Glycogen synthesis from lactate in skeletal muscle of the lizardDipsosaurus dorsalis

Summary The capacity of skeletal muscle to synthesize glycogen from lactate was tested in the iliofibularis muscle of the desert iguana,Dipsosaurus dorsalis. Like other reptiles,Dipsosaurus accumulates significant lactic acid concentrations following vigorous exercise. After 5 min of progressively faster treadmill running at 35°C (final speed=2.2 km/h), blood lactate concentration increased over 14 mM, which decreased 11 mM after 2 h of recovery. Blood glucose concentration remained unchanged throughout at 8.6±0.46 mM. The role that muscle gluconeogenesis might play in the removal of post-exercise lactate was evaluated. Animals were run to exhaustion at 1.5 km/h on a treadmill thermostatted at 35°C. Animals (n=43) ran 6.9±0.75 min prior to exhaustion. Animals were sacrificed and iliofibularis muscles of both hindlimbs removed and stimulated at 2 Hz for 5 min, reducing twitch tension to 6% of prestimulus tension. Fatigued muscles were then split into red and white fiber bundles and incubated 2 h or 5 h at 35°C in Ringer solution or in Ringer plus 20 mM lactate. In muscles tested in August, red fiber bundles incubated in lactate demonstrated a rate of glycogen synthesis of approximately 1 mg/(g muscle·h). In muscles tested in December, red fiber bundles synthesized glycogen at a reduced rate that was not statistically different than in fiber bundles incubated in Ringer solution without lactate. Glycogen synthesis from lactate was not evident in white fiber bundles in either August or December. The period of peak gluconeogenic capacity coincides with the field active season ofDipsosaurus. In vivo rates of lactate removal and in vitro rates of glycogen synthesis suggest that muscle gluconeogenesis may potentially account for 20% of the lactate removed during recovery from exhaustive activity.Abbreviations IF iliofibularis - FG fast twitch, glycolytic - FOG fast twitch, oxidative-glycolytic     

Temperature adaptation and the contractile properties of live muscle fibres from teleost fish

Summary The contractile properties of swimming muscles have been investigated in marine teleosts from Antarctic (Trematomus lepidorhinus, Pseudochaenichthys georgianus), temperate (Pollachius virens, Limanda limanda, Agonis cataphractus, Callionymus lyra), and tropical (Abudefduf abdominalis, Thalassoma duperreyi) latitudes. Small bundles of fast twitch fibres were isolated from anterior myotomes and/or the pectoral fin adductor profundis muscle (m. add. p). Live fibre preparations were viable for several days at in vivo temperatures, but became progressively inexcitable at higher or lower temperatures. The stimulation frequency required to produce fused isometric tetani increased from 50 Hz in Antarctic species at 0°C to around 400 Hz in tropical species at 25°C. Maximum isometric tension (P_o) was produced at the normal body temperature (NBT) of each species (Antarctic, 0–2°C; North Sea and Atlantic, 8–10°C; Indo-West Pacific, 23–25°C). P₀ values at physiological temperatures (200–300 kN·m^–2) were similar for Antarctic, temperate, and tropical species. A temperature induced tension hysteresis was observed in muscle fibres from some species. Exposure to <0°C in Antarctic and <2°C in temperate fish resulted in the temporary depression of tension over the whole experimental range, an effect reversed by incubation at higher temperatures. At normal body temperatures the half-times for activation and relaxation of twitch and tetanic tension increased in the order Antarctic>temperate>tropical species. Relaxation was generally much slower at temperatures <10°C in fibres from tropical than temperate fish. Q₁₀ values for these parameters at NBTs were 1.3 2.1 for tropical species, 1.7–2.6 for temperate species, and 1.6–3.5 for Antarctic species. The forcevelocity (P-V) relationship was studied in selected species using iso-velocity releases and the data below 0.8 P₀ iteratively fitted to Hill's equation. The P-V relation at NBT was found to be significantly less curved in Antarctic than temperate species. The unloaded contraction velocity (V_max) of fibres was positively correlated with NBT increasing from about 1 muscle fibre length·s^–;1 in an Antarctic fish (Trematomus lepidorhinus) at 1°C to around 16 muscle fibre lengths·s^–1 in a tropical species (Thalassoma duperreyi) at 24°C. It is concluded that although muscle contraction in Antarctic fish shows adaptations for low temperature function, the degree of compensation achieved in shortening speed and twitch kinetics is relatively modest.Abbreviations ET environmental temperature - m. add. p major adductor profundis - m. add. s. major adductor superficialis - NBT normal body temperature - P ₀ maximum isometric tension - P-V force velocity - SR sarcoplasmic reticulum - T 1/2 a half activation time - T 1/2 r half relaxation time - V _max unloaded contraction     

