Single muscle fiber adaptations with marathon training.

The purpose of this investigation was to characterize the effects of marathon training on single muscle fiber contractile function in a group of recreational runners. Muscle biopsies were obtained from the gastrocnemius muscle of seven individuals (22 +/- 1 yr, 177 +/- 3 cm, and 68 +/- 2 kg) before, after 13 wk of run training, and after 3 wk of taper. Slow-twitch myosin heavy chain [(MHC) I] and fast-twitch (MHC IIa) muscle fibers were analyzed for size, strength (P(o)), speed (V(o)), and power. The run training program led to the successful completion of a marathon (range 3 h 56 min to 5 h 35 min). Oxygen uptake during submaximal running and citrate synthase activity were improved (P < 0.05) with the training program. Muscle fiber size declined (P < 0.05) by approximately 20% in both fiber types after training. P(o) was maintained in both fiber types with training and increased (P < 0.05) by 18% in the MHC IIa fibers after taper. This resulted in >60% increase (P < 0.05) in force per cross-sectional area in both fiber types. Fiber V(o) increased (P < 0.05) by 28% in MHC I fibers with training and was unchanged in MHC IIa fibers. Peak power increased (P < 0.05) in MHC I and IIa fibers after training with a further increase (P < 0.05) in MHC IIa fiber power after taper. These data show that marathon training decreased slow-twitch and fast-twitch muscle fiber size but that it maintained or improved the functional profile of these fibers. A taper period before the marathon further improved the functional profile of the muscle, which was targeted to the fast-twitch muscle fibers.     

Effect of resistance training on single muscle fiber contractile function in older men.

The purpose of this study was to examine single cell contractile mechanics of skeletal muscle before and after 12 wk of progressive resistance training (PRT) in older men (n = 7; age = 74 +/- 2 yr and weight = 75 +/- 5 kg). Knee extensor PRT was performed 3 days/wk at 80% of one-repetition maximum. Muscle biopsy samples were obtained from the vastus lateralis before and after PRT (pre- and post-PRT, respectively). For analysis, chemically skinned single muscle fibers were studied at 15 degrees C for peak tension [the maximal isometric force (P(o))], unloaded shortening velocity (V(o)), and force-velocity parameters. In this study, a total of 199 (89 pre- and 110 post-PRT) myosin heavy chain (MHC) I and 99 (55 pre- and 44 post-PRT) MHC IIa fibers were reported. Because of the minimal number of hybrid fibers identified post-PRT, direct comparisons were limited to MHC I and IIa fibers. Muscle fiber diameter increased 20% (83 +/- 1 to 100 +/- 1 microm) and 13% (86 +/- 1 to 97 +/- 2 microm) in MHC I and IIa fibers, respectively (P < 0.05). P(o) was higher (P < 0.05) in MHC I (0.58 +/- 0.02 to 0.90 +/- 0.02 mN) and IIa (0.68 +/- 0.02 to 0.85 +/- 0.03 mN) fibers. Muscle fiber V(o) was elevated 75% (MHC I) and 45% (MHC IIa) after PRT (P < 0.05). MHC I and IIa fiber power increased (P < 0.05) from 7.7 +/- 0.5 to 17.6 +/- 0.9 microN. fiber lengths. s(-1) and from 25.5 to 41.1 microN. fiber lengths. s(-1), respectively. These data indicate that PRT in elderly men increases muscle cell size, strength, contractile velocity, and power in both slow- and fast-twitch muscle fibers. However, it appears that these changes are more pronounced in the MHC I muscle fibers.     

Single muscle fiber contractile properties during a competitive season in male runners

