Slip of rabbit striated muscle in rigor or AMPPNP

Single glycerol-extracted rabbit psoas muscle fibres have been slowly extended either in rigor or in the unhydrolysable ATP analogue, AMPPNP, and their sarcomere length, sarcomere structure and tension measured. The length of regularly arrayed sarcomeres, measured by optical diffraction, increased continuously as the muscle was stretched; the maximum sarcomere extension seen was approximately 6%. In the electron microscope sarcomeres from extended muscle fixed in rigor or AMPPNP remained regular in their internal structure, without rupture or obvious lengthening around the Z line. During steady extension at 0.024% per min the tension in the muscle fibre rose until it reached a limiting value [Tm] when the sarcomeres had stretched by 0.8-1.6% and then remained constant with continued extension, while the sarcomeres continued to stretch. Provided that a novel form of preparation of the glycerol-extracted fibres was employed, Tm in rigor was a large fraction of the tension expected from an active isometric muscle fibre. In the presence of AMPPNP Tm was reduced by a factor of 2 to 3. Step extension by 0.08% at 5-min intervals gave the same pattern of mechanical response with similar values of Tm. The isometric tension decay in the interval between the steps was very rapid at first and slowed continuously until the next step. The average speed of tension fall between 30 and 300 s after stretch was measured at each step and plotted relative to the tension in the muscle. The relationship approached linearity, although with a significant upward curvature at high tension.(ABSTRACT TRUNCATED AT 250 WORDS)