Chemical energetics of slow- and fast-twitch muscles of the mouse

The energy utilization associated with contraction was measured in isolated slow- and fast-twitch muscles of the mouse at 20 degrees C. The extent of this utilization was estimated from either the extent of high-energy phosphate splitting occurring during contraction (the initial chemical change, delta approximately P init) or from the extent of recovery resynthesis calculated from the observed oxygen consumption and lactate production occurring during the recovery period (recovery chemical resynthesis, delta approximately P rec). For short tetani, the cost to maintain isometric tension in the fast-twitch extensor digitorum longus (EDL) was approximately threefold greater than that in the slow-twitch soleus. With prolonged stimulation, however, the energy cost in the EDL diminished so that after 12 s of stimulation, the energy cost in the EDL was only 50% greater than that of the soleus. For both the slow-twitch soleus and the fast-twitch EDL and for all tetanus durations (up to 15 s), the extent of the initial chemical change was identical with the amount of recovery chemical resynthesis, showing that a biochemical energy balance existed in these muscles.     

Effects of caffeine at different temperatures on contractile properties of slow-twitch and fast-twitch rat muscles

The slow-twitch soleus muscle (SOL) exhibits decreased twitch tension (cold depression) in response to a decreased temperature, whereas the fast-twitch extensor digitorum longus (EDL) muscle shows enhanced twitch tension (cold potentiation). On the other hand, the slow-twitch SOL muscle is more sensitive to twitch potentiation and contractures evoked by caffeine than the fast-twitch EDL muscle. In order to reveal the effects of these counteracting conditions (temperature and caffeine), we have studied the combined effects of temperature changes on the potentiation effects of caffeine in modulating muscle contractions and contractures in both muscles. Isolated muscles, bathed in a Tyrode solution containing 0.1-60 mM caffeine, were stimulated directly and isometric single twitches, fused tetanic contractions and contractures were recorded at 35 degrees C and 20 degrees C. Our results showed that twitches and tetani of both SOL and EDL were potentiated and prolonged in the presence of 0.3-10 mM caffeine. Despite the cold depression, the extent of potentiation of the twitch tension by caffeine in the SOL muscle at 20 degrees C was by 10-15 % higher than that at 35 degrees C, while no significant difference was noted in the EDL muscle between both temperatures. Since the increase of twitch tension was significantly higher than potentiation of tetani in both muscles, the twitch-tetanus ratio was enhanced. Higher concentrations of caffeine induced contractures in both muscles; the contracture threshold was, however, lower in the SOL than in the EDL muscle at both temperatures. Furthermore, the maximal tension was achieved at lower caffeine concentrations in the SOL muscle at both 35 degrees C and 20 degrees C compared to the EDL muscle. These effects of caffeine were rapidly and completely reversed in both muscles when the test solution was replaced by the Tyrode solution. The results have indicated that the potentiation effect of caffeine is both time- and temperature-dependent process that is more pronounced in the slow-twitch SOL than in the fast-twitch EDL muscles.     

Correlated reduction of velocity of shortening and the rate of energy utilization in mouse fast-twitch muscle during a continuous tetanus

Isometric tetani of slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles of the mouse were studied at 20 degrees C. The total energy cost for 3- and 9-s isometric tetani was measured as a function of length above L0 and partitioned into a filament overlap-dependent fraction and a smaller filament overlap-independent fraction. In both muscles, the rate of filament overlap-independent energy cost did not change with tetanic duration. In the EDL, but not in the soleus, the rate of filament overlap-dependent energy utilization was greater in a 3-s tetanus than in a 9-s tetanus. The force-velocity relationships were studied after 3 and 9 s of isometric tetanus. In the soleus, Vmax was 2 fiber lengths/s and was not dependent on the duration of isometric tetanus. In contrast, in the EDL, Vmas decreased from 5.9 fiber lengths/s at 3 s to 3.9 fiber lengths/s at 9 s. The velocity of unloaded shortening (Vus) was examined by the slack test method as a function of the duration of isometric tetanus duration over the range of 1-15 s. In the soleus, Vus did not change, whereas in the EDL, Vus declined progressively from 6.4 to 3.2 fiber lengths/s after an isometric tetanus of increasing duration from 1 to 15 s. These results cannot exclude the hypothesis that in a maintained tetanus there is a decrease in the intrinsic cross- bridge turnover rate in the fast-twitch EDL, but not in the slow-twitch soleus muscle.     

