Contractile properties and myosin expression in rats born and reared in hypergravity

The effects of hypergravity (HG) on soleus and plantaris muscles were studied in Long Evans rats aged 100 days, born and reared in 2-g conditions (HG group). The morphological and contractile properties and the myosin heavy chain (MHC) content were examined in whole muscles and compared with terrestrial control (Cont) age-paired rats. The growth of HG rats was slowed compared with Cont rats. A decrease in absolute muscle weight was observed. An increase in fiber cross-sectional area/muscle wet weight was demonstrated, associated with an increase in relative maximal tension. The soleus muscle changed into a slower type both in contractile parameters and in MHC content, since HG soleus contained only the MHC I isoform. The HG plantaris muscle presented a faster contractile behavior. Moreover, the diversity of hybrid fiber types expressing multiple MHC isoforms (including MHC IIB and MHC IIX isoforms) was increased in plantaris muscle after HG. Thus the HG environment appears as an important inductor of muscular plasticity both in slow and fast muscle types.     

Preparation and characterization of troponin C from bullfrog skeletal muscle.

Troponin C was isolated from the skeletal muscle of bullfrog (Rana catesbeiana), and its relative molecular mass was estimated to be 18,000 by SDS/polyacrylamide gel electrophoresis. In its amino acid composition, bullfrog troponin C was similar to that of the frog (Rana esculenta) but different from that of rabbit. Its ultraviolet spectrum was consistent with its amino acid composition. The ultraviolet difference spectrum of the Ca(2+)-loaded form vs. the metal-free form indicated that the single Tyr residue and some Phe residues in the bullfrog troponin C molecule were affected by the conformational change associated with Ca2+ binding. On electrophoresis in polyacrylamide gel in 14 mM Tris and 90 mM glycine, the metal-free and Mg(2+)-loaded forms migrated slower than the Ca(2+)-loaded form. The property is shared by rabbit troponin C but not parvalbumins or calmodulin. The ATPase activity of CDTA-treated myofibrils reconstituted with bullfrog troponin C showed the same Ca(2+)- and Sr(2+)-sensitivity as that of those reconstituted with rabbit troponin C. Bullfrog troponin C is, thus, physiologically the same as rabbit troponin C, in spite of several marked differences in their physicochemical properties.     

Expression and functional properties of four slow skeletal troponin T isoforms in rat muscles

We investigated the expression and functional properties of slow skeletal troponin T (sTnT) isoforms in rat skeletal muscles. Four sTnT cDNAs were cloned from the slow soleus muscle. Three isoforms were found to be similar to sTnT1, sTnT2, and sTnT3 isoforms described in mouse muscles. A new rat isoform, with a molecular weight slightly higher than that of sTnT3, was discovered. This fourth isoform had never been detected previously in any skeletal muscle and was therefore called sTnTx. From both expression pattern and functional measurements, it appears that sTnT isoforms can be separated into two classes, high-molecular-weight (sTnT1, sTnT2) and low-molecular-weight (sTnTx, sTnT3) isoforms. By comparison to the apparent migration pattern of the four recombinant sTnT isoforms, the newly described low-molecular-weight sTnTx isoform appeared predominantly and typically expressed in fast skeletal muscles, whereas the higher-molecular-weight isoforms were more abundant in slow soleus muscle. The relative proportion of the sTnT isoforms in the soleus was not modified after exposure to hindlimb unloading (HU), known to induce a functional atrophy and a slow-to-fast isoform transition of several myofibrillar proteins. Functional data gathered from replacement of endogenous troponin complexes in skinned muscle fibers showed that the sTnT isoforms modified the Ca(2+) activation characteristics of single skeletal muscle fibers, with sTnT2 and sTnT1 conferring a similar increase in Ca(2+) affinity higher than that caused by low-molecular-weight isoforms sTnTx and sTnT3. Thus we show for the first time the presence of sTnT in fast muscle fibers, and our data show that the changes in neuromuscular activity on HU are insufficient to alter the sTnT expression pattern.     

Isoform switching in myofibrillar and excitation-contraction coupling proteins contributes to diminished contractile function in regenerating rat soleus muscle.