The purpose of this investigation was to examine the contractile properties of individual myofibers in response to periodized training periods throughout a collegiate cross-country season in male runners. Muscle biopsies of the gastrocnemius were taken after a summer base training phase (T1), an 8-wk intense training period (T2), and a 4-wk taper phase (T3). Five runners (n = 5; age = 20 +/- 1 yr; wt = 65 +/- 4 kg; ht = 178 +/- 3 cm) completed all three time points. A total of 328 individual muscle fibers [myosin heavy chain (MHC) I = 66%; MHC IIa = 33%; hybrids = 1%] were isolated and studied at 15 degrees C for their contractile properties. Diameter of MHC I fibers was 3% smaller (P < 0.05) at T2 compared with T1 and an additional 4% smaller (P < 0.05) after the taper. Cell size was unaltered in the MHC IIa fibers. MHC I and IIa fiber strength increased 18 and 11% (P < 0.05), respectively, from T1 to T2. MHC I fibers produced 9% less force (P < 0.05) after the taper, whereas MHC IIa fibers were 9% stronger (P < 0.05). Specific tension increased 38 and 26% (P < 0.05) for MHC I and IIa fibers, respectively, from T1 to T2 and was unchanged with the taper. Maximal shortening velocity (Vo) of the MHC I fibers decreased 23% (P < 0.05) from T1 to T2 and 17% (P < 0.05) from T2 to T3, whereas MHC IIa Vo was unchanged. MHC I peak power decreased 20% (P < 0.05) from T1 to T2 and 25% (P < 0.05) from T2 to T3, whereas MHC IIa peak power was unchanged. Power corrected for cell size decreased 15% (P < 0.05) from T2 to T3 and was 24% (P < 0.05) lower at T3 compared with T1 for the MHC I fibers only. These data suggest that changes in run training alter myocellular physiology via decreases in fiber size, Vo, and power of MHC I fibers and through increases in force per cross-sectional area of slow- and fast-twitch muscle fibers.     

Single muscle fiber contractile properties of young competitive distance runners.

The purpose of this investigation was to characterize the contractile properties of individual slow- and fast-twitch myofibers from highly trained distance runners. Muscle biopsies were obtained from the gastrocnemius of eight competitive runners (Run) and eight recreationally active individuals (Rec). Slow-twitch [myosin heavy chain (MHC) I] and fast-twitch (MHC IIa) myofibers were isolated and analyzed for diameter (microm), peak force (Po; mN), unloaded contraction velocity (Vo; fiber lengths/s), and power. Maximum oxygen uptake was higher (P<0.05) in Run (71+/-1 vs. 47+/-2 ml.kg(-1).min(-1)). Diameter of MHC I and MHC IIa fibers from Run subjects was approximately 20% greater (P<0.05) than Rec. Peak force of the MHC IIa fibers was 31% higher (P<0.05) in Run, whereas Po of MHC I fibers was not different between groups. No differences for specific tension (Po/cross-sectional area) were present between groups for either fiber type. Vo was higher (P<0.05) in MHC I (+70%) and MHC IIa (+18%) fibers from Run subjects. In vitro peak absolute power (microN.s(-1)) of both fiber types was greater (P<0.05) in Run (131 and 85% for MHC I and MHC IIa, respectively). Additionally, normalized power (W/l) of the MHC I fibers was 64% higher in Run, whereas no differences were noted for normalized power of MHC IIa fibers. These data indicate that highly trained endurance runners have elevated contraction velocity in both slow- and fast-twitch myofibers. These characteristics of the fast-twitch muscle fibers have not been previously reported in competitive endurance athletes and may contribute to the high level of running performance in these athletes.     

Functional properties of slow and fast gastrocnemius muscle fibers after a 17-day spaceflight.

The purpose of this investigation was to study the effects of a 17-day spaceflight on the contractile properties of individual fast- and slow-twitch fibers isolated from biopsies of the fast-twitch gastrocnemius muscle of four male astronauts. Single chemically skinned fibers were studied during maximal Ca2+-activated contractions with fiber myosin heavy chain (MHC) isoform expression subsequently determined by SDS gel electrophoresis. Spaceflight had no significant effect on the mean diameter or specific force of single fibers expressing type I, IIa, or IIa/IIx MHC, although a small reduction in average absolute force (P(o)) was observed for the type I fibers (0.68 +/- 0.02 vs. 0.64 +/- 0.02 mN, P < 0.05). Subject-by-flight interactions indicated significant intersubject variation in response to the flight, as postflight fiber diameter and P(o) where significantly reduced for the type I and IIa fibers obtained from one astronaut and for the type IIa fibers from another astronaut. Average unloaded shortening velocity [V(o), in fiber lengths (FL)/s] was greater after the flight for both type I (0.60 +/- 0.03 vs. 0.76 +/- 0.02 FL/s) and IIa fibers (2.33 +/- 0.25 vs. 3.10 +/- 0.16 FL/s). Postflight peak power of the type I and IIa fibers was significantly reduced only for the astronaut experiencing the greatest fiber atrophy and loss of P(o). These results demonstrate that 1) slow and fast gastrocnemius fibers show little atrophy and loss of P(o) but increased V(o) after a typical 17-day spaceflight, 2) there is, however, considerable intersubject variation in these responses, possibly due to intersubject differences in in-flight physical activity, and 3) in these four astronauts, fiber atrophy and reductions in P(o) were less for slow and fast fibers obtained from the phasic fast-twitch gastrocnemius muscle compared with slow and fast fibers obtained from the slow antigravity soleus [J. J. Widrick, S. K. Knuth, K. M. Norenberg, J. G. Romatowski, J. L. W. Bain, D. A. Riley, M. Karhanek, S. W. Trappe, T. A. Trappe, D. L. Costill, and R. H. Fitts. J Physiol (Lond) 516: 915-930, 1999].     

