Pressure-induced changes in the isometric contractions of single intact frog muscle fibres at low temperatures

Summary Effects of increased hydrostatic pressure (range 0.1–10 MPa) on isometric twitch and tetanic contractions of single intact muscle fibres, isolated from frog tibialis anterior muscle, were examined at 4–12° C. The tension changes produced on exposure to steady high pressures are compared with those produced on exposure to low concentrations of caffeine (0.5 mm, subthreshold for contracture) and when pressure is rapidly released during a contraction. The peak twitch tension was potentiated by pressure accompanied by increased rate of tension rise and increased duration; the pressure sensitivity of twitch tension was 8% mPa^-1. The correlation between the rate of tension rise and peak tension in caffeine-induced twitch tension potentiation was quantitatively similar to that in pressure-induced twitch potentiation. Experiments involving the rapid release of pressure (2 ms) during twitch contractions demonstrate that high pressure need only be maintained for a brief period during the early part of tension development to elicit full twitch potentiation. The tetanic tension was depressed by pressure (1% MPa^-1). Results demonstrate that the major effect of increased hydrostatic pressure on intact muscle fibres, which results in tension potentiation, is complete very early during contraction and is similar to that of caffeine.     

Mechanical properties of the isolated inferior oblique muscle of the rabbit

The passive-mechanical and dynamic properties of the rabbit inferior oblique muscle IO were studied in vitro at 35°C. The influence of length on the resting tension and isometric contractions were determined. Maximum twitch tension and fusion tension were developed at optimum length (L _o) an extension of the muscle to about 1.15 timesL _R, the resting length of the IO in situ. A linear relation was found between length and tension in the activated muscle. An increase in stimulation frequency induced a parallel shift in the curves to higher tension but the slope of the curves remained unchanged. On an average the IO had in response to direct massive stimulation a twitch contraction time of 6.4 ms and a half-relaxation time of 7.0 ms. At stimulation with 300 Hz or above the tetanus fused. Stimulus frequencies above fusion frequency increased the rate of tension rise but not the maximum tetanic tension. The maximum tetanic tension was about 6.4 N/cm², and the twitch: tetanus ratio was 0.1. To prolonged tetanic stimulations the IO exhibited a high fatigue resistance. Cooling the muscle to 25°C was followed by an increase in the time parameters of single twitches and tetanic contractions, a decrease of the tension developed in a fused tetanus and a small potentiation of the twitch. Following a repetitive stimulation a small post-tetanic potentiaion of the twitch was observed.     

Twitch potentiation after voluntary versus electrically induced isometric contractions in human knee extensor muscles

Twitch potentiation in knee extensor (KE) muscles after a 7-s conditioning isometric maximal voluntary contraction (MVC trial), submaximal (25% MVC) voluntary contraction (SVC trial) and submaximal tetanic contraction (25% MVC) induced by percutaneous electrical stimulation at 100 Hz (PES trial) was compared in 12 men aged 19–25 years. Isometric twitch characteristics of KE muscles were measured before conditioning contraction and following 10-min recovery by supramaximal electrical stimulation of the femoral nerve. During MVC trial, twitch peak torque (Pt) potentiated (P < 0.05) immediately after the conditioning contraction with sharp decline during the first and third minute of recovery. No significant potentiation of twitch Pt was observed in SVC trial. During PES trial, twitch Pt was potentiated (P < 0.05) within 3–10 min of recovery. The time-course of isometric twitch was not significantly altered by conditioning contractions. It was concluded that twitch potentiation in the KE muscles differed markedly following the three conditioning contractions.     

