Latency-relaxation in single muscle fibres

Mechanism of latency relaxation in frog skeletal muscle

The latency relaxation is a small drop of tension before skeletal muscle begins to develop active tension. This phenomenon was found nearly one century ago but its origin has not been clarified. In this review, the hypotheses for its mechanism are discussed in terms of the recent experimental results using X-ray diffraction. The latency relaxation takes place almost simultaneously as the structural change of the regulatory protein troponin, an unspecified structural change of the thick filament, and increase in stiffness. It seems difficult to associate all of these with the latency relaxation by assuming a simple mechanism.

A structural origin of latency relaxation in frog skeletal muscle

A time-resolved x-ray diffraction study at a time resolution of 0.53 ms was made to investigate the structural origin of latency relaxation (LR) in frog skeletal muscle. Intensity and spacing measurements were made on meridional reflections from the Ca-binding protein troponin and the thick filament and on layer lines from the thin filament. At 16 degrees C, the intensity and spacing of all reflections started to change at 4 ms, simultaneously with the LR. At 0 degrees C, the intensity of the troponin reflection and the layer lines from the thin filament and the spacing of the 14.3-nm myosin meridional reflection, but not the spacing of other myosin meridional reflections, began to change at approximately 15 ms, when the LR also started. Intensity of myosin-based reflections started to change later. When the muscle was stretched to non-overlap length, the intensity and spacing changes of the myosin reflections disappeared. The simultaneous spacing change of the 14.3-nm myosin meridional reflection with the LR suggests that detachment of myosin heads that are bound to actin in the resting muscle is the cause of the LR.

Earliest mechanical evidence of cross-bridge activity after stimulation of single skeletal muscle fibers

The stiffness of single fibers from frog skeletal muscle was measured by the application of small 2-kHz sinusoidal length oscillations during twitch and tetanic contractions at a range of initial sarcomere lengths. The earliest mechanical signs of activation were a fall in tension (latency relaxation) and a rise in stiffness. The earliest stiffness increase and the earliest tension fall occurred simultaneously at all sarcomere lengths. This suggests a cross-bridge origin for the latency relaxation. The lead of stiffness over tension seen during the rise of tension was substantially established during the latent period. Reducing the size of the twitch by reducing calcium release with D-600 (methoxyverapamil) reduced the latency relaxation and the stiffness development during latency much less than it reduced the twitch tension. For very small twitches the peak of the stiffness response occurred during the latent period and the times of onset of both latency relaxation and stiffness rise were delayed, but remained coincident. This suggests a strong connection between the latency relaxation and the rise of stiffness during the latent period, whereas the connection between these events and positive tension generation appears to be less strong.

Influence of deuterium oxide on calcium transients and myofibrillar responses of frog skeletal muscle

The influence of substituting D2O for H2O on calcium transients and on contraction was studied in intact single skeletal muscle fibres injected with aequorin and in mechanically skinned fibres from frogs. Most experiments were carried out at 10 degrees C. Experiments performed in vitro established that the calcium concentration-effect curve for aequorin is depressed and the rate of change of aequorin light emission after sudden changes of [Ca2+] is decreased when D2O is substituted for H2O. D2O substitution greatly reduces the amplitude of the aequorin signals of intact fibres both in twitches and in tetani. This is partly the result of the influence of D2O on aequorin, but the amplitude of the calcium transients is reduced as well. In both H2O and D2O Ringer solutions, the amplitude and time course of the sarcoplasmic calcium transient during a tetanus vary greatly with the stimulus frequency. In H2O, frequencies high enough to produce mechanical fusion normally produce cytoplasmic calcium concentrations high enough to saturate the myofibrils, with the result that stimulus frequency has very little influence on the mechanical response. Saturation does not occur in D2O, and the amplitude and form of fully fused tetani are greatly influenced by the frequency of stimulation. Aequorin was used as a calcium indicator to estimate the influence of D2O substitution on the apparent equilibrium association constant for the calcium-EGTA complex. The constant was reduced by more than one order of magnitude in D2O at pD = pH = 7.0. Experiments on mechanically skinned muscle fibres showed that D2O substitution decreased the apparent sensitivity of the myofibrils to calcium, but increased the force of contraction at saturating calcium concentrations. The latter effect is probably responsible for the potentiation of tetanic tension that is observed in certain D2O/H2O mixtures or in H2O solutions immediately after a switch from D2O. In intact muscle fibres the potentiation of twitch or tetanic tension after a switch from D2O to H2O declines with a half-time of about 25 s; this probably reflects the time course of exchange of intracellular D2O for H2O. The effects of D2O on the intracellular calcium transient appeared and disappeared with a half-time of less than 1 s; this time is of the same order of magnitude as that calculated for the exchange of H2O and D2O in the T-tubules. We conclude that D2O affects calcium release at a site in the T-tubule which is more accessible to the extracellular space than to the intracellular space.

