Potentiation of the isometric twitch and mechanism of tension recruitment in mammalian skeletal muscle.

Changes produced by bromide, nitrate, and iodide in the isometric myogram of the fast-twitch extensor digitorum longus and the slow-twitch soleus muscles of the rat were studied in vitro at 35°C. Replacement of chloride with bromide, nitrate, or iodide produced only small changes in the isometric tetanic tension, but it produced a reversible potentiation of the isometric twitch tension. The order of the potentiating influence was iodide > nitrate > bromide. The tension potentiation was associated with increases in rate of tension rise and the time to peak of the isometric twitch. The results from the two muscles were essentially similar and they were qualitatively similar to those reported from frog twitch muscle. The form of tension enhancement is qualitatively similar to that obtained during summation of mechanical responses, and the results predict with sufficient accuracy the characteristics of the fully summated two-stimuli mechanical response. We suggest that changes in excitation-contraction coupling, similar to those produced by these anions, occur in mammalian skeletal muscle fiber at 35 to 38°C during summation of isometric mechanical responses.     

Motor unit function in skeletal muscle autografts of rats

Standard and nerve-intact grafts of the extensor digitorum longus (EDL) muscles of rats do not fully recover control values for maximum tetanic tension (P_o). We compared the physiologic properties of motor units from standard and nerve-intact EDL grafts of rats with those of control EDL muscles. Standard grafts were completely removed and replaced in their original site. For nerve-intact grafts all physical connections were severed except for the nerve. Isometric contractile properties of whole muscles and single motor units were measured in situ 75 and 50 days after surgery for standard and nerve-intact grafts, respectively. Motor units from both types of grafts showed a mean and distribution for time-to-peak twitch tension (TPT) not different from control motor units. The absolute P_o of nerve-intact grafts showed a greater recovery than the standard grafts, but was still significantly lower than the control value. The average decrease in the P_o of motor units from nerve-intact grafts of 16% was not different from the decrease in the P_o of the total EDL graft which suggests no loss of motor units in nerve-intact grafts. In standard grafts, the 57% decrease in P_o for the whole muscle was attributable to a 20% decrease in the average tension development per motor unit and a 45% decrease in the number of motor units. These differences suggest a more complete reinnervation of the nerve-intact grafts than standard grafts.     

Influence of temperature on isometric tension development in mouse fast- and slow-twitch skeletal muscles

The influence of temperature (range 35 to 20°C) on the isometric contractile properties of fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles of the male C57 mice was studied in vitro, with direct stimulation. Cooling from 35 to 25°C resulted in an average twitch tension potentiation of 42% in EDL and 13% in SOL muscles. Further cooling to 20°C did not produce further potentiation in EDL muscles. The time-to-peak tension and the time to half-relaxation increased 2.4 to 3.2 times for a 10°C cooling in both muscles. The maximum tetanic tension was little changed in cooling from 35 to 30°C, but was depressed 16 to 19% in cooling to 20°C in both muscles. These results from mouse fast and slow muscles were compared with previously published data from muscles of the rat and the cat. The behaviors of the slow SOL muscles in the three species were found to be significantly different, but they could be explained on the basis of their muscle fiber-type composition.     

Survival, reinnervation, and histochemical maturation of skeletal muscle xenografts in the nude mouse.

Sensory neurons of rats survive and induce the formation of taste buds in vallate papillae when grafts of rat ganglion and mouse tongue are combined in the anterior chamber of the eyes of the immunologically deficient nude mouse. The present study was undertaken to determine if cross-species neuromuscular reinnervations also occur in grafts to nude mice. The intact extensor digitorum longus muscle was taken from 20-day-old normal rats and transplanted into the site formerly occupied by the excised extensor digitorum longus muscle of nude mice. Rat muscles were placed into an innervated or denervated graft site of nude mice, into an innervated graft site of normal mice, or into an innervated graft site of nude mice immunologically reconstituted by a thymus gland graft. Autotransplantation of the extensor digitorum longus muscle to an innervated graft site was also performed in some nude mice. Many rat muscle fibers survived 60 days in nude mice, became reinnervated, and developed into histochemically defined muscle fiber types. Some rat muscle fibers survived in nude mice in a denervated graft site but these fibers were atrophic and did not form fiber types. All rat muscle fibers were rejected by normal mice or by immunologically reconstituted nude mice. Muscle fibers in autografts of nude mouse muscle also survived, became reinnervated, and formed fiber types. The present demonstration of cross-species neuromuscular reinnervation means that different animals have similar cell recognition mechanisms involved in the formation of a neuromuscular junction and in the neurotrophic induction and maintenance of muscle fiber types. Because it is quite likely that human muscle would survive in the nude mouse, this animal might be useful for elucidating the origin or reversibility of diseased human muscle.     