Contractile properties of the human triceps surae with some observations on the effects of temperature and exercise

Summary The contractile properties of the triceps of five healthy male subjects (mean age 22 years) during electrically stimulated and voluntary isometric muscle contractions were investigated and some observations made on the effects of muscle heating and cooling and dynamic exercise. The times to peak twitch tension (TPT) and half relaxation time (1/2RT) were 111±20 ms and 83±13 ms respectively. Heating and prior exercise decreased, and cooling severely prolonged, TPT and 1/2RT. Exercise and heating had no effect on supramaximal twitch tension (P_t0) but cooling the muscle to a temperature of 24.3‡ C reduced it by 52%. The effects of repetitive stimuli on P_t0 were dependent on frequency; at 0.2 Hz potentiation was observed but at 2 Hz, P_t0 was depressed under control conditions. Heating had no effect on these responses but cooling reversed the 2 Hz and abolished 0.2 Hz response. Post-tetanic potentiation of the twitch was observed under all conditions of measurement. At submaximal stimulation voltages, heating and exercise enhanced twitch and tetanic tensions, but at supramaximal voltages heating reduced tetanic tension (P₀) at 10 Hz (by 115N), though not at 20 Hz. Exercise decreased P₀ at both frequencies of stimulation. Cooling significantly reduced the maximal voluntary contraction and P₀ at 20 Hz. At submaximal voltages, heating enhanced and cooling severely depressed tetanic tensions at high frequency (100 Hz) stimulation. A 2-min fatigue test was unaffected by heating but cooling reduced force generation at the onset of, and the decline of force during, the test. It was concluded that studies of the contractile properties of human muscle in vivo should be based on supramaximal stimulation, and that temperature and prior exercise should be carefully standardised in order to obtain reliable and meaningful results.     

Effects of hydrostatic pressure on fatiguing frog muscle fibres

Summary Effects of increased hydrostatic pressure (range 0.1–10 MPa) on isometric twitch and tetanic contractions of single, intact, frog muscle fibres were examined at 4, 11 and 21°C and at different stages of fatigue. Twitch tension was potentiated by pressure at all temperatures, but the extent of potentiation was more pronounced at higher temperatures (34% MPa^-1 at 21°C, compared to 8% MPa^-1 at 4°C). Tetanic tension was depressed by pressure at 4°C (0.7% MPa^-1) but was potentiated by pressure at 21°C (0.4% MPa^-1). The effect of hydrostatic pressure on the tetanic tension was dependent on the fatigue status of the muscle fibre: during the early stages of fatigue (when tetanic tension was depressed by <20%), high pressure produced a tension depression (as in an unfatigued muscle fibre), whilst during the later stages of fatigue high pressure induced a significant potentiation of tetanic tension. Our results support the suggestion that excitation-contraction coupling and contractile activation are impaired during late fatigue. Pressure-effects were basically similar to caffeine-effects under a variety of conditions, suggesting that an enhancement of Ca²⁺ release may be contributory to potentiation of twitch tension and, in severely, fatigued muscle, potentiation of tetanic tension. In the rested state and during early fatigue the main effect of pressure is an inhibition of the crossbridge cycle.     

Effects of external cadmium ions on excitation–contraction coupling in rat soleus fibres

 The actions of external Cd²⁺ on the twitch and tetanic contractions, action potentials and potassium (K⁺) contractures of rat soleus muscle fibre bundles have been investigated. Cd²⁺ at 1–1.5 mM did not significantly alter tetanic tension, but increased twitch tension and increased the duration and overshoot of action potentials. At ≥3 mM, Cd²⁺ (1) depressed tetanic contractions and initially potentiated but later depressed twitches, (2) abolished the action potential overshoot, and (3) shifted peak K⁺ contracture tension to more positive membrane potentials. Twitch and tetanic contractions, and action potentials remained depressed when Cd²⁺ was washed out of the bath. The effects of Cd²⁺ on the twitch, tetanus and action potential were mimicked by Zn²⁺, while La³⁺ and Co²⁺ at 3 mM – or Mg²⁺ and Ca²⁺ at 30 mM – depressed peak twitch and tetanic tension, but did not potentiate twitches. The results suggest that: (1) Cd²⁺ and Zn²⁺ potentiate twitch tension by prolonging action potential depolarisation; (2) Cd²⁺ depresses twitch and tetanic tension by reducing the action potential overshoot, and causing a positive shift in the voltage dependence of contraction; and (3) the irreversible depression of action potential amplitude in rat soleus muscle is a specific property of Cd²⁺ and Zn²⁺ that is not shared by Co²⁺, Mg²⁺ or Ca²⁺. Received: 14 April 1998 / Received after revision and accepted: 3 September 1998     

