REDISTRIBUTION OF SARCOMERE-LENGTH DURING ISOMETRIC CONTRACTION OF FROG-MUSCLE FIBERS AND ITS RELATION TO TENSION CREEP

Changes in length of successive 0.5-0.8 mm segments along single muscle fibers of R. temporaria were recorded during 3 s isometric (fixed fiber ends) tetani at 2.15 and 2.60 .mu.m sarcomere length. The measurements were performed by means of a photo-electric detector system which recorded the distance between opaque markers (ca. 60 .mu.m in width) that were attached to the upper surface of the fiber. The segment length change had an initial rapid phase (1) which coincided with the steep rise of force and a subsequent slow phase (2) which coincided with the upper, rounded portion of the force myogram and the plateau of the tetanus. At 2.15 .mu.m sarcomere length the majority of the central segments (comprising .apprx. 90% of the fiber) shortened to various degrees during phase 1. A considerable redistribution of length occurred during phase 2 in that some segments shortened at the expense of others which were forcibly stretched. The central region, taken as a whole, shortened by 0.1-0.5% during phase 2. The end sements were consistently found to elongate during phase 1. They were able to hold the tension, without further elongation, during phase 2. The pattern of length changes within the central region of the fiber observed at 2.15 .mu.m sarcomere spacing remained largely the same after increasing the sarcomere length to 2.60 .mu.m. In contrast to the situation at 2.15 .mu.m sarcomere length there was an over-all (0.4-1.5%) elongation of the central region of the fiber during phase 2 at the great fiber length. This elongation of the central region was assoicated with marked shortening of the end segments. The sarcomere length of the end segments (s.l.e) was compared to that of the central region of the fiber (s.l.c) at various fiber rest lengths. There was no significant difference between s.l.e and s.l.c when the fiber was just taut, i.e., at .apprx. 2.1 .mu.m sarcomere length. The following relationship between s.l.e and s.l.c was found to apply for values of s.l.c ranging between 2.2 and 2.7 .mu.m: s.l.e = 0.636 s.l.c + 0.744 (correlation coefficient, 0.93). The possibility was explored that redistribution of sarcomere length along the fiber causes the slow climb of force (tension creep) that occurs during a tetanus at great (> 2.2 .mu.m) sarcomere lengths. Tension creep could be reproduced, after peak force was attained, during an isometric tetanus by releasing the fiber to shorten within the range 2.6-2.3 .mu.m sarcomere length. Under these conditions shortening at a low, constant velocity caused a steady increase of force whereas shortening against a high, constant load caused acceleration of the fiber. Similar shortening protocols performed within the plateau of the length-tension relation did not enhance the contractile performance. The duration and magnitude of the tension creep increased with the sarcomere length at which the tetanus was initiated. Peak force occurred when the end segments had shortened to just below 2.2 .mu.m sarcomere length. This is explained by the fact that the end segments, being the main generators of tension creep, do not further improve their strength by shortening once they have reached the plateau of the length-tension relation. Tension creep could be eliminated, at any fiber rest length, by holding a small (0.5-0.7 mm) segment at constant length during the tetanus. This finding provides evidence that tension creep is not a feature of the contractile process at sarcomere level. A computer model was used to simulate segment length changes and tension creep during fixed-end-tetani.