Isometric contractile properties and instantaneous stiffness of amphibian skeletal muscle in the temperature range from 0 to 20 degrees C

The isometric contractile properties of frog (Rana pipiens) and toad (Bufo bufo) sartorii have been studied over the temperature range from 0 to 20 degrees C. The isometric twitch tension was found to vary considerably between these two species and between muscles in the same species. Between 0 and 4 degrees C there was very little change in maximum isometric twitch tension. Between 4 and 12 degrees C several muscles from frog or toad showed a potentiation of twitch tension whereas others showed a decline. Over this temperature range the toad sartorii consistently demonstrated a greater potentiation. By 12 degrees C a steady decline in twitch tension in both muscles was seen as the temperature range the toad sartorii consistently demonstrated a greater potentiation. By 12 degrees C a steady decline in twitch tension in both muscles was seen as the temperature approached 20 degrees C. The maximum isometric tetanic tension recorded between 18 and 20 degrees C increased fractionally to an average of 1.504 +/- 0.029 (n = 4) for frog sartorii and to 1.377 +/- 0.008 (n = 5) for toad sartorii. The time to peak twitch tension and the half-relaxation time decreased markedly with an increase in temperature. Moreover, the half-relaxation time was reduced by a greater proportion than the time to peak twitch tension. Measurements of instantaneous stiffness by controlled velocity releases from the plateau of isometric tetani revealed that the large increase in isometric tetanus tension as the muscle was warmed was not accompanied by a corresponding increase in the total number of active cross-bridges. The possibility that a decreased availability of intracellular Ca2+ ions at the contractile sites contributing to the fall of isometric twitch tension at elevated temperatures is discussed. The possibility exists that at elevated temperatures a change inthe intrinsic contractile ability of the muscle occurs which produces an increased tension per cross-bridge.     

Changes in isometric contractile properties of extensor digitorum longus and soleus muscles of C57BL/6J mice following denervation

In this study, conducted on mice of the C57BL/6J+/+ strain, we investigated the differential effects of denervation on the isometric contractile properties of the extensor digitorum longus (EDL) and soleus (SOL) muscles. The contractile properties were studied at 1, 28, 84, and 210 days following unilateral section of the sciatic nerve at 12 weeks of age. When isometric tetanus tension was expressed relative to wet weight, the denervated SOL showed an earlier and more pronounced loss in tension generating capacity than the EDL. Both the denervated SOL and EDL showed potentiation of the twitch tension at 28 days postdenervation. The time to peak twitch tension (TTP) and the time to half-relaxation (1/2RT) were prolonged by 28 days postdenervation in both muscles. This trend continued to the oldest age-groups studied in the EDL, but reached an apparent plateau in the SOL at 84 days postdenervation. In response to fatigue, the denervated SOL showed a marked decrease in resistance to fatigue at 1 day but a relatively normal response thereafter, whereas the denervated EDL showed an increase in resistance to fatigue at and beyond the 28-day period. In spite of the fact that the total contraction time of both muscles increased following denervation, the predominantly oxidative SOL remained a slower contracting muscle than the more glycolytic EDL.     

The effect of muscle fatigue on the isometric contractile characteristics of skeletal muscle in the cat

The rise time of an isometric twitch, the tetanic tension, the twitch tetanus ratio, the frequency-tension relationship, and the height of the MUAP (motor unit action potential) were measured in fast twitch (medial gastrocnemius) and slow twitch (soleus) muscles of the cat immediately before, in the middle, and immediately after fatiguing isometric contractions at tensions of 30, 50 and 80% of each muscle's initial strength (tetanic tension recorded from the unfatigued muscle). Although the twitch-tetanus ratio was always less for the soleus than for the medial gastrocnemius muscles, the twitch-tetanus ratio for any one muscle was constant throughout the duration of fatiguing isometric contractions at any of the tensions examined. In contrast, the twitch tension and tetanic tension of the muscles were both less after the contractions, the largest reduction occurring for both muscles during contractions sustained at the lowest isometric tensions. The time to peak tension of an isometric twitch was prolonged for both muscles following the contractions. This was associated with a corresponding shift in the frequency tension relationship such that at the point of muscular fatigue, the muscles tetanized at lower frequencies of stimulation than did the unfatigued muscle. In contrast, the amplitude of the MUAP showed only a modest reduction throughout the duration of the fatiguing contractions.     