Mechanical and electrical correlates of isometric muscle fatigue in skeletal muscle in the cat

The amplitude and duration of motor unit action potentials, the rise time, peak tension, and half relaxation time of an isometric twitch, and forcevelocity relationship, and tetanic tension were measured at the beginning (fresh muscle) and at the end of a fatiguing isometric contraction at a tension of either 40 or 70% of the initial strength in the soleus (a slow twitch muscle) and the medial gastrocnemius (a fast twitch muscle) of the cat. These same parameters were also measured at set intervals following these contractions to assess their rate of recovery to pre-exercise values. At The end of a fatiguing contraction examined,V _mx, twitch tension and tetanic tension, were all reduced while there was a prolonged twitch duration and duration of the motor unit action potential for both types of muscle. The height of the motor unit action potential was only marginally effected by muscle fatigue. Following the fatiguing contraction, the endurance required several hours to recover in the medial gastrocnemius muscle but recovered fully within 15 min after either tension in the soleus muscle. Tetanic tension and twitch tension both required less than 10 min for full recovery in the medial gastrocnemius muscle but recovered fully to the pre-exercise values within 3 min following fatiguing isometric contractions in the soleus muscles.V _mx, and the height and duration of the motor unit action potential both recovered within 1 min following the end of the exercise.     

Muscle weakness following sustained and rhythmic isometric contractions in man

Summary The effects of sustained and rhythmically performed isometric contractions on electrically evoked twitch and tetanic force generation of the triceps surae have been investigated in 4 healthy male subjects. The isometric contractions were performed separately and on different occasions at 30%, 60% and 100% of the force of maximal voluntary contraction (MVC). The area under the maximal voluntary contraction (MVC) force/ time curve during the rhythmic and sustained contractions was the same for each experiment. The results showed that following rhythmic isometric exercise there was a small decrease in low (10 and 20 Hz) and high (40 Hz) frequency tetanic tension which was associated with % MVC. However, there was no change in the 20/40 ratio of tetanic forces, MVC or the contraction times and force of the maximal twitch. In contrast, following sustained isometric exercise tetanic forces were markedly reduced, particularly at low frequencies of stimulation. The 20/40 ratio decreased and the induced muscle weakness was greater at 30% than 60% or 100% MVC. The performance of sustained isometric contractions also effected a decrease in contraction time of the twitch and MVC. The results are in accord with previous findings for dynamic work (Davies and White 1982), and show that if isometric exercise is performed rhythmically the effect on tetanic tensions is small and there is no evidence of a preferential loss of electrically evoked force at either high or low frequencies of stimulation following the contractions. For sustained contractions, however, the opposite is true, the ratio of 20/40 Hz forces is markedly reduced and following 30% sustained MVC there is a significant (p<0.05) change in the time to peak tension (TPT) of the maximal twitch.     

Postactivation potentiation of knee extensor muscles in power- and endurance-trained, and untrained women

This study compared postactivation potentiation (PAP) in knee extensor muscles after a 10 s conditioning isometric maximal voluntary contraction (MVC) in female power- (PT, n = 12) and endurance-trained (ET, n = 12) athletes, and untrained (UT, n = 12) women aged 20–24 years. Isometric twitch characteristics of the knee extensor muscles were assessed in pre-MVC condition and during 15 min post-MVC period using supramaximal electrical stimulation of the femoral nerve by rectangular pulses of 1 ms duration. A significant (P < 0.05) potentiation of twitch peak torque (Pt, 30–51% in different groups), maximal rates of torque development (50–125%) and relaxation (76–124%) occurred immediately (2 s) post-MVC. PAP declined sharply at 1–3 min of recovery, whereas a significant potentiation of twitch Pt was still present for ET athletes at 1 min, and for UT women and PT athletes at 5 min of recovery, respectively. There were no significant (P > 0.05) changes in twitch contraction and half-relaxation times after a 10 s conditioning MVC. We concluded that PAP in knee extensor muscles is enhanced in PT but not in ET female athletes. The magnitude of PAP was greater when measured immediately after the conditioning MVC and its decline was slower in PT compared with ET athletes. Immediately post-MVC, twitch speed-related characteristics were potentiated to a greater extent than twitch Pt. The time-course of isometric twitch was not significantly altered by conditioning MVC.     

