The visco-elasticity of resting intact mammalian (rat) fast muscle fibres

Summary Tension responses induced by ramp stretches (amplitude of 1–2% fibre length and speeds of 0.01–15 L _o s^-1) were examined in resting intact muscle fibre bundles isolated from the extensor digitorum longus (a fast muscle) of the rat; sarcomere length of a 2 mm region was monitored near the tension transducer end by means of a He-Ne laser diffractometer. The experiments were done at 10°C. During a ramp stretch, the tension rose rapidly (P₁) and then slowly (P₂) to reach a peak; after completion of the ramp, the tension decayed in a complex manner to a steady level (P₃) at approximately constant sarcomere length. At stretch velocities higher than 1–2 L _o s^-1, P₁ tension increased in direct proportion to stretch velocity, indicating that it is due to viscous resistance; in a half sarcomere, the viscous resistance to filament sliding may be about 5 × 10⁸ N s m^-3. The steady tension level after the ramp (P₃ tension) was independent of stretch velocity indicating that it represents an elastic tension. The amplitude of the slow tension rise (P₂ tension corrected for P₃) increased with stretch velocity up to a plateau (as in a visco-elastic component); the calculated relaxation time was 5–13 ms. Amplitudes of all three components were larger at longer sarcomere length (range 2.4–3 m). The presence of 5–10 mm BDM which abolished the twitch and markedly depressed the tetanic responses, produced little or no change in the tension components. Our results show that none of the tension components to stretch in relaxed mammalian muscle fibres is due to active, cycling cross-bridges; the possibility that the resting sarcomeric visco-elasticity (net P₂) resides in the connectin (=titin) containing gap filament is discussed.     

Sarcomeric visco-elasticity of chemically skinned skeletal muscle fibres of the rabbit at rest

The giant muscle protein titin (connectin), contained in the gap filament that connect a thick filament to the Z-line in a sarcomere, is generally considered to be responsible for the passive force (tension) and visco-elasticity in resting striated muscle. However, whether it can account for all the features of the resting tension response remains unclear. In this paper, we examine the basic features of the ‘sarcomeric visco-elasticity’ in a single resting mammalian muscle fibre and attempt to account for various tension components on the basis of known structural features of a sarcomere. At sarcomere length of ∼2.6 μm, the force response to a ramp stretch of 2–5% is complex but can be resolved into four functionally different components. The behaviour displayed by the components ranges from pure viscous type (directly proportional to stretch velocity, ranging from 0.1 to 30 lengths s⁻¹) to predominantly elastic type (insensitive to stretch velocity at 1–2 s time scale); simulations show two components of visco-elasticity with characteristically different relaxation times. The velocity-sensitive components (only) are enhanced by filament lattice compression (dextran – 500 kD) and by increased medium viscosity (dextran – 12 kD); also, the relaxation time of visco-elasticity is longer with increased medium viscosity. Amplitude of all the components and the relaxation time of visco-elasticity are increased at longer sarcomere length (range ∼2.5 – 3.0 μm). The study, and quantitative analyses, extend our previous work on intact muscle fibres and suggest that the velocity-sensitive tension components in intact sarcomere arise from interactions between sarcomeric filaments, filament segments and inter-filamentary medium; the two components of visco-elasticity arise from distinct regions of titin (connectin) molecules. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spontaneous tension oscillation in skinned bovine cardiac muscle

