Activity influences on soleus muscle myosin during rodent hindlimb suspension.

This study examined the effect of stationary ground support (2 and 4 h/day) and uphill running (1.5 h/day, 20 m/min, 30% grade) activity patterns on soleus muscle atrophy and slow myosin loss during 4 wk of rodent hindlimb unweighting by tail suspension. We also examined the effect of uphill running during the last 4 wk of an 8-wk hindlimb unweighting program and during 4 wk of cage recovery after 4 wk of hindlimb unweighting. All forms of activity partially spared soleus muscle weight (mg), myofibril protein (mg/muscle pair and microgram/mg muscle), and relative and absolute slow myosin (SM) isoform content (% of total and mg/muscle pair, P less than 0.05). Relative to the normal control soleus muscle, the uphill running regimens resulted in 1) increased fast myosin isoform content and 2) diminished recovery of SM isoform content when coupled with cage activity recovery. Four weeks of cage recovery after 4 wk of hindlimb unweighting resulted in recovery of the relative SM isoform content to proportions exceeding normal control values, suggesting an apparent degradation of any normally existing fast myosin. These results indicate that, in the context of the hindlimb unweighting model, the mechanisms controlling the expression of soleus muscle SM and fast myosin genes can be affected differently by the diverse activities of stationary ground support, unrestricted cage activity, and programmed uphill running.

Isomyosin distributions in rodent muscles: effects of altered thyroid state.

In this study we examined the effects of 6-8 wk of thyroid hormone manipulation on striated muscle isomyosin expression in adult female rats. Animals were randomly assigned to one of three groups: 1) euthyroid controls, 2) thyroid deficient (propylthiouracil treated), and 3) hyperthyroid (triiodothyronine treated). Thyroid deficiency resulted in a marked increase in the low-adenosinetriphosphatase V3 isoform by 20- and 49-fold in the left and right ventricle, respectively. Conversely, hyperthyroidism induced a modest (3-11%) but significant increase in the high-adenosinetriphosphatase V1 isoform in both ventricles. The thyroid-deficient rats exhibited significant increases in slow myosin in both soleus (8%) and red gastrocnemius (24%), with concomitant reductions in intermediate myosin in both muscles. Interestingly, while the slow-myosin isoform was decreased in both the soleus (-19%) and the red gastrocnemius (-43%) of the hyperthyroid group, the intermediate-myosin isoform was affected differentially in the two muscles, with a fivefold increase in the former vs. a 16% decrease in the latter. Furthermore, hyperthyroidism increased the fast myosins in the red gastrocnemius while exerting no effect on the same isoforms in the white gastrocnemius. Collectively these data suggest both different specificity and sensitivity among the myosin genes of different striated muscle types in response to thyroid hormone.

Myosin heavy chain expression in rodent skeletal muscle: effects of exposure to zero gravity.

This study ascertained the effects of 9 days of zero gravity on the relative (percentage of total) and calculated absolute (mg/muscle) content of isomyosin expressed in both antigravity and locomotor skeletal muscle of ground control (CON) and flight-exposed (FL) rats. Results showed that although there were no differences in body weight between FL and CON animals, a significant reduction in muscle mass occurred in the vastus intermedius (VI) (P < 0.05) but not in the vastus lateralis (VL) or the tibialis anterior. Both total muscle protein and myofibril protein content were not different between the muscle regions examined in the FL and CON groups. In the VI, there were trends for reductions in the relative content of type I and IIa myosin heavy chains (MHCs) that were offset by increases in the relative content of both type IIb and possibly type IIx MHC protein (P > 0.05). mRNA levels were consistent with this pattern (P < 0.05). The same pattern held true for the red region of the VL as examined at both the protein and mRNA level (P < 0.05). When the atrophy process was examined, there were net reductions in the absolute content of both type I and IIa MHCs that were offset by calculated increases in type IIb MHC in both VI and red VL. Collectively, these findings suggest that there are both absolute and relative changes occurring in MHC expression in the "red" regions of antigravity skeletal muscle during exposure to zero gravity that could affect muscle function.

Subunit composition of rodent isomyosins and their distribution in hindlimb skeletal muscles.

