Frequency response model of skeletal muscle and its association with contractile properties of skeletal muscle

The aims of the present study were to develop a mathematical model of the skeletal muscle based on the frequency transfer function, referred to as frequency response model, and to presume the relationship between the model elements and skeletal muscle contractile properties. Twitch force in elbow flexion was elicited by applying a single electrical stimulation to the motor point of biceps brachii muscles, and then analyzed visually by the Bode gain and phase diagram of the force signal. The frequency response model was represented by a frequency transfer function consisting of five basic control elements (proportional element, dead time element, and three first-order lag elements). The model element constants were estimated by best-fitting to the Bode gain and phase diagram of the twitch force signal. The proportional constant and the dead time in the frequency response model correlated significantly with the peak torque and the latency in the actual twitch force, respectively. In addition, the time constants of the three first-order lag elements in the model correlated strongly with the contraction time and the half relaxation time in the actual twitch force. The results suggested a possibility that the individual elements in the frequency response model would reflect the biochemical and biomechanical properties in the excitation–contraction coupling process of skeletal muscle.     

Thermal dependence of isotonic contractile properties of skeletal muscle and sprint performance of the lizardDipsosaurus dorsalis

Contractile properties of the fast-twitch glycolytic (FG) portion of the iliofibularis muscle and sprint running performance were studied at approximately 5 degrees C intervals from 15-44 degrees C in the lizard Dipsosaurus dorsalis. Maximal running velocity (VR) and stride frequency (f) were both greatest when body temperature (Tb) was 40 degrees C, the field-active Tb in Dipsosaurus. At 40 degrees C VR was 4.3 +/- 0.2 m/s and f was 13.5 +/- 0.5 s-1. Between 25 and 40 degrees C, the thermal dependencies of VR and f were approximately constant (Q10's of 1.31 and 1.36 got VR and f, respectively). Below 25 degrees C performance declined more markedly with decreasing temperature. At 20 degrees C strides were qualitatively normal, but VR was only half of the value at 25 degrees C. At 15 degrees C the lizards were substantially incapacitated, and VR was 10% of the value at 20 degrees C. Stride length was approximately 0.33 m and changed very little with Tb from 20-44 degrees C. The time dependent contractile properties of FG muscle were affected more by temperature than was sprint performance. The maximal velocity of shortening at zero load (VO) was 18.7 0/s at 40 degrees C and had a Q10 of 1.7 from 25-40 degrees C. Maximal power output (Wmax) determined from the force-velocity curve was 464 W/kg at 40 degrees C. Below 40 degrees C max varied with temperature with a Q10 of 2-3. The shape of the force-velocity curve changed little with temperature (Wmax/POVO = 0.11). Between 25 and 40 degrees C a relatively temperature-independent process must modulate the effects of temperature on the contractile properties of the muscles that supply the power for burst locomotion. Storage and recovery of elastic energy appears to be a likely candidate for such a process. Below 25 degrees C, however, the contraction time is prolonged to such an extent that the f attainable is limited by the minimum time taken to contract and relax the muscles.     

Length-dependent twitch contractile characteristics of skeletal muscle

The length dependence of force development of mammalian skeletal muscles was evaluated during twitch, double-pulse, and tetanic contractions, and the relation between muscle length and the time-dependent characteristics of twitch and double-pulse contractions were determined. In situ isometric contractions of the rat gastrocnemius muscle were analyzed at seven different lengths, based on a reference length at which the maximal response to double-pulse contractions occurred (Lopt-2P). Twitch and double-pulse contractions were analyzed for developed tension (DT), contraction time (tC), average rate of force development (DT-tC(-1)), half-relaxation time (t50%R), peak rate of relaxation (DT x dtmin(-1)), and 90%-relaxation time (t90%R). Considering the length at which maximal tetanic DT occurred to be the optimal length (Lo-TET), the peak DT for twitch contractions and double-pulse contractions was observed at Lo-TET + 0.75 mm (p < 0.05) and Lo-TET + 0.1 mm (p > 0.05), respectively. When measured at the length for which maximal twitch and double-pulse contractions were obtained, tetanic DT was 95.2 +/- 3 and 99.0 +/- 2% of the maximal value, respectively. These observations suggest that double-pulse contractions are more suitable for setting length for experimental studies than twitch contractions. Twitch and double-pulse contraction tC were 15.53 +/- 1.14 and 25.0 +/- 0.6 ms, respectively, at Lopt-2P, and increased above Lopt-2P and decreased below Lopt-2P. Twitch t50%R was 12.18 +/- 0.90 ms at Lopt-2P, and increased above Lopt-2P and below Lopt-2P. Corresponding changes for double-pulse contractions were greater. Stretching the muscle leads to slower twitch contractions and double-pulse contractions, but the mechanisms of this change in time course remain unclear.     