Twitch contraction properties of plantar flexor muscles in pre- and post-pubertal boys and men

This study compared electrically evoked twitch contraction characteristics of the plantar flexor muscles in pre-pubertal (11-year-old) and post-pubertal (16-year old) boys, and young (19- to 23-year-old) men. The posterior tibial nerve was stimulated by supramaximal square-wave pulses of 1 ms duration at rest and after brief (5 s) isometric maximal voluntary contraction (MVC) of the plantar flexor muscles, i.e. during post-activation potentiation. Men had higher MVC force than boys and post-pubertal boys higher than pre-pubertal boys. Pre-pubertal boys had lower peak twitch forces (P _t) at rest and when potentiated compared with post-pubertal boys and men, whereas no significant differences were found between post-pubertal boys and men. Pre-pubertal boys had higher ratios of P _t at rest and potentiated P _t to MVC force than post-pubertal boys and men. No age-related differences were obtained in post-activation potentiation, rest and potentiated twitch contraction and half-relaxation time, and MVC force relative to body mass. The main findings of the study were that puberty is characterized by increased muscle force-generating capacity with no change in twitch potentiation and time-course characteristics, and that twitch force-generating capacity develops in an adult-like pattern after puberty. Accepted: 10 April 2000     

Myosin light chain phosphorylation and posttetanic potentiation in fatigued skeletal muscle

Myosin light chain (P-LC) phosphorylation, which is thought to be the principle mechanism for twitch potentiation in skeletal muscle, is significantly decreased during staircase in fatigued muscle. Attenuated phosphorylation of P-LC could be due to either depressed Ca²⁺ transients in fatigue, or to some inhibitory influence of contractile activity on myosin light chain kinase (MLCK). Tetanic stimulation, which would presumably result in maximal activation of MLCK, could be used to evaluate these potential mechanisms. P-LC phosphorylation and twitch developed tension (DT) were assessed at 20 and 120 s following a tetanic contraction in either rested or fatigued rat gastrocnemius muscle in situ. P-LC phosphorylation was significantly lower in fatigued muscles (39.7 ± 3.2% vs 54.8 ± 3.5%, 20 s after a 2-s tetanic contraction), while posttetanic potentiation (PTP) was similar in fatigued (189.1 ± 6.5%) versus rested muscle (169.5 ± 2.6%). Tetanic DT was reduced following the fatigue protocol and, thus, the assumption that the MLCK system was fully activated by Ca²⁺ may not be valid. The potentiation-phosphorylation relationships were linear for both rested and fatigued muscles; however this relationship was shifted markedly leftward in fatigued muscles. It appears that during PTP, equivalent potentiation is attained with correspondingly lower levels of P-LC phosphorylation in fatigued muscle. This enhanced relative potentiation for a given level of phosphorylation could be expected if Ca²⁺ transients were attenuated in the fatigued muscle. However the results do not rule out the possibility that other factors contribute to potentiation under these circumstances. Received: 3 May 1995/Received after revision: 20 September 1995/Accepted: 15 November 1995     

Quantitative analysis of muscle hardness in tetanic contractions induced by electrical stimulation in rats