Three adult skeletal muscle sarcomeric myosin heavy chain (MHC) genes have been identified in the rat, suggesting that the expressed native myosin isoforms can be differentiated, in part, on the basis of their MHC composition. This study was undertaken to ascertain whether the five major native isomyosins [3 fast (Fm1, Fm2, Fm3), 1 slow (Sm), and 1 intermediate (Im)], typically expressed in the spectrum of adult rat skeletal muscles comprising the hindlimb, could be further differentiated on the basis of their MHC profiles in addition to their light chain composition. Results show that in muscles comprised exclusively of fast-twitch glycolytic (FG) fibers and consisting of Fm1, Fm2, and Fm3, such as the tensor fasciae latae, only one MHC, designated as fast type IIb, could be resolved. In soleus muscle, comprised of both slow-twitch oxidative and fast-twitch oxidative-glycolytic fibers and expressing Sm and Im, two MHC bands were resolved and designated as slow/cardiac beta-MHC and fast type IIa MHC. In muscles expressing a mixture of all three fiber types and a full complement of isomyosins, as seen in the plantaris, the MHC could be resolved into three bands. Light chain profiles were characterized for each muscle type, as well as for the purified isomyosins. These data suggest that Im (IIa) consists of a mixture of fast and slow light chains, whereas Fm (IIb) and Sm (beta) isoforms consist solely of fast- and slow-type light chains, respectively. Polypeptide mapping of denatured myosin extracted from muscles expressing contrasting isoform phenotypes suggests differences in the MHC primary structure between slow, intermediate, and fast myosin isotypes. These findings demonstrate that 1) Fm, Im, and Sm isoforms are differentiated on the bases of both their heavy and light chain components and 2) each isomyosin is distributed in a characteristic fashion among rat hindlimb skeletal muscles. Furthermore, these data suggest that the ratio of isomyosins in a given muscle or muscle region is of physiological importance to the function of that muscle during muscular activity.

Effect of anabolic steroids on overloaded and overloaded suspended skeletal muscle.

The purpose of this study was to ascertain whether anabolic steroids act synergistically with functional overload in terms of increasing muscle weight and subcellular protein content of normal overloaded and suspended overloaded rodent plantaris muscle. Female rats were randomly assigned to six groups (7 rats/group) for 6 wk: 1) normal control (NC), 2) overload (OV), 3) overload steroid (OV-S), 4) normal suspended (N-SUS), 5) overload suspended (OV-SUS), and 6) overload suspended steroid (OV-SUS-S). Rats receiving anabolic steroid were administered 0.3 mg nandrolone decanoate (Deca-Durabolin) per day. Relative to control values, overload induced 1) sparing of muscle weight of the OV-SUS group as well as larger absolute and normalized (mg muscle/g body wt) muscle weight of the OV group (P less than 0.05), 2) greater protein content (mg/muscle, P less than 0.05), and 3) an increased relative expression of slow myosin in both the OV and OV-SUS groups (P less than 0.05). Although anabolic steroid treatment of overload animals (OV-S) did not alter further the pattern of response of any parameter analyzed for the OV group, it did induce larger absolute and normalized muscle weight (P less than 0.05) as well as a greater protein content (mg/muscle, P less than 0.05) of the OV-SUS-S group compared with control values. However, anabolic steroid treatment did not alter the pattern of isomyosin expression observed in the overload (OV-S vs. OV) or overload suspended (OV-SUS-S vs. OV-SUS) groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of anabolic steroids on skeletal muscle mass during hindlimb suspension.

The efficacy of anabolic steroid treatment [0.3 or 0.9 mg nandrolone decanoate (Deca-Durabolin) per day] was examined in the context of sparing rodent fast-twitch plantaris and slow-twitch soleus muscle weight, sparing subcellular protein, and altering isomyosin expression in response to hindlimb suspension. Female rats were assigned to four groups (7 rats/group for 6 wk): 1) normal control (NC), 2) normal steroid (NS), 3) normal suspension (N-SUS), and 4) suspension steroid (SUS-S). Compared with control values for the plantaris and soleus muscles, suspension induced 1) smaller body and muscle weight (P less than 0.05), 2) losses in myofibril content (mg/muscle, P less than 0.05), and 3) shifts in the relative expression (expressed as %of total isomyosin) of isomyosins which favored lesser slow myosin and greater fast myosin isotypes (P less than 0.05). Steroid treatment of suspended animals (SUS-S vs. N-SUS) partially spared body and muscle weight (P less than 0.05) and spared plantaris but not soleus myofibril content (mg/muscle, P less than 0.05). However, steroid treatment did not modify the isomyosin pattern induced by suspension. In normal rats (NS vs. NC), steroid treatment enhanced body and plantaris muscle weight but not soleus weight (P less than 0.05) and did not alter isomyosin expression in either muscle type. Collectively these data suggest that in young female rats anabolic steroids 1) enhance the body weight and the weight of a fast-twitch ankle extensor in normal rats, 2) ameliorate the loss in body weight, fast-twitch muscle weight and protein content and slow-twitch muscle weight associated with hindlimb suspension.(ABSTRACT TRUNCATED AT 250 WORDS)

Time course adaptations in rat skeletal muscle isomyosins during compensatory growth and regression.

