Mechanical properties of demembranated muscle fibres in the presence of MgAMPPNP.

In view of the current disagreement about the degree of tension maintenance in rabbit fibres in AMPPNP, I have restudied the mechanics of glycerol-extracted rabbit psoas and Lethocerus flight muscle in rigor and in MgAMPPNP. Insect fibres elongated reversibly when AMPPNP was added; the effect required little nucleotide. Rabbit fibres showed only a relatively small reversible elongation on adding AMPPNP; my experimental evidence for mechanochemical equilibrium is therefore only certain for the insect muscle. At low muscle tension extension of either muscle type produced little tension decay; the stress-relaxation seen ('visco-elasticity') followed a power law of low order. At greater tension both muscles showed a second form of tension decay ('yield') which dominated at high tension, causing within a few minutes a near-total loss of added tension due to stretch. Below this tension the isometric tension decay was relatively slow, and considerable tension remained overnight. Yield dominated at a much lower tension in AMPPNP than in rigor, but relative to this changed level the speed of the yield process did not appear to alter greatly; again below a certain level tension decay became very slow. The effect of AMPPNP on yield was reversible and appeared similar in the two tissues. No obvious change occurred in any of these properties when the AMPPNP was purified before use or when a myokinase inhibitor and ADP-removing enzyme system were added. Thus in both preparations tension is maintained for a time long relative to the expected lifetime of an individual crossbridge, and its decay during yield is not accelerated by shortening that lifetime. These phenomena indicate that some process other than crossbridge detachment is limiting crossbridge slip; I am seeking an explanation in terms of interaction between neighbouring crossbridges.