The purpose of this study was to investigate the in vivo relation between muscle hardness during an electrically induced contracting state and neuromuscular functions (M-wave and developed tension). Sixteen Sprague-Dawley rats were deeply anesthetized with urethane. Muscle hardness was measured quantitatively at the mid-portion of the gastrocnemius (GS) muscle during tetanic contractions induced by electrical stimulation (50 Hz, 100 μs duration) of the sciatic nerve or of the muscle directly. The M-wave was recorded with a pair of wire electrodes inserted into the muscle, and the developed tension was monitored with a push–pull gauge. Muscle hardness, M-wave amplitude and developed tension increased rapidly with the onset of nerve stimulation. Similar but intensity-dependent increases in muscle hardness and tension were observed following direct tetanic stimulation of the muscle. The hardness measured during nerve stimulation was correlated with the amplitude of the M-wave (r = 0.62, P < 0.0001) and the developed tension (r = 0.85, P < 0.0001). These phenomena were suppressed by pancuronium treatment (2 mg/ml, i.v.). These results suggest that muscle tension might be the most important factor for transcutaneously measured muscle hardness induced by tetanic muscle contraction.     

The relations between sarcomere length and characteristics of isometric twitch contractions of frog sartorius muscle

1. Relations between sarcomere length, tension and time course of isometric twitches at 20 degrees C were determined for thirty-two sartorius muscles from Rana temporaria.2. The maximum isometric twitch tension per unit cross-sectional area of muscle ranged from 0.56 to 2.2 kg/cm(2) at 20 degrees C and initial sarcomere length about 2.1 mu. This variation was not correlated with the corresponding measure of tetanic tension.3. The maximum isometric twitch tension per unit cross-sectional area of muscle at 2.1 mu sarcomere length was directly correlated with twitch contraction time and inversely correlated with the cross-sectional area of the muscle.4. The isometric twitch contraction was potentiated with increase in initial sarcomere length from about 2.1 to about 2.8 mu at 20 degrees C, and the degree of potentiation was inversely correlated with the maximum isometric twitch tension per unit cross-sectional area of muscle at 2.1 mu sarcomere length.5. The sarcomere length: twitch tension relation is labile and may be altered by changes in temperature and the after-effects of repetitive stimulation.6. Variation in twitch contractions at different sarcomere lengths are discussed in connexion with excitation-contraction coupling and the degree of activation of muscle fibres during twitch contractions at 20 degrees C.     

Electrically evoked contractions of the triceps surae during and following 21 days of voluntary leg immobilization

Summary The effects of 21 days voluntary leg (plaster) immobilization on the mechanical properties of the triceps surae have been studied in 11 young female subjects, mean age 19.4 years. The results show that during the period of immobilization the mean time to peak tension (TPT) and half relaxation time (1/2RT) and tension (P_t) of the maximal twitch increased significantly (p<0.001) but the effects were short lived. Maximal tension and contraction times of the twitch recovered within 2–14 days following the removal of the plaster cast. The electrically evoked tetanic tensions at 10 Hz and 20 Hz did not change significantly (p>0.1) during immobilization, but the 50 Hz tetanic tension (P_°50) and maximal voluntary contraction (MVC) were reduced (p<0.05). The fall in P_°50 and MVC was associated with 10% decrease in the estimated muscle (plus bone) cross-sectional area. The relative (%) change in P_°50 and MVC following immobilization was related to the initial physiological status (as indicated by the response of the triceps surae to a standard fatigue test prior to immobilization) of the muscle. The rate of rise and recovery fall of the tetanus were slightly but significantly (p<0.01) reduced on day 7 of immobilization, but thereafter remained constant. The isokinetic properties of the triceps surae as reflected in the measured torque/velocity relation of the muscle in 4 subjects did not change significantly if account was taken of the slight degree of atrophy present following immobilization. It was concluded that short term voluntary leg immobilization produces atrophy and some loss of isometric twitch and tetanic function, but has little effect on the isokinetic properties of the triceps surae. The changes in the twitch characteristics during and immediately following immobilization may be indicative of a prolongation of the active state of the muscle.     