The purpose of this study was to ascertain the time course of change during both compensatory growth (hypertrophy) and subsequent growth regression on myosin isoform expression in rodent fast-twitch plantaris muscle in response to functional overload (induced by removal of synergists). Peak hypertrophy of the plantaris muscle (92%) occurred after 9 wk of overload. After 7 wk of overload regression (induced by a model of hindlimb unweighting), muscle weight returned to within 30% of control values. Myofibril protein content (mg/g muscle) remained relatively constant throughout the overload period but became significantly depressed relative to control values after 7 wk of regression. However, when expressed on a per muscle basis (mg/muscle) no differences existed at this time point (t = 7 wk regression). The distribution of native myosin isoforms in the myofibril protein pool of the overloaded plantaris muscle reflected a progressive increase (23% at t = 9 wk; P less than 0.001) in the relative proportion of slow myosin (Sm). This change was also accompanied by increases in intermediate myosin (Im) as well as the repression of the fast myosin one (Fm1) isoform (P less than 0.001). These shifts in Sm and Fm1 isoform expression were gradually reversed during the regression period, whereas Im remained elevated relative to control values. These adaptive changes in myosin isoform expression during both hypertrophy and regression were further supported by concomitant shifts in both myosin adenosinetriphosphatase (ATPase) activity (decreased during overload) and slow myosin light chain (SLC) expression. However, during regression the changes in myosin isoform expression and myosin ATPase were not as synchronous as they were during overload. Estimation of the mixed myosin heavy chain (MHC) half-life (t 1/2), using a linear model that assumes zero-order synthesis and first-order degradation kinetics, revealed t 1/2 values of approximately 19 and 10 days for the overload and regression periods, respectively. Collectively these data suggest that 1) skeletal muscle myosin isoforms and corresponding ATPase activity are in a dynamic state of change, although not completely synchronous, in response to altered muscle stress, and 2) the kinetics of change in the mixed MHC protein pool are slower during compensatory growth compared with regression of growth.

Interaction of compensatory overload and hindlimb suspension on myosin isoform expression.

The purpose of this study was to investigate the role of chronic weight-bearing activity as the primary inducer of compensatory muscle growth and changes in myosin isoform expression in rodent fast-twitch plantaris muscle. Thus, female rats were subjected to the independent and simultaneous exposure of functional overload (induced via synergist removal) and hindlimb unweighting (suspension) for 6 wk. Groups (n = 7/group) consisted of normal-control (NC); overload (OV); normal-suspension (N-SUS); and overload-suspension (OV-SUS). Body weight of both suspension groups was significantly less than both the NC and OV groups (P less than 0.001). Compared with the NC group, normalized plantaris weight (mg/g body wt) of both the OV and OV-SUS groups was greater, whereas that of the N-SUS was lower (P less than 0.001). However, normalized plantaris weight was greater in OV compared with OV-SUS by 35% (P less than 0.001). Myofibril protein content (mg/g) and Ca2+-regulated myofibril adenosinetriphosphatase (ATPase) specific activity were similar for all groups except that ATPase was lower in the OV group compared with the other groups (P less than 0.05). Native myosin isoform analysis revealed a significant increase in the expression of slow and intermediate myosin and the repression of fast myosin 1 (Fm1) in OV compared with NC. This shift in expression was not as pronounced in the OV-SUS group. Interestingly, only traces of slow myosin were observed in the N-SUS group compared with the other groups. These results suggest that weight bearing is an essential component of the overload model for inducing significant increases in both muscle mass and slow myosin isoform expression. Second, lack of weight bearing, while not markedly affecting fast myosins, appears to repress the expression of slow myosin.

Time course of soleus muscle myosin expression during hindlimb suspension and recovery.

