Light diffraction studies of active muscle fibres as a function of sarcomere length

This investigation has established the following points. (1) Activation of single intact frog muscle fibres or mechanically skinned fibres results in a decrease in the intensity of the first-order line at all sarcomere lengths at which filament overlap is present. (2) At long sarcomere lengths (>3.6 μm) the intensity decrease upon stimulation of intact fibres diminishes until above 3.9 μm no decrease is seen. In the skinned preparation, no intensity change is seen at sacromere lengths above 3.6 μm. (3) The intensity decrease seen in the intact fibres in the 3.6-3.9 μm sarcomere length region may be due to contraction of sarcomeres near the tendons, which have a shorter sarcomere length than those illuminated by the laser beam, and thus may not be stretched beyond filament overlap. (4) No intensity decrease is observed upon activation at very long (>4.25 μm) sarcomere lengths. (5) Mechanically skinned fibres show a graded intensity response to free calcium. (6) Scans of first-order line width show no broadening upon activation, indicating that sarcomere length dispersion in the illuminated region of the fibre does not increase.