Fibre-related nitric oxide synthase (NOS) in Duchenne muscular dystrophy.

Nitric oxide (NO) mediates fundamental physiological actions on skeletal muscle. The loss of NO synthase (NOS) from the sarcolemma was assumed to be associated with development of Duchenne muscular dystrophy (DMD). We have, however, recently reported that, in contrast to the commonly accepted view, NOS expression in DMD myofibres is up-regulated. This poses the question of the fibre type-specific NOS expression in DMD muscles and how the NOS expression is related to the regeneration or degeneration status. To address this issue, we examined localization of NOS isoforms I, II and III in skeletal muscles of DMD patients employing immunohistochemical labelling with tyramide signal amplification complemented with enzyme histochemistry. We found that NOS immunolabelling as well as metabolic enzyme activity in DMD muscles were heterogeneously distributed along the fibre length of DMD muscle fibres revealing regenerating and degenerate (hypercontracted) fibres as well as normal segments. Like in normal muscles, positive NOS immunoreactivity was found to be associated with fast-oxidative glycolytic (FOG) phenotype. The regeneration status of NOS-positive segments was deduced from the presence of neonatal and developmental myosin heavy chains. High NOS expression in regenerating DMD muscle fibres can be well reconciled with reports about the protective role of endogenous NO in inflammatory diseases and in muscle repair.

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.

The effect of a unilateral muscle transplantation on the muscle fiber type and the MyHC isoform content in unoperated hind limb slow and fast muscles of the inbred Lewis rats.

To reveal the effect of foreign innervation and altered thyroid status on fiber type composition and the myosin heavy chain (MyHC) isoform expression in the rat slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles, a method of heterochronous isotransplantation was developed. In this experimental procedure, the SOL or EDL muscles of young inbred Lewis rats are grafted either into the host EDL or SOL muscles of adult rats of the same strain with normal or experimentally altered thyroid status. To estimate the extent of fiber type transitions in the transplanted muscles, the SOL and EDL muscle from the unoperated leg and unoperated muscles from the operated leg could be legitimately used as controls, but only when the experimental procedure itself does not affect these muscles. To verify this assumption, we have compared the fiber type composition and the MyHC isoform content of unoperated contralateral SOL and EDL muscles and ipsilateral unoperated SOL muscle of experimental rats after unilateral isotransplantation into the host EDL muscle with corresponding muscles of the naive rats of the same age and strain. We provide compelling evidence that the unilateral heterochronous isotransplantation has no significant effect on the fiber type composition and the MyHC isoform content of unoperated muscles of experimental animals. Hence, these muscles can be used as controls in our grafting experiments.

Effects of artificial obstruction on the function of the upper urinary tract of Guinea pigs, rats and pigs.

PURPOSE: Four different experiments in animals were performed to evaluate the influence of pyelo-ureteral surgery on the function of the upper urinary tract. METHODS: Experiment I: In 17 female guinea pigs pyelo-ureteral anastomosis was performed microsurgically. Three months later, the ureteral peristalsis was investigated by measuring the intraureteral pressure and the in vitro activity of the renal pelvic and ureteric wall was analysed. Experiment II: 10 rats were used for microsurgical uretero-ureteral anastomosis. One month after surgery the pyelo-ureteral peristalsis was examined by videomicroscopy while simultaneously measuring the renal pelvic and intravesical pressure. Subsequently the kidneys were removed for histological examination. Experiment III: In 2 pigs unilateral pyeloplasty was performed. Using an implanted transmitter the intravesical and the renal pelvic pressures were recorded continuously over a time interval of 3 months. Five months after surgery the pyelo-ureteral peristalsis was investigated by pyelography. The kidneys were then removed for histological and biomechanical examinations. Experiment IV: A partial artificial obstruction was performed in 16 guinea pigs by implanting the ureter into the psoas muscle. Two to six months following surgery their upper urinary tracts were removed for analysis of in vitro activity as well as histological and immunohistochemical investigations of the ureter and renal pelvis. RESULTS: Experiment I: Ultrasound investigation showed in all cases a significant dilation of the renal pelvis. The ureteral contraction frequency distally was decreased in vivo as well as in vitro (p <0.05) compared with the controls. Experiment II: Videomicroscopic imaging showed in eight out of nine cases an interruption of the peristaltic wave below the anastomosis; the ureteral peristalsis was restored distally by ureteral contractions with a decreased frequency. Retroperistalsis was seen in the lower part of the ureter. The frequency of renal pelvic and ureteral contractions were decreased (p <0.05). Renal pelvic baseline pressure as well as contraction amplitude were irregularly changed. Histological examinations showed increased connective tissue within the renal pelvic wall in all cases. Experiment III: In both pigs an intermittent change in contraction frequency of the renal pelvis was found, associated with a changing contraction amplitude. Five months after surgery an interruption of the peristaltic wave was detected in both pigs. Histological examinations showed increased connective tissue within the renal pelvic wall. The stiffness of caliceal and pelvic tissue was lower following the pyeloplasty compared to the controls. Experiment IV: Following artificial partial ureteral obstruction in all guinea pigs the in vitro investigations showed an increased spontaneous activity of the upper urinary tract except in the proximal part of the ureter. Ureteral obstruction produced a change in contraction pattern of the proximal ureter and a decrease in contraction frequency of the distal ureter. Immunohistochemical investigations revealed rarefication and disorientation of nerve fibres within the proximal ureteric wall. CONCLUSIONS: Surgical interruption of the ureteral continuity and re-anastomosis cause a temporary disruption of the peristaltic wave at the anastomosis site. Ureteral peristalsis is restored by ureteral contractions associated with retroperistalsis as well as a decreased contraction frequency. Uretero-ureteral anastomosis in rats, pyelo-ureteral anastomosis in guinea pigs and pyeloplasty in pigs seem to influence the upper urinary tract similarly to a chronical functional obstruction, causing changes in pyelo-ureteral motility and spontaneous muscular activity of the renal pelvic and ureteral wall as well as biomechanical and histological characteristics.

