, the greater the magnitude of changes in the insulin sensitivity to a negative direction and lipid markers to a positive direction.

Contraction-Induced Loss of Plasmalemmal Electrophysiological Function Is Dependent on the Dystrophin Glycoprotein Complex

Weakness and atrophy are key features of Duchenne muscular dystrophy (DMD). Dystrophin is one of the many proteins within the dystrophin glycoprotein complex (DGC) that maintains plasmalemmal integrity and cellular homeostasis. The dystrophin-deficient mdx mouse is also predisposed to weakness, particularly when subjected to eccentric (ECC) contractions due to electrophysiological dysfunction of the plasmalemma. Here, we determined if maintenance of plasmalemmal excitability during and after a bout of ECC contractions is dependent on intact and functional DGCs rather than, solely, dystrophin expression. Wild-type (WT) and dystrophic mice (mdx, mL172H and Sgcb-/- mimicking Duchenne, Becker and Limb-girdle Type 2E muscular dystrophies, respectively) with varying levels of dystrophin and DGC functionality performed 50 maximal ECC contractions with simultaneous torque and electromyographic measurements (M-wave root-mean-square, M-wave RMS). ECC contractions caused all mouse lines to lose torque (p<0.001); however, deficits were greater in dystrophic mouse lines compared to WT mice (p<0.001). Loss of ECC torque did not correspond to a reduction in M-wave RMS in WT mice (p=0.080), while deficits in M-wave RMS exceeded 50% in all dystrophic mouse lines (p≤0.007). Moreover, reductions in ECC torque and M-wave RMS were greater in mdx mice compared to mL172H mice (p≤0.042). No differences were observed between mdx and Sgcb-/- mice (p≥0.337). Regression analysis revealed ≥98% of the variance in ECC torque loss could be explained by the variance in M-wave RMS in dystrophic mouse lines (p<0.001) but not within WT mice (R 2=0.211; p=0.155). By comparing mouse lines that had varying amounts and functionality of dystrophin and other DGC proteins, we observed that (1) when all DGCs are intact, plasmalemmal action potential generation and conduction is maintained, (2) deficiency of the DGC protein β-sarcoglycan is as disruptive to plasmalemmal excitability as is dystrophin deficiency and, (3) some functionally intact DGCs are better than none. Our results highlight the significant role of the DGC plays in maintaining plasmalemmal excitability and that a collective synergism (via each DGC protein) is required for this complex to function properly during ECC contractions.