Lower skeletal muscle capillarization and VEGF expression in aged vs. young men.

Recently, we observed that muscle capillarization, vascular endothelial growth factor (VEGF) protein, and the VEGF mRNA response to acute exercise were lower in aged compared with young women (Croley AN, Zwetsloot KA, Westerkamp LM, Ryan NA, Pendergast aged men, Hickner RC, Pofahl WE, and Gavin TP. J Appl Physiol 99: 1875-1882, 2005). We hypothesized that similar age-related differences in muscle capillarization and VEGF expression would exist between young and aged men. Skeletal muscle biopsies were obtained from the vastus lateralis before and at 4 h after a submaximal exercise bout for the measurement of morphometry, capillarization, VEGF, KDR, and Flt-1 in seven aged (mean age 65 yr) and eight young (mean age 21 yr) sedentary men. In aged compared with young men, muscle capillary contacts and capillary-to-fiber perimeter exchange index were lower regardless of fiber type. Muscle VEGF mRNA and protein were lower in aged men both at rest and 4 h postexercise. Exercise increased muscle VEGF mRNA and protein and KDR mRNA independent of age group. There were no effects of exercise or age on muscle Flt-1 mRNA or protein or KDR protein. These results confirm that skeletal muscle capillarization and VEGF expression are lower in aged compared with young men.     

Lower capillary density but no difference in VEGF expression in obese vs. lean young skeletal muscle in humans.

Obesity is associated with lower skeletal muscle capillarization and lower insulin sensitivity. Vascular endothelial growth factor (VEGF) is important for the maintenance of the skeletal muscle capillaries. To investigate whether VEGF and VEGF receptor [kinase insert domain-containing receptor (KDR) and Flt-1] expression are lower with obesity, vastus lateralis muscle biopsies were obtained from eight obese and eight lean young sedentary men before and 2 h after a 1-h submaximal aerobic exercise bout for the measurement of VEGF, KDR, Flt-1, and skeletal muscle fiber and capillary characteristics. There were no differences in VEGF or VEGF receptor mRNA at rest between lean and obese muscle. Exercise increased VEGF (10-fold), KDR (3-fold), and Flt-1 (5-fold) mRNA independent of group. There were no differences in VEGF, KDR, or Flt-1 protein between groups. Compared with lean skeletal muscle, the number of capillary contacts per fiber was the same, but lower capillary density (CD), greater muscle cross sectional area, and lower capillary-to-fiber area ratio were observed in both type I and II fibers in obese muscle. Multiple linear regression revealed that 49% of the variance in insulin sensitivity (homeostasis model assessment) could be explained by percentage of body fat (35%) and maximal oxygen uptake per kilogram of fat-free mass (14%). Linear regression revealed significant relationships between maximal oxygen uptake and both CD and capillary-to-fiber perimeter exchange. Although differences may exist in CD and capillary-to-fiber area ratio between lean and obese skeletal muscle, the present results provide evidence that VEGF and VEGF receptor expression are not different between lean and obese muscle.     

Human skeletal sarcoplasmic reticulum Ca2+ uptake and muscle function with aging and strength training.

This study investigated the adaptations of skeletal muscle sarcoplasmic reticulum (SR) Ca2+ uptake, relaxation, and fiber types in young (YW) and elderly women (EW) to high-resistance training. Seventeen YW (18-32 yr) and 11 EW (64-79 yr) were assessed for 1) electrically evoked relaxation time and rate of the quadriceps femoris; and 2) maximal rates of SR Ca2+ uptake and Ca2+-ATPase activity and relative fiber-type areas, analyzed from muscle biopsies of the vastus lateralis. EW had significantly slower relaxation rates and times, decreased SR Ca2+ uptake and Ca2+-ATPase activity, and a larger relative type I fiber area than did YW. A subgroup of 9 young (YWT) and 10 elderly women (EWT) performed 12 wk of high-resistance training (8 repetition maximum) of the quadriceps and underwent identical testing procedures pre- and posttraining. EWT significantly increased their SR Ca2+ uptake and Ca2+-ATPase activity in response to training but showed no alterations in speed of relaxation or relative fiber-type areas. In YWT none of the variables was altered after resistance training. These findings suggest that 1) a reduced SR Ca2+ uptake in skeletal muscle of elderly women was partially reversed with resistance training and 2) SR Ca2+ uptake in the vastus lateralis was not the rate-limiting mechanism for the slowing of relaxation measured from electrically evoked quadriceps muscle of elderly women.     

