Sprint-training effects on some contractile properties of single skinned human muscle fibres

The effects of sprint training on the contractile properties of human muscle fibres obtained by needle biopsy were investigated. Individual fibres were mechanically skinned and activated by Ca²⁺- and Sr²⁺-buffered solutions at pH 7.1, and allocated to distinct populations on the basis of their contractile characteristics. The majority of fibres sampled pre-training could be separated into the three major fibre groups: Populations I (24/70, 34%), II (25/70, 36%) and III (18/70, 26%), which exhibited characteristics similar to those of histochemically classified type I, IIA and IIB fibres, respectively. The remainder (3/70, 4%) represented another fibre group, with intermediate characteristics. The muscle fibres were also activated by Ca²⁺ at a reduced pH of 6.6, to mimic the intracellular acidification that occurs during intense exercise. Lowering pH increased the threshold for contraction by Ca²⁺, reduced Ca²⁺ sensitivity, and increased the steepness of the force-pCa relationship, in all fibres sampled from the three major fibre groups. Maximum force was not significantly reduced in any fibre population. In the post-training sample, the three major fibre types were present in different proportions: Populations I (10/52, 19%), II (20/52, 38.5%) and III (11/52, 21%). Three other fibre groups sampled in low numbers exhibited contractile characteristics intermediate between Population I and Population II. Following sprint training all of the three main fibre populations exhibited higher thresholds for contraction by, and lower sensitivities to, Sr²⁺ but not Ca²⁺, compared with the fibres sampled pre-training. Maximum force was significantly lower in Population II fibres after sprint training. At pH 6.6, post-trained Population III fibres exhibited even lower Ca²⁺ sensitivity, with concomitant increases in the threshold for contraction and force-pCa curve steepness.     

Endurance exercise effects on the contractile properties of single,skinned skeletal muscle fibres of young rats

Single fibres were isolated from the extensor digitorum longus (EDL) and the soleus (SOL) muscles of the hindlimb from young male Wistar rats which had undergone a 10-week programme of endurance swimming from the age of 2 weeks. Fibres were mechanically skinned and activated with Ca²⁺- and Sr²⁺ -buffered solutions. Muscle fibres were classified by means of well-defined criteria concerning various aspects of their contractile behaviour. Most fibres could be allocated into specific groups; however, a significant proportion (13% of the sampled population) did not fit these rigid classifications but displayed contractile activation characteristics common to more than one fibre type. In these cases models which used a combination of both fast- and slow-twitch contractile and regulatory properties were used to characterise the activation behaviour of fibres. It is proposed that the exercise, initiated at a young age, induced changes in the contractile characteristics of the single fibres by modifying protein isoforms of the contractile apparatus.     

Effects of chronic nicotine exposure on contractile enzyme-histochemical and biochemical properties of fast- and slow-twitch skeletal muscles in the rat

Nicotine-exposed and control rats were compared with respect to contractile, enzyme-histochemical and biochemical properties of fast- and slow-twitch skeletal muscles in order to elucidate the mechanisms underlying previously observed effects of tobacco smoking on skeletal muscle. The nicotine was administered in drinking water, since this approach has been shown to result in a plasma nicotine pattern similar to that seen in tobacco smokers. In a pilot study, fibre-type proportions and mitochondrial enzyme activities tended to change in the slow-twitch soleus muscle after 9 weeks of nicotine exposure in a way similar to that previously reported in tobacco smokers. In the present study, the duration of nicotine exposure was more prolonged (18 weeks) and the number of studied animals was increased. In this series neither contractile, enzyme-histochemical nor biochemical properties were affected by the nicotine exposure. It is thus concluded that prolonged nicotine exposure has no significant effect on the skeletal muscle characteristics studied, and that other aetiological agent(s) for the observed differences in such characteristics between smokers and non-smokers should be searched for.     

