Z-line structural diversity in frog single muscle fiber in the passive state

The structural changes of the Z-line between small square net (ss) and basket weave (bw) cross-sectional patterns were examined using intact single fibers and mechanically skinned fibers in the passive state to determine if the pattern is related to the sarcomere length (SL) and if the pattern undergoes a reversible transition in low- and high-osmotic medium. Frog single fibers were isolated from the anterior tibial muscle in Ringer's solution. Entirely or partially skinned single fibers were prepared in relaxing solution (also called low-osmotic medium). The high osmotic medium contained 10% polyvinylpyrrolidone (PVP) in relaxing solution. The sarcomere length (SL) of each fiber was measured directly by use of a laser beam or indirectly from electron micrographs with use of a correction factor. The ss and bw forms in cross sections were quantified by analysis of electron micrographs. The results show that the structural change of Z-line occurs around bw << 2.3-2.4 microns << ss (n = 25) and bw << 3.1-3.2 microns << ss (n = 13) in intact single fibers and skinned fibers, respectively. With the quick freeze-freeze substitution method, an intact single fiber with a SL of 2.35 microns showed almost 100% of ss form. The structural transition in cross section was also confirmed in four partially skinned fibers, where patterns went from mostly ss form (intact portion) to mostly bw form (skinned portion) at the SL between 2.40 to 3.20 microns. The reversibility of the change between ss and bw was proved by using low- and high-osmotic medium. The transition and reversion of cross-sectional patterns both occur in the passive state.