 Skinned fibres from bovine ventricles exhibited spontaneous tension oscillations when MgADP and inorganic phosphate (Pi) were added to the solution bathing fibres in the relaxed state (ADP-SPOC). A similar type of oscillation was observed at intermediate concentrations of free Ca²⁺ in the absence of MgADP and Pi (Ca-SPOC). To investigate the correlation between ADP-SPOC and Ca-SPOC, we constructed two-dimensional state diagrams of cardiac muscle using different concentrations of Pi (0–20 mM) and free Ca²⁺ [pCa=around 5 (+Ca²⁺), pCa=5.15–6.9 and +EGTA (–Ca²⁺)], with varying concentrations of MgADP (0–10 mM), with 2 mM MgATP and 2 mM free Mg²⁺ maintaining ionic strength at 0.15±0.01 M, pH 7.0, 25 °C. The three-dimensional (pCa-Pi-MgADP) state diagram thus obtained was divided into three regions, i.e. the contraction region in which tension oscillation was undetectable, the spontaneous tension oscillation (SPOC) region and the relaxation region. We found that the regions of ADP-SPOC and Ca-SPOC were continuously connected by a single oscillation region sandwiched between the contraction and relaxation regions. The state diagram, which encompasses physiological conditions, shows that the probability of SPOC is higher in cardiac muscle than in skeletal muscle. From these results, we suggest that, despite distinct ionic conditions, the molecular state of cross-bridges during SPOC is common to both ADP-SPOC and Ca-SPOC. Received 19 February 1996 / Received after revision: 16 July 1996 / Accepted: 14 August 1996     

The effects of ramp stretches on active contractions in intact mammalian fast and slow muscle fibres

The effects of a ramp stretch (amplitude < 6% muscle fibre length (L ₀), speed < 13L ₀ s^–1) on twitch tension and twitch tension re-development were examined in intact mammalian (rat) fast and slow muscle fibre bundles. The experiments were done in vitro at 20°C and at an initial sarcomere length of 2.68 m. In both fibre types, a stretch applied during the rising phase of the twitch response (including the time of stimulation) increased the re-developed twitch tension (15–35%). A stretch applied before the stimulus had little or no effect on the twitch myogram in fast muscle fibres, but it increased the twitch tension (5%) in slow muscle fibres. A similar stretch had little or no effect on tetanic tension in either muscle fibre type. In general, the results indicate that the contractile-activation mechanism may be stretch sensitive and this is particularly pronounced in slow muscle fibres. Recorded at a high sampling rate and examined at an appropriate time scale, the transitory tension response to a stretch rose in at least two phases; an initial rapid tension rise to a break (break point tension, P ₁ ^a) followed by a slower tension rise (apparent P ₂ ^a) to a peak reached at the end of the stretch. Plotted against stretch velocity, P ₁ ^a tension increased in direct proportion to stretch velocity (viscous-like) whereas, P ₂ ^a tension (calculated as peak tension minus P ₁ ^a tension) increased with stretch velocity to a plateau (visco-elastic). Examined at the peak of a twitch, P ₁ ^a tension had a slope (viscosity coefficient) of 1.8 kNm^–2 per L ₀ s^–1 in fast fibres and 4.7 kNm^–2 per L ₀ s^–1 in slow muscle fibres. In the same preparations, P ₂ ^a tension had a relaxation time of 8 ms in the fast muscle fibres and 25 ms in the slow muscle fibres. The amplitudes of both tension components scaled with the instantaneous twitch tension in qualitatively the same way as the instantaneous fibre stiffness. These fast/slow fibre type differences probably reflect differences in their cross-bridge kinetics.     

Comparison of the tension responses to ramp shortening and lengthening in intact mammalian muscle fibres: crossbridge and non-crossbridge contributions

We examined the tension responses to ramp shortening and lengthening over a range of velocities (0.1–5 L ₀/s) and at 20°C and 30°C in tetanized intact fibre bundles from a rat fast (flexor hallucis brevis) muscle; fibre length (L ₀) was 2.2 mm and sarcomere length ∼2.5 μm. The tension change during ramp releases as well as ramp stretches showed an early transition (often appearing as an inflection) at 1–4 ms; the tension change at this transition and the length change at which it occurred increased with velocity. A second transition, indicated by a more gradual reduction in slope, occurred when the length had changed by 14–28 nm per half-sarcomere; the tension at this transition increased with lengthening velocity towards a plateau and it decreased with shortening velocity towards zero tension. The velocity dependence of the time to the transitions and the length change at the transitions showed some asymmetries between shortening and lengthening. Based on analyses of the velocity dependence of the tension and modelling, we propose that the first transition reflects the tension change associated with the crossbridge power stroke in shortening, or with the reversal of the power stroke in lengthening. Modelling shows that the reduction in slope at the second transition occurs when most of the crossbridges (myosin heads) that were attached at the start of the ramp become detached. After the second transition, the tension reaches a steady level in the model whereas the tension continues to increase during lengthening and continues to decrease during shortening in the experiments; this continuous tension change is seen at a wide range of initial sarcomere lengths and when active force is reduced by the myosin inhibitor, BTS. The continuous tension decline during shortening is not abolished by caffeine, but the rate of decline is reduced when the active force is depressed by BTS. We propose that stiffening of non-crossbridge visco-elastic elements upon activation contributes to the continuous tension rise during lengthening and the release of such tension and Ca-insensitive deactivation contribute to the tension decline during shortening in muscle fibres.     