Calcium sensitivity and myofibrillar protein isoforms of rat skinned skeletal muscle fibres

We investigated the calcium sensitivity for tension generation of different fibre types and the possible correlation between calcium sensitivity and the presence of distinct regulatory protein and myosin light chain (MLC) isoforms in rat skinned skeletal muscle fibres. Fibre types 1, 2A and 2B were identified by electrophoretic analysis of myosin heavy chain (MHC) isoforms. Fibres showing more than one MHC isoform were discarded. Type 1 fibres from the soleus showed a higher pCa (–log₁₀ [Ca], where [ ] denotes concentration) threshold and a lower slope of pCa/tension curve than type 2 extensor digitorum longus (EDL) fibres; between type 2 fibres, type 2B showed the higher slope of pCa/tension curve. Type 1 fibres from different muscles showed similar calcium sensitivities when containing only the slow set of regulatory proteins and MLC; when both slow and fast isoforms were present, calcium sensitivity shifted toward fast type fibre values. Type 2A fibres from different muscles showed a similar calcium sensitivity, independently of the set (purely fast or mixed) of regulatory proteins and MLC. It is suggested that when both fast and slow isoforms of regulatory proteins and of MLC are present in a muscle fibre, calcium sensitivity is dictated mainly by the fast isoforms.     

Effects of ageing and gender on contractile properties in human skeletal muscle and single fibres

Aim: The objective of this study is to improve our understanding of the mechanisms underlying the ageing‐ and gender‐related muscle weakness. Methods: Ageing‐ and gender‐related differences in regulation of muscle contraction have been studied in knee‐extensor muscles at the whole muscle and single muscle fibre levels in young and old sedentary men and women. In vivo knee‐extensor muscle function was measured at slow (30° s^?1) and faster (180 ° s^?1) speeds of movement. Maximum velocity of unloaded shortening (V₀) and maximum force normalized to cross‐sectional area (CSA) [specific tension (ST)] were measured in single ‘skinned’ skeletal muscle fibre segments. Results: Significant ageing‐ and gender‐related differences were observed in muscle torque. A 33–55% ageing‐related decline (P < 0.001) in maximum torque was observed irrespective of gender. At the single muscle fibre level, the ageing‐related decline in knee‐extensor muscle function was accompanied by a 20–28% decline in ST in muscle fibres expressing the type I MyHC isoform in both men and women, and a 29% decline in type IIa muscle fibre CSA, but the decreased fast‐twitch fibre size was restricted to the men. Furthermore, in both men and women, V₀ decreased in muscle cells expressing the type I and IIa MyHC isoforms. Conclusion: The present results provide evidence of specific ageing‐ and gender‐related differences in regulation of muscle contraction at the cellular level. It is suggested that these cellular changes have a significant impact on muscle function and the ageing‐related motor handicap.     

Shortening induced deactivation of skinned fibres of frog and mouse striated muscle

The depressant effect of active shortening, previously established in intact muscle fibres, was studied during calcium induced contractures of chemically skinned fibres from the semitendinosus muscle of Rana temporaria and the psoas muscle of the mouse. The decrease in contractile activity was determined by comparing the rate of force redevelopment (at a given tension level) after a large (test) and a small (control) release step. Under standard experimental conditions (ionic strength: frog 135 mM, mouse 190 mM; Ca²⁺ 3.0 μM; Mg²⁺: frog 25 μM, mouse 100 μM; MgATP^2-: frog 1.0 mM, mouse 2.0 mM) active shortening of 0.15 μm per sarcomere (in excess of control release) reduced the contractile activity by approximately 50% of the control in both frog and mouse muscle fibres. Full contractile activity was regained within 4 s during isometric activity after the shortening phase. The depressant effect of shortening was steadily reduced, to almost complete disappearance of the effect, by increasing the free calcium concentration within the range 1.5–12.0 μM. Similarly, an increase in ionic strength from 105 to 235 mM reduced the depressant effect by approximately 40%. In contrast, there was a progressive enhancement of the shortening effect as the magnesium ion concentration was increased from 25 to 590 μM. It is proposed that interaction between the myosin cross-bridges and the thin filament during sarcomere shortening leads to a decrease in troponin-calcium binding resulting in a temporary deactivation of the contractile system.     

