The effect of procaine on snake twitch muscle fibres

1. The effect of procaine on the contractile responses of voltage clamped snake twitch muscle fibres has been investigated. Procaine hydrochloride shortened the duration of contractures produced by long depolarizing pulses. It also reduced the amount of inactivating outward current.2. The rate of repriming was greatly slowed by procaine. At -80 mV, for example, the time to half restoration of contractile response was 6 sec in procaine (5 x 10(-3), w/v) as compared to 1.15 sec in tetrodotoxin (TTX), (1 x 10(-7) w/v). At -100 mV, repriming was still much slower in procaine. However, at -150 mV, repriming in procaine was quite rapid, the time to half recovery being about 0.4 sec.3. After prolonged exposure to procaine, subthreshold depolarizing pulses produced a marked diminution in the contractile response to a subsequent supermaximal depolarization. In some fibres complete inactivation could be produced in the presence of procaine without any contractile response having occurred.     

Re-innervation of twitch and slow muscle fibres of the frog after crushing the motor nerves.

1. The conduction velocities of individual motor axons innervating twitch and slow muscle fibres of the frog were determined by intracellular recording of junctional potentials elicited by stimulating the motor nerves at two different points. 2. In normal pyriformis muscles twitch and slow fibres were found to be innervated by two distinct populations of motor axons. Twitch fibre axons conducted at 10-18-7 m/sec, while the conduction velocities of slow fibre axons ranged from 0-5 to 5 m/sec (at 7-9 degrees C). The thresholds for electrical stimulation were significantly lower in the fast than in the slow axons population. 3. Following denervation by crushing the sciatic nerve fast axons which re-innervated the muscle had lower conduction velocities than normal but could still be identified. These lower conduction velocities were measured proximal to the site of the crush and did not recover over a period of 446 days. 4. Fast motor axons regenerated more quickly than slow axons and re-innervated twitch as well as slow muscle fibres non-selectively. About 1 month later slow axons re-established synaptic contacts with slow (and some twitch) muscle fibres. Simultaneous re-innervation by fast and slow motor axons was occasionally observed in slow muscle fibres. Finally, the slow muscle fibres were innervated by slow axons only, while synapses of fast axons could no longer be found in this type of muscle fibre. 5. Action potentials were observed in denervated as well as in re-innervated slow muscle fibres; they disappeared as re-innervation progressed. 6. It is concluded that non-selective re-innervation of slow muscle fibres is present in the frog; it is, however, a transient phenomenon followed by restoration of the original innervation pattern.     

Contractile repriming in snake twitch muscle fibres

1. Contractile repriming has been studied in voltage-clamped snake twitch muscle fibres. Maintained depolarization causes a contractile response which inactivates after a few seconds. Repolarization of the fibre can restore its ability to contract to a subsequent depolarization. This restoration, or repriming, depends on the magnitude and the duration of the repolarization. At -100 mV the minimal period of repolarization which restores contractile response is 0.38 sec. The time for recovery to half maximal tension is about 0.68 sec, and restoration is complete at about 4 sec.2. Repolarization to smaller levels of membrane potential results in a slower rate of repriming. For example, at -60 mV the mean minimal time for repriming was 2.89 sec, and nearly 17 sec of repolarization was required for full restoration of contractile response.3. The rate of repriming was not influenced by lowering the external sodium concentration.4. Repriming could be produced by repetitive, brief pulses of repolarization.5. The restoration of contractile response and of outward inactivating current showed similar time courses.     

Relationships between motor nerve conduction velocities and motor unit contraction characteristics in a slow twitch muscle of the cat

1. Isometric contractions of cat soleus muscles and of functionally isolated motor units have been measured under conditions similar to those of Bagust, Knott, Lewis, Luck & Westerman (1973).2. The muscle lengths at which motor unit twitch and tetanus tensions were maximal were close to the optimal lengths of the parent muscles.3. Motor unit contraction times and tensions were shown to be related to the conduction velocities of the nerve axons supplying them. These relationships were obscured when a number of small samples from several experiments were grouped together.4. The ratios of the twitch to the tetanic tensions of the motor units were directly related to their times to peak tension.     