Sexual dimorphism in histological characteristics and contractility of the iliofibularis muscle in the lizard Sceloporus torquatus

The iliofibularis is a hindlimb muscle used in lizard locomotion that is composed of at least three types of fibres: fast‐twitch‐glycolytic (FG), fast‐twitch‐oxidative‐glycolytic fibre (FOG) and slow‐twitch‐oxidative (SO). The striated skeletal muscle is a highly plastic tissue undergoing phenotypic change in response to activity. The lizard Sceloporus torquatus has sexual differences associated with microhabitat use, which can be reflected in the physiology and anatomy of the muscle, and thus, in our study, we analysed the morphological and contractile characteristics of the iliofibularis muscle (IF) of S. torquatus males and females. We found a larger prevalence of FOG compared with FG and SO fibres in the muscle of both sexes. We also found that males show larger areas of the three types of fibres, develop greater strength but also faster fatigue than females, suggesting that strength is a key functional feature that enables males to perform faster movements (but for shorter periods), associated with the demands of territoriality.     

The physiological basis of slow locomotion in chamaeleons

The African chamaeleon, Chamaeleo senegalensis, will not move faster than approximately 0.1 m/second at 23 degrees C, whereas the lizard Agama agama, like most lizards its size, runs at speeds more than 10X as fast. To account for this difference, we measured various physiological parameters of the iliofibularis muscle of both lizards. The maximum speed of tetanic contraction of unloaded Chamaeleo muscle was half as fast as that of Agama muscle (2.5 vs. 5.8 resting lengths per second). Heavily loaded Chamaeleo iliofibularis contracted at nearly 1/4 the speed of Agama muscle. Time to peak isometric twitch tension and time to half relaxation were twice as long in Chamaeleo as in Agama (122 vs. 58 msec, and 168 vs. 81 msec). Much more of the Chamaeleo muscle consisted of tonic muscle fibers, and the Chamaeleo muscle, compared to Agama muscle, showed physiological evidence of having a significant amount of tonic fibers (potassium contracture and high tetanus to twitch ratios). Finally, the myofibrillar ATPase activity of the Chamaeleo muscle was 1/3 that of Agama muscle. Thus, these results show that the slow locomotion of old world chamaeleons can, in part, be explained by the physiology, biochemistry, and fiber-type distribution of their muscles.     

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

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

Lactate: a substrate for reptilian muscle gluconeogenesis following exhaustive exercise

