EDL Fibers Research Articles

Effects of membrane potential on just detectable movement in rat skeletal muscle: effects of denervation.

The potential, Vt, at which a brief test depolarization first elicited movement was determined using two-microelectrode point voltage clamp. We expected that inactivation of excitation-contraction coupling at conditioning potentials between -60 and 0 mV would shift Vt to more positive potentials, and that fibers would become inactivatable with less conditioning depolarization in EDL than soleus. The curve relating Vt to conditioning potential had a negative slope (which was insensitive to addition of 1 mM cobalt or replacement of calcium with 20 mM CaEGTA) between -60 and -35 mV and a steep positive slope with further depolarization Unexpectedly, fibers became inactivatable with less conditioning depolarization in soleus than in EDL when Vt was measured with 50 msec test pulses. However, the positive shift in Vt became less steep as test pulse duration lengthened in soleus fibers. When Vt obtained with test pulses approaching rheobase (10 msec in EDL and 500 msec in soleus) was compared, EDL fibers became inactive with less conditioning depolarization than soleus fibers. The increase in Vt became steeper with 1 mM cobalt or 20 mM CaEGTA and was shifted to more positive potentials by denervation in soleus fibers. We conclude that inactivation (i) does not strongly influence threshold contractions at conditioning potentials between -60 and -40 mV and (ii) influences Vt between -40 and 0 mV in a manner that depends on test pulse duration.

The effects of endurance exercise on dystrophic mdx mice. II. Contractile properties of skinned muscle fibres.

Dystrophic (mdx) mice were subjected to a 15 week exercise programme consisting of endurance swimming. Single fibres from the extensor digitorum longus (EDL, fast-twitch) and soleus (SOL, mixed fast- and slow-twitch) muscles were attached to a sensitive force-recording apparatus, and activated in Ca(2+)- and Sr(2+)-buffered solutions. In addition to the normal well-defined fibre types in these muscles, a small number of fibres were also sampled from the soleus of both experimental groups, which were 'Intermediate' to the other two SOL fibre types. Type IIB fibres from the EDL and type IIA fibres from the soleus of the Swim group were significantly less sensitive to Ca2+ and Sr2+ compared with those fibres sampled from the sedentary (Sedent) group, suggesting that endurance exercise was able to modify Ca(2+)- and Sr(2+)-activated contractile characteristics. The swim-trained (Swim) group's increased incidence of SOL fibres with characteristics intermediate to those of the fast- and slow-twitch fibre types suggests a possible exercise-induced fibre type transformation as an adaptation to the functional demand.

Charge movements and transverse tubular ultrastructure in organ cultured skeletal muscle

A study was made of charge movements and the transverse tubular systems in rat EDL and soleus muscle fibres maintained for up to five days in organ culture. In the cultured EDL muscle the maximum amount of charge moved was about one third of that in innervated muscle. Charge movements in innervated soleus fibres are small, less than 10 nC/microF, and difficult to resolve. They remain small following organ culturing. The ultrastructural study examined the concentration of junctional feet because of their proposed key role in excitation-contraction coupling. The general architecture of the triads and the spacing of the feet in both muscle types was largely unchanged by culturing. In cultured EDL muscles the small changes in feet concentration did not parallel the large fall in charge movement. The results reported here support a previous conclusion that, in mammalian muscle, there is not a simple relation between charge and feet. The stimulation of cultured soleus muscles with a fast twitch pattern of electrical activity produced no observable changes in morphology.

Effects of 2,3-butanedione monoxime on the contractile activation properties of fast- and slow-twitch rat muscle fibres.