Actions of lyotropic anions on the mechanical properties of fast and slow twitch rat muscles at different temperatures.

The effects of lyotropic (swelling) anions (Cl(-), Br(-), NO(3)(-) and I(-)) on contractile properties of fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles were investigated in vitro at 20 degrees C and 35 degrees C. Isolated muscles bathed in anionic Tyrode solution were stimulated directly and isometric single twitches and fused tetanic contractions were recorded. In a Cl(-)Tyrode solution a decrease of the bathing temperature led to a cold potentiation of the twitch tension (P(t)) in EDL muscles, however, to a cold depression in SOL muscles, in both muscles combined with a prolongation of contraction (CT) and half relaxation (HRT) times. The extent and order of the potentiating effect of lyotropic anions on the P(t), CT and HRT in EDL and SOL were quite similar and increased in the order: Cl(-)< Br(-)< NO(3)(-)< I(-). Since the lyotropic anions did not influence tetanic tensions, the twitch-tetanus ratio (TTR) was increased in NO(3)(-) and I(-)solutions. All effects of the anions were rapidly and completely reversed in both muscles when the test solution was replaced by the normal one. The temperature decrease caused no significant alteration in the potentiation capacity of the anions or in the kinetics of their action and reversibility.

[Neuromuscular reinnervation of mimetic muscles. A histomorphologic study of the zygomatic muscle of the rabbit]. / Neuromuskuläre Reinnervation mimischer Muskulatur. Eine histomorphologische Untersuchung am M. zygomaticus des Kaninchens.

To evaluate occurrences of reinnervation and degeneration, the portio auricularis of the zygomatic muscle was reinnervated (either primary or secondary reinnervation) with an interval of 6 weeks, using a neuromuscular transplant of the sternohyoid muscle, which was left at the ansa hypoglossi. Histological examination was performed 6 and 12 weeks later. Enzymatic stains for myofibrillar ATPase were used for differentiation of muscle fibre types. Histomorphological features of denervation and reinnervation were analysed. Six weeks after denervation and primary neuromuscular transplantation, type grouping in regions close to the transplanted muscle were detectable as signs of reinnervation. After a period of 12 weeks, the progressing reinnervation was proved by type grouping in distal areas of the muscle far from the area of transplantation. At the same time, differentiation of fibre typing in the type grouping areas increased. All denervated and reinnervated muscles had pathological variations of fibre diameter. Primary reinnervated muscles showed tendency of normalization in variation of diameter. Compared with primary reinnervated muscles, secondary reinnervated muscles with an interval of 6 weeks showed pronounced polymorphology of fibres and a high pathological increase of the variation of fibre diameters. The interfascicular space was significantly spread. The tendency of reinnervation with incidence of fibre type grouping was comparable to primary reinnervated muscles and also occurred 6 weeks after transplantation.