Exercise adaptation attenuates VEGF gene expression in human skeletal muscle

Angiogenesis is a component of the multifactoral adaptation to exercise training, and vascular endothelial growth factor (VEGF) is involved in extracellular matrix changes and endothelial cell proliferation. However, there is limited evidence supporting the role of VEGF in the exercise training response. Thus we studied mRNA levels of VEGF, using quantitative Northern analysis, in untrained and trained human skeletal muscle at rest and after a single bout of exercise. Single leg knee-extension provided the acute exercise stimulus and the training modality. Four biopsies were collected from the vastus lateralis muscle at rest in the untrained and trained conditions before and after exercise. Training resulted in a 35% increase in muscle oxygen consumption and an 18% increase in number of capillaries per muscle fiber. At rest, VEGF/18S mRNA levels were similar before (0.38 +/- 0.04) and after (1.2 +/- 0.4) training. When muscle was untrained, acute exercise greatly elevated VEGF/18S mRNA levels (16.9 +/- 6.7). The VEGF/18S mRNA response to acute exercise in the trained state was markedly attenuated (5.4 +/- 1.3). These data support the concept that VEGF is involved in exercise-induced skeletal muscle angiogenesis and appears to be subject to a negative feedback mechanism as exercise adaptations occur.     

Effect of acute exercise and exercise training on VEGF splice variants in human skeletal muscle

The present study investigated the effect of an acute exercise bout on the mRNA response of vascular endothelial growth factor (VEGF) splice variants in untrained and trained human skeletal muscle. Seven habitually active young men performed one-legged knee-extensor exercise training at an intensity corresponding to approximately 70% of the maximal workload in an incremental test five times/week for 4 wk. Biopsies were obtained from the vastus lateralis muscle of the trained and untrained leg 40 h after the last training session. The subjects then performed 3 h of two-legged knee-extensor exercise, and biopsies were obtained from both legs after 0, 2, 6, and 24 h of recovery. Real-time PCR was used to examine the expression of VEGF mRNA containing exon 1 and 2 (all VEGF isoforms), exon 6 or exon 7, and VEGF(165) mRNA. Acute exercise induced an increase (P < 0.05) in total VEGF mRNA levels as well as VEGF(165) and VEGF splice variants containing exon 7 at 0, 2, and 6 h of recovery. The increase in VEGF mRNA was higher in the untrained than in the trained leg (P < 0.05). The results suggest that in human skeletal muscle, acute exercise increases total VEGF mRNA, an increase that appears to be explained mainly by an increase in VEGF(165) mRNA. Furthermore, 4 wk of training attenuated the exercise-induced response in skeletal muscle VEGF(165) mRNA.     

Muscle fiber type-specific response of Hsp70 expression in human quadriceps following acute isometric exercise.

To investigate the time course of fiber type-specific heat shock protein 70 (Hsp70) expression in human skeletal muscle after acute exercise, 10 untrained male volunteers performed single-legged isometric knee extensor exercise at 60% of their maximal voluntary contraction (MVC) with a 50% duty cycle (5-s contraction and 5-s relaxation) for 30 min. Muscle biopsies were collected from the vastus lateralis before (Pre) exercise in the rested control leg (C) and immediately after exercise (Post) in the exercised leg (E) only and on recovery days 1 (R1), 2 (R2), 3 (R3), and 6 (R6) from both legs. As demonstrated by Western blot analysis, whole muscle Hsp70 content was unchanged (P > 0.05) immediately after exercise (Pre vs. Post), was increased (P < 0.05) by approximately 43% at R1, and remained elevated throughout the entire recovery period in E only. Hsp70 expression was also assessed in individual muscle fiber types I, IIA, and IIAX/IIX by immunohistochemistry. There were no fiber type differences (P > 0.05) in basal Hsp70 expression. Immediately after exercise, Hsp70 expression was increased (P < 0.05) in type I fibers by approximately 87% but was unchanged (P > 0.05) in type II fibers (Pre vs. Post). At R1 and throughout recovery, Hsp70 content in E was increased above basal levels (P < 0.05) in all fiber types, but Hsp70 expression was always highest (P < 0.05) in type I fibers. Hsp70 content in C was not different from Pre at any time throughout recovery. Glycogen depletion was observed at Post in all type II, but not type I, fibers, suggesting that the fiber type differences in exercise-induced Hsp70 expression were not related to glycogen availability. These results demonstrate that the time course of exercise-induced Hsp70 expression in human skeletal muscle is fiber type specific.     

