LATENCY RELAXATION AND A THEORY OF MUSCULAR MECHANO-CHEMICAL COUPLING

Latency relaxation in frog skeletal muscle under hypertonic conditions

The effect of exposure to hypertonic conditions on latency relaxation (LR), twitch tension and resting tension in frog toe muscle was studied. Measurements were carried out in standard and hypertonic conditions (1 to 3.6 times standard tonicity) and following transversal stimulation. In hypertonic conditions, the LR showed increased duration and decreased amplitude. Both time course and amplitude showed the same dependency on sarcomere length as in standard conditions. The fall in amplitude and extension of the time course was proportional to the increase in tonicity up to 2.2 times the tonicity of the standard. At higher tonicities the time course extended earlier and diverged from the linear dependency, the amplitude falling later with increasing tonicity. The LR was abolished at tonicities exceeding 3 times the standard. In hypertonic conditions the twitch tension was reduced and extended, but showed the same dependency on sarcomere length as in standard conditions. The amplitude showed the same dependency on tonicity as the LR amplitude. The time course extended in a non-linear way with growing tonicity. In hypertonic conditions the resting tension was higher than or the same as in standard conditions, and the dependency on sarcomere length was the same.

Increase of resistance to stretch during the latent period in single muscle fibres of the frog

The time from stimulation to the first change of the extensibility in an isolated skeletal muscle fibre was measured by subjecting the fibre to a rapid, small stretch at various times during the latent period. The experiments were performed at constant temperatures in the range 1-16 degrees C. Irrespective of the temperature, the first increase of the resistance to stretch occurred after the onset of the latency relaxation. At a temperature or 10 degrees C and a sarcomere length of 3 microgram the resistance started to increase 5.5 ms after stimulation, i.e. 1.5-2 ms after the onset of the drop in tension, and then increased nearly linearly with time. Corresponding to the end of the latent period, i.e. when the tension recrosses the resting level, the amplitude of the response was about 1.5 times its resting value. There was an interval between the onset of increase f the resistance to stretch and the time when the rate of drop in tension had passed its maximum value. The results are compatible with the hypothesis that the latency relaxation arises from a minute lengthening of the thin filaments as proposed by Haugen & Sten-Knudsen, and that attachment and generation of force take place at separate steps of the cross-bridge cycle.

Latency-relaxation in single muscle fibres

1. Latency relaxation and twitch tension were recorded simultaneously in single isolated muscle fibres of Xenopus laevis. 2. During low frequency (0.6 or 1 pulse/sec) repetitive stimulation, three successive phases of twitch tension were observed: negative staricase (a slight drop in tension), positive staircase (about 15% increase in tension) and fatigue. At the same time the amplitude of latency relaxation decreased monotonically, and near the peak of positive staircase, the amplitude decreased almost to an undetectable level. 3. The application of caffeine (0.1--1.5 mM) increased peak twitch tension by 15--200%, but decreased the latency relaxation amplitude by 30--93%. 4. The application of Ca-release inhibitors, deuterium oxide and dantrolene sodium, caused a 43--89% decline in peak twitch tension but no change in latency relaxation amplitude. 5. The lack of correlation between changes in peak twitch tension and latency relaxation amplitude suggests that latency relaxation is associated with the mechanism which triggers Ca2+ relase rather than with Ca2+ release itself.

Fatigue and caffeine effects in fast-twitch and slow-twitch muscles of the mouse

In excised, curarized and massively stimulated fast-twitch mouse gastrocnemius muscles the early twitch tension enhancements (treppe) during 1/s activity between 10 and 36 degrees C increase and affect more contractions as temperature increases. Tension output eventually declines at a temperature-independent rate. Half-relaxation time lengthens below 25 degrees C and shortens above 25 degrees C. During 1/0.63s twitches half-relaxation time lengthens even at 25 degrees C. In slow-twitch soleus muscles activity decreases twitch tension and half-relaxation time regardless of temperature. Activity shortens contraction times in both muscles. Oxygen lack induced by NaN3 cannot account satisfactorily for these results. Activation is apparently more plastic in the gastrocnemius than in the soleus, and the relationship between the rates of their activation and relaxation processes and the temperature sensitivities of these rates also seem to differ. In both muscles caffeine can convert activity-induced shortened of half-relaxation times into prolongations. In the soleus this effect is more pronounced at 30 than at 25 degrees C. At high temperature and twitch rates caffeine reduces treppe amplitude and duration without affecting the eventual twitch tension decline in the gastrocnemius while it greatly accelerates twitch tension decline in the soleus. In both muscles intrafiber Ca2+ movements are apparently major determinants of fatigue behavior.

The dependence of tension relaxation in skeletal muscle on the number of sarcomeres in series

Mulieri (1972) has reported that the depth of the latency relaxation, tension relaxation, changes only insignificantly when about half of a fibre bundle is wound tightly around a small glass rod. From this observation he concludes that the tension relaxation is not correlated with the number of sarcomeres in series in a frog muscle fibre. The dependency of the tension relaxation on the number of sarcomeres in series has been reinvestigated using both Mulieri's method and other independent methods. When Mulieri's experimental procedure was repeated, his result was reproduced. In the other methods used the tension relaxation was found to be proportional to the number of sarcomeres in series which were activated initially. This result was also obtained by introducing a slight modification to the procedure used by Mulieri. The results could equally well be understood if the latency relaxation was caused by a definite elongation rather than an increase in compliance of the fibre.

The dependence of the latency relaxation on sarcomere length and other characteristics of isolated muscle fibres

1. The latency relaxation has been examined in single fibres from frog striated muscle with particular attention given to its possible relation to Ca(2+) release during excitation-contraction coupling.2. Latency relaxations were recorded at 19-23 degrees C from massively stimulated (0.2 msec pulses) single fibres using two selected RCA 5734 transducer tubes in a bridge circuit.3. The depth of the latency relaxation has its full value when stimulus strength is between 40 and 400% above twitch threshold. Stronger stimuli reversibly diminish the latency relaxation.4. The variation in depth of latency relaxation with sarcomere length was found similar to that reported previously for multifibre preparations but in single fibres the peak of the curve consists of a plateau between sarcomere lengths of 2.8 mu and 3.2 mu.5. Sucrose hypertonicity increases the depth of the latency relaxation at sarcomere lengths below 2.8 mu but above this length it has either no effect or a depressant effect depending on the degree of hypertonicity.6. The maximal depth of the latency relaxation (measured at 3 mu) averaged 0.23% of the maximal tetanus tension (measured at 2.2 mu) and was strongly correlated (r = 0.87) with the latter in forty-five single fibres.7. The maximal depth of the latency relaxation is not correlated with the number of sarcomeres in series in a fibre.8. The results of this study are shown to fully support and extend Sandow's (1966) hypothesis that the latency relaxation is caused by release of activator Ca(2+) from the sarcoplasmic reticulum.

Latent Period of Relaxation

The latent period of relaxation of molluscan myocardium due to anodal current is much longer than that of contraction. Although the rate and the grade of relaxation are intimately related to both the stimulus condition and the muscle tension, the latent period of relaxation remains constant, except when the temperature of the bathing fluid is changed.