Force-velocity properties and myosin light chain isoform composition of an identified type of skinned fibres from rat skeletal muscle

Force-velocity relations, myosin heavy chain (MHC) and myosin light chain (MLC) isoform composition of single skinned fibres from rat plantaris muscle were determined. In fibres containing the same (2X) isoform of myosin heavy chain, several parameters derived from the force-velocity relation and isometric force (Po) were tested for relation with the fibre content in alkali myosin light chain (MLC) isoforms. Whereas maximum shortening velocity was found to be proportional to the relative content in the 3f isoform of alkali MLC, velocity of shortening at 5% relative load, maximum power output, and Po were not. These results strengthen the idea that, in mammalian skeletal fibres, alkali MLC isoforms modulate shortening velocity at zero load, but suggest that they do not control the contractile behaviour at loads higher than zero.     

Compared properties of the contractile system of skinned slow and fast rat muscle fibres

Chemically skinned fibres from soleus and plantaris rat muscles were used to compare the contractile properties of slow and fast muscles. The maximal isometric tension appeared larger in plantaris than in soleus fibres. The apparent Ca²⁺ threshold for activation was lower in slow than in fast fibres while Ca²⁺ concentrations required to obtain either the maximal tension or half maximal tension (pCa₅₀) were lower in fast than in slow fibres. This apparent difference in Ca²⁺ sensitivity will be discussed. As could be expected from other studies, a faster force development in plantaris than in soleus fibres occurred. However, one interesting new result showed that in soleus, the kinetics of the tension development estimated by the t _max parameter were slightly dependent on the Ca²⁺ concentration whereas the t ₅₀ parameter changed significantly with the Ca²⁺ concentration. In plantaris, both t _max and t ₅₀ parameters were found to depend strongly on the Ca²⁺ concentration. Finally, the plantaris muscle showed a greater caffeine sensitivity than the soleus muscle. All the results suggested that the Ca-regulatory mechanism in the slow fibres was essentially different from that in the fast fibres.     

Force and force transients in skeletal muscle fibres of the frog skinned by freeze-drying

A freeze-drying method is described by which single skinned skeletal muscle fibres or fibre bundles can readily be obtained. Skinned fibre segments of the ileofibularis and semitendinous muscles of the frog — activated by means of a rapid increase in the Ca-concentration — showed very stable and reproducible contractions. Complete activation occurred at a Ca-concentration of 1.6·10^–6 M and the mid-point of the pCa-tension curve occurred at 6.3·10^–7 M. Addition of phosphate (10^–2 M) had a depressing effect on the speed of the Ca-activated tension development as well as on the maximum tension reached.Addition of caffeine (10^–2 M) had no effect on the tension generation, indicating that the sarcoplasmic reticulum, if present, was not active. The force responses due to rapid length changes applied to the Ca-activated fibre preparations were found to be qualitatively similar to the force responses on intact tissue. This skinning technique might be employed on human biopsies, enabling the measurement of physiological parameters such as for example force and shortening velocity.     

The high-force region of the force-velocity relation in frog skinned muscle fibres

The force-velocity relation has been studied during calcium-induced contracture of chemically skinned fibres from the semitendinosus muscle of Rana temporaria with special interest focused on the high-load region. The force-velocity curve was hyperbolic at low and intermediate loads but departed below the hyperbola as the load exceeded about 80% of the isometric force (P_o). The force intercept (P*_o) of the hyperbola derived from force-velocity data truncated at 0.78 P_o was higher than P_o (P*_o /P_o = 1.14±0.04). At submaximum Ca²⁺ concentration, where the isometric force of the fibre was 65–75% of the maximum value, the force-velocity data still departed below the hyperbola at high loads (P_o /P_o = 1.09±0.04). The departure of the force-velocity data from the hyperbola at high force was also found at high ionic strength (250 mM), but not at low ionic strength (150 mM) (P_o /P_o = 1.09±0.03 and 0.98±0.03, respectively). The force-velocity relations derived under different experimental conditions could be fitted well by a modified version of Hill's (1938) hyperbolic equation (Edman 1988) using similar numerical values of k₁ and k₂ in the latter equation. The results indicate that the force-velocity relation in skinned muscle fibres is biphasic, and that the two curvatures, as in intact muscle fibres, are closely related to one another. Furthermore the evidence supports the hypothesis that the altered shape of the force-velocity relation at high loads is not related to the force level per se but rather to the speed of shortening of the contractile system (Edman 1992).     