Bed rest reduces metabolic protein content and abolishes exercise-induced mRNA responses in human skeletal muscle

The aim was to test the hypothesis that 7 days of bed rest reduces mitochondrial number and expression and activity of oxidative proteins in human skeletal muscle but that exercise-induced intracellular signaling as well as mRNA and microRNA (miR) responses are maintained after bed rest. Twelve young, healthy male subjects completed 7 days of bed rest with vastus lateralis muscle biopsies taken before and after bed rest. In addition, muscle biopsies were obtained from six of the subjects prior to, immediately after, and 3 h after 45 min of one-legged knee extensor exercise performed before and after bed rest. Maximal oxygen uptake decreased by 4%, and exercise endurance decreased nonsignificantly, by 11%, by bed rest. Bed rest reduced skeletal muscle mitochondrial DNA/nuclear DNA content 15%, hexokinase II and sirtuin 1 protein content ～45%, 3-hydroxyacyl-CoA dehydrogenase and citrate synthase activity ～8%, and miR-1 and miR-133a content ～10%. However, cytochrome c and vascular endothelial growth factor (VEGF) protein content as well as capillarization did not change significantly with bed rest. Acute exercise increased AMP-activated protein kinase phosphorylation, peroxisome proliferator activated receptor-γ coactivator-1α, and VEGF mRNA content in skeletal muscle before bed rest, but the responses were abolished after bed rest. The present findings indicate that only 7 days of physical inactivity reduces skeletal muscle metabolic capacity as well as abolishes exercise-induced adaptive gene responses, likely reflecting an interference with the ability of skeletal muscle to adapt to exercise.     

A single bout of exercise with high mechanical loading induces the expression of Cyr61/CCN1 and CTGF/CCN2 in human skeletal muscle.

High mechanical loading was hypothesized to induce the expression of angiogenic and/or lymphangiogenic extracellular matrix (ECM) proteins in skeletal muscle. Eight men performed a strenuous exercise protocol, which consisted of 100 unilateral maximal drop jumps followed by submaximal jumping until exhaustion. Muscle biopsies were taken 30 min and 48 h postexercise from the vastus lateralis muscle and analyzed for the following parameters: mRNA and protein expression of ECM-associated CCN proteins [cysteine-rich angiogenic protein 61 (Cyr61)/CCN1, connective tissue growth factor (CTGF)/CCN2], and mRNA expression of vascular endothelial growth factors (VEGFs) and hypoxia-inducible factor-1alpha. The mRNA expression of Cyr61 and CTGF increased 30 min after the exercise (14- and 2.5-fold, respectively; P < 0.001). Cyr61 remained elevated 48 h postexercise (threefold; P < 0.05). The mRNA levels of VEGF-A, VEGF-B, VEGF-C, VEGF-D, or hypoxia-inducible factor-1alpha did not change significantly at either 30 min or 48 h postexercise; however, the variation between subjects increased markedly in VEGF-A and VEGF-B mRNA. Cyr61 protein levels were higher at both 30 min and 48 h after the exercise compared with the control (P < 0.05). Cyr61 and CTGF proteins were localized to muscle fibers and the surrounding ECM by immunohistochemistry. Fast fibers stained more intensively than slow fibers. In conclusion, mechanical loading induces rapid expression of CCN proteins in human skeletal muscle. This may be one of the early mechanisms involved in skeletal muscle remodeling after exercise, since Cyr61 and CTGF regulate the expression of genes involved in angiogenesis and ECM remodeling.     

Myogenic gene expression at rest and after a bout of resistance exercise in young (18-30 yr) and old (80-89 yr) women.