Contraction parameters, myosin composition and metabolic enzymes of the skeletal muscles of the etruscan shrew Suncus etruscus and of the common European white-toothed shrew Crocidura russula (Insectivora: soricidae).

In the Etruscan shrew, the isometric twitch contraction times of extensor digitorum longus (EDL) and soleus muscles are shorter than in any other mammal, allowing these muscles to contract at outstandingly high contraction frequencies. This species has the highest mass-specific metabolic rate of all mammals and requires fast skeletal muscles not only for locomotion but also for effective heat production and for an extremely high ventilation rate. No differences could be detected in the fibre type pattern, the myosin heavy and light chain composition, or in the activity of the metabolic enzymes lactate dehydrogenase and citrate synthase of the two limb muscles, the EDL and the soleus, which in larger mammalian species exhibit distinct differences in contractile proteins and metabolic enzymes. All properties determined in EDL and soleus muscles of Suncus etruscus, as well as in the larger Crocidura russula, are typical for fast-oxidative fibres, and the same holds for several other skeletal muscles including the diaphragm muscle of S. etruscus. Nevertheless, the EDL and soleus muscles showed different mechanical properties in the two shrew species. Relaxation times and, in C. russula, time to peak force are shorter in the EDL than in the soleus muscle. This is in accordance with the time course of the Ca(2+) transients in these muscles. Such a result could be due to different parvalbumin concentrations, to a different volume fraction of the sarcoplasmic reticulum in the two muscles or to different Ca(2+)-ATPase activities. Alternatively, the lower content of cytosolic creatine kinase (CK) in the soleus compared with the EDL muscle could indicate that the observed difference in contraction times between these shrew muscles is due to the CK-controlled activity of their sarcoplasmic reticulum Ca(2+)-ATPase.

Effects on skeletal muscle fibres of diabetes and Ginkgo biloba extract treatment.

Combined cytophotometric and morphometric analysis of muscle fibre properties and myosin heavy chain electrophoresis were performed on extensor digitorum longus and soleus muscles from healthy rats and rats with streptozotocin-induced diabetes. Moreover, the protective effect of Ginkgo biloba extract, a potent oxygen radical scavenger, on diabetic muscles was investigated. Changes in fibre type-related enzyme activities, fibre type distribution, fibre cross areas and myosin isoforms were found. In muscles of diabetic rats, a metabolic shift was measured mainly in fibres with oxidative metabolism. Fast-oxidative glycolytic fibres showed a shift to more glycolytic metabolism and about a third transformed into fast-glycolytic fibres. Slow-oxidative fibres became more oxidative. Fibre atrophy was measured in diabetic muscles dependent on fibre type and muscle. Different fibre types atrophied to a different degree. Therefore, a decreased area percentage of slow fibres and an increased area percentage of fast fibres of the whole muscle cross section in both muscles were found. This is supported by reduced slow and increased fast myosin heavy chain isoforms. These alterations of diabetic muscle fibres could be due to less motion of diabetic rats and diabetic neuropathy. After treatment with Ginkgo biloba extract, enzyme activities were increased mainly in oxidative fibres of diabetic muscles, which was interpreted as protective effect. Generally, the soleus muscle with predominant oxidative metabolism was more vulnerable to diabetic alterations and Ginkgo biloba extract treatment than the extensor digitorum longus muscle with predominant glycolytic metabolism.

Cardiomyoplasty - improvement of muscle fibre type transformation by an anabolic steroid (metenolone).

Dynamic cardiomyoplasty, a method to support ventricular function by the chronically stimulated latissimus dorsi muscle wrapped around the heart is accompanied by a loss of mass and force of the transplanted muscle. These effects and the fast-to-slow transformation of the muscle could be possibly influenced by the additional administration of anabolic steroids. In this study, the left latissimus dorsi muscles of 12 sheep were electrically conditioned (group A). In 12 other animals (group B), stimulation was combined with the administration of metenolone (100 mg/week). Biopsies were taken from the right and left muscles at the beginning and after 6 and 12 weeks of treatment, frozen and cross-sectioned. The muscle fibre type composition was studied enzymhistochemically (SDH-staining and Myosin-ATPase-reaction) and immunocytochemically (using antibodies against different myosin heavy chains, MHC). Furthermore, the expression of different MHC isoforms was investigated electrophoretically. The untreated latissimus dorsi muscle contains 20% type I fibres expressing slow MHC and 80% type II fibres expressing fast MHC. After 6 weeks, the respective fibre type composition was 42 and 58% (group A) and 80 and 20% (group B). After 12 weeks, the percentage of the type I fibres rose in group A to 59% and in group B to 98%. In accordance with these morphological results, the MHC pattern determined electrophoretically showed a corresponding shift from the fast to the slow isoform. Therefore, the administration of metenolone avoids severe muscle atrophy, and improves and accelerates fast to slow fibre type conversion necessary for successful cardiomyoplasty.