Postnatal development of skeletal muscle occurs through the progressive transformation of diverse biochemical, metabolic, morphological, and functional characteristics from the embryonic to the adult phenotype. Since muscle regeneration recapitulates postnatal development of muscle fiber, it offers an appropriate experimental model to investigate the existing relationships between diverse muscle functions and the expression of key protein isoforms, particularly at the single-fiber level. This study was carried out in regenerating soleus muscle 14 days after injury. At this intermediate stage, the regenerating muscle exhibited a recovery of mass greater than its force generation capacity. The lower specific tension of regenerating muscle suggested intrinsic defective excitation-contraction coupling and/or contractility processes. The presence of developmental isoforms of both the voltage-gated Ca(2+) channel (alpha(1)C) and of ryanodine receptor 3, paralleled by an abnormal caffeine contracture development, confirms the immature excitation-contraction coupling of the regenerating muscle. The defective Ca(2+) handling could also be confirmed by the lower sarcoplasmic reticulum caffeine sensitivity of regenerating single fibers. Also, regenerating single fibers revealed a lower maximal specific tension, which was associated with the residual presence of embryonic myosin heavy chains. Moreover, the fibers showed a reduced Ca(2+) sensitivity of myofibrillar proteins, particularly those simultaneously expressing the slow and fast isoforms of troponin C. The present results indicate that the expression of developmental proteins determines the incomplete functional recovery of regenerating soleus.     

Replacement of three troponin components with cardiac troponin components within single glycerinated skeletal muscle fibers.

The tension of single glycerinated rabbit skeletal muscle fiber was desensitized to a Ca(2+)-concentration after treatment with an excessive amount of bovine cardiac troponin T and reached a level of about 70% of the maximum tension of the untreated fiber. A SDS-gel electrophoretic examination indicated that troponin C.I.T complex in the fiber was replaced with the added cardiac troponin T. The Ca(2+)-sensitivity of the tension of the troponin T-treated fiber was then recovered by the addition of bovine cardiac troponins I and C. The rabbit skeletal muscle fiber thus hybridized with bovine cardiac troponin C.I.T showed the same cooperativity of Ca(2+)-activation as the cardiac muscle.     

Coupled expression of troponin T and troponin I isoforms in single skeletal muscle fibers correlates with contractility

Striated muscle contraction is powered by actin-activated myosin ATPase. This process is regulated by Ca(2+) via the troponin complex. Slow- and fast-twitch fibers of vertebrate skeletal muscle express type I and type II myosin, respectively, and these myosin isoenzymes confer different ATPase activities, contractile velocities, and force. Skeletal muscle troponin has also diverged into fast and slow isoforms, but their functional significance is not fully understood. To investigate the expression of troponin isoforms in mammalian skeletal muscle and their functional relationship to that of the myosin isoforms, we concomitantly studied myosin, troponin T (TnT), and troponin I (TnI) isoform contents and isometric contractile properties in single fibers of rat skeletal muscle. We characterized a large number of Triton X-100-skinned single fibers from soleus, diaphragm, gastrocnemius, and extensor digitorum longus muscles and selected fibers with combinations of a single myosin isoform and a single class (slow or fast) of the TnT and TnI isoforms to investigate their role in determining contractility. Types IIa, IIx, and IIb myosin fibers produced higher isometric force than that of type I fibers. Despite the polyploidy of adult skeletal muscle fibers, the expression of fast or slow isoforms of TnT and TnI is tightly coupled. Fibers containing slow troponin had higher Ca(2+) sensitivity than that of the fast troponin fibers, whereas fibers containing fast troponin showed a higher cooperativity of Ca(2+) activation than that of the slow troponin fibers. These results demonstrate distinct but coordinated regulation of troponin and myosin isoform expression in skeletal muscle and their contribution to the contractile properties of muscle.     

Adaptation in synergistic muscles to soleus and plantaris muscle removal in the rat hindlimb.

Although the soleus muscle comprises only 6% of the ankle plantar flexor mass in the rat, a major role in stance and walking has been ascribed to it. The purpose of this study was to determine if removal of the soleus muscle would result in adaptations in the remaining gastrocnemius and plantaris muscles due to the new demands for force production imposed on them during stance or walking. A second purpose was to determine whether the mass or the fiber type of the muscle(s) removed was a more important determinant of compensatory adaptations. Male Sprague-Dawley rats underwent bilateral removal of soleus muscle, plantaris muscle, or both muscles. For comparison, compensatory hypertrophy was induced in soleus and plantaris muscles by gastrocnemius muscle ablation. After forty days, synergist muscles remaining intact were removed. Mass, and oxidative, glycolytic, and contractile enzyme activities were determined. Despite its role in stance and slow walking, removal of the soleus muscle did not elicit a measurable alteration in muscle mass, or in citrate synthase, lactate dehydrogenase, or myofibrillar ATPase activity in gastrocnemius or plantaris muscles. Similarly, removal of the plantaris muscle, or soleus and plantaris muscles, had no effect on the gastrocnemius muscle, suggesting that this muscle was able to easily meet the new demands placed on it. These results suggest that amount of muscle mass removed, rather than fiber type, is the most important stimulus for compensatory hypertrophy. They also suggest that slow-twitch motor units in the gastrocnemius muscle play an important role during stance and locomotion in the intact animal.     