The effect of heat stress on skeletal muscle contractile properties

An elevated heat-shock protein (HSP) content protects cells and tissues, including skeletal muscles, from certain stressors. We determined if heat stress and the elevated HSP content that results is correlated with protection of contractile characteristics of isolated fast and slow skeletal muscles when contracting at elevated temperatures. To elevate muscle HSP content, one hindlimb of Sprague–Dawley rats (21–28 days old, 70–90 g) was subjected to a 15 min 42 °C heat-stress. Twenty-four hours later, both extensor digitorum longus (EDL) and soleus muscles were removed, mounted in either 20 °C or 42 °C Krebs-Ringer solution, and electrically stimulated. Controls consisted of the same muscles from the contra-lateral (non-stressed) hindlimbs as well as muscles from other (unstressed) animals. Isolated muscles were twitched and brought to tetanus every 5 min for 30 min. As expected, HSP content was elevated in muscles from the heat-stressed limbs when compared with controls. Regardless of prior treatment, both EDL and soleus twitch tensions were lower at 42 °C when compared with 20 °C. In addition, when incubated at 42 °C, both muscles showed a drop in twitch tension between 5 and 30 min. For tetanic tension, both muscles also showed an increase in tension between 5 and 30 min when stimulated at 20 °C regardless of treatment but when stimulated at 42 °C no change was observed. No protective effect of an elevated HSP content was observed for either muscle. In conclusion, although heat stress caused an elevation in HSP content, no protective effects were conferred to isolated contracting muscles.     

Spectrophotometric studies on the pH of frog skeletal muscle. PH change during and after contractile activity

The spectral characteristics of the pH-sensitive dyes neutral red (NR) and bromcresol purple (BCP) were utilized for studies of the changing intracellular pH (pHi) of sartorius muscles from Rana pipiens, both during the course of an isometric twitch and during recovery metabolism subsequent to a train of twitches. The information from the two dissimilar dyes correlated to confirm the methodology. Neither the fast realkalinization observed during a twitch nor the slow alkalizing phase of recovery metabolism was affected in an obvious manner when phosphocreatine (PC) hydrolysis was blocked by 1-fluoro-2,4-dinitrobenzene (FDNB). Iodoacetic acid (IAA) did inhibit the slow acidic phase of recovery metabolism. The conclusion is made that alkalizing reactions other than PC breakdown must be considered as operative at these levels of activity. Hypertonic solutions altered twitch tension and time course without altering the pHi shifts observed until approximately 75% of the twitch amplitude was abolished. Multiple effects of hypertonic solutions as the muscle approach tonic equilibrium are proposed.     

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.     

Intracellular pH during sequential, fatiguing contractile periods in isolated single Xenopus skeletal muscle fibers.

The purpose of the present study was to test the hypothesis that a preceding contractile period in isolated single skeletal muscle fibers would attenuate the decrease in pH during an identical, subsequent contractile period, thereby reducing the rate of fatigue. Intact single skeletal muscle fibers (n = 9) were isolated from Xenopus lumbrical muscle and incubated with the fluorescent cytosolic H+ indicator 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) AM for 30 min. Two identical contractile periods were performed in each fiber, separated by a 1-h recovery period. Force and intracellular pH (pHi) fluorescence were measured simultaneously while fibers were stimulated (tetanic contractions of 350-ms trains with 70-Hz stimuli at 9 V) at progressively increasing frequencies (0.25, 0.33, 0.5, and 1 contraction/s) until the development of fatigue (to 60% initial force). No significant difference (P < 0.05) was observed between the first and second contractile periods in initial force development, resting pHi, or time to fatigue (5.3 +/- 0.5 vs. 5.1 +/- 0.6 min). However, the relative decrease in the BCECF fluorescence ratio (and therefore pHi) from rest to the fatigue time point was significantly greater (P < 0.05) during the first contractile period (to 65 +/- 4% of initial resting values) compared with the second (77 +/- 4%). The results of the present study demonstrated that, when preceded by an initial fatiguing contractile period, the rise in cytosolic H+ concentration in contracting single skeletal muscle fibers during a second contractile period was significantly reduced but did not attenuate the fatigue process in the second contractile period. These results suggest that intracellular factors other than H+ accumulation contribute to the fall in force development under these conditions.     