Contractile properties of rabbit psoas muscle fibres inhibited by beryllium fluoride

The structure of truncated, recombinant Dictyostelium myosin motor domain complexed with Mg · ADP and slowly dissociating analogues of Pi has previously been characterized as two main states (S1-MgADP plus BeF vs. AlF ₂ ^– or Vi). The BeF bound state is thought to mimic the weak actin-binding M · ATP complex, while the states with AlF ₂ ^– or Vi bound mimic the M · ADP · Pi state. While the effects of AlF ₂ ^– and Vi on fibre mechanics have been previously described (Chase etal., 1994, 1993), the effects of BeF have not been characterized in detail. At pCa 4.5 (12°C), we measured (i) steady-state isometric tension, (ii) stiffness (K_S; 1 kHz sinusoids), and (iii)unloaded shortening velocity (V_u; slack test) in single skinned muscle fibres from rabbit psoas. Results were compared when tension was inhibited with either BeF or 2,3-butanedione-monoxime (BDM) or modulated by altering myoplasmic [Ca²⁺]. With 3 mM total fluoride, 1 mM BeF inhibited both tension and K_S by 50% (compared to 7–10 mM BDM and 50–100 M AlF ₂ ^– ). Increasing [BeF] to 10 mM further reduced tension to 15% P₀, but had little further effect on K_S; with BDM and altered [Ca²⁺], K_S scaled more proportionately with tension. Inhibition of tension and K_S by BeF was more rapidly reversible, compared with slow recovery from tension inhibition with AlF ₂ ^– or Vi. V_u exhibited a complex dependence on [BeF], being relatively unaffected by concentrations 1 mM, and becoming inhibited steeply for [BeF] above this level. With BDM, V_u co-varied more directly with force. Our results suggest that BeF may induce a different cross-bridge state in fibres than do AlF ₂ ^– or Vi, but all three analogues of Pi form complexes that mimic crossbridge states that follow ATP hydrolysis.     

Effect of phosphate and temperature on force exerted by white muscle fibres from dogfish

Effects of Pi (inorganic phosphate) are relevant to the in vivo function of muscle because Pi is one of the products of ATP hydrolysis by actomyosin and by the sarcoplasmic reticulum Ca²⁺ pump. We have measured the Pi sensitivity of force produced by permeabilized muscle fibres from dogfish (Scyliorhinus canicula) and rabbit. The activation conditions for dogfish fibres were crucial: fibres activated from the relaxed state at 5, 12, and 20°C were sensitive to Pi, whereas fibres activated from rigor at 12°C were insensitive to Pi in the range 5–25 mmol l⁻¹. Rabbit fibres activated from rigor were sensitive to Pi. Pi sensitivity of force produced by dogfish fibres activated from the relaxed state was greater below normal body temperature (12°C for dogfish) in agreement with what is known for other species. The force-temperature relationship for dogfish fibres (intact and permeabilized fibres activated from relaxed) showed that at 12°C, normal body temperature, the force was near to its maximum value.     