Human vastus lateralis and soleus muscles display divergent cellular contractile properties

The purpose of this study was to investigate potential differences in single-fiber contractile physiology of fibers with the same myosin heavy chain isoform (MHC I and MHC IIa) originating from different muscles. Vastus lateralis (VL) and soleus biopsies were obtained from 27 recreationally active females (31 +/- 1 yr, 59 +/- 1 kg). A total of 943 single fibers (MHC I = 562; MHC IIa = 301) were isolated and examined for diameter, peak tension (Po), shortening velocity (Vo), and power. The soleus had larger (P < 0.05) fibers (MHC I +18%; MHC IIa +19%), higher MHC I Vo (+13%), and higher MHC I Po (+18%) compared with fibers from the VL. In contrast, fibers from the VL had higher (P < 0.05) specific tension (MHC I +18%; MHC IIa +20%), and MHC I normalized power (+25%) compared with the soleus. There was a trend for MHC IIa soleus fibers to have higher Vo [MHC IIa +13% (P = 0.058)], whereas VL MHC IIa fibers showed a trend for higher normalized power compared with soleus fibers [MHC IIa +33% (P = 0.079)]. No differences in absolute power were detected between muscles. These data highlight muscle-specific differences in single-fiber contractile function that should serve as a scientific basis for consideration when extending observations of skeletal muscle tissue from one muscle of interest to other muscles of origin. This is important when examining skeletal muscle adaptation to physical states such as aging, unloading, and training.     

Reduction in hybrid single muscle fiber proportions with resistance training in humans.

The purpose of this investigation was to examine the effects of 12 wk of progressive resistance training (PRT) on single muscle fiber myosin heavy chain (MHC; I, I/IIa, I/IIa/IIx, IIa, IIa/IIx, IIx) isoform proportions in young individuals. Young, untrained men (YM; n = 6) and women (YW; n = 6) (age = 22 +/- 1 and 25 +/- 2 yr for YW and YM, respectively) received pre- and post-PRT muscle biopsies from the right vastus lateralis for single muscle fiber MHC distribution by electrophoretic analysis (192 +/- 5 pre- and 183 +/- 6 post-fibers/subject analyzed; 4,495 fibers total). Data are presented as percentages of the total fibers analyzed per subject. The PRT protocol elicited an increase in the pure MHC IIa (Delta = + 24 and + 27; YW and YM, respectively; P < 0.05) with no change in the pure MHC I distribution. The hybrid MHC distributions decreased I/IIa/IIx (Delta = -2; YM and YW; P < 0.05), IIa/IIx (Delta = -13 and -19 for YM and YW, respectively; P < 0.05), and total hybrid fiber proportion (I/IIa + I/IIa/IIx + IIa/IIx) decreased (Delta = -19 and -30 for YM and YW, respectively; P < 0.05) with the training, as did the MHC IIx distribution (Delta = -2; YW only; P < 0.05). Alterations in the predominance of MHC isoforms within hybrid fibers (decrease in MHC I-dominant I/IIa and nondominant MHC IIa/IIx, increase in MHC IIa-dominant IIa/IIx; P < 0.05) appeared to contribute to the increase in the MHC IIa proportion. Electrophoresis of muscle cross sections revealed an approximately 7% increase (P < 0.05) in MHC IIa proportion in both groups, whereas the MHC IIx decrease by 7.5 and 11.6% post-PRT in YW and YM, respectively. MHC I proportions increase in YM by 4.8% (P < 0.05) post-PRT. These findings further support previous resistance training data in young adults with respect to the increase in the MHC IIa proportions but demonstrate that a majority of the change can be attributed to the decrease in single-fiber hybrid proportions.     