Octopamine action on the contractile system of crustacean skeletal muscle

1. In the opener muscle of walking legs of crayfish (Astacus leptodactylus) octopamine (OA) greatly enhances the contractions resulting from brief applications of L-glutamate or of elevated K-concentrations. Synephrine is as effective as OA. 2. In the case of potentiation of responses to high-K applications a presynaptic component of the OA action was excluded by first desensitising the muscle fibres to the action of the natural transmitter, using a high concentration (1 mM) of glutamate. 3. The Ca-antagonists Co, Ni and Mn (1 mM) reduced the effects of glutamate and of elevated K to about one-half. In preparations treated with OA, the same Ca-antagonists also depressed the potentiated contractural responses to glutamate and to elevated K, again to about one-half. 4. OA also enhanced contractions resulting from the application of caffeine. 5. With 5-hydroxytryptamine (5-HT) application, the same postsynaptic effects were obtained as described for OA, except that the 5-HT actions were much weaker. 6. With OA, maximal effects were obtained with concentrations of 5 x 10(-6)-10(-5) M; maximally effective concentrations of 5-HT were around 10(-5) M. 7. The lowest effective concentrations of OA were around 10(-8) M; those of 5-HT were around 10(-7) M. 8. In the same preparation, 5-HT is far more effective in enhancing transmitter release (presynaptic action) than OA, the lowest effective concentration being around 10(-11) M while no presynaptic effects of OA were seen at concentrations below 10(-8) M, in some cases even below 10(-5) M.     

Plasticity of TOM complex assembly in skeletal muscle mitochondria in response to chronic contractile activity

We investigated the assembly of the TOM complex within skeletal muscle under conditions of chronic contractile activity-induced mitochondrial biogenesis. Tom40 import into mitochondria was increased by chronic contractile activity, as was its time-dependent assembly into the TOM complex. These changes coincided with contractile activity-induced augmentations in the expression of key protein import machinery components Tim17, Tim23, and Tom22, as well as the cytosolic chaperone Hsp90. These data indicate the adaptability of the TOM protein import complex and suggest a regulatory role for the assembly of this complex in exercise-induced mitochondrial biogenesis.     

Soluble miniagrin enhances contractile function of engineered skeletal muscle

Neural agrin plays a pleiotropic role in skeletal muscle innervation and maturation, but its specific effects on the contractile function of aneural engineered muscle remain unknown. In this study, neonatal rat skeletal myoblasts cultured within 3-dimensional engineered muscle tissue constructs were treated with 10 nM soluble recombinant miniagrin and assessed using histological, biochemical, and functional assays. Depending on the treatment duration and onset time relative to the stage of myogenic differentiation, miniagrin was found to induce up to 1.7-fold increase in twitch and tetanus force amplitude. This effect was associated with the 2.3-fold up-regulation of dystrophin gene expression at 6 d after agrin removal and enhanced ACh receptor (AChR) cluster formation, but no change in cell number, expression of muscle myosin, or important aspects of intracellular Ca(2+) handling. In muscle constructs with endogenous ACh levels suppressed by the application of α-NETA, miniagrin increased AChR clustering and twitch force amplitude but failed to improve intracellular Ca(2+) handling and increase tetanus-to-twitch ratio. Overall, our studies suggest that besides its synaptogenic function that could promote integration of engineered muscle constructs in vivo, neural agrin can directly promote the contractile function of aneural engineered muscle via mechanisms distinct from those involving endogenous ACh.     