Effects of removal of weight-bearing function on contractility and myosin isoform composition in single human skeletal muscle cells

The purpose of this study was to investigate the effects of a 6-week period without weight bearing, achieved by bed rest, on the contractile behaviour, myosin isoform expression and myofibrillar protein content of single human muscle fibres. Percutaneous biopsied specimens of the quadriceps muscle were taken from three healthy male volunteers before and at the end of the experimental period. Maximum force normalised to cross-sectional area (specific tension), maximum velocity of unloaded shortening (V  ₀), and myosin heavy chain (MyHC) and light chain (MyLC) isoform composition were measured in single membrane-permeabilised muscle cells obtained from these specimens. At the end of the experimental period, specific tension was reduced (P < 0.001) by 40% and there was a parallel decline in myofibrillar protein content per muscle cell volume. V  ₀ did not change significantly in response to bed rest when data from all muscle cells were pooled. In two of the subjects, however, V  ₀ decreased (P < 0.01–0.001) in muscle cells expressing the β/slow (type I) MyHC isoform, but there was no change in fibres expressing type IIA or a combination of type IIA and IIB MyHCs. The slowing in type I MyHC fibres was associated with a change in the isoform composition of the regulatory MyLC. Received: 5 October 1995/Received after revision and accepted: 3 January 1996     

Association of muscle hardness with muscle tension dynamics: a physiological property

This study aimed to investigate the relationship between muscle hardness and muscle tension in terms of length–tension relationship. A frog gastrocnemius muscle sample was horizontally mounted on the base plate inside a chamber and was stretched from 100 to 150% of the pre-length, in 5% increments. After each step of muscle lengthening, electrical field stimulation for induction of tetanus was applied using platinum-plate electrodes positioned on either side of the muscle submerged in Ringer’s solution. The measurement of muscle hardness, i.e., applying perpendicular distortion, was performed whilst maintaining the plateau of passive and tetanic tension. The relationship between normalised tension and normalised muscle hardness was evaluated. The length–hardness diagram could be created from the modification with the length–tension diagram. It is noteworthy that muscle hardness was proportional to passive and total tension. Regression analysis revealed a significant correlation between muscle hardness and passive and total tension, with a significant positive slope (passive tension: r = 0.986, P < 0.001; total tension: r = 0.856, P < 0.001). In conclusion, our results suggest that muscle hardness depends on muscle tension in most ranges of muscle length in the length–tension diagram.     

Mechanism of force enhancement during and after lengthening of active muscle: a temperature dependence study

The aim of the present study was to examine the temperature dependence of active force in lengthening and shortening muscle. Experiments were done, in vitro, on bundles of intact fibres (fibre length L₀?~2?mm; sarcomere length?~2.5???m) isolated from a rat fast muscle (flexor hallucis brevis) and a ramp length change of 5?C7?% L₀ was applied on the plateau of an isometric tetanic contraction. Ramp lengthening increased and ramp shortening decreased the muscle tension to new approximately steady levels in a velocity-dependent way. The isometric tension and the lower steady tension reached at a given shortening velocity, increased with warming from 10 to 35?°C and the relation between tension and reciprocal absolute temperature was sigmoidal. However, the tension?Ctemperature curve of shortening muscle was sharper and shifted to higher temperature with increased velocity. In contrast, the enhanced steady tension during lengthening at a given velocity was largely temperature-insensitive within the same temperature range; we hypothesize that the tension?Ctemperature curve may be shifted to lower temperatures in lengthening muscle. Consequently, when normalised to the isometric tension at each temperature, the tension during lengthening at a given velocity decreased exponentially with increase of temperature. The residual force enhancement that remains after ramp lengthening showed a similar behaviour and was markedly reduced in warming from 10 to 35?°C. The findings are consistent with the thesis that active force generation in muscle is endothermic and strain-sensitive; during shortening with a faster crossbridge cycle it becomes more pronounced, but during lengthening it becomes depressed as the cycle slows in a velocity-dependent way. The residual force enhancement may be caused by the same process in addition to non-crossbridge mechanism(s).     