Differentiation of motor nerve terminals formed in the absence of muscle fibres

During reinnervation of frog skeletal muscle, axons form functional nerve terminals at original synaptic sites on denervated myofibres. When muscle is damaged as well as denervated, myofibres decompose but their sheaths of basal lamina (BL) survive. Despite the absence of myofibres, axons regenerate to contact BL and there acquire clusters of synaptic vesicles and membrane-associated dense patches that resemble active zones; BL regulates this differentiation. We show here that these BL-associated axonal segments appear smaller and contain fewer active zones than terminals on intact myofibres in the same preparation. However, terminals formed on BL sheaths are capable of activity-dependent recycling of synaptic vesicles (demonstrated by tracer uptake), and bear an antigen normally present in terminals but not preterminal axons (demonstrated by immunofluorescence). Thus, axons can acquire functional and biochemical, as well as morphological, characteristics of normal motor nerve terminals in the absence of a postsynaptic cell.     

Acetylcholinesterase content in both motor nerve and muscle is correlated with twitch properties

The activity and molecular forms of acetylcholinesterase (AChE) were studied in the rat soleus muscle and its nerve, as compared to their fast-twitch counterparts. The soleus muscle and its nerve exhibited both significantly lower AChE activity and less of the G₄ (10S) molecular form. In addition, the soleus muscle displayed a specific increase in the A_s (13S) and A₄ (8.8S) asymmetric forms, not seen in any of the fast-twitch muscles examined. These results indicate that the AChE content of a muscle and its nerve are linked and depend on the twitch properties, and that the slow-twitch muscle is characterized by a specific set of AChE molecular forms.     

The relation of membrane changes to contraction in twitch muscle fibres

1. Contractile responses in short twitch-type snake muscle fibres have been studied. These fibres are sufficiently short to allow fairly uniform changes in membrane potential along their length when current is passed through an intracellular micropipette. Active sodium permeability changes were blocked with tetrodotoxin (TTX), procaine, or by using solutions low in sodium. Current and voltage micropipettes were used to voltage-clamp these fibres. Depolarization steps to about -40 mV evoked contractile responses, maximal tension being developed between -10 and 0 mV. The relation between contraction and membrane potential was sigmoid.

2. Depolarization beyond a critical threshold produced an increment of outward current which inactivated with time. The threshold for this delayed rectification was normally similar to the threshold for contractile activation. Fibres exposed to high potassium showed a reversal of this inactivating current to slightly super-threshold depolarizing pulses. At membrane potentials near 0 mV, no inactivating current was noted, while stronger depolarizing pulses produced an inactivating current in the normal direction. Fibres in high potassium show the same threshold for initiation of contraction as in normal solution.

3. Thiocyanate, nitrate, and caffeine shifted the relation between membrane potential and contraction toward higher levels of membrane potential. The threshold for inactivating rectifying current failed to shift to a corresponding extent, although some shift in rectification which did not inactivate was evident.

4. When depolarization was maintained, contractile tension was maximal for several seconds, then gradually disappeared. The rate of this contractile inactivation depended upon the level of depolarization.

     

Collagen of slow twitch and fast twitch muscle fibres in different types of rat skeletal muscle

Summary The appearance of collagen around individual fast twitch (FT) and slow twitch (ST) muscle fibres was investigated in skeletal muscles with different contractile properties using endurance trained and untrained rats as experimental animals. The collagenous connective tissue was analyzed by measuring hydroxyproline biochemically and by staining collagenous material histochemically in M. soleus (MS), M. rectus femoris (MRF), and M. gastrocnemius (MG). The concentration of hydroxyproline in the ST fibres dissected from MS (2.72±0.35 g·mg^–1 d.w.) was significantly higher than that of the FT fibres dissected from MRF (1.52±0.33 g·mg^–1 d.w.). Similarly, the concentration of hydroxyproline was higher in ST (2.54±0.51 g·mg^–1 d.w.) than in FT fibres (1.60±0.43 g·mg^–1 d.w.), when the fibres were dissected from the same muscle, MG. Histochemical staining of collagenous material agreed with the biochemical evidence that MS and the slow twitch area of MG are more collagenous than MRF and the fast twitch area of MG both at the level of perimysium and endomysium. The variables were not affected by endurance training. When discussing the role of collagen in the function of skeletal muscle it is suggested that the different functional demands of different skeletal muscles are also reflected in the structure of intramuscular connective tissue, even at the level of endomysial collagen. It is supposed that the known differences in the elastic properties of fast tetanic muscle compared to slow tonic muscle as, e.g., the higher compliance of fast muscle could at least partly be explained in terms of the amount, type, and structure of intramuscular collagen.This study was supported by grants from the Finnish Research Council for Physical Education and Sport (Ministry of Education) and the Academy of Finnland     