Regulation analysis of contractile ATPase flux in skeletal muscle

The [Ca²⁺] regulation of contractile ATPase flux, J _p, in skeletal muscle was analysed by computation of the Response R ^Jp _Ca2+ for a 10 Hz range of electrical stimulation frequencies. Results of our analysis of the kinetic controls in ATP free energy metabolism in a network model of contracting muscle (J.A.L. Jeneson, H.V. Westerhoff and M.J. Kushmerick (2000) Am. J. Physiol. 279, C813–C832) formed the basis for the computations of R ^Jp _Ca2+. We found that neural regulation of sustained force generation via simple [Ca²⁺]_cyto frequency encoding in the network was robust for frequencies up to 2 Hz. Above 2 Hz, however, this regulation design broke down because of a shift in contractile ATPase flux control from the Ca²⁺-sensitive contractile filaments to mitochondria with low Ca²⁺ sensitivity. The role of glyco(geno)lytic ATP production at high contraction workloads is discussed in the context of this result     

Procedures for rat in situ skeletal muscle contractile properties

There are many circumstances where it is desirable to obtain the contractile response of skeletal muscle under physiological circumstances: normal circulation, intact whole muscle, at body temperature. This includes the study of contractile responses like posttetanic potentiation, staircase and fatigue. Furthermore, the consequences of disease, disuse, injury, training and drug treatment can be of interest. This video demonstrates appropriate procedures to set up and use this valuable muscle preparation. To set up this preparation, the animal must be anesthetized, and the medial gastrocnemius muscle is surgically isolated, with the origin intact. Care must be taken to maintain the blood and nerve supplies. A long section of the sciatic nerve is cleared of connective tissue, and severed proximally. All branches of the distal stump that do not innervate the medial gastrocnemius muscle are severed. The distal nerve stump is inserted into a cuff lined with stainless steel stimulating wires. The calcaneus is severed, leaving a small piece of bone still attached to the Achilles tendon. Sonometric crystals and/or electrodes for electromyography can be inserted. Immobilization by metal probes in the femur and tibia prevents movement of the muscle origin. The Achilles tendon is attached to the force transducer and the loosened skin is pulled up at the sides to form a container that is filled with warmed paraffin oil. The oil distributes heat evenly and minimizes evaporative heat loss. A heat lamp is directed on the muscle, and the muscle and rat are allowed to warm up to 37°C. While it is warming, maximal voltage and optimal length can be determined. These are important initial conditions for any experiment on intact whole muscle. The experiment may include determination of standard contractile properties, like the force-frequency relationship, force-length relationship, and force-velocity relationship. With care in surgical isolation, immobilization of the origin of the muscle and alignment of the muscle-tendon unit with the force transducer, and proper data analysis, high quality measurements can be obtained with this muscle preparation.     

A highly non-ideal solution: the contractile system of skeletal muscle

The contractile system is a highly non-ideal solution. The activities of its components must be determined in order to achieve a meaningful representation of cross-bridge kinetics and of chemio-mechanical transduction. Osmotic techniques may help in this respect. A few examples are presented. Protein osmotic pressure influences cross-bridges by determining (1) their free energy minimum, (2) their stiffness and (3) their contractile force.     

Interaction of flavonoid compounds with contractile proteins of skeletal muscle

Flavonoids (quercetin, rutin) influence ATPase activity and actomyosin superprecipitation. Low concentrations (below 20 mumol/l) of flavonoids were found to cause conformational changes in the myosin structure accompanied by an increase in ATPase activity. At higher concentrations an inhibitory action of flavonoids on both ATPase activity and actomyosin superprecipitation occurred. Conformational changes are likely to be due to flavonoids binding to regulatory site near the active centre of the myosin head. The effect of quercetin was stronger than that of rutin.     

Effect of contractile activity on rat skeletal muscle beta-adrenoceptor properties

The effect of fiber type and endurance exercise training on skeletal muscle beta-adrenoceptor properties were assessed using a direct radioligand binding technique. Six separate muscles, composed of a variety of different fiber types, were examined in treadmill trained and sedentary rats. In trained animals, sarcolemmal preparations from heart and slow twitch soleus muscle exhibited a significantly greater receptor concentration than membranes from white fast twitch glycolytic fibers of the vastus lateralis. No significant changes were observed between trained and sedentary rat muscle beta-adrenoceptor density (beta max, fmole/mg protein) or affinity (Kd, nM) within each muscle type, despite significantly increased myocardial/body weight ratios and skeletal muscle enzyme adaptations associated with the exercise program. These results suggest that muscle beta-adrenoceptor properties may be influenced in part by the motor nerve innervation to that muscle, and are further discussed with respect to a possible relationship between exercise intensity and receptor regulation.