The purpose of this study was to investigate mRNA expression of several key skeletal muscle myogenic controllers; myogenic differentiation factor (MyoD), muscle regulatory factor 4 (MRF4), myogenic factor 5 (Myf5), myogenin, myostatin, and myocyte enhancer factor 2 (MEF2) at rest and 4 h after a single bout of resistance exercise (RE) in young and old women. Eight young women (YW; 23 +/- 2 yr, 67 +/- 5 kg) and six old women (OW; 85 +/- 1 yr, 67 +/- 4 kg) performed 3 sets of 10 repetitions of bilateral knee extensions at 70% of one repetition maximum. Muscle biopsies were taken from the vastus lateralis before and 4 h after RE. Using real-time RT PCR, mRNA from the muscle samples was amplified and normalized to GAPDH. At rest, OW expressed higher (P < 0.05) levels of MyoD, MRF4, Myf5, myogenin, and myostatin compared with YW. In response to RE, there was a main time effect (P < 0.05) for the YW and OW combined in the upregulation of MyoD (2.0-fold) and MRF4 (1.4-fold) and in the downregulation of myostatin (2.2-fold). There was a trend (P = 0.08) for time x age interaction in MRF4. These data show that old women express higher myogenic mRNA levels at rest. The higher resting myogenic mRNA levels in old women may reflect an attempt to preserve muscle mass and function. When challenged with RE, old women appear to respond in a similar manner as young women.     

Rapid determination of myosin heavy chain expression in rat, mouse, and human skeletal muscle using multicolor immunofluorescence analysis

Skeletal muscle is a heterogeneous tissue comprised of fibers with different morphological, functional, and metabolic properties. Different muscles contain varying proportions of fiber types; therefore, accurate identification is important. A number of histochemical methods are used to determine muscle fiber type; however, these techniques have several disadvantages. Immunofluorescence analysis is a sensitive method that allows for simultaneous evaluation of multiple MHC isoforms on a large number of fibers on a single cross-section, and offers a more precise means of identifying fiber types. In this investigation we characterized pure and hybrid fiber type distribution in 10 rat and 10 mouse skeletal muscles, as well as human vastus lateralis (VL) using multicolor immunofluorescence analysis. In addition, we determined fiber type-specific cross-sectional area (CSA), succinate dehydrogenase (SDH) activity, and α-glycerophosphate dehydrogenase (GPD) activity. Using this procedure we were able to easily identify pure and hybrid fiber populations in rat, mouse, and human muscle. Hybrid fibers were identified in all species and made up a significant portion of the total population in some rat and mouse muscles. For example, rat mixed gastrocnemius (MG) contained 12.2% hybrid fibers whereas mouse white tibialis anterior (WTA) contained 12.1% hybrid fibers. Collectively, we outline a simple and time-efficient method for determining MHC expression in skeletal muscle of multiple species. In addition, we provide a useful resource of the pure and hybrid fiber type distribution, fiber CSA, and relative fiber type-specific SDH and GPD activity in a number of rat and mouse muscles.     

Gender differences in muscle inflammation after eccentric exercise.

Unaccustomed exercise is followed by delayed-onset muscle soreness and morphological changes in skeletal muscle. Animal studies have demonstrated that women have an attenuated response to muscle damage. We studied the effect of eccentric exercise in untrained male (n = 8) and female (n = 8) subjects using a unilateral exercise design [exercise (Ex) and control (Con) legs]. Plasma granulocyte counts [before (Pre) and 48 h after exercise (+48h)] and creatine kinase activity [Pre, 24 h after exercise (+24h), +48h, and 6 days after exercise (+6d)] were determined before (Pre) and after (+24h, +48h, +6d) exercise, with biopsies taken from the vastus lateralis of each leg at +48h for determination of muscle damage and/or inflammation. Plasma granulocyte counts increased for men and decreased for women at +48h (P < 0.05), and creatine kinase activity increased for both genders at +48h and +6d (P < 0.01). There were significantly greater areas of both focal (P < 0.001) and extensive (P < 0.01) damage in the Ex vs. Con leg for both genders, which was assessed by using toluidine blue staining. The number of leukocyte common antigen-positive cells/mm(2) tissue increased with exercise (P < 0.05), and men tended to show more in their Ex vs. Con leg compared with women (P = 0.052). Men had a greater total (Ex and Con legs) number of bcl-2-positive cells/mm(2) tissue vs. women (P < 0.05). Atrophic fibers with homogeneous bcl-2-positive staining were seen only in men (n = 3). We conclude that muscle damage is similar between genders, yet the inflammatory response is attenuated in women vs. men. Finally, exercise may stimulate the expression of proteins involved in apoptosis in skeletal muscle.     

