A-Band Shortening in Isolated Myofibrils of Rabbit Psoas Muscles

Abstract

Muscle contraction is explained by cyclical interactions between actin and myosin filaments. These filaments are typically assumed to be rigid and thus are not thought to contribute to muscle contraction and force production. However, previous works have observed reduction in A-band width, an indicator of myosin length, in activated single fibers. As individual sarcomeres and A-bands cannot be resolved accurately in single fibers the purpose of this study was to determine A-band widths in a preparation that allows such resolution i.e., isolated myofibrils. Isolated myofibrils (n=13, 10-15 sarcomeres long) of rabbit psoas muscles were placed in a bath containing relaxing solution under an inverted microscope. Myofibrils were secured in two segments with two glass micro needles and a silicon nitrate nano lever. Mean sarcomere length (SL) of the control segment between the glass needle and the cantilever was set to ∼2.4µm and the mean SL of the experimental segment between the needles was varied between 2.0µm and 3.2µm. Upon Ca2+-activation, myofibrillar forces were calculated in the control segment from the cantilever deflection (186±112nN). A-band widths and SL were measured from video continually for 5min following activation. Reduction in A-band width (range=0.0-0.6µm) correlated (p=-0.7884) with the post-activation mean SL (range=2.1-3.2µm) of the experimental segment in a bi-linear fashion with an inflection at ∼2.8µm. A-bands shorten upon activation at SL below 2.8µm, but remain constant at SL above 2.8µm. This result contradicts long held beliefs about myosin filament rigidity and may challenge our current thinking about the molecular mechanisms of contraction. However, the slow speed of A-band shortening raises questions about its functional relevance in everyday muscle activity and its occurrence at short SL only may suggest a role in adjusting the length of the molecular spring titin.