This study examined the time course of adult rodent soleus muscle myofibril and myosin isoform protein expression after 4, 8, 16, 28, and 56 days of hindlimb unweighting by tail suspension (S). The time course of soleus muscle recovery (R) was also examined after 28 days of hindlimb unweighting with an additional 4, 8, 16, and 28 days of unrestricted cage activity. During suspension, soleus muscle myofibril protein rapidly decreased from 34.3 +/- 3.1 (1.96SE) mg/pair in the control (C) group to 6.9 +/- 1.4 (1.96SE) mg/pair in S (t = 56 days). The calculated first-order degradation rate constant for this loss was kd = 0.17 days-1 [half time (t1/2) = 4.1 days]. The estimated slow myosin (SM) isoform content decreased from 13.4 +/- 2.0 (1.96SE) mg/pair in C to 2.1 +/- 0.2 (1.96SE) mg/pair in S (kd = 0.19 days-1, t1/2 = 3.6 days). The relative proportion of other myosin isoforms was increased at 28 and 56 days of suspension, reflecting an apparent de novo synthesis and the loss of SM. Recovery of contractile protein after 28 days of suspension was slower for both the myofibril protein and the SM isoform (kd = 0.07 days-1, t1/2 = 10 days). These data suggest that loss of weight bearing specifically affected the mechanisms of contractile protein expression reflected in soleus muscle protein degradation processes. In addition, the expression of the myosin isoforms were apparently differentially affected by the loss of weight-bearing activity.

Myosin isozyme distribution in rodent hindlimb skeletal muscle.

The purpose of this study was to examine the distribution of myosin isozymes in rodent (Rattus norvegicus) hindlimb skeletal muscles and regions of muscle known to have contrasting fiber-type composition. Muscle samples were analyzed for Ca2+-regulated myofibril adenosine triphosphatase (ATPase) activity, Ca2+-activated myosin ATPase activity, myosin isozyme profile, and myosin light chain profile. Four isozymes of myosin were identified based on native protein and light chain electrophoresis patterns: one associated primarily with slow-twitch muscle (SM) and three associated primarily with fast-twitch muscle (FM). Multiple linear regression analysis of Ca2+-regulated myofibril ATPase activity (pCA 4) vs. measured isozyme profile was used to estimate the myofibril ATPase activities of the individual isozymes (FM1 = 0.86, FM2 = 0.52, FM3 = 0.31, and SM = 0.15 mumol Pi formed . mg myofibril protein-1 . min-1 at 25 degrees C, n = 180, P less than 0.001). Differences in the native isozyme profiles and myofibril ATPase activities between muscles and muscle regions of similar fiber type composition indicate that a given fiber type may not necessarily express a single isozyme profile. These data are consistent with the hypothesis that, among rodent hindlimb skeletal muscles and inherently their motor units, a range of myosin isozyme profiles exists that may provide a broad range of mechanical expression.

Substrate oxidation capacity in rodent skeletal muscle: effects of exposure to zero gravity.

A study was conducted, as part of the integrated National Aeronautics and Space Administration Space Life Sciences 1 mission flown in June of 1991, to ascertain the effects of 9 days of exposure to zero gravity on the capacity of rodent skeletal muscle fiber types to oxidize either [14C]pyruvate or [14C]palmitate under state 3 metabolic conditions, i.e., nonlimiting amounts of substrate and cofactors. In addition, activity levels of marker enzymes of the tricarboxylic acid cycle, malate shuttle, and beta-oxidation were measured. Results showed that significant differences in muscle weight occurred in both the predominantly slow vastus intermedius and predominantly fast vastus lateralis of flight vs. control groups (P < 0.05). Total protein content of the muscle samples was similar between groups. Both pyruvate oxidation capacity and the marker oxidative enzymes were not altered in the flight relative to control animals. However, the capacity to oxidize long-chain fatty acids was significantly reduced by 37% in both the high- and low-oxidative regions of the vastus muscle (P < 0.05). Although these findings of a selective reduction in fatty acid oxidation capacity in response to spaceflight are surprising, they are consistent with previous findings showing 1) an increased capacity to take up glucose and upregulate glucose transporter proteins and 2) a marked accumulation of triglycerides in the skeletal muscles of rats subjected to states of unloading. Thus, skeletal muscle of animals exposed to non-weight-bearing environments undergo subcellular transformations that may preferentially bias energy utilization to carbohydrates.