Summary The pattern of lactate and glycogen metabolism in red and white muscle fibers was examined in fasted, cannulated lizards (Dipsosaurus dorsalis) run on a treadmill to exhaustion. The white and red portions of the iliofibularis (wIF, rIF) muscle of the hindlimb were analyzed post-exercise and at intervals over 120 min of recovery for lactate and glycogen changes. Five min of exercise resulted in lactate concentrations of from 35 mM (rIF) to 48 mM (wIF) while blood lactate concentrations were elevated to 21 mM from resting levels of 1.8 mM. Glycogen depletion was significant (p<0.05) in whole hindlimb (–30%) and in wIF (–42%) but not in rIF (–25%). Metabolite changes were consistent with a pattern of fiber type recruitment favoring fast-twitch glycolytic (FG) fibers during high intensity locomotion. Glycogen replenishment during recovery was fiber typespecific. After 2 h recovery, whole hindlimb glycogen concentration had increased 24% above pre-exercise levels (p<0.05). Rates of glycogen resynthesis during recovery were significant only in oxidative fibers of the red iliofibularis. Animals were infused with either [U-¹⁴C]-lactate or [U-¹⁴C]-glucose at the point of exhaustion, and label incorporation into muscle glycogen was used to estimate the substrate preference for glycogenesis during recovery. Lactate uptake and incorporation occurred in both wIF and rIF. Glucose uptake and incorporation into glycogen was greatest in the rIF, where it equalled 9% of the rate of lactate incorporation. The rate of lactate incorporation could account for 67% of the rate of glycogen synthesis that occurred in oxidative fibers of the rIF. The data indicate that in contrast to mammalian muscle, reptilian muscle replenishes glycogen while it removes lactate, utilizing lactate directly as a gluconeogenic substrate. The data also suggest that lactate produced by FG fibers during exercise is utilized by oxidative fiber types post-exercise to synthesize glycogen in excess of pre-exercise levels.Abbreviations wIF, rIF white, red portions of iliofibularis muscle - FG fast-twitch, glycolytic muscle fiber - FOG fast-twitch, oxidative, glycolytic muscle fiber - HPLC high performance liquid chromatography - SA specific activity - [LA] lactate concentration - GLU glucose - ANOVA analysis of variance - C.I. confidence interval     

Twitch and tetanic tension during culture of mature Xenopus laevis single muscle fibres

Investigation of the mechanisms of muscle adaptation requires independent control of the regulating factors. The aim of the present study was to develop a serum-free medium to culture mature single muscle fibres of Xenopus laevis. As an example, we used the culture system to study adaptation of twitch and tetanic force characteristics, number of sarcomeres in series and fibre cross-section. Fibres dissected from m. iliofibularis (n = 10) were kept in culture at a fibre mean sarcomere length of 2.3 microm in a culture medium without serum. Twitch and tetanic tension were determined daily. Before and after culture the number of sarcomeres was determined by laser diffraction and fibre cross-sectional area (CSA) was determined by microscopy. For five fibres twitch tension increased during culture and tetanic tension was stable for periods varying from 8 to 14 days ('stable fibres'), after which fibres were removed from culture for analysis. Fibre CSA and the number of sarcomeres in series remained constant during culture. Five other fibres showed a substantial reduction in twitch and tetanic tension within the first five days of culture ('unstable fibres'). After 7-9 days of culture, three of these fibres died. For two of the unstable fibres, after the substantial force reduction, twitch and tetanic tension increased again. Finally at day 14 and 18 of culture, respectively, the tensions attained values higher than their original values. For stable fibres, twitch contraction time, twitch half-relaxation time and tetanus 10%-relaxation time increased during culture. For unstable fibres these parameters fluctuated. For all fibres the stimulus threshold fluctuated during the first two days, and then remained constant, even for the fibres that were cultured for at least two weeks. It is concluded that the present culture system for mature muscle fibres allows long-term studies within a well-defined medium. Unfortunately, initial tetanic and twitch force are poor predictors of the long-term behaviour of the fibres.     

The peripheral nerve allograft in the primate immunosuppressed with Cyclosporin A: II. Functional evaluation of reinnervated muscle.