Anabolic steroids (metenolone) improve muscle performance and hemodynamic characteristics in cardiomyoplasty.

The loss of force and mass in the conditioned latissimus dorsi muscle are principal reasons for the poor improvement in hemodynamic functioning attained by cardiomyoplasty. Using 24 sheep, we investigated the effect of anabolic steroids on the hemodynamic, histologic, and myophysiologic characteristics in the setting of cardiomyoplasty. In 12 of the animals (group A), the latissimus dorsi muscles were electrically conditioned with an Itrel pulse generator; in the remaining 12 animals (group B), the electrical conditioning was combined with the administration of an anabolic hormone (metenolone; 100 mg/week). The hemodynamic measurements were performed during isolated perfusion of the subclavian artery (maintenance of pressure in the muscles), while all other circulation variables were held at the exact and reproducible value of zero by inducing ventricular fibrillation. Maximum force and muscle mass showed a significant increase in group B (maximum force: group A, 4.23 +/- 0.55 kp, and group B, 6.0 +/- 3.14 kp; muscle mass: group A, +11.07% +/- 1.06%, and group B, +79.9% +/- 40.8%). The ratio of type I to type II fibers after 12 weeks was 65.2% to 34.8% in group A and 96.7% to 3.3% in group B, as opposed to 19.9% to 80.1% in the control group. No side effects of the anabolic steroids were observed during the experiment. In the hemodynamic studies, we were able to demonstrate a further significant increase in the left ventricular pressure, fractional fiber shortening value, ejection fraction, stroke volume, cardiac output, and stroke work when using conditioned latissimus dorsi muscles that were additionally treated with metenolone.(ABSTRACT TRUNCATED AT 250 WORDS)

The force-velocity relation of the rabbit inferior oblique muscle; influence of temperature.

The contractile properties of the rabbit inferior oblique muscle (IO) were studied in vitro with direct stimulation at temperatures between 20 and 35 degrees C. Isovelocity releases were used to determine the force/velocity relation. Cooling the muscle from 35 degrees C to 20 degrees C increased contraction and half-relaxation times of single twitches with a temperature coefficient (Q10) of 0.4, but did not affect significantly the twitch tension. The tetanic tension increases with increasing temperature (Q10 = 1.32). Cooling decreased the maximum shortening velocity of the IO with a Q10 of 1.6 and the maximum mechanical power with a Q10 of 2.3. At 35 degrees C, the maximum speed of shortening of the muscle (19 +/- 2 muscle lengths/s, mean +/- SEM) corresponded to a maximum shortening velocity of the sarcomeres of 57 +/- 6 microns/s. This value is similar to data obtained for extraocular muscles (EOM) of smaller rodents (mice and rats). In comparison with mammalian limb muscles the isometric and force-velocity properties of mammalian EOM appear to be virtually independent of the size of the animal. Thus, IO is a fast-twitch muscle endowed with a maximum velocity of shortening higher than that of fast-twitch skeletal muscle, but using a tetanic mechanical power lower than that produced by slow-twitch muscle: the combination of these properties makes it ideally suited to move an ocular globe of low mass at high velocity.

Effect of an anabolic steroid (Metenolon) on contractile performance of the chronically stimulated latissimus dorsi in sheep.