Satellite cell content is specifically reduced in type II skeletal muscle fibers in the elderly

Satellite cells (SC) are essential for skeletal muscle growth and repair. Because sarcopenia is associated with type II muscle fiber atrophy, we hypothesized that SC content is specifically reduced in the type II fibers in the elderly. A total of eight elderly (E; 76 +/- 1 yr) and eight young (Y; 20 +/- 1 yr) healthy males were selected. Muscle biopsies were collected from the vastus lateralis in both legs. ATPase staining and a pax7-antibody were used to determine fiber type-specific SC content (i.e., pax7-positive SC) on serial muscle cross sections. In contrast to the type I fibers, the proportion and mean cross-sectional area of the type II fibers were substantially reduced in E vs. Y. The number of SC per type I fiber was similar in E and Y. However, the number of SC per type II fiber was substantially lower in E vs. Y (0.044 +/- 0.003 vs. 0.080 +/- 0.007; P < 0.01). In addition, in the type II fibers, the number of SC relative to the total number of nuclei and the number of SC per fiber area were also significantly lower in E. This study is the first to show type II fiber atrophy in the elderly to be associated with a fiber type-specific decline in SC content. The latter is evident when SC content is expressed per fiber or per fiber area. The decline in SC content might be an important factor in the etiology of type II muscle fiber atrophy, which accompanies the loss of skeletal muscle with aging.     

A single bout of exercise activates matrix metalloproteinase in human skeletal muscle.

The aims of this study were 1) to characterize changes in matrix metalloproteinase (MMP), endostatin, and vascular endothelial growth factor (VEGF)-A expression in skeletal muscle in response to a single bout of exercise in humans; and 2) to determine if any exchange of endostatin and VEGF-A between circulation and the exercising leg is associated with a change in the tissue expression or plasma concentration of these factors. Ten healthy males performed 65 min of cycle exercise, and muscle biopsies were obtained from the vastus lateralis muscle at rest and immediately and 120 min after exercise. In the muscle biopsies, measurements of mRNA expression levels of MMP-2, MMP-9, MMP-14, and tissue inhibitor of metalloproteinase; VEGF and endostatin protein levels; and MMP activities were performed. Femoral arterial and venous concentrations of VEGF-A and endostatin were determined before, during, and 120 min after exercise. A single bout of exercise increased MMP-9 mRNA and activated MMP-9 protein in skeletal muscle. No measurable increase of endostatin was observed in the skeletal muscle or in plasma following exercise. A concurrent increase in skeletal muscle VEGF-A mRNA and protein levels was induced by exercise, with no signs of peripheral uptake from the circulation. However, a decrease in plasma VEGF-A concentration occurred following exercise. Thus 1) a single bout of exercise activated the MMP system without any resulting change in tissue endostatin protein levels, and 2) the increased VEGF-A protein levels are due to changes in the skeletal muscle tissue itself. Other mechanisms are responsible for the observed exercise-induced decrease in VEGF-A in plasma.     

Human skeletal muscle fiber type alteration with high-intensity intermittent training

The response of muscle fiber type proportions and fiber areas to 15 weeks of strenuous high-intensity intermittent training was investigated in twenty-four carefully ascertained sedentary (14 women and 10 men) and 10 control (4 women and 6 men) subjects. The supervised training program consisted mainly of series of supramaximal exercise lasting 15 s to 90 s on a cycle ergometer. Proportions of muscle fiber type and areas of the fibers were determined from a biopsy of the vastus lateralis before and after the training program. No significant change was observed for any of the histochemical characteristics in the control group. Training significantly increased the proportion of type I and decreased type IIb fibers, the proportion of type IIa remained unchanged. Areas of type I and IIb fibers increased significantly with training. These results suggest that high-intensity intermittent training in humans may alter the proportion of type I and the area of type I and IIb fibers and in consequence that fiber type composition in human vastus lateralis muscle is not determined solely by genetic factors.     