Effects of removal and reconstitution of myosin regulatory light chain and troponin C on the Ca(2+)-sensitive ATPase activity of myofibrils from scallop striated muscle

In order to examine the involvement of troponin-linked Ca(2+)-regulation, in addition to well-known myosin-linked Ca(2+)-regulation, in the contraction of molluscan striated muscle, myofibrils from Ezo-giant scallop striated muscle were desensitized to Ca(2+) by removing both myosin regulatory light chain and troponin C by treatment with a strong divalent cation chelator, CDTA. The ATPase level in the desensitized myofibrils was about half the maximum level in intact myofibrils regardless of the Ca(2+)-concentration at 25 and 15 degrees C. In the absence of Ca(2+), the ATPase of the desensitized myofibrils was suppressed by myosin regulatory light chain but not affected by troponin C at either temperature. The ATPase was activated at higher Ca(2+)-concentrations by both myosin regulatory light chain and troponin C, but the activating effects of these two proteins were affected differently by temperature. The activation of ATPase by myosin regulatory light chain was much greater than that by troponin C at 25 degrees C, whereas the activation by troponin C was much greater than that by myosin regulatory light chain at 15 degrees C. The maximum activation was only obtained in the presence of both myosin regulatory light chain and troponin C at these temperatures. These findings strongly suggest that the contraction of scallop striated muscle is regulated through both myosin-linked and troponin-linked Ca(2+)-regulation, and that the troponin-linked Ca(2+)-regulation is more significant at lower temperature.     

Structure-function relationships of molluscan troponin T revealed by limited proteolysis

Molluscan troponin regulates muscle contraction through a novel Ca(2+)-dependent activating mechanism associated with Ca(2+)-binding to the C-terminal domain of troponin C. To elucidate the further details of this regulation, we performed limited chymotryptic digestion of the troponin complex from akazara scallop striated muscle. The results indicated that troponin T is very susceptible to the protease, compared to troponin C or troponin I. The cleavage occurred at the C-terminal extension, producing an N-terminal 33-kDa fragment and a C-terminal 6-kDa fragment. This extension is conserved in various invertebrate troponin T proteins, but not in vertebrate troponin T. A ternary complex composed of the 33-kDa fragment of troponin T, troponin I, and troponin C could be separated from the 6-kDa troponin T fragment by gel filtration. This complex did not show any Ca(2+)-dependent activation of the Mg-ATPase activity of rabbit-actomyosin-scallop-tropomyosin. In addition, the actin-tropomyosin-binding affinity of this complex was significantly decreased with increasing Ca(2+) concentration. These results indicate that the C-terminal extension of molluscan troponin T plays a role in anchoring the troponin complex to actin-tropomyosin filaments and is essential for regulation.     

Regulation of contraction kinetics in skinned skeletal muscle fibers by calcium and troponin C

The influences of [Ca(2+)] and Ca(2+) dissociation rate from troponin C (TnC) on the kinetics of contraction (k(Ca)) activated by photolysis of a caged Ca(2+) compound in skinned fast-twitch psoas and slow-twitch soleus fibers from rabbits were investigated at 15 degrees C. Increasing the amount of Ca(2+) released increased the amount of force in psoas and soleus fibers and increased k(Ca) in a curvilinear manner in psoas fibers approximately 5-fold but did not alter k(Ca) in soleus fibers. Reconstituting psoas fibers with mutants of TnC that in solution exhibited increased Ca(2+) affinity and approximately 2- to 5-fold decreased Ca(2+) dissociation rate (M82Q TnC) or decreased Ca(2+) affinity and approximately 2-fold increased Ca(2+) dissociation rate (NHdel TnC) did not affect maximal k(Ca). Thus the influence of [Ca(2+)] on k(Ca) is fiber type dependent and the maximum k(Ca) in psoas fibers is dominated by kinetics of cross-bridge cycling over kinetics of Ca(2+) exchange with TnC.     