Isometric contractile function was evaluated in primates receiving peripheral nerve allografts and autografts. Twelve adult male cynomolgus monkeys received both sural nerve allografts and autografts to the ulnar nerve in opposite forearms. Half the animals received Cyclosporin A (CsA) immunosuppression (25 mg/kg per day); the remaining animals received placebo. One year following nerve engraftment, isometric contractile muscle function was evaluated in reinnervated abductor digiti quinti and intact abductor pollicis brevis muscles. Maximal twitch tension (Pt), tetanic tension (P(o)), time to peak tension (tpt), rate of rise of twitch tension (DP/dt), and muscle fatigue were evaluated at optimal muscle length (L(o)). All reinnervated muscles distal to nerve autografts and allografts in both Cyclosporin A-immunosuppressed and placebo-treated animals generated equivalent maximal twitch tension, tetanic tension, and time to peak tension, with no significant difference between groups (p > 0.05 by ANOVA). There was a tendency toward increased muscle fatiguability in Cyclosporin A-treated animals (p > 0.05). However, the rate of rise of twitch tension was significantly faster in the reinnervated and intact muscles of Cyclosporin A-treated primates (p < 0.05). Evidence of excellent functional reinnervation across nerve allografts and autografts similar to that seen in histologic and electrophysiologic studies was noted. Cyclosporin A immunosuppression did not significantly enhance recovery of muscle function distal to nerve allografts in this model.     

Influence of electrical stimulation on a fast-twitch muscle in aging rats.

Recently we observed that the flexor digitorum longus muscle of the Fischer 344 rat, which is comprised primarily of type IIb muscle, does not change in size, fiber type, or physiological characteristics during senescence [Am. J. Physiol. 258 (Cell Physiol. 27): C1031-C1035, 1990]. This muscle was utilized to determine whether a predominantly fast-twitch glycolytic muscle would respond to tonic electrical stimulation (ES) with the same degree of fiber-type transformation in aging and young rats. The extent of transformation was quantified by measuring the contractile and metabolic properties, as well as the fiber-type composition, of the flexor digitorum longus muscle after ES (10 Hz, 8 h/day) imposed on the tibial nerve for periods of 0-90 days in young adult (YG; 6-8 mo), middle-aged (MA; 16-18 mo), and senescent (SN; 26-28 mo) male Fischer 344 rats. Although ES induced a IIb-to-IIa fiber-type shift in all groups, in the SN rats the shift was significantly less pronounced at the intermediate time points and remained incomplete after 90 days, compared with YG and MA rats. ES resulted in a reduction in tetanic tension (Po), which in the YG and MA rats was due to a reduction in muscle cross-sectional area. In the SN rats the reduced Po was due to a combined loss of cross-sectional area and specific tension (Po, N/cm2). Contraction and half-relaxation times were largely unaffected by ES, and maximal velocity of unloaded shortening declined throughout ES in all groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Temperature,contractility and high energy phosphates in anoxic fish heart muscle

Summary Isolated, electrically paced ventricular tissue of rainbow trout, Oncorhynchus mykiss, was examined at 20 and 10°C for the effects of different metabolic inhibitions on isometric force development and cellular content of phosphocreatine, creatine, ATP, ADP and AMP. At 20 relative to 10°C, twitch force was the same, but both twitch development and relaxation occurred over a shorter time and at a considerably higher maximal rate. Inhibition of cellular respiration caused twitch force and phosphocreatine to decrease, both about twice as fast at 20 as at 10°C. This doubling of energy degradation, i.e. in decrease of phosphocreatine, ATP, and loss of twitch force also occurred in preparations in which the energy liberation was totally blocked by iodoacetate in combination with N₂ and cyanide; both anaerobic energy degradation and anaerobic energy liberation expressed as lactate production were doubled. The similar effect of temperature on degradation and liberation of energy might explain why loss of twitch force during a 1-h period of anoxia was the same at both temperatures. The latter result was also found in the myocardium of eel Anguilla anguilla. In spite of its large influence on the time-course of twitch force development, the difference in temperature had no evident effects on the relationship between twitch force and phosphocreatine.Abbreviation Cr_t total creatine (creatine and phosphocreatine) - EDTA ethylenediminetetra-acetate - IAA iodoacetate - PCr phosphocreatine - TPT time-to-peak force - TR ₇₅ time for relaxation - V _F maximal rate of force development - V _R maximal rate of relaxation     

Reptilian skeletal muscle: contractile properties of identified, single fast-twitch and slow fibers from the lizard Dipsosaurus dorsalis