Respective effects of anabolic/androgenic steroids and physical exercise on isometric contractile properties of regenerating skeletal muscles in the rat

We examined the respective effects of anabolic-androgenic steroids and physical exercise on the contractile properties of regenerating fast and slow hindlimb skeletal muscles. Degeneration/regeneration of the left extensor digitorum longus muscles (EDL) and soleus of young Wistar male rats was induced by a snake venom (Notechis scutatus scutatus) injection. During muscle regeneration, experimental rats were either treated with nandrolone (NAN, nortestosterone, im, 2 mg X kg(-1) X week(-1), or endurance exercised on a treadmill (EXE, 60 min x day(-1), 10-40 m X min(-1). Twenty-one days after injury, isometric contractile properties of regenerating muscles were studied in situ. Neither the nandrolone treatment nor the physical exercise program was able to change significantly muscle contraction parameters both in twitch and tetanus in both regenerating EDL and soleus (p > 0.05). However, we observed a greater peak twitch tension in NAN versus grouped control and EXE EDL (p < 0.01). In conclusion, endurance exercise program or anabolic-androgenic steroid (nortestosterone) treatment did not significantly improve isometric contractile properties of regenerating slow and fast muscles in the male young rats.     

肌球蛋白抑制剂BDM对骨骼肌收缩功能的非特异性作用

目的:探讨萎缩骨骼肌单位面积上等长收缩最大张力(Pt)降低的机理.方法:采用肌球蛋白ATP酶抑制剂BDM(Butanedione monoxime)灌流,观测其对离体骨骼肌肌条等长收缩功能的影响.结果:研究表明,BDM可使比目鱼肌(SOL)与趾长伸肌(EDL)等长收缩Pt明显降低,BDM对骨骼肌收缩功能的抑制呈剂量依赖性关系,且完全可逆.低浓度BDM(1 mmol/L)仅降低骨骼肌等长收缩的Pt而不影响其收缩时程,高浓度(10 mmol/L)下使收缩时程明显缩短.与SOL相比,在10mmol/LBDM作用下,使EDL等长收缩Pt降低一半的时间明显加快.无论在低浓度还是高浓度下,BDM对EDL肌球蛋白ATP酶活性的抑制作用均大于SOL.在相同浓度下,BDM对Pt的抑制程度远远大于对肌球蛋白ATP酶活性的抑制.结论:这些结果提示骨骼肌横桥功能降低可能是其等长收缩pt下降的原因之一;BDM并非特异型肌球蛋白ATP酶抑制剂,可对兴奋-收缩偶联的多个环节产生影响.     

Skeletal muscle fatigue in vitro is temperature dependent

Our purpose was to determine the effect of temperature on the fatigability of isolated soleus and extensor digitorum longus (EDL) muscles from rats during repeated isometric contractions. Muscles (70-90 mg) were studied at 20-40 degrees C in vitro. Fatigability was defined with respect to both the time and number of stimuli required to reach 50% of the force (P) developed at the onset of the fatigue test. Fatigue was studied during stimulation protocols of variable [force approximately 70% of maximum force (Po)] and constant frequency (28 Hz). Results for soleus and EDL muscles were qualitatively similar, but fatigue times were longer for soleus than for EDL muscles. During the variable-frequency protocol, development of approximately 70% of Po required an increase in stimulation frequency as temperature increased. During stimulation at these frequencies, fatigue time shortened as temperature increased. For both fatigue protocols, the relationship between temperature and the number of stimuli required to reach fatigue followed a bell-shaped curve, with maximum values at 25-30 degrees C. The temperature optimum for maximizing the number of isometric contractions to reach fatigue reflects direct effects of temperature on muscle function.     

Deficiency of alpha-sarcoglycan differently affects fast- and slow-twitch skeletal muscles

Alpha-sarcoglycan (Sgca) is a transmembrane glycoprotein of the dystrophin complex located at skeletal and cardiac muscle sarcolemma. Defects in the alpha-sarcoglycan gene (Sgca) cause the severe human-type 2D limb girdle muscular dystrophy. Because Sgca-null mice develop progressive muscular dystrophy similar to human disorder they are a valuable animal model for investigating the physiopathology of the disorder. In this study, biochemical and functional properties of fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles of the Sgca-null mice were analyzed. EDL muscle of Sgca-null mice showed twitch and tetanic kinetics comparable with those of wild-type controls. In contrast, soleus muscle showed reduction of twitch half-relaxation time, prolongation of tetanic half-relaxation time, and increase of maximal rate of rise of tetanus. EDL muscle of Sgca-null mice demonstrated a marked reduction of specific twitch and tetanic tensions and a higher resistance to fatigue compared with controls, changes that were not evident in dystrophic soleus. Contrary to EDL fibers, soleus muscle fibers of Sgca-null mice distinctively showed right shift of the pCa-tension (pCa is the negative log of Ca2+ concentration) relationships and reduced sensitivity to caffeine of sarcoplasmic reticulum. Both EDL and soleus muscles showed striking changes in myosin heavy-chain (MHC) isoform composition, whereas EDL showed a larger number of hybrid fibers than soleus. In contrast to the EDL, soleus muscle of Sgca-null mice contained a higher number of regenerating fibers and thus higher levels of embryonic MHC. In conclusion, this study revealed profound distinctive biochemical and physiological modifications in fast- and slow-twitch muscles resulting from alpha-sarcoglycan deficiency.     