Denervation and reinnervation of fast and slow muscles. A histochemical study in rats.

A histochemical study, using myosin-adenosine triphosphatase activity at pH 9.4, was conducted in soleus and plantaris muscles of adult rats, after bilateral crushing of the sciatic nerve at the sciatic notch. The changes in fiber diameter and per cent composition of type I and type II fibers plus muscle weights were evaluated along the course of denervation-reinnervation curve at 1, 2, 3, 4 and 6 weeks postnerve crush. The study revealed that in the early denervation phase (up to 2 weeks postcrush) both the slow and fast muscles, soleus and plantaris, resepctively, atrophied similarly in muscle mass. Soleus increased in the number of type II fibers, which may be attributed to "disuse" effect. During the same period, the type I fibers of soleus atrophied as much or slightly more than the type II fibers; whereas the type II fibers of plantaris atrophied significantly more than the type I fibers, reflecting that the process of denervation, in its early stages, may affect the two fiber types differentially in the slow and fast muscles. It was deduced that the type I fibers of plantaris may be essentially different in the slow (soleus) and fast (plantaris) muscles under study. The onset of reinnervation, as determined by the increase in muscle weight and fiber diameter of the major fiber type, occurred in soleus and plantaris at 2 and 3 weeks postcrush, respectively, which confirms the earlier hypotheses that the slow muscles are reinnervated sooner than the fast muscles. It is suggested that the reinnervation of muscle after crush injury may be specific to the muscle type or its predominant fiber type.     

Changes in muscle proteins and spermidine content in response to unloading and clenbuterol treatment

Anabolic agents such clenbuterol (Cb) are useful tools for probing the mechanisms by which muscles respond to disuse. Cb was examined under different loading conditions with respect to its effects on muscle mass, protein (myofibrillar and cytosolic), and spermidine content in mature male rats. Compared with control treatment, Cb significantly increased loaded and unloaded soleus, plantaris, and extensor digitorum longus (EDL) mass. Likewise, Cb significantly increased loaded and unloaded soleus (24.8 and 21.6%, respectively), plantaris (12.1 and 22.9%, respectively), and EDL (22.4 and 13.3%, respectively) myofibrillar protein content. After unloading, cytosolic proteins significantly increased in the EDL but decreased in the soleus and plantaris. Cb significantly increased cytosolic protein levels in all loaded muscles, while only causing increases in unloaded soleus. When compared with controls, unloading caused significant reductions in spermidine levels in the soleus (40.4%) and plantaris (35.9%) but caused increases in the EDL (54.8%). In contrast, Cb increased spermidine levels in unloaded soleus (42.9%), plantaris (102.8%), and EDL (287%). In loaded muscles, Cb increased spermidine levels in all three muscles, but to a lesser degree than under unloading conditions. Nonlinear regression analyses indicated that the plantaris behaves like a slow-twitch muscle under unloading conditions and like a fast-twitch muscle when loaded. This suggests that the responses of these muscles to unloading and (or) Cb treatment might be influenced by factors beyond fiber type alone.     

Hindlimb unloading-induced muscle atrophy and loss of function: protective effect of isometric exercise.

The primary objective of this study was to determine the effectiveness of isometric exercise (IE) as a countermeasure to hindlimb unloading (HU)-induced atrophy of the slow (soleus) and fast (plantaris and gastrocnemius) muscles. Rats were assigned to either weight-bearing control, 7-day HU (H7), H7 plus IE (I7), 14-day HU (H14), or H14 plus IE (I14) groups. IE consisted of ten 5-s maximal isometric contractions separated by 90 s, administered three times daily. Contractile properties of the soleus and plantaris muscles were measured in situ. The IE attenuated the HU-induced decline in the mass and fiber diameter of the slow-twitch soleus muscle, whereas the gastrocnemius and plantaris mass were not protected. These results are consistent with the mean electromyograph recordings during IE that indicated preferential recruitment of the soleus over the gastrocnemius and plantaris muscles. Functionally, the IE significantly protected the soleus from the HU-induced decline in peak isometric force (I14, 1.49 +/- 0.12 vs. H14, 1.15 +/- 0.07 N) and peak power (I14, 163 +/- 17 vs. H14, 75 +/- 11 mN.fiber length.s-1). The exercise protocol showed protection of the plantaris peak isometric force at H7 but not H14. The IE also prevented the HU-induced decline in the soleus isometric contraction time, which allowed the muscle to produce greater tension at physiological motoneuron firing frequencies. In summary, IE resulted in greater protection from HU-induced atrophy in the slow soleus than in the fast gastrocnemius or plantaris.     