Contractile properties and innervation patterns were determined in identified single fibers from the iliofibularis muscle of the desert iguana, Dipsosaurus dorsalis. Single fibers from both the red and white regions of the iliofibularis muscle were dissected along their length under oil and a portion was mounted on transducers for determination of maximum isometric tension (Po) and unloaded shortening velocity (Vmax) using the slack test method. Fibers were chemically skinned and activated by high Ca++. The remaining portion of the muscle fiber was mounted on a glass slide and histochemically treated to demonstrate myosin ATPase activity. Fibers studied functionally could therefore be classified as fast or slow according to their myosin ATPase activity, and they could also be classified metabolically according to the region of the muscle from which they were dissected. Fast-twitch glycolytic (FG) fibers from the white region and fast-twitch oxidative, glycolytic (FOG) and slow fibers from the red region had shortening velocities at 25 degrees C of 7.5, 4.4, and 1.5 l X s-1, respectively. Po did not differ in the three fiber types, averaging 279 kN X m-2. In a second experiment, 10 microns sections were examined every 30 microns through the proximal-most 7.5 mm of the iliofibularis muscle for motor endplates. Sections were stained to demonstrate regions of acetylcholinesterase activity. Fibers with visible endplates were classified in serial sections by histochemical treatment for myosin ATPase and succinic dehydrogenase. All slow fibers examined (n = 22) exhibited multiple endplates, averaging one every 725 microns.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fatigue and contraction of slow and fast muscles in hypokinetic/hypodynamic rats

This investigation examined the effects of hypokinesia/hypodynamia (H/H) on fatigability and contractile properties of rat soleus (S) and gastrocnemius (G) muscles. Whole-body suspension for 1 wk was used to eliminate hindlimb load-bearing functions and simultaneously permit voluntary isotonic contractions. Train stimulations (45/min, 16 min) resulted in significantly (P less than 0.05) faster rates of fatigue to lower asymptotes in G from H/H rats. Fatigue in the S was minimal at this stimulation frequency and differences between H/H and control animals were not significant. Contractile properties (twitch and tetanic) were measured before and after train stimulations. H/H suspension resulted in an increased twitch tension in G. However, H/H did not change train or tetanic tensions per gram or other G contractile properties. Peak twitch, train, and tetanic tensions, time to peak tension, one-half relaxation time, and twitch and tetanic peak rates of tension development and decline were unchanged by H/H in S muscles. These results indicate that 1 wk of H/H-induced muscle atrophy significantly increases fatigability in G but does not effect contractile properties of fast-twitch (G) or slow-twitch (S) muscles.     

A comparison of quantitative ultrastructural and contractile characteristics of muscle fibre types of the perch,Perca fluviatilis L.

Summary Quantitative ultrastructural and physiological parameters were investigated in three types of muscle fibres ofPerca fluviatilis: white fibres from the m. levator operculi anterior, pink (intermediate) fibres of the m. hyohyoideus and deep red fibres of the m. levator operculi anterior. Times to peak tension and half relaxation times of isometric twitches increased in the mentioned order. The extent of contact between the T system and the sarcoplasmic reticulum and the relative volume and surface area of the terminal cisternae showed an inverse relation with the time to peak tension of the twitch. The maximal isometric tetanic force per unit cross section area was similar for all three investigated types. The inverse relation between the time to peak tension of the twitch and the relative length of contact between T system and SR is in agreement with data obtained for fast- and slow twitch muscle fibres of the carp,Cyprinus carpio L.Abbreviations LOPA musculus levator operculi anterior - HH musculus hyohyoideus - SR Sarcoplasmic reticulum     

Supercooling and freeze-tolerance in the European wall lizard,Podarcis muralis,with a revisional history of the discovery of freeze-tolerance in vertebrates