Effects of training on biochemical and functional properties of rodent neonatal heart.

This study was undertaken to determine biochemical and functional (in vivo) adaptations of the rodent neonatal heart in response to a training program of endurance running. Ten day-old rats were progressively trained on a treadmill (final intensity, 21 m/min, 30% grade, 1 h/day) until 75 days of age. The training program induced 14, 57, and 24% increases in relative heart mass, skeletal muscle citrate synthase activity, and whole-body maximal O2 uptake, respectively (P less than 0.05). Cardiac myosin (ATPase) and Ca2+-regulated myofibril ATPase were both reduced by approximately 15% in trained vs. sedentary animals (P less than 0.05). In the majority of trained hearts examined, the myosin isozyme profile reflected an estimated 14 +/- 3% shift toward the V3 or low ATPase isozyme. Left ventricular functional indices during submaximal exercise, derived from a fluid-filled indwelling cannula, indicated that the trained animals maintained similar left ventricular (LV) systolic pressure, LV + the time derivative of pressure, and systemic arterial mean blood pressure compared with their sedentary counterparts. These functional parameters were maintained even though the trained animals performed with lower submaximal exercise heart rate. These findings suggest that maximal exercise capacity can be enhanced in neonatal rats even though the biochemical potential for ATP degradation in the cardiac contractile system is lowered. We speculate that the trend to maintain the myosin isozyme pattern further in the direction of the V3 isozyme in the trained neonatal rat heart may reflect a means to economize cross-bridge cycling while maintaining normal levels of ventricle performance at a given submaximal work load.

Effects of endurance exercise on isomyosin patterns in fast- and slow-twitch skeletal muscles.

Although endurance training has been shown to profoundly affect the oxidative capacity of skeletal muscle, little information is available concerning the impact of endurance training on skeletal muscle isomyosin expression across a variety of muscle fiber types. Therefore, a 10-wk running program (1 h/day, 5 days/wk, 20% grade, 1 mile/h) was conducted to ascertain the effects of endurance training on isomyosin expression in the soleus, vastus intermedius (VI), plantaris (PLAN), red and white medial gastrocnemius (RMG and WMG), and red and white vastus lateralis muscles (RVL and WVL). Evidences of training were noted by the presence of a resting and a submaximal exercise bradycardia, as well as an enhancement in peak O2 consumption in the trained rodents relative to the nontrained controls. No evidence for skeletal muscle hypertrophy was observed subsequent to training when muscle weight was normalized to body weight. Shifts in the isomyosin profile of the trained VI, RMG, RVL, and PLAN were seen relative to the nontrained controls. Specifically, training affected the slow myosin (SM) composition of the VI by decreasing the relative content of the SM2 isoform by 14% while increasing that of the SM1 isoform (P less than 0.05). In addition, training elicited various degrees of a fast to slower myosin transformation in the RMG, RVL, and PLAN. All three muscles showed a significant reduction in the fast myosin 2 isoform (P less than 0.05), with significant increases in intermediate myosin in the RVL and PLAN along with elevations in SM2 in the RMG and PLAN (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of thyroid state on cardiac myosin P-light chain phosphorylation during exercise.

This study ascertained the effects of thyroid deficiency (TD) and hyperthyroidism (H) on in vivo cardiovascular functional capacity in the context of cardiac myosin light chain 2 phosphorylation [P-LC(P)], a proposed modulator of myocardial function, at rest and during exercise. Compared with normal controls (NC), Ca2(+)-regulated myofibril adenosinetriphosphatase was reduced by 39% in TD and increased by 9% in H rats. This response was associated with a 20-fold increase in the V3 isoform and an 11% increase in the V1 isoform in TD and H rats, respectively. Submaximal treadmill exercise elicited significant elevations in all myocardial functional indexes examined in H rats compared with the NC group, whereas the opposite occurred for the TD group. Despite the marked contrast in cardiac function among the three groups, intrinsic levels of P-LC(P) were similar at rest among the groups and were significantly reduced in both TD and H groups relative to controls during exercise. These data suggest that although thyroid state exerts a profound impact on intrinsic myocardial functional state, it exerts little control over cellular processes regulating P-LC(P) during rest and exercise.

A new animal model for modulating myosin isoform expression by altered mechanical activity.

