Activation delays in frog twitch muscle fibres.

Abstract

SUMMARY1. 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 'C.2. The onset of the reactivation response showed little (< 1x X 1O-7 N) or no latency relaxation.3. Reactivation latency was minimal (2-8 msec) and constant at 1P9-21 ,tm sarcomere length and it increased by about 3 msec with sarcomere extension to 3-2,sm.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 jum sarcomere length and was delayed about 4 msec with increase in sarcomere length to 3-2 ,um.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 ,um.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 near 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 ,gsec 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.

Birefringence signal and early mechanical changes at normal and increased tonicities in frog skeletal muscle.

“…However, this concept is difficult to reconcile with the opposite changes in amplitude of the pre-relaxation contraction and twitch with increasing osmolarity and time of exposure to 2T Ringer solution. Close's (1981) attempts to measure contraction onset undisturbed by latency relaxation yielded (at 15 TC and IT) a minimal interval of about 3 ms between the stimulus and onset of positive tension upon restimulation (the second stimulus was applied a few milliseconds before the peak of the twitch). The delays (corrected for different temperatures by a Q1o of 2 1, see Fig.…”

Arsenazo III calcium transients and latency relaxation in frog skeletal muscle fibres at different sarcomere lengths.

“…The equipment and general procedures for recording and analysing latency relaxation and isometric contractions have been described earlier (Close, 1981): briefly, tension was recorded using a transducer having an output of about 0 13 V/mN, compliance 5 x 10-4 m/N and natural frequency 5 kHz in air. Data collection was controlled by a Hewlett Packard 21MX computer and sampling was carried out with a Hewlett Packard 5610 analog-to-digital converter paced by an internal 8 MHz crystal oscillator.…”

“…As the first three events take at least 2-3 ms at 15 0C (Close, 1981), these processes no doubt limit the rate of rise of the Ca2+ transients that are recorded as average responses of the whole sarcomere. Regarding the kinetic properties of the Ca2+ -Arsenazo III complex, association and dissociation rate constants of about 4.4 x 106 M-1 s-1 and 85 s-1, respectively, have been reported for about 17 /LMArsenazo III (KDCaAIII about 19 jUm; Ogawa, Harafugi & Kurebayashi, 1980), but no information is available on the rate constants corresponding to lower dissociation constants at millimolar dye concentrations comparable to those used for our intracellular recordings.…”

“…The dye-related absorbances of resting fibres at 2-4 and 3 0 gm sarcomere lengths Increase in sarcomere length from 2-4 to 3 0 ,m had little or no effect on the delay to onset of either the Ca2+ transient (above) or latency relaxation (Haugen & Sten-Knudsen, 1976;Close, 1981); as the amplitudes of the two signals show opposite dependence on length over that range, the most sensitive comparison of starting times is obtained using the Ca2+ transient at short lengths and the latency relaxation recorded at longer length. Such a comparison for one fibre (Fig.…”

“…Recently several lines of indirect evidence have suggested that the time of onset of Ca2+ release, like the onset of latency relaxation, is virtually independent of sarcomere length (reviewed by Close, 1981) but crucial information about the time relations of the two events had not been obtained. The aim of the present investigation was to record the early phases of mechanical and Ca2+ transients in twitch responses at different sarcomere lengths and to test the hypothesis that the onset of latency relaxation signals the arrival of activation Ca2+ in the myofibrillar space.…”