The effect of dantrolene on the enhancement and diminution of tension evoked by staircase and by tetanus in rat muscle

1. The effect of Dantrolene on the potentiation of isometric twitch tension was examined during and after the staircase (5/sec, 250 stimuli) and after the tetanus (167/sec, 250 stimuli) in the extensor digitorum longus muscle of adult Lewis rats at 37-38 degrees C.2. The study confirmed that Dantrolene decreased the twitch tension much more than the tetanic tension. The drug shortened the contraction time of the twitch. The rate of force development of the twitch was diminished by only half that of the twitch tension. The findings may suggest that the drug both shortened and diminished the activation of the muscle during the twitch.3. Dantrolene decreased the potentiation produced by a given number of stimuli early in the staircase. At the 250th stimulus the staircase was about 25% larger after than before application of the drug. Dantrolene increased the potentiation 2 sec after the tetanus by about 60%.4. After application of Dantrolene, the decay of potentiation after the staircase indicated that the process that diminished the twitch during the staircase, present before application of the drug, was absent. The size of the process causing potentiation was the same with and without the drug. Both events of potentiation after the tetanus were increased by Dantrolene. Both after the staircase and after the tetanus, Dantrolene increased the slow phase of decay by 60-70%. The fast rate of decay after the tetanus was unchanged by the drug.5. The contraction time was prolonged less in the potentiated twitch after than before application of Dantrolene. This was presumably due to a greater relative increase in activation rather than to a prolongation of the time during which the muscle was activated.6. A model is proposed where the delay in potentiation during the staircase and the increase in potentiation after the tetanus are due to the proportion of sites in the excitation-contraction coupling occupied by Dantrolene being reduced by repetitive depolarizations of the transverse tubules during trains of repetitive stimuli.

Activation delays in frog twitch muscle fibres

1. The length dependence of the mechanical latent period and early tension development, uncomplicated by latency relaxation, was determined from responses of single muscle fibres to restimulation at the peak of the isometric twitch at 15 degrees C. 2. The onset of the reactivation response showed little (less than 1.5 X 10(-7) N) or no latency relaxation. 3. Reactivation latency was minimal (2.8 msec) and constant at 1.9-2.1 micrometers sarcomere length and it increased by about 3 msec with sarcomere extension to 3.2 micrometers. 4. Reactivation responses showed two stages of early tension development, an initial phase in which tension acceleration increased, and a phase of maximum responsiveness in which tension acceleration was constant; the transition between the two phases occurred about 4.5 msec after the start of the stimulus at 2.2 micrometers sarcomere length and was delayed about 4 msec with increase in sarcomere length to 3.2 micrometers. 5. The square root of the maximum tension acceleration was directly proportional to the degree of overlap of thick and thin filaments in the sarcomere length range 2.3-3.2 micrometers. 6. It is proposed that the onset of the phase of constant tension acceleration marks the end of the period during which the activator, calcium, is distributed throughout the sarcomere. 7. Analysis of early tension transients in relation to myofibril structure showed that length-dependent changes in reactivation latency and time of onset of constant tension acceleration were probably brought about mainly by alteration of the kinetics of distribution of activator within the myofibril and by changes in the diffusion distance between activator-release sites ner the end of the sarcomere and the tension-generating sites. 8. There was a 2 msec myofibril priming delay in the rise of tension in twitch responses that was not seen in reactivation responses; the possible origin of that delay is discussed in relation to structural changes accompanying activation and to competition between calcium-binding structures. 9. The onset of twitch latency relaxation occurred within about 250 microseconds after the time corresponding to latency of the earliest reactivation responses and appeared to signal the start of a process that took place after the arrival of calcium among the myofilaments. The origin of latency relaxation is discussed.

Action potentials of isolated single muscle fibers recorded by potential-sensitive dyes

Light transmission changes upon massive stimulation of single muscle fibers of Xenopus were studied with the potential-sensitive nonpermeant dyes, merocyanine rhodanine (WW375) and merocyanine oxazolone (NK2367). Upon stimulation an absorption change (wave a) occurred, which probably represents the sum of action potentials in the transverse tubules and surface membrane. In WW375-stained fibers wave a is a decrease in transmission over the range of 630 to 730 nm (with NK2367, over the range of 590 to 700 nm) but becomes an increase outside this range, thus showing a triphasic spectral pattern. This spectrum differs from that of the squid axon, in which depolarization produces only an increase in transmission over the whole range of wavelengths (Ross et al. 1977. J. Membr. Biol. 33:141-183). When wave a was measured at the edge of the fiber to obtain more signal from the surface membrane, the spectrum did not seem to differ markedly from that obtained from the entire width of the fiber. Thus, the difference in the spectrum between the squid axon and the vertebrate muscle cannot be attributed to the presence of the tubular system.