A factor modifying the shortening response of muscle fibre bundles to ATP

Abstract

There is present in muscle a factor discovered by Marsh (1951) which modifies its response to addition of ATP. Addition of ATP to a suspension of muscle fibres in which glycolysis is just complete brings about an increase in fibre volume, as measured by centrifugation at about 3000 rev/min. This increase is followed, as ATP is enzymatically removed, by a decrease to about the original volume, and this effect can be repeated apparently indefinitely by fresh additions of ATM Marsh showed that this increase of fibre volume in the presence of ATP occurs only with fresh muscle, or muscle which has just passed into rigor mortis. Addition of ATP to a brei made from muscle removed from the animal 24 to 30 h previously, or to a brei which has been washed free of substances soluble in 0·16M-KCl, invariably results in an immediate and irreversible synaeresis, similar to that occurring in the actomyosin preparations of Szent-Györgyi. However, with fresh preparations, readdition of the washings to the washed fibres, followed by ATP addition, gives rise once more to the increase in fibre volume. The factor present in the washings is heat- and acid-labile, cannot be dialyzed without loss of activity and cannot, therefore, be separated by classical methods such as ammonium sulphate fractionation which involves dialysis. I have now shown that this factor probably consists of a non-dialyzable protein; and a fairly large heat-labile dialyzable component of molecular weight about 10000. So far, further identification has proved impossible. The factor will not operate below an ionic strength of 0·05, and is strongly inhibited by calcium ions at a molarity of 0·005 or above.