Ca2+ movements in sarcoplasmic reticulum of rat soleus fibers after hindlimb suspension.

The functional capacity of skeletal muscle sarcoplasmic reticulum (SR) was examined in the slow soleus of rats submitted to 15 days of disuse produced by hindlimb suspension (HS). By using caffeine-induced contractions of single skinned fibers, Ca2+ uptake, Ca2+ release, and passive Ca2+ leakage through the SR membrane were investigated. In the SR of atrophied muscles, the amounts of Ca2+ uptake and Ca2+ release were significantly higher than in the control muscles and were close to those found for a fast muscle, the plantaris. Moreover, the study of the Ca2+ leakage showed that the time required to empty the SR previously loaded with Ca2+ was reduced by a factor of two after HS. Such disturbances of the Ca2+ movements in the SR suggested that alterations of the SR membrane occurred after HS. The results supported the idea that after hindlimb unweighting the slow soleus muscle acquired SR properties that were very much like those of a faster muscle.     

Age effects on myosin subunit and biochemical alterations with skeletal muscle hypertrophy.

The purpose of this study was to determine whether skeletal muscle mass, myofibrillar adenosinetriphosphatase activity, and the expression of myosin heavy (MHC) and light chain subunits are differentially affected in juvenile (4 wk) and young adult (12 wk) rats by a hypertrophic growth stimulus. Hypertrophy of the plantaris or soleus was studied 4 wk after ablation of either two [gastrocnemius (GTN) and soleus or plantaris] or one (GTN) synergistic muscle(s). There was no difference in the relative magnitude of hypertrophy because of age. Plantaris myofibrillar adenosinetriphosphatase activity was decreased 21 and 12% in juvenile and adult rats, respectively, as a result of ablation of both the GTN and soleus. Slow myosin light chain isoforms (1s and 2s) were expressed to a greater extent in hypertrophied plantaris muscles of both ages, but the increase in 1s was greater in juvenile rats. The relative expression of slow beta-MHC in hypertrophied plantaris muscles increased by 470 and 350%, whereas MHC IIb decreased by 70 and 33% in juvenile and adult rats, respectively. The relative expression of MHC IIa increased (56%) in the plantaris after ablation in juvenile rats only. These shifts in myosin subunit expression and the increases in mass were generally about one-half the magnitude when only the GTN was removed. There were no detectable myosin shifts in hypertrophied soleus muscles. Although the extent of muscle hypertrophy is similar, the shifts in myosin subunits were greater in juvenile than in young adult rats.     

Depressed contractile performance and reduced fatigue resistance in single skinned fibers of soleus muscle after long-term disuse in rats

Long-term disuse results in atrophy in skeletal muscle, which is characterized by reduced functional capability, impaired locomotor condition, and reduced resistance to fatigue. Here we show how long-term disuse affects contractility and fatigue resistance in single fibers of soleus muscle taken from the hindlimb immobilization model of the rat. We found that long-term disuse results in depression of caffeine-induced transient contractions in saponin-treated single fibers. However, when normalized to maximal Ca(2+)-activated force, the magnitude of the transient contractions became similar to that in control fibers. Control experiments indicated that the active force depression in disused muscle is not coupled with isoform switching of myosin heavy chain or troponin, or with disruptions of sarcomere structure or excessive internal sarcomere shortening during contraction. In contrast, our electronmicroscopic observation supported our earlier observation that interfilament lattice spacing is expanded after disuse. Then, to investigate the molecular mechanism of the reduced fatigue resistance in disused muscle, we compared the inhibitory effects of inorganic phosphate (Pi) on maximal Ca(2+)-activated force in control vs. disused fibers. The effect of Pi was more pronounced in disused fibers, and it approached that observed in control fibers after osmotic compression. These results suggest that contractile depression in disuse results from the lowering of myofibrillar force-generating capacity, rather than from defective Ca(2+) mobilization, and the reduced resistance to fatigue is from an enhanced inhibitory effect of Pi coupled with a decrease in the number of attached cross bridges, presumably due to lattice spacing expansion.     

