Meeting Abstract
The relationship between length and tension (LT curve) observed in skeletal muscle is attributed to the requirement for overlap between actin and myosin. However, the LT curve varies with contractile conditions; optimum length is inversely correlated with activation level. We previously suggested that this is a consequence of compliance in series with actin and myosin during force transmission. Here we test this hypothesis by artificially varying the series compliance of the frog sartorius muscle, and show that optimum muscle length decreases with increasing series compliance. We suggest that the fiber shortening that occurs, even in nominally isometric contractions, in the presence of series compliance may be responsible for the decrease in optimum length. Shortening induced force depression is well established in skeletal muscle. The molecular mechanism underlying this phenomena is unknown. However, one current theory is that the internal strain induced by force production reduces the probability of myosin binding. This would prevent the formation of new crossbridges, and development of additional force, at the post-shortening length. This phenomena is likely to be length dependent, decreasing in effect size with decreasing length, and so may explain the shift towards shorter optimum length with increased series compliance and fiber shortening. We have developed a refined spatial model of the half sarcomere in which tropomyosin strain due to force production inhibits crossbridge binding. We use this to explore the combined hypothesis that shortening induced force depression is a result of strain-mediated crossbridge inhibition, and is responsible for the decrease in optimum length with increasing series compliance.