Functional properties of human muscle fibers after short-term resistance exercise training

The aim of this study was to assess the relationships between human muscle fiber hypertrophy, protein isoform content, and maximal Ca(2+)-activated contractile function following a short-term period of resistance exercise training. Six male subjects (age 27 +/- 2 yr) participated in a 12-wk progressive resistance exercise training program that increased voluntary lower limb extension strength by >60%. Single chemically skinned fibers were prepared from pre- and posttraining vastus lateralis muscle biopsies. Training increased the cross-sectional area (CSA) and peak Ca(2+)-activated force (P(o)) of fibers containing type I, IIa, or IIa/IIx myosin heavy chain by 30-40% without affecting fiber-specific force (P(o)/CSA) or unloaded shortening velocity (V(o)). Absolute fiber peak power rose as a result of the increase in P(o), whereas power normalized to fiber volume was unchanged. At the level of the cross bridge, the effects of short-term resistance training were quantitative (fiber hypertrophy and proportional increases in fiber P(o) and absolute power) rather than qualitative (no change in P(o)/CSA, V(o), or power/fiber volume).     

Distribution of myosin heavy chain isoforms in non-weight-bearing rat soleus muscle fibers

Talmadge, Robert J., Roland R. Roy, and V. Reggie Edgerton.Distribution of myosin heavy chain isoforms in non-weight-bearing rat soleus muscle fibers. J. Appl.Physiol. 81(6): 2540-2546, 1996.The effects of14 days of spaceflight (SF) or hindlimb suspension (HS) (Cosmos 2044)on myosin heavy chain (MHC) isoform content of the rat soleus muscleand single muscle fibers were determined. On the basis ofelectrophoretic analyses, there was a de novo synthesis of type IIx MHCbut no change in either type I or IIa MHC isoform proportions aftereither SF or HS compared with controls. The percentage of fiberscontaining only type I MHC decreased by 26 and 23%, and the percentageof fibers with multiple MHCs increased from 6% in controls to 32% inHS and 34% in SF rats. Type IIx MHC was always found in combinationwith another MHC or combination of MHCs; i.e., no fibers contained typeIIx MHC exclusively. These data suggest that the expression of thenormal complement of MHC isoforms in the adult rat soleus muscle isdependent, in part, on normal weight bearing and that the absence ofweight bearing induces a shift toward type IIx MHC protein expression in the preexisting type I and IIa fibers of the soleus.

     

Effect of long-term muscle paralysis on human single fiber mechanics.

This study compared human muscles following long-term reduced neuromuscular activity to those with normal functioning regarding single fiber properties. Biopsies were obtained from the vastus lateralis of 5 individuals with chronic (>3 yr) spinal cord injury (SCI) and 10 able-bodied controls (CTRL). Chemically skinned fibers were tested for active and passive mechanical characteristics and subsequently classified according to myosin heavy chain (MHC) content. SCI individuals had smaller proportions of type I (11 +/- 7 vs. 34 +/- 5%) and IIa fibers (11 +/- 6 vs. 31 +/- 5%), whereas type IIx fibers were more frequent (40 +/- 13 vs. 7 +/- 3%) compared with CTRL subjects (P < 0.05). Cross-sectional area and peak force were similar in both groups for all fiber types. Unloaded shortening velocity of fibers from paralyzed muscles was higher in type IIa, IIa/IIx, and IIx fibers (26, 65, and 47%, respectively; P < 0.01). Consequently, absolute peak power was greater in type IIa (46%; P < 0.05) and IIa/IIx fibers (118%; P < 0.01) of the SCI group, whereas normalized peak power was higher in type IIa/IIx fibers (71%; P < 0.001). Ca(2+) sensitivity and passive fiber characteristics were not different between the two groups in any fiber type. Composite values (average value across all fibers analyzed within each study participant) showed similar results for cross-sectional area and peak force, whereas maximal contraction velocity and fiber power were more than 100% greater in SCI individuals. These data illustrate that contractile performance is preserved or even higher in the remaining fibers of human muscles following reduced neuromuscular activity.     