Vascular endothelial growth factor mRNA expression and arteriovenous balance in response to prolonged,submaximal exercise in humans

Angiogenesis, the growth of new blood vessels from existing ones, occurs in the skeletal muscle as an adaptive response to exercise that satisfies the increased requirement of this tissue for oxygen delivery and metabolic processes. Of the factors that have been identified to regulate this process, the endothelial cell mitogen vascular endothelial growth factor (VEGF) has been proposed to play a key role. The aim of this study was to measure the skeletal muscle VEGF mRNA content and arteriovenous protein balance across the working leg in response to a single bout of prolonged, submaximal exercise. Seven physically active males completed 3 h of two-legged kicking ergometry. Muscle biopsies were collected from the vastus lateralis muscle from both working legs, and blood samples were collected from one femoral artery and femoral vein before, during, and in recovery from exercise. We show that the exercise stimulus elicited a decrease in VEGF protein arteriovenous balance across the exercising leg (P = 0.007), and a ninefold elevation in skeletal muscle VEGF mRNA expression (P < 0.001). The changes in VEGF protein balance and mRNA content were most pronounced 1 h after the cessation of exercise. In conclusion, these findings demonstrate that submaximal exercise, suitable for humans with low CV fitness, induces a decrease in VEGF arteriovenous balance that is likely to be of clinical significance in promoting angiogenic effects.     

Co-Expression of SERCA Isoforms,Phospholamban and Sarcolipin in Human Skeletal Muscle Fibers

Sarcolipin (SLN) and phospholamban (PLN) inhibit the activity of sarco(endo)plasmic reticulum Ca²⁺-ATPases (SERCAs) by reducing their apparent affinity for Ca²⁺. A ternary complex between SLN, PLN, and SERCAs results in super-inhibition of SERCA activity. Analysis of skeletal muscle homogenate has limited our current understanding of whether SLN and PLN regulate SERCA1a, SERCA2a, or both in skeletal muscle and whether SLN and PLN are co-expressed in skeletal muscle fibers. Biopsies from human vastus lateralis were analyzed through single fiber Western blotting and immunohisto/fluorescence staining to circumvent this limitation. With a newly generated SLN antibody, we report for the first time that SLN protein is present in human skeletal muscle. Addition of the SLN antibody (50 µg) to vastus lateralis homogenates increased the apparent Ca²⁺ affinity of SERCA (K _Ca, pCa units) (-Ab, 5.85 ± 0.02 vs. +Ab, 5.95 ± 0.02) and maximal SERCA activity (μmol/g protein/min) (-Ab, 122 ± 6.4 vs. +Ab, 159 ± 11) demonstrating a functional interaction between SLN and SERCAs in human vastus lateralis. Specifically, our results suggest that although SLN and PLN may preferentially regulate SERCA1a, and SERCA2a, respectively, physiologically they both may regulate either SERCA isoform. Furthermore, we show that SLN and PLN co-immunoprecipitate in human vastus lateralis homogenate and are simultaneously expressed in 81% of the fibers analyzed with Western blotting which implies that super-inhibition of SERCA may exist in human skeletal muscle. Finally, we demonstrate unequivocally that mouse soleus contains PLN protein suggesting that super-inhibition of SERCA may also be important physiologically in rodent skeletal muscle.     

Exercise alters the profile of phospholipid molecular species in rat skeletal muscle.

We have determined the effect of two exercise-training intensities on the phospholipid profile of both glycolytic and oxidative muscle fibers of female Sprague-Dawley rats using electrospray-ionization mass spectrometry. Animals were randomly divided into three training groups: control, which performed no exercise training; low-intensity (8 m/min) treadmill running; or high-intensity (28 m/min) treadmill running. All exercise-trained rats ran 1,000 m/session for 4 days/wk for 4 wk and were killed 48 h after the last training bout. Exercise training was found to produce no novel phospholipid species but was associated with significant alterations in the relative abundance of a number of phospholipid molecular species. These changes were more prominent in glycolytic (white vastus lateralis) than in oxidative (red vastus lateralis) muscle fibers. The largest observed change was a decrease of approximately 20% in the abundance of 1-stearoyl-2-docosahexaenoyl-phosphatidylethanolamine [PE(18:0/22:6); P < 0.001] ions in both the low- and high-intensity training regimes in glycolytic fibers. Increases in the abundance of 1-oleoyl-2-linoleoyl phopshatidic acid [PA(18:1/18:2); P < 0.001] and 1-alkenylpalmitoyl-2-linoleoyl phosphatidylethanolamine [plasmenyl PE (16:0/18:2); P < 0.005] ions were also observed for both training regimes in glycolytic fibers. We conclude that exercise training results in a remodeling of phospholipids in rat skeletal muscle. Even though little is known about the physiological or pathophysiological role of specific phospholipid molecular species in skeletal muscle, it is likely that this remodeling will have an impact on a range of cellular functions.     