Mechanical properties and myosin heavy chain isoform composition of skinned skeletal muscle fibres from a human biopsy sample

 Experiments were conducted to investigate the mechanics of contraction of chemically skinned muscle fibre segments of a biopsied sample of single human quadriceps muscle. Subsequently, the isoforms of the myosin heavy chain (MHC) were analysed by sodium dodecyl sulphate (SDS) gel electrophoresis.Of the 41 fibres, 26 contained MHCI (type I), 11 of the fibres contained MHCIIa (type IIA), and 4 of the fibres contained both MHCI and MHCIIa (of which MHCIIa was always slightly predominant (type IIC)). Distinct differences between fibre types were found in terms of the kinetics of force responses following stepwise length changes (order of velocity: IIA > IIC > I). The differences in maximal shortening velocity and in the kinetics of Ca²⁺-dependent activation were of the same order, but much less pronounced. Type I fibres had significantly greater fibre diameters than type IIA fibres. No significant differences were found among different fibre types in terms of isometric tension, resting sarcomere length or the length change needed to discharge the elasticity of maximally Ca²⁺-activated fibres (y _o value). The distribution of shortening velocity and kinetics of stretch activation values suggest that two muscle fibre subtypes may exist in human type I fibres. Received: 15 April 1997 / Received after revision 19 May 1997 / Accepted: 20 May 1997     

Effects of perchlorate on myofibrillar calcium sensitivity in rat skinned skeletal muscles

The effects of perchlorate (1–20 mm ) on myofibrillar calcium responsiveness have been tested in Triton X-100-skinned fibre bundles from rat soleus (slow-twitch) and extensor digitorum longus (fast-twitch) skeletal muscles. In extensor digitorum longus and soleus, perchlorate dose-dependently shifted the pCa (-log[Ca²⁺])/tension relationship towards lower free calcium concentration (sensitizing effect) and maximal tension was unchanged. The degree of sensitization was greater in extensor digitorum longus than in soleus bundles. Reversibility after exposure to 12 mm perchlorate was complete in soleus but not in extensor digitorum longus muscles. In fact, the ‘return’ pCa/tension relationship in extensor digitorum longus was shifted to higher free calcium concentration (desensitizing effect) compared with control. Perchlorate (12 mm ) also enhanced myofibrillar calcium responsiveness of frog semitendinosus skinned skeletal fibres. Assuming a passive distribution of perchlorate across the sarcolemma, this sensitizing effect is probably not involved in perchlorate-induced potentiation of contractile responses of intact muscles and thereby supports the specificity of perchlorate as an agonist of the excitation/calcium release sequence in skeletal muscle fibres.     

Changes in tension generation and ATPase activity in skinned muscle fibres of the carp following temperature acclimation

Summary Common carp (Cyprinus carpio L.) were acclimated to either 7°C or 23°C for 1–2 months. Skinned fibre preparations were isolated from the white myotomal muscle, and ATPase activity measured during maximal isometric contractions. At 7°C, fibres from the cold acclimated fish were found to generate more force than those from warm acclimated fish (123.1 and 97.2kN m^–2 respectively), and more work (force.time integral) was obtained for each ATP hydrolysed. ATP turnover per myosin head in fibres from cold-acclimated fish was lower than in fibres from warm-acclimated fish (1.85 and 2.84 ATP S ₁ ^–1 s^–1.     