Summary Wall lizards were collected in the fall of 1988 from a population introduced in 1951 into Cincinnati, OH. They were acclimated to 5 °C for several weeks prior to testing at sub-zero temperatures. Eleven super-cooled lizards were removed from the cooling chamber prior to crystallization after between 15 min and 26 h at body temperatures ranging from -2.2 to -5.9 °C. With the exception of one individual supercooled to-5.0 °C, all lizards recovered fully. The crystallization temperatures of 15 lizards which froze ranged from -0.6 to -6.4 °C. Frozen lizards were stiff with a distinct blue color, which faded upon thawing at 3 °C. The ice contents of frozen lizards were determined calorimetrically and/or estimated from a theoretical model, the two methods being generally in close agreement. Remarkably, five individuals recovered fully from exposures as long as 2 h and with as much as 28% of their body water frozen. Although these animals are not as tolerant as certain other vertebrates they are clearly able to withstand freezing under some circumstances. Failure to survive freezing was attributed either to excessive ice accumulation during a prolonged freeze or to excessive supercooling prior to freezing, which induced a large initial surge of ice formation upon crystallization. Our results accord with those of Weigmann (1929). We accordingly recognize him as the first to demonstrate freeze-tolerance in vertebrates, and we further recognize P. muralis as the first vertebrate known to survive freezing.     

Electrical and mechanical changes in immobilized human muscle

After forearm fracture, the human thumb was unilaterally immobilized in eight subjects for 6 wk in a standard plaster cast. Changes of contraction properties were studied in the adductor pollicis muscle. The contralateral muscle remained unrestrained and served as control. After immobilization, the maximal voluntary contraction was reduced by 55% (P less than 0.05), and the electrically evoked maximal tetanic contraction (Po) was reduced by 33% (P less than 0.05). The decrease of Po was associated with increased maximal rate of tension development (10%) and decreased maximal rate of tension relaxation (22%). The twitch times to peak and to half relaxation were increased by 16 and 14%, respectively, but the twitch tension (Pt) was not significantly changed and the Pt/Po ratio was increased by 43% after immobilization. The muscle surface action potential presented an increase of its duration (19%) and a decrease of the amplitude and the total area (15 and 26%, respectively). The comparison of the electrical and mechanical alterations recorded during voluntary contractions, and in contractions evoked by electrical stimulation of the motor nerve, suggests that immobilization not only modifies the peripheral processes associated with contraction but also changes central and/or neural command of the contraction. At peripheral sites, it is proposed that the intracellular processes of contraction play the major role in the contractile impairment recorded during immobilization.     

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

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 (Pt) of the maximal twitch increased significantly (p less than 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 greater than 0.1) during immobilization, but the 50 Hz tetanic tension (Po50) and maximal voluntary contraction (MVC) were reduced (p less than 0.05). The fall in Po50 and MVC was associated with 10% decrease in the estimated muscle (plus bone) cross-sectional area. The relative (%) change in Po50 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 less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)     

甲状腺功能亢进大鼠比目鱼肌肌浆网Ca2+-ATP酶活性增高可加速强直收缩疲劳

甲状腺功能亢进(甲亢)时甲状腺素分泌增加,不仅使具有神经支配的慢缩型肌纤维向快缩型转化,而且改变骨骼肌的强直收缩功能.因此,甲亢性肌病的肌肉乏力可能与骨骼肌强直收缩易发生疲劳有关.本实验在离体条件下,观测甲亢4周引起的大鼠慢缩肌--比目鱼肌(soleus, SOL)单收缩与间断强直收缩功能的变化.结果显示,甲亢4周大鼠体重明显低于同步对照组[(292±13)g vs (354±10)g],但SOL湿重没有明显改变[(107.3±8.6)mg vs (115.1±6.9)mg].甲亢大鼠SOL单收缩张力达到峰值的时间(time to peak tension, TPT)、从峰值降至75%舒张时间(time from peak tension to 75% relaxation, TR75)均明显缩短;强直收缩的TR75也明显缩短[(102.8±4.1)ms vs (178.8±15.8)ms];强直收缩的最适频率从对照组的100Hz增加到140Hz;间断强直收缩期间容易发生疲劳.甲亢大鼠SOL肌浆网Ca2 -ATP酶(sarcoplasmic-reticulum Ca2 -ATPase, SERCA)活性增高.采用SERCA特异性抑制剂CPA (1.0μmol/L)处理后,对照组与甲亢大鼠SOL间断强直收缩的TR75均延长,同时不易出现疲劳.5.0μmol/L CPA灌流虽可进一步抵抗甲亢大鼠SOL间断强直收缩引起的疲劳,但强直收缩期间的静息张力却明显升高.将CPA浓度增至10.0μmol/L,甲亢大鼠SOL间断强直收缩又趋向易发生疲劳.这些结果提示,与心肌相同,骨骼肌肌纤维SERCA活性亦可影响单收缩与强直收缩的舒张时间,SERCA活性升高可加速间断强直收缩发生疲劳.     