Effect of bovine serum albumin on the calcium release channel of sarcoplasmic reticulum from rabbit skeletal muscle

The effect of bovine serum albumin (BSA) on the activity of the calcium release channel of the sarcoplasmic reticulum from rabbit skeletal muscle was investigated using both tension recording from skinned fibres and electrophysiological recording of unitary channel currents from planar lipid membranes. BSA had no effect on the Ca²⁺ affinity of the contractile proteins, elicited no tension per se in Ca²⁺-loaded skinned fibres, but potentiated caffeine-induced tension. Maximum potentiation was observed with 0.05–0.5% BSA. BSA (0.1%) had no detectable effect on the basal activity of the Ca²⁺-release channel incorporated in lipid bilayer. However, channel stimulation elicited by either caffeine (2 mM ) or ATP (60 μM ) was further enhanced by BSA (0.1%), as indicated by significant increases in P_o, the open probability of the channel. These results suggest that BSA can modulate the response of the skeletal muscle SR Ca²⁺-release channel to different activators such as caffeine and ATP.     

The cation selectivity of the sarcoball Ca2+ channel in frog muscle fibres

 We have measured single-channel currents from sarcoplasmic reticulum (SR) blebs (sarcoballs) of frog skeletal muscle fibres using conventional patch-clamp electrodes with excised patches. With both the pipette and bath solutions containing 50 mM Ca(gluconate)₂ the slope conductance of the single channels was 39.2 pS for the most commonly seen state, with a reversal potential of –0.4 mV. The cation selectivity of this channel was investigated by replacing the bathing solution with either gluconate or HEPES salts of selected cations. The Goldman permeability ratios, calculated from the reversal potentials, were found to be P(Ca²⁺)/P(K⁺)=2.4, P(Ca²⁺)/ P(Na⁺)=2.7, P(Ca²⁺)/P(Tris⁺)=3.1, P(Ca²⁺)/P(Mg²⁺)=1.0 and P(Ca²⁺)/P(Ba²⁺)=1.1. Each value for the monovalent ions was found to be less than the corresponding value reported for the SR ryanodine receptor channel from skeletal and cardiac muscle. Single-channel activity could be recorded when the preparation was bathed in symmetrical 50 mM Mg(gluconate)₂ solutions, and these channels had a similar conductance and open probability to that measured when the preparation was bathed in symmetrical Ca(gluconate)₂ solution. The channel activity in symmetrical 50 mM Ca(gluconate)₂ solution was insensitive to bath-applied caffeine (5 mM) and ryanodine (10 μM). The results are in agreement with the conclusion that the sarcoball Ca²⁺ channel is not the ryanodine receptor release channel, but possibly a form of the SR Ca²⁺-ATPase which is uncoupled from the catalytic events of the pump and acts as a passive ion channel. Received: 13 February 1998 / Received after revision: 6 April 1998 / Accepted: 7 April 1998     

Free calcium in sheep cardiac tissue and frog skeletal muscle measured with Ca2+-selective microelectrodes

Microelectrodes filled with neutral carrier selective to Ca²⁺ were used to measure the free intracellular Ca²⁺ concentration ([Ca²⁺]_i) in sheep cardiac tissue and frog skeletal muscle. Calibration of the electrodes was performed in the presence of a solution resembling the cationic composition of the cytoplasm. [Ca²⁺]_i at rest in normal physiological saline (20–22° C) was 240 nM in Purkinje fibres, 270 nM in ventricular muscle, and 52 nM in skeletal muscle. In Purkinje fibres, elevation of [Ca²⁺]_o from 1.8 mM to 5.4 mM produced a 1.7-fold increase in [Ca²⁺]_i. Elevation of [Ca²⁺]_o from 1.8 mM to 18 mM induced a 2.6-fold increase in [Ca²⁺]_i. Exposure to Na⁺-free solution (Li⁺-substituted) gave rise to elevation of [Ca²⁺]_i by factors of 5.8 and 14 in ventricular muscle and Purkinje fibres, respectively. These latter changes in [Ca²⁺]_i were associated with the development of contractures which reached 34% and 172% of the corresponding twitch tension.     