Series elastic properties of skinned muscle fibres in contraction and rigor

Summary Isometric tension of skinned fibres from the frog semitendinosus muscle is sigmoidally related to Ca²⁺ concentration betweenpCa 7 and 6. Stiffness measurements showed that the Ca²⁺-activated tension may be due to recruitment of attached cross-bridges. In the absence of ATP (rigor solution) the skinned fibre develops a rigor tension which reaches about 80–110% of the maximum Ca²⁺-activated tension.However, stiffness measurements showed that in rigor many more cross-bridges are attached to actin at any one moment than in contraction. It was concluded that the force per cross-bridge is 37% smaller in rigor than in contraction.     

Lactate in blood,mixed skeletal muscle,and FT or ST fibres during cycle exercise in man

The relationship between muscle and blood lactate levels during progressively step-wise incrementing cycle exercise has been investigated in 10 male subjects. Steps between power outputs during exercise were 50 W and each stage, from loadless pedalling until voluntary exhaustion, lasted 4 min. Blood samples and biopsies (m. vastus lateralis) were taken for lactate determination at each power output beginning with the exercise intensity perceived by the subject as being “rather moderate”. The ratio muscle: blood lactate was greater than one at all power outputs and increased most markedly at the power output closest to that eliciting 4 mmol × I^-1 blood lactate (W_OBLA). At W_OBLA. blood lactate was positively correlated to muscle lactate concentrations which covaried widely among subjects (mean 8.3. range 4.5–14.4 mmol × kg^-l wet weight). Muscle fibres from the W_OBLA biopsy in 6 subjects were dissected out and identified as fast twitch (FT) or slow twitch (ST). No significant difference in lactate concentration was observed between pools of FT or ST fibres.     

Effects of age and gender on shortening velocity and myosin isoforms in single rat muscle fibres

The maximum velocity of unloaded shortening (V₀) and the myosin heavy chain (MyHC) and light chain (MyLC) isoform composition were determined in single fibres from soleus and extensor digitorum longus (EDL) muscles of male and female rats 3–6 and 22–24 months old. In the soleus muscle, the β/slow (type I MyHC) isoform predominated in both young and old animals, irrespective of gender. In the EDL, fibres expressing type IIX MyHC or a combination of IIX and IIB (IIXB) MyHC isoforms were predominant in old rats, while type IIB MyHC fibres predominated in young individuals of both genders. The V₀ of soleus fibres expressing the type I MyHC isoform decreased (P < 0.001) by 40% with age in spite of an unchanged MyLC composition. In the EDL, the V₀ of fibres expressing IIX, IIXB and IIB MyHC isoforms did not change with age or differ between males and females. In conclusion, similar age-related changes in V₀ and MyHC composition were observed in single muscle cells from both male and female rats. The present results demonstrate that the relationship between V₀ and MyHC isoform composition at the single fibre level is similar in male and female rats, and that similar qualitative changes take place during ageing in both genders.     

Contractile properties of two varieties of twitch muscle fibres in Xenopus laevis

The contractile properties of twitch muscle fibres in the iliofibularis muscle of Xenopus laevis with characteristic differences in light-microscopical appearance have been studied under isometric and isotonic conditions. Type 1 fibres (large. pale) have a short contraction time (t_e), a shoulder in the relaxation phase, and a high twitch-tetanus ratio. In type 2 fibres (medium-to-large with abundant mitochondria and lipid droplets) t_e is longer, half-relaxation time equals t_e, and the twitch-tetanus is lower. The capillary density is about 4 times higher for type 2 than for type 1. Computer-fitted hyperbolic force-velocity curves gave the following Hill constants (20°C) for type 1 (2) fibres: P_o*/P_o 1.03 (l.15), a/P_o* 0.48 (0.26), b 3.51 (133) I_o/s; extrapolated V_max (0–0.8P_o) was 7.60 (6.27) I_o/s. Lowered temperature (10°C) increases the curvature of the P-V relation in type 1 fibres, little effect was seen in type 2. Increased P_co2 depressed the isometric tension in both types; in type I fibres the P-V relation became less curved, in type 2 fibres no change in curvature was observed. The results suggest that type 1 and type 2 fibres might contain myosin isozymes with slightly different temperature- and pH-sensitivities.     

