EFFECTS OF PRESSURE ON EQUATORIAL X-RAY FIBER DIFFRACTION FROM SKELETAL-MUSCLE FIBERS

When skeletal muscle fibers are subjected to a hydrostatic pressure of 10 MPa (1 00 atmospheres), reversible changes in tension occur. Passi ve tension from relaxed muscle is unaffected, rigor tension rises, and active tension falls. The effects of pressure on muscle structure are unknown: therefore a pressure-resistant cell for x-ray diffraction ha s been built, and this paper reports the first study of the low-angle equatorial patterns of pressurized relaxed, rigor, and active muscle f ibers, with direct comparisons from the same chemically skinned rabbit psoas muscle fibers at 0.1 and 10 MPa. Relaxed and rigor fibers show little change in the intensity of the equatorial reflections when pres surized to 10 MPa, but there is a small, reversible expansion of the l attice of 0.7 and 0.4%, respectively. This shows that the order and st ability of the myofilament lattice is undisturbed by this pressure. Th e rise in rigor tension under pressure is thus probably due to axial s hortening of one or more components of the sarcomere. Initial results from active fibers at 0.1 MPa show that when phosphate is added the la ttice spacing and equatorial intensities change toward their relaxed v alues. This indicates cross-bridge detachment, as expected from the re duction in tension that phosphate induces. 10 MPa in the presence of p hosphate at ll-degrees-C causes tension to fall by a further 12%, but no change is detected in the relative intensity of the reflections, on ly a small increase in lattice spacing. Thus pressure appears to incre ase the proportion of attached cross-bridges in a low-force state.