The effect of noradrenaline on the end-plate potential in twitch fibres of the frog

1. The action of noradrenaline in increasing the amplitude of the end-plate potential (e.p.p.) has been studied in magnesium-blocked frog skeletal muscle.2. Noradrenaline (10(-5)M) increased the e.p.p. by about 20% without causing any comparable change in the amplitude of the miniature end-plate potentials (m.e.p.p.s). It was concluded that noradrenaline acts by potentiating the release of acetylcholine by the nerve impulse, rather than by increasing the sensitivity of the end-plate.3. In support of this, it was found that the increase in e.p.p. amplitude was associated with a reduction in the variability of successive e.p.p.s, as is to be expected if the quantal content of the e.p.p. became larger. Further, noradrenaline was without effect on the response to iontophoretically-applied acetylcholine, although it potentiated depolarizations elicited by acetylcholine applied for much longer periods in the bathing fluid.4. Noradrenaline increased the frequency of occurrence of m.e.p.p.s, and unusually large m.e.p.p.s. were occasionally observed.     

Tension minima and activation in frog twitch muscle fibres

Responses to restimulation at different times during isometric relaxation were recorded from single muscle fibres at 15°C. The reactivation latency was constant over a wide range of tensions but the time (t _min) between restimulation and the subsequent tension minimum always varied by several milliseconds. Variation int _min was directly correlated with the rate of relaxation at the time of restimulation and was inversely related to tension throughoug most of the phase of relaxation. The results are discussed in relation to excitation-contraction coupling and it is shown that the tension minima did not collectively represent the time course of activation of the sarcomere.     

Diazepam, a highly effective twitch potentiator in isolated muscle fibres of the frog

The contractile effects of diazepam (10-200 microM) were studied on twitch and tetanus responses of isolated fibres of the semitendinosus muscle of Rana temporaria (3.5-5.0 degrees C). Diazepam (100-200 microM) enhanced the twitch amplitude to 95-100% of maximum tetanic force and increased the rate of rise of force, the time to peak twitch force and the total duration of the relaxation phase. The maximum tetanic output was unaffected by diazepam but the drug increased the rate of rise of the tetanic force and delayed the onset of force decay after the last stimulus. However, the kinetics of relaxation was unaffected by the drug. Diazepam had no effect on either threshold, submaximum or maximum contracture responses to caffeine and to increased potassium concentration. Diazepam in concentrations producing full twitch potentiation caused only a moderate (ca 30%) increase in action potential duration. The results are in line with the idea that 1: diazepam enhances the twitch response by increasing the rate of release of activator calcium without affecting the rate of calcium resequestration and 2: diazepam acts by modulating a mechanism in the excitation-contraction coupling that responds specifically to membrane excitation.     

The spread of the action potential through the T-system in hagfish twitch muscle fibres.

The input impedance of twitch muscle fibres of the Atlantic hagfish has been measured with sinusoidal transmembrane currents. The apparent specific membrane resistance and capacitance decreased markedly with frequency, and were relatively independent of fibre diameter. A model of the T-system based on anatomical observations, was used to predict the input impedance in the normal solution (artificial sea water). The changes in input impedance produced by glycerol treatment, low chloride solution, reduced pH and isotonic solutions with low ionic strength were easily interpreted in terms of the same model. The model predicts severe attenuation of the action potential if conducted electrotonically by the transverse tubules towards the center of the fibre.     

Effect of barium ions on excitation-contraction coupling of frog twitch muscle fibres

The mechanism for the Ba2(+)-induced potentiation of the twitch tension in frog skeletal muscle was investigated. No significant difference in the peak amplitude of the calcium transients evoked by voltage-clamp depolarizing pulse was found between the fibres bathed in Ca2+ (control) and Ba2+ Ringer's solution, whereas the calcium transients evoked by the action potentials from the fibres in Ba2+ Ringer's solution were increased by about 64%, compared with control. In comparison with control, the action potentials of the fibres in Ba2+ Ringer's solution had a similar overshoot but a significantly longer time course. Our results suggest that excitation-contraction coupling in frog twitch muscle fibres is not altered by replacing Ca2+ with Ba2+. The Ba2(+)-induced potentiation of contraction may be accounted for by broadening of the action potentials.