Nandrolone decanoate treatment induces changes in contractile responses of rat untrained fast‐twitch skeletal muscle

This investigation was designed to examine whether short‐term administration of anabolic–androgenic steroids (AAS) (nandrolone decanoate) could produce changes in contractile responses of untrained rat fast‐ (edl) and slow‐ (soleus) twitch skeletal muscle. Twenty male rats were divided into two groups, one group received weekly (for 6 weeks) an intramuscular injection of AAS, nandrolone decanoate (15 mg kg^–1) and the second group received weekly the similar doses of vehicle (sterile peanut oil). In edl intact isolated small bundles (two to four cells), it was found that nandrolone decanoate treatment increases the K⁺ contracture tension (146 mM ) relative to maximum tension by 56%, whereas no change was observed in the time to peak tension and in the time constant of relaxation. By contrast, in treated soleus muscle, compared with control, no significant modification was found in the K⁺ contracture characteristics. The change in edl contractile responses was associated with a shift to more negative potential of the voltage‐dependence activation and the steady‐state inactivation curves which also shifted leftward in treated soleus fibres. Furthermore, in edl skinned Triton X‐100 fibres, the Ca²⁺ sensitivity of contractile proteins (pCa₅₀) was increased, while electrophoresis analysis indicates no significant effect of nandrolone decanoate treatment on myosin heavy chain (MHC) isoforms. The present results show that nandrolone decanoate treatment produces more pronounced changes in untrained fast muscle function rather than soleus by acting at different levels of the excitation–contraction coupling mechanism without changes in the MHC isoforms and that contractile responses became similar to those found in soleus muscle.     

Structure-function relationship of soleus muscle fibres from the rhesus monkey

Functional and structural properties of rhesus monkey skinned fibres were studied in order to examine the relationship between calcium/strontium (Ca/Sr) activation characteristics and protein composition. The fibres were classified according to their Ca/Sr affinity into slow (61%) and fast groups (39%). According to the myosin isoform composition, two additional hybrid types were defined. Thus, four profiles were characterized: two corresponding to slow (S) and fast (F) isoforms and two corresponding to a mixed proportion of slow and fast isoforms. They were called hybrid slow (HS) or hybrid fast (HF) based on the predominant myosin isoform. Tension/pCa parameters and maximal shortening velocities were determined. S fibres showed a higher pCa threshold and affinity as well as shallower slopes of their tension/pCa curve than did F fibres. HS and HF fibres exhibited tension/pCa curves which were positioned close to those of S and F fibres, respectively. No significant difference was observed between S and HS fibres or between F and HF fibres. Maximal shortening velocity values were higher for fibres expressing predominantly fast myosin isoforms. We suggest than when both S and F isoforms of myofibrillar proteins are expressed in a muscle fibre, the functional properties are mainly governed by the predominant isoform.     

Shortening velocity and force/pCa relationship in skinned crab muscle fibres of different types

Single fibres of three different types, which had been characterized histochemically with regard to differences in myofibrillar adenosine triphosphatase (ATPase) activity and its pH stability, were microdissected from freeze dried preparations of the closer muscle in walking legs of the crab Eriphia spinifrons. Shortening velocities were determined in slack tests and under constant load conditions in maximally Ca²⁺-activated skinned muscle fibres. Force/pCa relationships were also measured for the different types of fibres. Compared with data on vertebrate muscles, all crab muscle fibres required large length changes to reach zero force and showed low Ca²⁺ sensitivity for isometric force generation. The length/time relationship obtained from slack tests had a biphasic course. Maximal velocity of filament sliding differed in the three types of fibres investigated. The filament sliding of type IV fibres was about 3 times faster than that of type I fibres. The values obtained for type II fibres ranged in between. These data are positively correlated with myofibrillar ATPase activity determined histochemically. Ca²⁺ sensitivity of force generation was lowest in the fast type IV fibres. It was high in the slow type I and the faster contracting type II fibres. Ca²⁺ sensitivity in crab muscle seems not to be correlated with speed of shortening.