Visco-elastic properties of the rapidly adapting stretch receptor muscle of the crayfish

The visco-elastic properties of the receptor muscle associated with the rapidly adapting stretch receptor organ of crayfish (Pacifastacus Leniusculus) were studied by recording the tension responses to various length changes. Steady-state length changes resulted in a non-linear tension development in the receptor muscle. The tension increased slowly for small extensions and more rapidly when extension increased. Muscle tension responses to ramp-and-hold extension were characterized by a transient peak followed by a gradual non-exponentional decline in tension. At the onset of the ramp the tension increased rapidly, similar to what has been observed in the muscle of the slowly adapting receptor (SM). The steeper rise in tension during the first part of the ramp indicating higher initial stiffness, resulted in a ‘hump’ when large extensions (> 15%) were applied. The results show that the rapidly adapting receptor muscle has a more pronounced dynamic component; the ratio between the amplitude of the peak and the steady state response was larger in the rapidly than in the slowly adapting receptor muscle. Accordingly, different values for the elements of a visco-elastic model of the muscle had to be set for the two types of receptors. The different properties of the rapidly and slowly adapting receptor muscles are in line with the differences in the overall adaptive behaviour of the organ and give further support to the idea that mechanical factors contribute to the adaptive properties.     

Effects of vanadate, phosphate and 2,3-butanedione monoxime (BDM) on skinned molluscan catch muscle

The effects of orthovanadate (V_i), inorganic phosphate (P_i) and 2,3-butanedione monoxime (BDM) on tension, force transients and the catch state (passive tension maintenance) were investigated in saponin-skinned fibre bundles of the anterior byssus retractor muscle (ABRM) of the bivalve mollusc Mytilus edulis at pH 6.7. During maximal Ca²⁺ activation isometric force was depressed by V_i (0.03–10 mM), P_i (10 mM) and BDM (50 mM). Force transients following quick stretches (0.1–0.3% of fibre length) were accelerated substantially by 1 mM V_i, 10 mM P_i or 50 mM BDM. These compounds also accelerated force responses in experiments in which ATP was released rapidly from caged ATP by flash photolysis at both pCa 4.7 (force rise) and at pCa>8 (force decline). The effects on the catch state were investigated in two types of experiments: (1) Ca²⁺ removal after maximal Ca²⁺ activation and (2) rapid ATP release during high-force rigor at pCa>8. In both cases rapid relaxation was followed by slow relaxation (slower than 2% of initial force per min). This later slow relaxation (catch) was insensitive to V_i (1–10 mM), P_i (10 mM) and BDM (50 mM) but was accelerated by 0.12 mM cAMP. Complete relaxation to almost zero force was attained by changing pH from 6.7 to 7.7 (pCa>8). We conclude that catch depends on cAMP- and pH-sensitive structures linking the myofilaments and not on the force-generating actomyosin cross-bridges that are sensitive to V_i, P_i and BDM.     

Cross-bridge stiffness in Ca2+-activated skinned single muscle fibres

Tension transients, in response to small and rapid length changes (completed within 40 s), were obtained from skinned single frog muscle fibres incubated in activating solutions with varying concentrations of Ca²⁺. The first 2 ms of these transients were described by a linear model in which the fibre is regarded as a rod composed of infinitesimally small, identical segments containing a mass, one undamped elastic element and in the case of relaxed fibres two damped elastic elements in series, or in the case of activated fibres three such elastic elements in series. The stiffness of activated fibres, expressed in elastic constants or apparent elastic constants, increased with increasing concentrations of Ca²⁺. All the damped elastic constants that were necessary to describe the tension responses of activated fibres were proportional to isometric tension. However, the undamped elastic constant did not increase linearly with increasing isometric tension. Equatorial X-ray diffraction patterns were obtained from single frog muscle fibres under similar conditions as under which the tension transients were obtained. The filament spacing (d ₁₀)of Ca²⁺-activated single frog muscle fibres decreased with increasing isometric force, whereas the intensity ratio (I ₁₁/I₁₀)increased linearly with increasing isometric force. From experiments in which dextran (MW 200000 Da) was added, it followed that such a change in filament spacing would modify passive stiffness. The d ₁₀value of relaxed fibres decreased and stiffness increased with increasing concentrations of the polymer dextran, whereas I ₁₁/I₁₀remained constant. The relation of stiffness and filament spacing with concentration of dextran was used to eliminate the effect of decreased filament spacing on stiffness of activated fibres. After correction for changes in filament spacing the undamped complicance C ₁, normalized to tension, was not constant, but increased with increasing isometric tension. If we assume that isometric tension is proportional to the number of force generating cross-bridges, this means that only part of the undamped complicance of activated fibres is located in the crossbridges.     