Measurement of rate constants for the contractile cycle of intact mammalian muscle fibers.

Small, random length changes were applied to bundles of intact fibers from rat and mouse extensor digitorum longus (EDL) and soleus muscles, while they were being tetanically stimulated. With increasing frequency of length changes, EDL muscle stiffness (tension change per unit change in length) increased, then decreased and increased again. The decrease was not seen in the soleus muscles. The EDL frequency-response could be well fitted by three exponential components with apparent rate constants of approximately 25, 150, and 500 s-1 at 20 degrees C. All rate constants increased steadily with temperature and for each 10 degrees C increase in temperature, the rates in the mouse EDL increased by a factor (Q10) between 1.8 and 2.4. With tetanic stimulation, force increased nearly exponentially to a steady level with a rate constant of 24 s-1 at 20 degrees C in mouse EDL muscles, and a Q10 of 2.4. These values correspond closely to the lowest frequency rate constant measured with length perturbations, which suggests that this process may limit the rate of rise of force in intact muscle fibers. During fatigue the high frequency and intermediate frequency rate constants declined, but the low frequency rate constant remained unchanged. These results are discussed in relation to current biochemical models for cross-bridge cycling.     

Phosphorylation of rabbit skeletal muscle myosin in situ

Myosin light chain (P light chain) is phosphorylated by Ca2+ X calmodulin-dependent myosin light chain kinase. Based on studies with rat skeletal muscles, it has been shown that P light chain phosphorylation correlated to the extent of potentiation of isometric twitch tension. It is not clear whether this correlation exists in rabbit skeletal muscle, which has been the primary source of contractile proteins for biochemical studies. Therefore, phosphorylation of myosin P light chain in rabbit slow-twitch soleus and fast-twitch plantaris muscles in situ was examined. Electrical stimulation (5 Hz, 20 seconds) of plantaris muscle produced an increase in the phosphate content of P light chain from 0.17 to 0.45 mol phosphate/mol P light chain. This increase in phosphate content was accompanied by a 58% increase in maximal isometric twitch tension. Tetanic stimulation (100 Hz, 15 seconds) of rabbit soleus muscle resulted in only a small increase in P light chain phosphate content from 0.02 to 0.10 mol phosphate/mol P light chain, and posttetanic twitch tension did not increase significantly. The correlation between potentiated isometric twitch tension and P light chain phosphorylation in rabbit fast-twitch muscle is similar to that observed in rat skeletal muscle. These results were consistent with the hypothesis that phosphorylation of rabbit skeletal muscle myosin, which results in an increase in actin-activated ATPase activity, may be related to isometric twitch potentiation.     

Generation of tension by skinned fibers and intact skeletal muscles from desmin-deficient mice

We have investigated the physiological role of desmin in skeletal muscle by measuring isometric tension generated in skinned fibres and intact skeletal muscles from desmin knock-out (DES-KO) mice. About 80% of skinned single extensor digitorum longus (EDL) fibres from adult DES-KO mice generated tensions close to that of wild-type (WT) controls. Weights and maximum tensions of intact EDL but not of soleus (SOL) muscles were lowered in DES-KO mice. Repeated contractions with stretch did not affect subsequent isometric tension in EDL muscles of DES-KO mice. Tension during high frequency fatigue (HFF) declined faster and this deficiency was compensated in DES-KO EDL muscles by 5 mM caffeine which had no influence on HFF in WT EDL. Furthermore, caffeine evoked twitch potentiation was higher in DES-KO than in WT muscles. We conclude that desmin is not essential for acute tensile strength but rather for optimal activation of intact myofibres during E-C coupling.     

