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Unloaded contraction of a single sarcomere was studied on fiber fragments (1-micron thick, 4- to 6-microns wide, and less than 50 microns long) from glycerinated scallop striated muscle. The fragment was activated from rigor by a rapid solution exchange method. Sarcomere length (SL) was measured in the intensity profile caused by scanning the phase-contrast image of the striation of the fragment every 5.4 ms using an optical scanner-photomultiplier system with an accuracy of 0.04 micron. The sarcomere started to contract with a latency of 44 +/- 11 ms (mean +/- SD, n = 21) after the solution change. Averaging a few SLs in a fragment clearly revealed a phase, in which shortening was close to zero, preceded and followed by a rapid shortening phase. The contraction of a single sarcomere appeared to occur in an oscillatory manner with one or more of the zero-velocity phases during the observed period (up to 250 ms) when the data points were approximated with a polynominal curve. The distance between the near zero-velocity phases was 0.17 +/- 0.09 micron (n = 18). As the contraction was approximated with a staircase pattern consisting of a rapid shortening and a zero-velocity phase, the duration of the zero-velocity phase was 44 +/- 18 ms (n = 23). The velocity and the extent of the rapid shortening phase were 19.0 +/- 7.9 microns/sarcomere (n = 29) and 0.16 +/- 0.06 micron (20 degrees C), respectively, both of which were independent of the SL examined (1.7-2.6 microns). The velocity in the shortening phase was independent of the extent of the shortening but decreased with increasing duration of the shortening phase. Probably because of the loss of light chains relating to the Ca-regulation from some myosins during preparation, an increase in the concentration of ATP in the absence of Ca2+ resulted in a single shortening step of the sarcomere with a latency of 82 +/- 23 ms, followed by a phase of little shortening; the velocity in and the extent of the shortening step were 12.8 +/- 8.4 microns/s/sarcomere (n = 5) and 0.12 +/- 0.04 micron, respectively. The mechanism underlying the observed oscillatory contraction was considered in terms of the cross-bridge mechanism.

Oscillatory Contraction of Single Sarcomere in Single Myofibril of Glycerinated, Striated Adductor Muscle of Scallop.