Neuromuscular partitioning, architectural design, and myosin fiber types of the M. vastus lateralis of the llama (Lama glama)

The llama (Lama glama) is one of the few mammals of relatively large body size in which three fast myosin heavy chain isoforms (i.e., IIA, IIX, IIB) are extensively expressed in their locomotory muscles. This study was designed to gain insight into the morphological and functional organization of skeletal musculature in this peculiar animal model. The neuromuscular partitioning, architectural design, and myosin fiber types were systematically studied in the M. vastus lateralis of adult llamas (n = 15). Four nonoverlapping neuromuscular partitions or compartments were identified macroscopically (using a modified Sihler's technique for muscle depigmentation), although they did not conform strictly to the definitions of "neuromuscular compartments." Each neuromuscular partition was innervated by primary branches of the femoral nerve and was arranged within the muscle as paired partitions, two in parallel (deep-superficial compartmentalization) and the other two in-series (proximo-distal compartmentalization). These neuromuscular partitions of the muscle varied in their respective architectural designs (studied after partial digestion with diluted nitric acid) and myosin fiber type characteristics (identified immunohistochemically with specific anti-myosin monoclonal antibodies, then examined by quantitative histochemistry and image analysis). The deep partitions of the muscle had longer fibers, with lower angles of pinnation, and higher percentages of fast-glycolytic fibers than the superficial partitions of the muscle. These differences clearly suggest a division of labor in the whole M. vastus lateralis of llamas, with deep partitions exhibiting features well adapted for dynamic activities in the extension of stifle, whereas superficial portions seem to be related to the antigravitational role of the muscle in preserving the extension of the stifle during standing and stance phase of the stride. This peculiar structural and functional organization of the llama M. vastus lateralis does not confirm the generalized idea that deep muscles or the deepest portions within the same muscles somehow develop postural and/or low-intensity isometric functions. Rather, it suggests a primacy of architecture over intramuscular location in determining fiber type composition and hence division of labor within the muscle. A compartmentalization in the distribution of the three fast-subtype fibers (IIA, IIX, and IIB) also occurred, and this could also be relevant functionally, since these fiber types differed significantly in size (IIA < IIX < IIB), oxidative capacity (IIA > IIX > IIB), and capillarization (IIA = IIX > IIB). Furthermore, a typical spatial pattern in fiber type distribution was encountered in llama muscle (i.e., fiber types were consistently ranked in the order I --> IIA --> IIX --> IIB from the center to the periphery of fascicles), suggesting again peculiar and not well understood functional adaptations in these species.     

Fiber type composition of the vastus lateralis muscle of young men and women.

This study presents data collected over the past 10 years on the muscle fiber type composition of the vastus lateralis muscle of young men and women. Biopsies were taken from the vastus lateralis muscle of 55 women (21.2+/-2.2 yr) and 95 men (21.5+/-2.4 yr) who had volunteered to participate in various research projects. Six fiber types (I, IC, IIC, IIA, IIAB, and IIB) were classified using mATPase histochemistry, and cross-sectional area was measured for the major fiber types (I, IIA, and IIB). Myosin heavy chain (MHC) content was determined electrophoretically on all of the samples from the men and on 26 samples from the women. With the exception of fiber Type IC, no significant differences were found between men and women for muscle fiber type distribution. The vastus lateralis muscle of both the men and women contained approximately 41% I, 1% IC, 1% IIC, 31% IIA, 6% IIAB, and 20% IIB. However, the cross-sectional area of all three major fiber types was larger for the men compared to the women. In addition, the Type IIA fibers were the largest for the men, whereas the Type I fibers tended to be the largest for the women. Therefore, gender differences were found with regard to the area occupied by each specific fiber type: IIA>I>IIB for the men and I>IIA>IIB for the women. These data establish normative values for the mATPase-based fiber type distribution and sizes in untrained young men and women.     

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.