The purpose of this study was to develop a new rodent model that is capable of delineating the importance of mechanical loading on myosin heavy chain (MHC) isoform expression of the plantar and dorsi flexor muscles of the ankle. The essential components of this system include 1) stimulating electrodes that are chronically implanted into a muscle, allowing for the control of the activation pattern of the target muscle(s); 2) a training apparatus that translates the moment of the ankle into a linear force; and 3) a computer-controlled Cambridge 310 ergometer. The isovelocity profile of the ergometer ensured that the medial gastrocnemius (MG) produced forces that were > 90% of maximal isometric force (Po), and the eccentric contractions of the tibialis anterior (TA) were typically 120% of Po. Both the concentric and eccentric training programs produced statistically significant increases in the muscle mass of the MG (approximately 15%) and TA (approximately 7%) as well as a decrease in myofibrillar adenosinetriphosphatase activity. Both the white and red regions of the MG and TA exhibited significant increases in the relative content of the type IIa MHC and concomitant decreases in type IIb MHC expression. Although the red regions of the MG and red TA contained approximately 10% type I MHC, the training programs did not affect this isoform. It appears that when a fast-twitch muscle is stimulated at a high frequency (100 Hz) and required to contract either concentrically or eccentrically under high loading conditions, the expression of the type IIa MHC isoform will be upregulated, whereas that of the type IIb MHC will be concomitantly downregulated.

Left ventricular functional capacity in the endurance-trained rodent.

Cardiac myosin P-light chain phosphorylation [P-LC(P)] has been proposed to augment myocardial force production. This study was undertaken to examine the potential for cardiac myosin P-LC(P) for both equivalent heart rate and work load in exercising endurance-trained and nontrained rodents. A 10-wk training protocol elicited a significant reduction in submaximal running O2 uptake while enhancing peak O2 uptake (-17 and 10%, respectively, P less than 0.05). Left ventricular functional index during submaximal exercise, obtained with a high-fidelity Millar ultraminiature pressure transducer, indicated that the trained animals were able to maintain peak left ventricular pressure (LVP) in comparison to their sedentary counterparts, even though both heart rate and rate of LVP development were significantly reduced (P less than 0.05). When expressed on the basis of equivalent submaximal heart rate, peak LVP was augmented in the trained animals. Cardiac myosin P-LC(P) was examined under two conditions known to produce disparate responses in trained vs. sedentary animals. For an equivalent work load, we observed parallel increases in P-LC(P) (20%) and systolic pressure (17%) in both groups, even though the trained animals exhibited significantly lower heart rates (P less than 0.05). For an equivalent heart rate, training evoked a significant increase in systolic pressure (26%, P less than 0.05) and caused a slight increase in P-LC(P) relative to the nontrained controls. Cardiac myosin adenosinetriphosphatase was reduced approximately 10% in the trained animals (P less than 0.05), commensurate with a 2.0-fold increase in the V3 (low adenosinetriphosphatase) isomyosin.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac biochemical and functional adaptations to exercise in sympathectomized neonatal rats.

This study was undertaken to examine the influence of guanethidine monosulfate-induced sympathectomy on exercise-induced adaptations of cardiac contractile protein and on acute hemodynamic responses to exercise involving female neonatal rats. Four groups of rats were studied: 1) normal sedentary (NS), 2) normal trained (NT), 3) sympathectomized sedentary (SS), and 4) sympathectomized trained (ST). The 9-wk running program, which began at 20 days of age, induced increases in whole-body maximal O2 consumption and skeletal-muscle citrate synthase activity in both NT and ST groups compared with NS (P less than 0.05). Submaximal exercise tests demonstrated circulatory adaptations for NT, SS, and ST groups compared with NC; however, the ST group demonstrated the greatest degree of altered cardiac function (decreased heart rate, left ventricular pressure, and contractility index) during exercise. Also, significant reductions in both myosin- and Ca2+-regulated myofibril adenosinetriphosphatase (ATPase) activity and increases in the relative content of the low ATPase myosin isozyme, V3, occurred in the hearts of the two trained groups (P less than 0.05). These findings suggest that chronic exercise involving normal and sympathectomized neonatal rats improves cardiac function without compromising maximal exercise capacity. Also, the exercise-related adaptation involving myosin isozyme shifts are exaggerated when involvement of the sympathetic nervous system is reduced during training.

Effect of spaceflight on skeletal muscle: mechanical properties and myosin isoform content of a slow muscle.