Contractile function of single muscle fibers after hindlimb unweighting in aged rats

Thompson, L. V., and J. A. Shoeman. Contractilefunction of single muscle fibers after hindlimb unweighting in aged rats. J. Appl. Physiol. 84(1):229-235, 1998.This investigation determined how muscle atrophyproduced by hindlimb unweighting (HU) alters the contractile functionof single muscle fibers from older animals (30 mo). After 1 wk of HU,small bundles of fibers were isolated from the soleus muscles and thedeep region of the lateral head of the gastrocnemius muscles. Singleglycerinated fibers were suspended between a motor lever and forcetransducer, functional properties were studied, and the myosin heavychain (MHC) composition was determined electrophoretically. After HU, the diameter of type I MHC fibers of the soleus declined (88 ± 2 vs. 80 ± 4 µm) and reductions were observed in peak active force (47 ± 3 vs. 28 ± 3 mg) and peak specific tension(P_o; 80 ± 5 vs. 56 ± 5 kN/m²). The maximal unloadedshortening velocity increased. The type I MHC fibers from thegastrocnemius showed reductions in diameter (14%), peak active force(41%), and P_o (24%), whereas thetype IIa MHC fibers showed reductions in peak active force andP_o. Thus 1 wk ofinactivity has a significant effect on the force-generating capacity ofsingle skeletal muscle fibers from older animals in a fibertype-specific manner (type I MHC > type IIa MHC > type I-IIa MHC).The decline in the functional properties of single skeletal musclefibers in the older animals appears to be more pronounced than what hasbeen reported in younger animal populations.

     

Progressive resistance training reduces myosin heavy chain coexpression in single muscle fibers from older men.

The purpose of this study was to examine myosin heavy chain (MHC) and myosin light chain (MLC) isoforms following 12 wk of progressive resistance training (PRT). A needle biopsy was taken from the vastus lateralis to determine fiber-type expression [ATPase (pH 4.54) and MHC/MLC] in seven healthy men (age = 74.0 +/- 1.8 yr). Subjects were also tested for 1-repetition maximum (1-RM), pre- and posttraining. The progressive knee extensor protocol consisted of three sets at 80% of 1-RM 3 days/wk for 12 wk. Freeze-dried, single muscle fibers were dissected for MHC and MLC analysis and then subjected to SDS-PAGE and silver staining, pre- and posttraining. MHC expression increased in the I (10.4%; P < 0.05) and decreased in I/IIa (9.0%; P < 0.05), I/IIa/x (0.9%; P < 0.05), and IIa/x (8.9%; P < 0.05) isoforms, with no change in the IIa and IIx isoforms, pre- vs. posttraining (total fibers = 3,059). The MLC(3f)-to-MLC(2) ratio did not change with the PRT in either the MHC I or MHC IIa isoforms (total fibers = 902), pre- to posttraining. ATPase fiber distribution did not significantly differ following training (I: 50. 4 +/- 6.7 vs. 51.9 +/- 7.9, IIa: 36.8 +/- 5.3 vs. 41.1 +/- 7.0, IIb: 12.8 +/- 5.6 vs. 7.0 +/- 4.0%; pre- vs. posttraining, respectively). 1-RM increased (51.9%; P < 0.05) from pre- to posttraining. The PRT provide a stimulus for alterations in MHC isoforms, which demonstrated a decrease in all hybrid isoforms and an increase in MHC I expression (not found in the ATPase results), unlike the MLC ratio (3:2), which was not altered with training.     

Enzyme‐ and immunohistochemical aspects of skeletal muscle fibers in brown bear (Ursus arctos)

To further elucidate the pattern of MHC isoform expression in skeletal muscles of large mammals, in this study the skeletal muscles of brown bear, one of the largest mammalian predators with an extraordinary locomotor capacity, were analyzed. Fiber types in longissimus dorsi, triceps brachii caput longum, and rectus femoris muscles were determined according to the myofibrillar ATPase (mATPase) histochemistry and MHC isoform expression, revealed by a set of antibodies specific to MHC isoforms. The oxidative (SDH) and glycolytic enzyme (α‐GPDH) capacity of fibers was demonstrated as well. By mATPase histochemistry five fiber types, i.e., I, IIC, IIA, IIAX, IIX were distinguished. Analyzing the MHC isoform expression, we assume that MHC‐I, ‐IIa, and ‐IIx are expressed in the muscles of adolescent bears. MHC‐I isoform was expressed in Type‐I fibers and coexpressed with presumably ‐IIa isoform, in Type‐IIC fibers. Surprisingly, two antibodies specific to rat MHC‐IIa stained those fast fibers, that were histochemically and immunohistochemically classified as Type IIX. This assumption was additionally confirmed by complete absence of fiber staining with antibody specific to rat MHC‐IIb and all fast fiber staining with antibody that according to our experience recognizes MHC‐IIa and ‐IIx of rat. Furthermore, quite high‐oxidative capacity of all fast fiber types and their weak glycolytic capacity also imply for MHC‐IIa and ‐IIx isoform expression in fast fibers of bear. However, in adult, full‐grown animal, only MHC‐I and MHC‐IIa isoforms were expressed. The expression of only two fast isoforms in bear, like in many other large mammals (humans, cat, dog, goat, cattle, and horse) obviously meets the weight‐bearing and locomotor demands of these mammals. J. Morphol., 2009. © 2008 Wiley‐Liss, Inc.     