Actions of lyotropic anions on the mechanical properties of fast and slow twitch rat muscles at different temperatures

The effects of lyotropic (swelling) anions (Cl(-), Br(-), NO(3)(-) and I(-)) on contractile properties of fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles were investigated in vitro at 20 degrees C and 35 degrees C. Isolated muscles bathed in anionic Tyrode solution were stimulated directly and isometric single twitches and fused tetanic contractions were recorded. In a Cl(-)Tyrode solution a decrease of the bathing temperature led to a cold potentiation of the twitch tension (P(t)) in EDL muscles, however, to a cold depression in SOL muscles, in both muscles combined with a prolongation of contraction (CT) and half relaxation (HRT) times. The extent and order of the potentiating effect of lyotropic anions on the P(t), CT and HRT in EDL and SOL were quite similar and increased in the order: Cl(-)< Br(-)< NO(3)(-)< I(-). Since the lyotropic anions did not influence tetanic tensions, the twitch-tetanus ratio (TTR) was increased in NO(3)(-) and I(-)solutions. All effects of the anions were rapidly and completely reversed in both muscles when the test solution was replaced by the normal one. The temperature decrease caused no significant alteration in the potentiation capacity of the anions or in the kinetics of their action and reversibility.     

Neural Control of Myofibrillar ATPase Activity in Rat Skeletal Muscle

THE limb muscles of mammals such as the cat and rat can be divided into the fast-twitch muscles and the slow-twitch muscles. While the absolute contraction speeds vary from species to species the isometric twitch time (the time taken from the start of contraction until the instant of peak tension development) of a slow-twitch muscle is always about three times longer than the isometric twitch time of a fast-twitch muscle. Thus, at 37° C, the isometric twitch time of cat soleus muscle (a slow-twitch muscle) is approximately 70 ms while the isometric twitch time of the flexor hallucis longus muscle (a fast-twitch muscle) is approximately 20 ms. In the rat, the contraction times of the corresponding muscles would be of the order of 36 ms and 12 ms respectively.     

Effect of temperature and Zn2+ on isometric contractile properties and electrical phenomena of frog (Rana) and Xenopus skeletal muscle fibers

Effects of temperature and Zn2+ on the isometric contractile properties of toe muscle fibers of Rana catesbeiana and Xenopus laevis were studied. The maximum twitch tension almost doubled when the temperature was lowered from 20 to 4 degrees C in Rana muscles but not in Xenopus muscles, although the duration of action potential in Xenopus muscle was increased slightly more than that seen in the Rana species. The maximum rate of rise of tension was greater in Xenopus muscle than in the Rana muscle, at 20 degrees C. The prolongation of the time-to-peak tension following exposure to low temperature (4 degrees C) was more pronounced in Rana than in Xenopus muscles. These results suggest that the speed of release and reuptake of Ca2+ by the sarcoplasmic reticulum (SR) differs in Rana and Xenopus muscles and that these factors may be related to differences in the SR and the T-tubular morphology. In Rana muscles, Zn2+ prolonged the falling phase of the action potential and potentiated the twitch tension. In Xenopus muscles, Zn2+ marginally prolonged the duration of action potential and the twitch tension was not markedly potentiated. These results indicate that Zn2+ potentiates the twitch by prolonging the action potential and that Rana muscles are more sensitive to the effects of Zn2+.     

Temperature effects on the MgATP-induced electron transfer between the nitrogenase proteins from Azotobacter vinelandii.

The temperature dependence of the pre-steady-state MgATP-dependent electron transfer from the MoFe protein to the Fe protein of the nitrogenase from Azotobacter vinelandii has been investigated between 6 degrees C and 31 degrees C by stopped-flow spectrophotometry. Below 14 degrees C, the data are consistent with a model in which interaction of MgATP with nitrogenase is fast and irreversible, and is followed by reversible electron transfer. From the extent and from the rate of the absorbance change, the rate constants for electron transfer from Fe protein to MoFe protein and of the reverse reaction were calculated. The direct rate constant increases with temperature (6-14 degrees C) from about 1 s-1 to about 26 s-1. The rate constant for the reverse reaction was found to be approximately 4 s-1 and invariant with the reaction temperature. Analysis of the data obtained in the temperature range between 6 degrees C and 12 degrees C within the framework of the transition-state theory show that electron transfer from the Fe protein to the MoFe protein occurs via a highly disordered transition state with activation parameters delta H(0) ++ = 289 kJ.mol-1 and delta S(0) ++ = 792 J.K-1.mol-1. The Eyring plot of the stopped-flow data displays an inflection point around 14 degrees C. From the stopped-flow data obtained between 18 degrees and 27 degrees C the activation parameters delta H(0) ++ and delta S(0) ++ for the reduction of the MoFe protein by Fe protein are calculated to be 90 kJ.mol-1 and 99 J.K-1.mol-1 respectively. A second inflection point in the Eyring plot could exist around 28 degrees C.     