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.     

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.     

Mitochondrial function in human skeletal muscle is not impaired by high intensity exercise

 The hypothesis that high-intensity (HI) intermittent exercise impairs mitochondrial function was investigated with different microtechniques in human muscle samples. Ten male students performed three bouts of cycling at 130% of peak O₂ consumption (V ^·O_2,peak). Muscle biopsies were taken from the vastus lateralis muscle at rest, at fatigue and after 110 min recovery. Mitochondrial function was measured both in isolated mitochondria and in muscle fibre bundles made permeable with saponin (skinned fibres). In isolated mitochondria there was no change in maximal respiration, rate of adenosine 5’-triphosphate (ATP) production (measured with bioluminescence) and respiratory control index after exercise or after recovery. The ATP production per consumed oxygen (P/O ratio) also remained unchanged at fatigue but decreased by 4% (P<0.05) after recovery. In skinned fibres, maximal adenosine 5’-diphosphate (ADP)-stimulated respiration increased by 23% from rest to exhaustion (P<0.05) and remained elevated after recovery, whereas the respiratory rates in the absence of ADP and at 0.1 mM ADP (submaximal respiration) were unchanged. The ratio between respiration at 0.1 and 1 mM ADP (ADP sensitivity index) decreased at fatigue (P<0.05) but after the recovery period was not significantly different from that at rest. It is concluded that mitochondrial oxidative potential is maintained or improved during exhaustive HI exercise. The finding that the sensitivity of mitochondrial respiration to ADP is reversibly decreased after strenuous exercise may indicate that the control of mitochondrial respiration is altered. Received: 17 June 1998 / Received after revision: 11 November 1998 / Accepted: 26 November 1998     

Effects of pH on spontaneous tension oscillation in skinned bovine cardiac muscle

Skinned cardiac muscle preparations exhibit spontaneous tension oscillations (spontaneous oscillatory contractions; SPOCs) in the absence of Ca²⁺, and in the presence of MgATP, MgADP and inorganic phosphate (Pi; ADP-SPOC). Similar oscillations occur in the presence of sub-micromolar concentrations of Ca²⁺ under normal activating conditions without MgADP and Pi (Ca-SPOC). In the study presented here, we investigated the effects of pH on both types of SPOC in skinned bovine cardiac ventricular muscle. First, a decrease in pH increased the MgADP concentration required to induce the half-maximal isometric tension that is obtained in the absence of Ca²⁺ and in the presence of MgATP (ADP-contraction). The inhibitory effect of Pi on ADP-contractions was not affected by pH. Second, ADP-SPOCs occurred upon the addition of Pi to the solution that resulted in ADP-contraction, and the relative amplitude and the period of the tension oscillation in the presence of 2 mM MgATP, 10 mM MgADP and 10 mM Pi were unchanged under all pH conditions examined (6.6, 7.0, 7.4). On the contrary, the relative amplitude and the period of the Ca-SPOCs were markedly diminished at pH 6.6. Finally, we constructed state diagrams showing the effects of pH on SPOC conditions. The state diagram shows that SPOCs occur less frequently under acidic conditions than at neutral pH. We suggest that the intermediate state of crossbridges that is required for SPOCs is more difficult to attain at a low pH. Received: 14 September 1998 / Received after revision: 23 February 1999 / Accepted: 10 March 1999