Comparison of twitch potentiation in the gastrocnemius of young and elderly men

Summary The capacity for twitch potentiation in the gastrocnemius muscle was determined following maximal voluntary contractions (MVC) in 11 elderly (¯x±SD; 66.9±5.3 years) and 12 young (25.7±3.8 years) men. Potentiation was observed by applying selective stimulation to the muscle belly, 2 s after a 5 s MVC. With this procedure, both groups showed significant (P<0.05) increases in twitch tension in the gastrocnemius (ratios of potentiated twitch to baseline were ¯x=1.68±0.40 for young vs ¯x=1.40±0.20 for the elderly,P<0.001). Time to peak tension of the twitch decreased from ¯x=101.5±17.9 ms to ¯x=88.0±15.8 ms in the young men following po tentiation; the respective values for the older men were 136.7±17.9 ms and 133.1±28.6 ms. These changes resulted in a greater rate of tension development in the potentiated state. The elderly gastrocnemius thus showed qualitatively similar changes in the isometric twitch following potentiation, but reduced and prolonged responses in comparison to young adults. Slowed muscle contraction and reduced capacity for potentiation may be physiological correlates of the reported morphological changes in aged skeletal muscle.     

The Effects of Dietary Creatine Supplements on the Contractile Properties of Rat Soleus and Extensor Digitorum Longus Muscles

Daily creatine supplements (0.258 g kg(-1) ) were administered to adult male Wistar rats (n = 7) in the drinking water. Age matched rats (n = 6) acted as controls. After 5-6 days, contractile properties were examined in soleus and extensor digitorum longus (EDL) muscle strips in vitro at 30 degrees C. In soleus muscles, creatine supplements decreased the half-relaxation time of the isometric twitch from 53.6 +/- 4.3 ms in control muscles to 48.4 +/- 5.5 ms but had no effect on twitch or tetanic tension or on twitch contraction time. In EDL muscles twitch tension, tetanic tension, twitch contraction and half-relaxation times were all unaffected by creatine supplements. Creatine supplements increased the fatigue resistance of the soleus muscles but had no effect on that of the EDL muscles. After a 5 min low-frequency fatigue test, tension (expressed as a percentage of initial tension) was 56 +/- 3 % in control soleus muscles, whereas that in the creatine-supplemented muscles was 78 +/- 6 % (P < 0.01). In the EDL muscles, the corresponding values were 40 +/- 2 % and 41 +/- 9 %, respectively. The force potentiation which occurred in the EDL muscles during the initial 20-30 s of the fatigue test was 170 +/- 10 % of initial tension in the control muscles 24 s after the initial stimulus train but was reduced (P < 0.01) to 130 +/- 20 % in the creatine-supplemented muscles. In conclusion, soleus muscle endurance was increased by creatine supplements. EDL endurance was unaffected but force potentiation during repetitive stimulation was decreased. Experimental Physiology (2001) 86.2, 185-190.     

Influence of ethanol and acetaldehyde on electro-mechanical coupling of skeletal muscle fibres

The effects of ethanol were studied on the time course of the isometric twitch and tetanus of isolated semitendinosus muscle fibres of Rana temporaria (at 2.8-4.5°C). Ethanol in concentrations between (0.1-0.4 M) suppressed the isometric twitch without significantly affecting the tetanus amplitude. A further increase in ethanol concentration (0.5 M) depressed the tetanic tension as well. The maximum rate of force development both of twitch and tetanus was reduced. These changes were fully reversible when ethanol was removed from the bathing fluid. In presence of ethanol there was a reduction, although not statistically significant, in the resting membrane potential and in the maximum rate of rise of the action potential. The overshoot and the maximum rate of fall of the action potential were significantly lowered leading to a prolongation of the duration of the action potential. Acetaldehyde in relatively low concentrations (0.9-1.8 mM) caused a marked potentiation of the twitch without significantly affecting the tetanic amplitude. These changes were, to a great extent, reversible. Higher concentrations (18 mM) of acetaldehyde reduced both the twitch and the tetanic tension and these effects were not reversible on removal of drug from the bathing solution. No detectable effects of the drug (1.8 mM) were observed on resting or action membrane potentials of the muscle fibre. The data presented supports the idea that ethanol suppresses the twitch response by inhibiting the calcium release mechanism and uncouples the excitation-contraction process. The twitch potentiation by acetaldehyde seems to be due to enhancement of calcium release from the storage sites.     