In 12 sheep the left latissimus dorsi muscles (LD) were conditioned by chronic electrostimulation with a pulse generator (Itrel, Medtronic). Six animals (group B) received a weekly intramuscular injection of an anabolic steroid (Metenolon). After 14 weeks the contraction parameters of the left LDs (group A and B) and right LDs (control group) were investigated. The increase in weight of the conditioned LDs was 11.07% (+/- 1.06%) in group A and 79.97% (+/- 40.8; P < 0.05) in group B. The force capacity under stimulation patterns which were just tetanic was 1.15 kp in group A and 4.13 kp in group B (P < 0.05); under supramaximal stimulation patterns it was 4.23 kp (A) and 6.0 kp (B) (P = ns). The force time relation (dF/dt) was 6.7 kp/s for the left LDs in group A versus 16.4 kp/s for the right LDs (P < 0.01); in group B it was 5.13 kp/s for the left LDs versus 15.8 kp/s for the control muscles (P < 0.05). The maximal force (Fmax) per 100 g muscle weight did not differ significantly (A: 2.42 kp/100 g; B: 2.52 kp/100 g). In conclusion, the LD muscles which were subjected to both anabolic therapy and electrical stimulation showed a significant increase in their force capacity due to an enormous increase in mass. Fibre type transformation was complete only in group B. No fibre deterioration was observable in either group. No anabolic side effects were detected in the animals. With the use of anabolic steroids, therefore, a clearer direct increase in contractility on the left ventricle should be expected ("squeezing" theory), as well as a contribution to reduction in wall tension and myocardial oxygen consumption, respectively, according to Laplace's Law (via the considerable increase in thickness).

Electrophoretic analysis of myosin heavy chain isoform patterns in extraocular muscles of the rat.

Six oculorotatory muscles and the levator palpebrae muscle of the rat were analysed by SDS-PAGE for their myosin heavy chain (MHC) isoform patterns. Oculorotatory muscles display a marked predominance of fast MHC isoforms. They contain, in addition to the slow (MHCI) and fast (MHCIIb, MHCIId, MHCIIa) skeletal MHCs, the neonatal MHCneo and the extraocular MHCeom. The levator palpebrae, generally assumed to be a member of the extraocular muscle group because of its innervation by the oculomotor nerve, does not contain MHCneo and MHCeom. It resembles a fast-twitch skeletal muscle with a predominance of MHCIId.

Site-dependent effects of experimental hypo- and hyperthyroidism on resident macrophages in extraocular muscles of rats: a quantitative immunohistochemical study.

It has been suggested that the effects of dysthyroidism on resident immunocompetent cells of the extraocular muscles may play a role in the pathogenesis of Graves' ophthalmopathy. The distribution of such cells was therefore studied in extraocular muscles of rats that were made hyper- or hypothyroid by the oral administration of thyroxine or propylthiouracil respectively. Skeletal muscles were studied for comparison. The cell distributions were analysed in cryostat cross-sections subjected to a two-step immunoperoxidase method using well-characterized monoclonal antibodies against T cells, B cells, macrophages and MHC class II antigens. The extraocular muscles of control (euthyroid) rats contained numerous macrophages, fewer MHC-II positive cells and T cells and no B cells. Differences in the distribution of immunocompetent cells were found in control rats, between skeletal and extraocular muscles as well as within the various recti eye muscles. This particular tissue distribution resembles that previously reported for human extraocular and skeletal muscles. Quantitative analysis showed that experimental dysthyroidism only affected cell populations in the extraocular muscles. Significant effects on the number of macrophages were observed in the inferior rectus muscle of both hypo- and hyperthyroid rats, this was most pronounced in the orbital layer of the muscles. Both hyper- and hypothyroidism appear to affect local cell distributions in a tissue-specific manner. The presently observed site-dependent effects of dysthyroidism on local immunocompetent cell populations may have relevance for the differential involvement of muscular tissues in Graves' ophthalmopathy.

Arrest of developmental conversion of type II to type I fibres after suspension hypokinesia.

The effects of hypokinesia and of the lack of gravity on muscle fibres, fibre type composition and myosin light chain pattern, as well as on muscle mechanoreceptors were investigated in the slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles of young growing and adult rats after suspension hypokinesia (SH) of their hind limbs. The animals were suspended by their tail so that their hind limbs were relieved of their normal weight-bearing function for 3-6 weeks. In normal 3- to 4-week-old rats the SOL contained about 50% type I fibres and their percentage increased up to about 80% until the 10th week, with simultaneous reduction of type IIA fibres. After 3 to 6 weeks of suspension treatment maintained from 3- to 4-week-old rats up to 6 to 10 weeks of age, the SOL still only contained about 50% of type I fibres. The content of fast LC1 and LC2 in the SOL of 6-week-old rats after 3 weeks of suspension was higher than that of control litter-mates reflecting the higher occurrence of IIA fibres in the suspended solei. No changes in fibre type composition were observed after SH performed in adult rats. SH thus leads, in young animals, to the arrest of conversion of type IIA to type I fibres resulting in the persistence of the fibre type composition and of the myosin light chain pattern corresponding to those present in the SOL at the time of the onset of suspension. In both young and adult rats, SH markedly decreased the mass and the mean cross-sectional area of the SOL, mainly due to the severe atrophy of type I fibres. We observed no signs indicating conversion of type I back to type IIA muscle fibres due to the SH either in young or adult animals. In contrast to profound changes in the SOL, no significant differences were found in the EDL in any of the parameters studied. No changes in the investigated parameters of muscle spindles and tendon organs were observed after SH, performed either in young or in adult rats. We thus conclude that SH leads to muscle atrophy and that it influences mainly or exclusively type I fibres in muscles with a postural function such as the SOL.(ABSTRACT TRUNCATED AT 400 WORDS)