Morphologic differences in skeletal muscle with age in normally active human males and their well-trained counterparts

In this study we elucidate the interaction of physical activity with aging as regards skeletal muscle fiber distribution and size. Thirty-three male athletes and 42 normally active counterparts served as subjects. They were assigned to younger (less than 25.5 years) and older (greater than 25.5 years) subgroups. Serial cross-sections from muscle biopsy samples (musculus vastus lateralis) were stained to distinguish fiber type: fast glycolytic (type IIb), fast oxidative-glycolytic (type IIa), or slow oxidative (type I). We also measured fiber diameters. A greater mean diameter of type I fibers was seen in older as opposed to younger athletes. Older controls had a smaller mean diameter of type IIb fibers than did younger controls. Athletes had a smaller mean percentage of type IIa fibers and a greater mean percentage of type I fibers than did controls. There was a greater mean percentage of type I fibers in older as opposed to younger controls, but this was not the case in athletes. Athletes may have larger fibers and a greater percentage of type I fibers at the expense of type IIa fibers. Atrophy of fibers with aging might be retarded by training, which might also reduce the age-associated rate of type IIb percentage loss and type I percentage gain.     

Fiber types in canine muscles: myosin isoform expression and functional characterization

This study was aimed to achieve a definitive and unambiguous identification of fiber types in canine skeletal muscles and of myosin isoforms that are expressed therein. Correspondence of canine myosin isoforms with orthologs in other species as assessed by base sequence comparison was the basis for primer preparation and for expression analysis with RT-PCR. Expression was confirmed at protein level with histochemistry, immunohistochemistry, and SDS-PAGE combined together and showed that limb and trunk muscles of the dog express myosin heavy chain (MHC) type 1, 2A, and 2X isoforms and the so-called "type 2dog" fibers express the MHC-2X isoform. MHC-2A was found to be the most abundant isoform in the trunk and limb muscle. MHC-2X was expressed in most but not all muscles and more frequently in hybrid 2A-2X fibers than in pure 2X fibers. MHC-2B was restricted to specialized extraocular and laryngeal muscles, although 2B mRNA, but not 2B protein, was occasionally detected in the semimembranosus muscle. Isometric tension (P(o)) and maximum shortening velocity (V(o)) were measured in single fibers classified on the basis of their MHC isoform composition. Purified myosin isoforms were extracted from single muscle fibers and characterized by the speed (V(f)) of actin filament sliding on myosin in an in vitro motility assay. A close proportionality between V(o) and V(f) indicated that the diversity in V(o) was due to the different myosin isoform composition. V(o) increased progressively in the order 1/slow < 2A < 2X < 2B, thus confirming the identification of the myosin isoforms and providing their first functional characterization of canine muscle fibers.     

Proteolytic mRNA expression in response to acute resistance exercise in human single skeletal muscle fibers.