Effect of 120 days of bed-rest with and without countermeasures on the mechanical properties of the triceps surae muscle in young women

The effect of a 120-day 6° head-down tilt (HDT) bed rest with and without countermeasures on the mechanical properties of the human triceps surae muscle was studied in eight healthy young women subjects. One group [n = 4, mean age 31.5 (SEM 1.7) years] underwent a 120-day HDT only and a second group [n = 4; mean age 28.0 (SEM 1.1) years] underwent HDT with countermeasures (physical training). The results showed that the contractile properties of the skeletal muscle studied changed considerably. After HDT without countermeasures the maximal voluntary contraction (MVC) had decreased by 36% (P < 0.05), and the electrically evoked tetanic tension at 150 Hz (P _o) and isometric twitch contraction (P _t) had decreased by 24% (P < 0.02) and 12% (P < 0.05), respectively. Time- to-peak tension (TPT) of the twitch had significantly increased by 14% (P<0.05), but half-relaxation time (1/2RT), and total contraction time (TCT) had decreased by 19% (P<0.05) and 18% (P<0.05), respectively. The difference between P _o and MVC expressed as a percentage of P _o and referred to as force deficiency (FD), was also calculated. The FD had increased by 40% (P<0.001). The rate of increase of voluntary contractions calculated according to a relative scale had significantly reduced, but for the electrically evoked contraction no substantial changes were observed. After HDT with countermeasures TPT, 1/2RT and TCT of the twitch had decreased by 4%, 7%, 19%, respectively in relation to the control condition. Training had caused a decrease of 3% (P>0.05) in MVC, and P _t, and in P _o of 14%, and of 9% (P>0.05), respectively. The FD had decreased significantly by 10% (P<0.02). The rate of increase of electrically evoked tetanic tension did not change significantly during HDT with countermeasures but the rate of increase in isometric voluntary tension development was increased. Physical training provided a reserve of neuromuscular function, which attenuated the effect of bed rest. The experimental findings indicated that neural as well as muscle adaptation occurred in response to HDT with countermeasures. Accepted: 7 November 1997     

Different effects of verapamil and low calcium on repetitive contractile activity of frog fatigue-resistant and easily-fatigued muscle fibres.

The effects of low calcium and verapamil on contractility of two muscle fibre types (m. iliofibularis, Rana temporaria) upon different stimulation protocols were been compared. Verapamil (0.02 mmol/l) induced temporal excitation-contraction coupling failure during single tetanic stimulation and enhanced the decline of tetanic force during 30 s repetitive tetanic stimulation in both fatigue-resistant fibres and easily-fatigued fibres. In contrast to verapamil, low extracellular calcium (0.02 mmol/l) only enhanced the decline of tetanic force in fatigue-resistant during repetitive tetanic stimulation but had no effect on easily-fatigued fibres. The effect of verapamil on the decline of tetanic force in fatigue-resistant fibres was more profound in low calcium conditions. Both verapamil and low calcium eliminated twitch facilitation that appeared after prolonged contractile activity in fatigue-resistant fibres. 4mmol/l Ni+2, used as calcium channel antagonist, had effects similar to low calcium medium. It could be concluded that (i) extracellular Ca2+-requirements for excitation-contraction coupling are different in fatigue-resistant and easily-fatigued fibres; (ii) the effects of verapamil on force performance are not entirely dependent upon calcium channel blockade.     

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

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