Isometric contractile properties of fast- and slow-twitch muscles in normal and spastic mice

Isometric contractile properties of the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles of 24 male C57 mice were studied in vitro at 35°C. Ten animals exhibited the hereditary movement disorder known as spasticity. EDL muscles of spastic and normal mice developed similar specific tensions and they had similar twitch tension:tetanic tension ratios. The time-to-peak tension and the time to half relaxation in the twitch of spastic EDL muscles were, on average, slightly longer. SOL muscles of spastic and normal mice also developed similar specific tensions and their twitch contractions had similar times to half-relaxation. The time-to-peak tension in the twitch and the twitch tension: tetanic tension ratio of spastic SOL muscles were, on average, higher than in normal SOL. The significance of our findings remains somewhat uncertain.     

Physiological and histochemical changes of the extensor digitorum longus and soleus muscles after lateral cordotomy in the albino rat

The physiological and histochemical effects of unilateral and bilateral cordotomy on the extensor digitorum longus and soleus muscles were studied in the albino rat. After unilateral cordotomy, the ipsilateral extensor digitorum longus and soleus muscles became slower and relatively faster, respectively, compared to the contralateral and normal muscles. No gross histological abnormalities were found, but changes in fiber diameter and typology were statistically significant in both muscle types compared to their contralateral muscles, and were in substantial agreement with the changes observed on the contractile force and kinetics of the isometric twitch. Bilateral cordotomy in adult animals, like unilateral cordotomy in immature animals, affected only the soleus muscle fiber types. The results have implications for the role of the upper motoneuron physiological and histochemical trophic influences in controlling slow and fast motor units.     

Effect of pentobarbital on contractility of mouse skeletal muscle

Pentobarbital is a hypnotic drug commonly used as anesthesia for in vivo studies in various animals. A direct effect of pentobarbital on the central nervous system and skeletal neuromuscular junction has been known for at least 30 years. A recent study using single fiber preparations from amphibian muscles indicated a significant acute and direct effect on muscle contractility at drug concentrations within the anesthetic range. The present study using whole muscles from mice demonstrated a similar augmentation of twitch tension and rate of tension development whereas tetanic tension was reduced by this drug at similar concentrations. In addition, most time parameters of contraction were prolonged. It is of interest that the slow (oxidative) muscles were considerably more sensitive to pentobarbital than the fast (primarily anaerobic) muscles. We suggest that pentobarbital should not be used as the anesthetic agent for in vivo studies of other interventions when conclusions are based on changes in muscle contractility.     

Influence of castration on the electrical excitability and contraction properties of the rat levator ani muscle

Castration of male rats at 2 (C₂), 20 (C₂₀), or 75 (C₇₅) days of age resulted in marked atrophy of the levator ani (LA) muscle by 105 days of age. Although the single twitch was prolonged, the twitch and tetanus tension were decreased in LA muscles from C₂ and C₂₀ rats but not from C₇₅ rats. However, the maximum force of contraction expressed per gram of muscle tissue was significantly increased after castration in all groups. Testosterone administration to rats castrated at adult age (C₇₅) reversed the changes induced by castration. The alterations induced by castration on properties of the extensor digitorum longus (EDL) muscle were quantitatively less evident than those detected for the LA muscle in the same group. However, the younger the animal was at castration, the more affected were the contraction properties of the EDL. The directly elicited action potentials of the muscle fiber in both muscles were not significantly affected by castration. These findings suggest that the effect of castration on the LA and to a lesser extent on the EDL are primarily related to the trophic influence of testosterone on the development and maintenance of the contractile system.     

Slowing of fast-twitch muscle in the dystrophic mouse

Isometric twitch tension was measured in fast-twitch and slow-twitch muscles of normal and dystrophic ( ) mice in vivo. In dystrophic mice more than 6 months old the fast-twitch extensor digitorum longus (EDL) showed a prolongation of the time to peak tension as well as the time to relax to one-half peak tension ( ) compared with age-matched controls. In younger dystrophic mice (4 to 6 weeks) the time to peak tension was prolonged but not significantly so. This apparent “slowing” of dystrophic fast-twitch muscle was accompanied by a reduction in both cooling potentiation and post-tetanic potentiation toward values typical of slow-twitch muscle. Slow-twitch soleus muscle (SOL) of old mice was almost unaffected by the dystrophic process with regared to its contractile characteristics. However, there appeared to be a slight, but significant “speeding” of young dystrophic SOL compared with age-matched control muscles. This was apparent in reduced times to peak tension and half-relaxation as well as an enhanced cooling potentiation. We suggest that the altered contractile characteristics result from a change in some intrinsic property of the muscle fibers rather than from extrinsic factors such as the additional perimysial connective tissue seen in these muscles.     