Impaired load dependence of diaphragm relaxation during congestive heart failure in the rabbit.

The load dependence (LD) of relaxation was studied in the diaphragm of rabbits with congestive heart failure (CHF). CHF (n = 15) was induced by combined chronic volume and pressure overload. Aortic insufficiency was induced by forcing a catheter through the aortic sigmoid valves, followed 3 wk later by abdominal aortic stenosis. Six weeks after the first intervention, animals developed CHF. Sham-operated animals served as controls (C; n = 12). Diaphragm mechanics were studied in vitro on isolated strips, at 22 degrees C, in isotonic and isometric loading conditions. Contractility was lower in the CHF group, as reflected by lower total tension: 1.11 +/- 0.10 in CHF vs. 2.38 +/- 0.15 N/cm(2) in C in twitch (P < 0.001) and 2.46 +/- 0.22 in CHF vs. 4.90 +/- 0.25 N. cm(-2) in C in tetanus (P < 0.001). The index LD was used to quantify the load dependence of relaxation: LD is <1 in load-dependent muscles and tends toward 1 in load-independent muscles. LD was significantly higher in CHF than in C rabbits, in both twitch (0.99 +/- 0.01 vs. 0.75 +/- 0.03; P < 0. 001) and tetanus (0.95 +/- 0.02 vs. 0.84 +/- 0.02; P < 0.001). In the CHF rabbits' diaphragm, the fall in total tension was linearly related to the fall in load dependence of relaxation. The decrease in load dependence of relaxation in CHF animals suggests sarcoplasmic reticulum abnormalities. Impairment of the sarcoplasmic reticulum may also partly account for the decrease in contractile performance of diaphragm in CHF animals.     

Chronic stimulation modifies the isotonic shortening velocity of denervated rat slow-twitch muscle

Rat soleus muscles were denervated and stimulated in vivo for periods of up to 104 days. Stimuli used were trains of 1 ms pulses at 100 Hz delivered for periods of 1 s; trains were repeated every 10-100 s. In a majority of animals the tension of the muscles was maintained at about 10% of normal, equivalent to muscles denervated but unstimulated for 20 days. At the longest periods the stimulated muscles developed ten times more tension than ones that were denervated but not stimulated. In denervated and denervated-stimulated muscles twitch contraction and relaxation times were prolonged, compared with controls, for up to 3 weeks. Thereafter both sets showed a speeding of the isometric twitch that was greater in the stimulated muscles. At the longest periods the twitch was as short as that of a denervated fast muscle. Stimulation did not affect contralateral denervated muscles. Twitch: tetanus ratios remained high despite stimulation, and muscles showed little post-tetanic potentiation. Tension developed more rapidly in the tetani of the stimulated muscles, even allowing for larger final values. Maximum velocity of shortening was increased in many of the stimulated muscles, and there was a proportional flattening of the force-velocity curve, i.e. a/P0 increased. Maximum velocity and a/P0 increased reciprocally with twitch time to peak, so that those muscles that had twitches most changed by stimulation also had their isotonic properties modified to the greatest extent. Even at the longest period of stimulation, twitch time course and tetanic tension were not converted to those of normal fast muscle.     

Energetics of Isometric and Isotonic Twitches in Toad Sartorius

Contractile energetics have been studied in twitches of toad sartorius muscle at 6-7°C. Isometric and isotonic energy production has been measured and plotted against a wide range of developed tensions and tension-time integrals. These parameters were varied by altering the isotonic load or by changing the preset isometric length. The isometric tension-independent heat was 1.12 ±0.18 (SD) mcal/g. The isometric heat coefficient Pl₀/H was 12.0 ±1.4 in muscles having twitch to tetanus ratios ranging from 0.4 to 0.6. Isometric enthalpy increased monotonically with tension or tension-time integral but the correlation between isometric heat and these parameters was poor. Isotonic enthalpy consumption was always higher than isometric enthalpy for any given tension or tension-time integral; however, isotonic heat production was consistently less than isometric heat production. The isotonic heat for the highest load (3 g) was not significantly different from the isometric tension-independent heat. Thus isotonic heat production first decreased and then increased with increasing tension or tension-time integral. In the discussion it is shown that the results conflict with all current interpretations of muscle energetics.