The first thin filament layer line decreases in intensity during an isometric contraction of frog skeletal muscle

During isometric contraction/activation of full-overlap and non-overlap live frog skeletal muscles, the intensity of the first thin filament layer line at the axial spacing of approximately 1/37 nm-1, when separated from the partially overlapping first thick filament layer-line at approximately 1/43 nm-1, remained unchanged in the inner radial region (0.02-0.08 nm-1) where a large intensity increase is observed in the rigor state. The intensity decreased in the outer radial region (0.08-0.18 nm-1) where this layer line is expected to peak in the resting state. The intensity decrease in the outer region became larger with increasing filament overlap; on activation of the non-overlap muscle, it was about half that of the full-overlap muscle. Thus the first thin filament layer line decreases in intensity and any indication of the rigor-like intensification is not observed at all during contraction. This intensity decrease can be attributed to the same structural changes giving rise to the intensity increase of the second thin filament layer line. The results indicate that the configuration of the myosin heads interacting with actin during contraction differs significantly from that of the rigor state. Four-fold rotational symmetry of the thin filament structure as a whole becomes more pronounced during isometric contraction of the overlap muscle.