1. The effects of 2,3-butanedione monoxime (BDM, 0.2-10 mmol/l) have been examined at different temperatures on calcium transients (measured with aequorin) and isometric force in intact bundles of fibres from soleus (slow-twitch) and extensor digitorum longus (EDL; fast-twitch) muscles of the rat and on Ca2+-activated isometric force of mechanically skinned soleus and EDL fibres. Ca2+ release and uptake by the sarcoplasmic reticulum has also been investigated directly on skinned EDL fibres at 1 mmol/l BDM at 23 degrees C. 2. BDM bound calcium with low affinity (KCaBDMapp = 55.9 +/- 4.3 M-1 at 22 degrees C, pH 7.10, ionic strength approximately 155 mmol/l) and did not affect the kinetics and quantum yield of the Ca2+-induced aequorin luminescence. 3. BDM reversibly reduced both force- and Ca2+-dependent aequorin light during the twitch and tetanus of all intact-muscle preparations in a dose-related manner. Twitch responses of soleus fibres were more sensitive to BDM than were EDL fibres. Higher concentrations of BDM were needed to abolish the tetanic force response than the twitch response in both muscle types. 4. The initial rate of rise and the decay rate constants of twitches in both soleus and EDL muscles were increased in the presence of BDM. Accordingly, a higher frequency of stimulation was required to produce fusion of the tetanus. This phenomenon could be explained by the reduced magnitude of the Ca2+ transient. 5. BDM reduced maximal Ca2+-activated force development in mechanically skinned soleus and EDL muscle fibres in a dose-dependent fashion. This depression of force was less sensitive to BDM than were the light transients in intact fibres. Furthermore, BDM affected EDL skinned fibres to a greater extent than soleus skinned fibres, in contrast with its effects on intact fibres. 6. At concentrations of BDM greater than 2.5 mmol/l, BDM significantly decreased the sensitivity of the contractile apparatus to calcium. The relative force-pCa (= -log [Ca2+]) curves for both soleus and EDL skinned fibres were shifted to the right (i.e. to higher calcium concentrations) in a dose-related manner. 7. BDM (10 mmol/l) slowed maximal Ca2+-activated force development of skinned preparations of both fibre types at all temperatures investigated (4-24 degrees C). 8. BDM reduced stiffness in skinned preparations maximally activated by calcium in proportion to the reduction in the isometric force response. BDM also inhibited rigor force of all fibres but only if it was present before the onset of rigor.(ABSTRACT TRUNCATED AT 400 WORDS)

Oxidative potential in developing rat diaphragm, EDL, and soleus muscle fibers.

To examine the effect of postnatal development on changes in oxidative potential of fibers of specific types (I, IIa, IIb, and IIc) in the rat diaphragm, determinations of succinate dehydrogenase (SDH) activity were made using microphotometric measures of optical density. Samples of the costal region of the diaphragm were extracted from 56 male Wistar rats ranging in age from 8 to 85 days and subgrouped into seven developmental periods (1, 2, 3, 4, 6, 9, and 12 wk). For type I fibers, increases of 17% (P less than 0.05) in SDH activity occurred during 2nd wk, remained elevated through 4th wk, and increased further (P less than 0.05) to 137% of 1-wk values by the end of 6th wk. No further increases were noted between 6 and 12 wk. A similar maturational trend was evident for type IIa fibers, although SDH activities remained higher throughout development when compared with type I fibers. In contrast, SDH in type IIb fibers, although increasing by 14% during the first two measurement weeks (P less than 0.05), declined from 6 to 9 wk before ultimately reaching a value similar to 3 wk. SDH activity was also assessed in a typical slow- (soleus) and fast-twitch (extensor digitorum longus, EDL) muscle of the hindlimb to contrast their development with that of the diaphragm. Generally, SDH in type I and IIa fibers was approximately 40 and 20% higher, respectively, in the diaphragm than in matched fiber types in the other muscles throughout development (diaphragm greater than EDL greater than soleus).(ABSTRACT TRUNCATED AT 250 WORDS)

Upper motor neurone modulation of the structure of the terminal cisternae in rat skeletal muscle fibres

There are fewer indentations on the flat surfaces of terminal cisternae in soleus (slow-twitch) than in extensor digitorum longus (EDL, fast-twitch) muscle fibres of rats. Following mid-thoracic spinal cord transection, there is an increase in the number of indentations in soleus fibres but no change in EDL fibres. The increase in the numbers of indentations after spinal cord transection is correlated with changes in the contractile and charge movement properties of the soleus fibres so that they resemble normal EDL fibres. The indentations appear to have an important role in excitation-contraction coupling.

Upper motor neurone modulation of charge movement and mechanical activation in rat skeletal muscle fibres

The characteristics of charge movement and contractile activation are different in soleus (slow-twitch) and extensor digitorum longus (EDL, fast-twitch) muscle fibres of rats. Following mid-thoracic spinal cord transection, both characteristics change in soleus fibres to resemble those of EDL fibres, showing that upper motor neurones modulate both contractile activation and charge movement.

Membrane constants of red and white muscle fibers in the rat.

The membrane constants of the EDL and SOL muscle fibers of the rat were measured by' square pulse analysis'. The transverse resistance of the fiber membrane at 27°C is about 540Ωcm2 for both muscles and the membrane capacitance is about 6μF/cm2 for the EDL and 4μF/cm2 for the SOL. Functional significance of the difference in the membrane capacitance between the two muscles has been discussed.