Maximal shortening velocities, isomyosins and fibre types in soleus muscle of mice, rats and guinea-pigs.

1. Guinea-pig soleus contains only type I fibres and slow isomyosin, SM2. Rat and mouse soleus contain about 70% of type I fibres and a mixture of isomyosins: slow, SM2 and intermediate, IM. Many rat soleus muscles contain a third isomyosin of a slow type, SM1. 2. The maximal velocity of unloaded shortening, V0, is largest in mouse soleus (6.11 Lf s-1), slowest in guinea-pig soleus (1.67 Lf s-1) and intermediate in rat soleus (4.16 Lf s-1) (Lf = fibre length). 3. In guinea-pig soleus, V0 is equal to the maximal velocity (Vmax) computed using the Hill force-velocity relationship; V0 is approximately twice as large as Vmax in mouse and rat soleus. 4. V0 measures the unloaded shortening velocity of the fastest fibres whereas Vmax is a function of the force-velocity characteristics of all the fibres contained in the muscle. 5. V0 increases according to the isomyosin composition of the fibres in the sequence SM2 less than SM1 + IM less than IM.

The influence of suspension hypokinesia on contractile properties of slow and fast twitch muscles of young growing and adult rats.

The influence of the lack of the gravity force on contractile properties of the slow-twitch soleus (SOL) and the fast-twitch extensor digitorum longus (EDL) muscles was investigated in 3 week or 6-month-old rats suspended by their tail. In both groups suspension hypokinesia (SH) lasting 3 weeks or longer leads to a marked decrease of the mass and of the mean cross-sectional area of the SOL. The SOL of suspended animals shows decreased contraction and half-relaxation times and a lower tension of the single twitch, a decreased maximum tetanic force output, an increased fusion frequency and a decreased tetanus-half-rise time. The force developed per unit cross-sectional area and the twitch-tetanus ratio are unchanged. The atrophy and the changes in contractile properties are more pronounced in young developing rats than in adult ones. The SOL of suspended young rats does not develop into a slow-twitch muscle and shows a posttetanic potentiation and a cooling potentiation of the single twitch, properties observed in fast-twitch muscles only. In comparison to controls SH has no influence on the wet weight, mean cross-sectional area, and contractile properties of the EDL of young or adult rats. The data presented here and supported by histochemical and biochemical results demonstrates the importance of the gravity force for slow-twitch muscles especially during development.

Temperature effects on isometric contractions of slow and fast twitch muscles of various rodents--dependence on fibre type composition: a comparative study.

The influence of temperature (range 15-37 degrees C) on the isometric contractions of the slow twitch soleus (SOL) and the fast twitch extensor digitorum longus (EDL) muscles of mice, rats and guinea pigs were investigated in vitro. Cooling of the bathing solution prolonged the time parameters of single twitches and tetanic contractions in a non-linear manner in both muscle types of all animals. In muscles containing predominately fast twitch fibres like the EDL of all animals cooling was followed by an increase of the single twitch tension (cold potentiation) with a maximum of 160-180% at about 20 degrees C. The influence of a decrease of the temperature on the twitch tension was different in the SOL of the three animals. In SOL of mice (containing about 50% slow twitch fibres) the twitch tension was virtually unchanged, in SOL of rats (about 70% slow twitch fibres) a moderate cold depression and in SOL of guinea pigs (composed by slow twitch fibres only) a strong cold depression was observed. The maximum tetanic tension decreased progressively on cooling in all muscles and independently of their fibre type composition. Cooling increased the twitch-tetanus-ratio in fast twitch and decreased it in pure slow twitch muscles. It is concluded that the temperature dependence of the single twitch and the twitch-tetanus-ratio can be used as a physiological measure of the fibre type composition of a given muscle.