Fasting-related autophagic response in slow- and fast-twitch skeletal muscle

This study investigated regulation of autophagy in slow-twitch soleus and fast-twitch plantaris muscles in fasting-related atrophy. Male Fischer-344 rats were subjected to fasting for 1, 2, or 3 days. Greater weight loss was observed in plantaris muscle than in soleus muscle in response to fasting. Western blot analysis demonstrated that LC3-II, a marker protein for macroautophagy, was expressed at a notably higher level in plantaris than in soleus muscle, and that the expression level was fasting duration-dependent. To identify factors related to LC3-II enhancement, autophagy-related signals were examined in both types of muscle. Phosphorylated mTOR was reduced in plantaris but not in soleus muscle. FOXO3a and ER stress signals were unchanged in both muscle types during fasting. These findings suggest that preferential atrophy of fast-twitch muscle is associated with induction of autophagy during fasting and that differences in autophagy regulation are attributable to differential signal regulation in soleus and plantaris muscle.     

Dual effect of deafferentation on contractile characteristics and sarcoplasmic reticulum properties in rat soleus fibers.

The neural message is known to play a key role in muscle development and function. We analyzed the specific role of the afferent message on the functional regulation of two subcellular muscle components involved in the contractile mechanism: the contractile proteins and the sarcoplasmic reticulum (SR). Rats were submitted to bilateral deafferentation (DEAF group) by section of the dorsal roots L(3) to L(5) after laminectomy. Experiments were carried out in single skinned fibers of the soleus muscle. The maximal force developed by the contractile proteins was increased in the DEAF group compared with control, despite a decrease in muscle mass by 17%. The tension-pCa relationship was shifted toward lower calcium (Ca(2+)) concentrations. Different functional properties of the SR of DEAF soleus were examined by using caffeine-induced contractions. The caffeine sensitivity of the Ca(2+) release was decreased after deafferentation and ryanodine receptor 1 isoform was expressed at a lower level. The rate of Ca(2+) uptake was only slightly increased. The results underlined the dual effect of the afferent input on the functional regulation of both contractile proteins and SR.     

Catecholamines inhibit Ca(2+)-dependent proteolysis in rat skeletal muscle through beta(2)-adrenoceptors and cAMP

Overall proteolysis and the activity of skeletal muscle proteolytic systems were investigated in rats 1, 2, or 4 days after adrenodemedullation. Adrenodemedullation reduced plasma epinephrine by 95% and norepinephrine by 35% but did not affect muscle norepinephrine content. In soleus and extensor digitorum longus (EDL) muscles, rates of overall proteolysis increased by 15-20% by 2 days after surgery but returned to normal levels after 4 days. The rise in rates of protein degradation was accompanied by an increased activity of Ca(2+)-dependent proteolysis in both muscles, with no significant change in the activity of lysosomal and ATP-dependent proteolytic systems. In vitro rates of Ca(2+)-dependent proteolysis in soleus and EDL from normal rats decreased by ~35% in the presence of either 10(-5) M clenbuterol, a beta(2)-adrenergic agonist, or epinephrine or norepinephrine. In the presence of dibutyryl cAMP, proteolysis was reduced by 62% in soleus and 34% in EDL. The data suggest that catecholamines secreted by the adrenal medulla exert an inhibitory control of Ca(2+)-dependent proteolysis in rat skeletal muscle, mediated by beta(2)-adrenoceptors, with the participation of a cAMP-dependent pathway.     

Effects of moderate heart failure and functional overload on rat plantaris muscle.

It is thought that changes in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) of skeletal muscle contribute to alterations in skeletal muscle function during congestive heart failure (CHF). It is well established that exercise training can improve muscle function. However, it is unclear whether similar adaptations will result from exercise training in a CHF patient. Therefore, the purpose of this study was to determine whether skeletal muscle during moderate CHF adapts to increased activity, utilizing the functional overload (FO) model. Significant increases in plantaris mass of the CHF-FO and sham-FO groups compared with the CHF and control (sham) groups were observed. Ca(2+) uptake rates were significantly elevated in the CHF group compared with all other groups. No differences were detected in Ca(2+) uptake rates between the CHF-FO, sham, and sham-FO groups. Increases in Ca(2+) uptake rates in moderate-CHF rats were not due to changes in SERCA isoform proportions; however, FO may have attenuated the CHF-induced increases through alterations in SERCA isoform expression. Therefore, changes in skeletal muscle Ca(2+) handling during moderate CHF may be due to alterations in regulatory mechanisms, which exercise may override, by possibly altering SERCA isoform expression.