Chronic phenytoin administration alters the metabolic profile of superficial gastrocnemius muscle fibers in dystrophic mice

Phenytoin is known to reduce neural overactivity (pseudomyotonia) affecting the hind limb musculature in $\text{C57B1}\text{6J}$ dystrophic ($\text{dy}{}^{\mathrm{2J}}\text{dy}{}^{\mathrm{2J}}$) mice. This study reports a change in the metabolic profile of superficial gastrocnemius muscle fibers from $\text{dy}{}^{\mathrm{2J}}\text{dy}{}^{\mathrm{2J}}$ animals after chronic phenytoin treatment. The superficial gastrocnemius muscle region from normal mice is composed of 98% fast-twitch glycolytic muscle fibers. In dystrophic mice these fibers (FG) show increased oxidative capacity without evidence of morphologic degeneration during the first few months ex utero. Many of these fibers also store abnormally large amounts of glycogen as determined by periodic acid-Schiff histochemistry. After 104 days of phenytoin treatment, the $\text{dy}{}^{\mathrm{2J}}\text{dy}{}^{\mathrm{2J}}$ FG muscle fibers showed a reduction in abnormally high oxidative capacity as monitored by succinic dehydrogenase activity; there was also a reduction of glycogen storage in a number of $\text{dy}{}^{\mathrm{2J}}\text{dy}{}^{\mathrm{2J}}$ fibers. One hypothesis suggests that the increase in oxidative capacity of the $\text{dy}{}^{\mathrm{2J}}\text{dy}{}^{\mathrm{2J}}$ superficial gastrocnemius muscle fibers is the expected result of overstimulation by the pseudomyotonia. Our experiments indicated that the abnormal metabolic profile observed in those fibers can be altered simply by a reduction in pseudomyotonia. These results mimic those seen after short-term denervation of the same $\text{dy}{}^{\mathrm{2J}}\text{dy}{}^{\mathrm{2J}}$ muscle. After phenytoin treatment the mean $\text{dy}{}^{\mathrm{2J}}\text{dy}{}^{\mathrm{2J}}$ superficial gastrocnemius muscle fiber cross-sectional area was significantly increased compared with untreated animals. Cursory examination of the degenerated deep region of this same muscle suggested that similar changes did not occur after drug treatment. This suggests that the pseudomyotonia was partially different from the factor(s) causing early degeneration of the oxidative muscle fibers in the $\text{dy}{}^{\mathrm{2J}}\text{dy}{}^{\mathrm{2J}}$ animals.     

Post-tetanic depression of twitch tension in the cat soleus muscle

Changes produced by a preceding tetanus in the isometric twitch myogram were examined in cat soleus (slow twitch) muscle in situ, at 37°C, and with supramaximal nerve stimulation. A tetanus produced a depression in the twitch tension (P) accompanied by a shortening of time to peak (TP) and time to half-relaxation (HR), and a slight depression in the maximum rate of tension rise (MR). Recovery of P, TP, and HR, after a standard tetanus of 100 Hz for 300 ms, showed an early-fast-phase (time constant of 0.8 to 0.9 s) and a late-slow-phase (time constant of 70 to 80 s), whereas that of MR showed only the late phase of recovery. Analysis of the data indicated the occurrence of two post-tetanic influences which produce different maxima of tension depression at zero test interval and have different rates of recovery. The twitch tension depression, when examined at the same test interval (0.7 s), increased with increase in the number of stimuli in the preceding tetanus. The relationship between the post-tetanic twitch tension and the number of stimuli in the tetanus was approximately logarithmic and was similar for tetanic frequencies of 50, 100, and 200 Hz. Unlike in the fast-twitch muscles, there was no clear evidence of a post-tetanic influence which may potentiate the twitch tension: the most prominent post-tetanic effect, in this muscle, was an acceleration of the tension relaxation in the twitch.     

A comparative study of contractile responses in diaphragm muscles of normal and dystrophic (C57BL6Jdy2Jdy2J) mice

Potassium and caffeine contractures of isolated small bundles (100 to 200 μm diameter) of muscle fibers isolated from the diaphragm of normal and dystrophic ($\text{C57BL}\text{6J}\text{dy}{}^{\mathrm{2J}}\text{dy}{}^{\mathrm{2J}}$) mice were compared. In diaphragms of pathologic mice (3 to 5 months old) the resting potential, the characteristics of the twitch, and some histological examinations were typical of dystrophic muscles. The slopes of the relationships between the steady membrane potential and log [K]₀ were similar for the two types of cells. In 110 mM and 146 mM K there were no significant differences in the time course of the contractures and reduction in [Ca]₀ decreased the time to peak and the time constant of relaxation to the same extent; the relative efficiency of [Mg]₀ compared with [Ca]₀ was equivalent. Repriming of K contractures at different external calcium concentrations indicated that the normal diaphragm did not have any special advantage. The exposure of isolated strips to a solution containing caffeine resulted in a similar increase of the strength of the regularly evoked twitch responses. However, the contractures elicited by 1.25 to 20 mM caffeine showed a subsensitivity of the dystrophic diaphragm (K_mDys = 9.3 K_mN) and the rate of relaxation was significantly slower than in normal muscle (in 20 mM caffeine, 50% decay time for normal muscle was 25.2 ± 7.6 s and for dystrophic muscle 54.8 ± 11.2 s. THese results suggest an absence of major alterations in the mechanism of excitation-contraction coupling associated with dystrophy, except for a change in the specific element of the sarcoplasmic reticulum where caffeine acts.