This study examined changes in contractile, biochemical, and histochemical properties of slow antigravity skeletal muscle after a 6-day spaceflight mission. Twelve male Sprague-Dawley rats were randomly divided into two groups: flight and ground-based control. Approximately 3 h after the landing, in situ contractile measurements were made on the soleus muscles of the flight animals. The control animals were studied 24 h later. The contractile measurements included force-velocity relationship, force-frequency relationship, and fatigability. Biochemical measurements focused on the myosin heavy chain (MHC) and myosin light chain profiles. Adenosine-triphosphatase histochemistry was performed to identify cross-sectional area of slow and fast muscle fibers and to determine the percent fiber type distribution. The force-velocity relationships of the flight muscles were altered such that maximal isometric tension (Po) was decreased by 24% and maximal shortening velocity was increased by 14% (P < 0.05). The force-frequency relationship of the flight muscles was shifted to the right of the control muscles. At the end of the 2-min fatigue test, the flight muscles generated only 34% of Po, whereas the control muscles generated 64% of Po. The flight muscles exhibited de novo expression of the type IIx MHC isoform as well as a slight decrease in the slow type I and fast type IIa MHC isoforms. Histochemical analyses of flight muscles demonstrated a small increase in the percentage of fast type II fibers and a greater atrophy of the slow type I fibers. The results demonstrate that contractile properties of slow antigravity skeletal muscle are sensitive to the microgravity environment and that changes begin to occur within the 1st wk. These changes were at least, in part, associated with changes in the amount and type of contractile protein expressed.

Microgravity-induced transformations of myosin isoforms and contractile properties of skeletal muscle.

This study examined the effects of microgravity (14 days) on 1) the contractile properties of the soleus (Sol), an antigravity skeletal muscle; and 2) the myosin heavy chain (MHC) protein and mRNA isoform content of the Sol, vastus intermedius (VI), plantaris (Plan), and tibialis anterior (TA) muscles. The force-velocity relationships of the flight Sol muscles had a significant reduction in maximal isometric tension (-37%) and a corresponding increase in maximal shortening velocity (+20%). Additionally, the force-frequency relationship of the flight Sol muscles was shifted to the right of the ground-based control group. Microgravity had the greatest effect on muscle fiber composition in the Sol muscle, with a reduction in slow muscle fibers and a corresponding increase in muscle fibers categorized as hybrid fibers. The estimated absolute MHC isoform content was altered to the greatest extent in the Sol and VI muscles, with significant decreases and elevations in the slow type I and fast type IIX MHC protein isoforms, respectively. Consistent with the protein data, both the flight Sol and VI muscles exhibited significant elevations in the fast type IIX MHC mRNA isoform. In contrast, however, the flight Plan and TA groups had significant increases in the fast type IIB MHC mRNA isoform content without corresponding changes at the protein level. The results of this study suggest that spaceflight of even short duration produces important changes in the contractile properties of antigravity skeletal muscle. These changes are mediated by alterations in MHC phenotype and reductions in muscle mass. In some instances, the alterations in MHC mRNA isoform content seemed to be uncoupled from those occurring at the protein level. This apparent uncoupling between mRNA and protein expression demonstrates that the effects of microgravity must be better understood at the transcriptional, translational, and post-translational levels.

Interaction of various mechanical activity models in regulation of myosin heavy chain isoform expression.

The purpose of this study was to determine the effects of a novel combination of mechanical activity paradigms on the isomyosin distribution in rat hindlimb muscles. Thirty female Sprague-Dawley rats were divided into five experimental groups as follows: normal control, functional overload (OV) of the plantaris, OV in conjunction with hindlimb suspension (OV-S), and a combination of OV-S and either static standing weight-bearing activity (OV-SS) or high-incline treadmill exercise (OV-SE). OV of the plantaris resulted in significant hypertrophy and significant fast-to-slow isomyosin shifts. These changes were completely inhibited by the addition of hindlimb suspension (OV-S). Also, neither of the two weight-bearing regimes (OV-SS and OV-SE) was able to attenuate the suspension-induced atrophy. In the vastus intermedius and vastus lateralis, however, OV-SS was able to partially retard the atrophy associated with suspension. In both the plantaris and vastus intermedius, only OV-SS was able to partially reverse the slow-to-fast isomyosin transitions associated with suspension. These results suggest that the type of mechanical activity is important in determining adaptation to altered loading conditions, with OV-SS appearing more effective than OV-SE in reversing the effects of unweighting.