What is fatigue?

Fatigue and potentiation are two forms of force modulation. A general definition of fatigue is "a circumstance where less than the anticipated contractile response is obtained." Fatigue is associated with depressed Ca2+ release and possibly decreased Ca2+ sensitivity. Potentiation results from increased Ca2+ sensitivity due to regulatory light chain phosphorylation. Muscle fatigue and potentiation can coexist, making it difficult to quantify these processes. With repetitive 10Hz stimulation, the developed tension first increases, then decreases. Is fatigue present when developed tension first begins to decrease or when it falls below the developed tension of the first response? Intermittent incompletely fused tetanic contractions for which peak developed tension first decreases, then increases, is another unusual example of fatigue. A third example is when twitch contractions following a tetanic contraction decrease to a level below the pretetanic twitch amplitude, indicating that fatigue may have been coexistent with posttetanic potentiation. These observations illustrate the complexity of detecting fatigue, based on the simple, but commonly accepted definition presented above. Care must be taken in interpreting "before vs. after" contractile responses. Even when the contraction amplitude is greater than the initial response, there is no guarantee that mechanisms associated with fatigue are not present.     

Reappraisal of the role of sodium ions in excitation-contraction coupling in frog twitch muscle

Summary Tetanic and twitch tension were recorded on isolated frog twitch fibres under experimental conditions modifying the influx of sodium ions. In current clamp conditions replacing Li⁺ for Na⁺ did not modify the electrical activity but drastically decreased the plateau of tetanic tension. In voltage clamp conditions replacing Li⁺ for Na⁺ did not modify the inward currents but induced a marked decrease of the plateau of the tetanic tension for depolarizations between the activation threshold and the reversal potential of sodium current. Under veratridine treatment, during tetanic depolarization, a slow inward sodium (or lithium) current developed. This induced a parallel increase of the tetanic tension which was much more pronounced in sodium than in lithium containing solution. The twitch tension obtained during short depolarization was increased by > 100% during veratridine treatment with a sizeable decrease (40%) of the delay between the end of depolarization and the beginning of tension. All these results could be reproduced in calcium-free solution. Our data confirm that the entry of sodium ions (and to a lesser extent of lithium ions) is able to modulate the release of calcium from the sarcoplasmic reticulum (SR). We discuss these results in terms of a model where sodium ions entering the compartment between the tubular membrane and the SR junctional membrane carry counter charges through the SR K⁺ channels and help to maintain the SR Ca²⁺ release. This could occur in particular during a physiological tetanic contraction where the junctional compartment is probably filled with Na⁺ ions and depleted of K⁺ ions.     

Comparison of the histochemical and contractile properties of human triceps surae

Summary Contractile and histochemical properties of the triceps surae were compared in 16 males and 4 females aged 20 to 49 years. Surface electrical stimulation was used to determine twitch, tetanic and fatigue parameters. From these tests, twitch tension (Pt), time to peak tension (TPT), half relaxation time 1/2 RT), tetanic tensions at 10, 20 and 50 Hz and an index of fatigue (FI) were calculated. A maximal voluntary contraction (MVC) was also performed. Muscle samples from the belly of the lateral gastrocnemius were obtained using the needle biopsy technique. The samples were treated histochemically for myosin ATPase and NADH-tetrazolium reductase in order to classify the fibres as either Type I, slow twitch (ST) or Type II, fast twitch (FT) and to determine fibre areas. Correlations were performed between the grouped male and female contractile and histochemical variables. The results demonstrated significant positive relationships between percentage of ST fibres (%ST) and TPT (r=0.49), and %ST and the ratio of tetanic forces at 10 Hz to 50 Hz (Po10/Po50) (r=0.55). No significant relationships were obtained for Pt, 1/2 RT, MVC or FI with any histochemical parameter. The results suggest that fibre type distribution determined using myosin ATPase is related to electrically stimulated isometric contractile speeds and not to voluntary force generation (MVC) or electrically induced fatigue.