The purpose of this study was to characterize changes in mRNA expression of select proteolytic markers in human slow-twitch [myosin heavy chain (MHC) I] and fast-twitch (MHC IIa) single skeletal muscle fibers following a bout of resistance exercise (RE). Muscle biopsies were obtained from the vastus lateralis of eight young healthy sedentary men [23 +/- 2 yr (mean +/- SD), 93 +/- 17 kg, 183 +/- 6 cm] before and 4 and 24 h after 3 x 10 repetitions of bilateral knee extensions at 65% of one repetition maximum. The mRNA levels of TNF-alpha, calpains 1 and 2, muscle RING (really interesting novel gene) finger-1 (MuRF-1), atrogin-1, caspase-3, B-cell leukemia/lymphoma (Bcl)-2, and Bcl-2-associated X protein (Bax) were quantified using real-time RT-PCR. Generally, MHC I fibers had higher (1.6- to 5.0-fold, P < 0.05) mRNA expression pre- and post-RE. One exception was a higher (1.6- to 3.9-fold, P < 0.05) Bax-to-Bcl-2 mRNA ratio in MHC IIa fibers pre- and post-RE. RE increased (1.4- to 4.8-fold, P < 0.05) MuRF-1 and caspase-3 mRNA levels 4-24 h post-RE in both fiber types, whereas Bax-to-Bcl-2 mRNA ratio increased 2.2-fold (P < 0.05) at 4 h post-RE only in MHC I fibers. These results suggest that MHC I fibers have a greater proteolytic mRNA expression pre- and post-RE compared with MHC IIa fibers. The greatest mRNA induction following RE was in MuRF-1 and caspase-3 in both fiber types. This altered and specific proteolytic mRNA expression among slow- and fast-twitch muscle fibers indicates that the ubiquitin/proteasomal and caspase pathways may play an important role in muscle remodeling with RE.     

Do skeletal muscle phenotypic characteristics of Xhosa and Caucasian endurance runners differ when matched for training and racing distances?

Although East African black athletes dominate endurance running events, it is unknown whether black and white endurance runners with similar racing ability, matched for training, may differ in their skeletal muscle biochemical phenotype. Thirteen Xhosa (XR) and 13 Caucasian (CR) endurance runners were recruited and matched for 10-km performance, average preferred racing distance (PRD(A)), and training volume. Submaximal and maximal exercise tests were done, and vastus lateralis muscle biopsies were taken. XR were significantly lighter and shorter than CR athletes but had similar maximum oxygen consumption corrected for body weight and peak treadmill speed (PTS). XR had lower plasma lactate concentrations at 80% PTS (P < 0.05) compared with CR. Also, XR had more type IIA (42.4 +/- 9.2 vs. 31.3 +/- 11.5%, P < 0.05) and less type I fibers (47.8 +/- 10.9 vs. 63.1 +/- 13.2%, P < 0.05), although oxidative enzyme activities did not differ. Furthermore, XR compared with CR had higher lactate dehydrogenase (LDH) activity in homogenate muscle samples (383 +/- 99 vs. 229 +/- 85 mumol.min(-1).g dry weight(-1), P < 0.05) and in both type IIa (P < 0.05) and type I (P = 0.05) single-fiber pools. A marked difference (P < 0.05) in the composition of LDH isoform content was found between the two groups with XR having higher levels of LDH(5-4) isoforms (skeletal muscle isozymes; LDH-M) than CR, which was not accounted for by fiber-type differences alone. These results confirm differences in muscle phenotype and physiological characteristics, particularly associated with high-intensity running.     

Ontogeny of the Pectoralis muscle in the little brown bat,Myotis lucifugus

The ontogeny of a primary flight muscle, the pectoralis, in the little brown bat (Myotis lucifugus: Vespertilionidae) was studied using histochemical, immunocytochemical, and electrophoretic techniques. In fetal and early neonatal (postnatal age 1–6 days) Myotis, histochemical techniques for myofibrillar ATPase (mATPase) and antibodies for slow and fast myosins demonstrated the presence of two fiber types, here called types I and IIa. These data correlated with multiple transitional myosin heavy chain isoforms and native myosin isoforms demonstrated with SDS-PAGE and 4% pyrophosphate PAGE. There was a decrease in the distribution and number of type I fibers with increasing postnatal age. At postnatal age 8–9 days, the adult phenotype was observed with regard to muscle fiber type (100% type IIa fibers) and myosin isoform profile (single adult MHC and native myosin isoforms). This “adult” fiber type profile and myosin isoform composition preceeded adult function by about 2 weeks. For example, little brown bats were incapable of sustained flight until approximately postnatal day 24, and myofiber size did not achieve adult size until approximately postnatal day 25. Although Myotis pectoralis is unique in being composed of 100% type IIa fibers, transitional fiber types and isoforms were present. These transitional forms had been observed previously in other mammals bearing mixed adult muscle fibers and which undergo transitional stages in muscle ontogeny. However, in Myotis pectoralis, this transition transpires relatively early in development. © 1994 Wiley-Liss, Inc.