The exchange of actin-bound ADP in muscular activity

Abstract

Szent-Gyorgyi and Prior (1966) provided evidence for the exchange of actin-bound ADP during superprecipitation of actomyosin and myofibrils. Tentative experimental support was given by Cheesman and Whitehead (1968) to the suggestion that the exchange also occurred in vivo as a result of muscular contraction. Further experiments with improved techniques have confirmed these findings. Injections of C-glucose subcutaneously into frogs leads to a rapid incorporation of label into the total ATP by synthesis de novo. The total ATP therefore assumes a high specific activity and, by an exchange of nucleotide, the bound ADP associated with actin also becomes labelled. In phasic muscles of active frogs the specific activity of the bound ADP soon exceeds that of the total ATP, which suggests that the myofibrils are in contact with a highly labelled nucleotide pool. The labelling of bound ADP in tonic muscles is much slower.Curarisation, and therefore muscular paralysis, before the injection of C-glucose produces an inhibition of uptake of radio active isotope, which provides the first support for the view that the exchange of nucleotide is attendant upon contraction.Stimulation, either by depolarisation with KC1 or by direct electrical excitation, brings the bound nucleotide into exchange with the pool. Since an exchange of nucleotide seems only to occur in vitro as a result of depolymerisation or a change in the conformation of actin, these results provide strong evidence for a similar change in the physical state of this protein during muscular activity.