Evidence for the Presence of AM-ADP Myosin Heads in Rigor Muscle Fibers: Its Implication of the State of Myosin Heads after the End of Powerstroke

Abstract

It is generally believed that, during muscle contraction, myosin heads (M) extending from myosin filaments bind with actin filaments (A) and perform powerstrokes associated with reaction, AM-ADP-Pi →AM+Pi + ADP. The presence of rigor or rigor-like myosin heads AM can not, however, be detected by X-ray diffraction studies on contracting muscle. To give information about the state of myosin heads after the end of powerstroke, we studied changes in the state of myosin heads when single Ca-activated skinned muscle fibers were transferred into high-Ca rigor solution (pCa,4), by applying quick releases (0.5-1% of Lo, complete in 1-2ms followed by restretch) at various times after the transfer of the fiber into high-Ca rigor solution. Unexpectedly, the fibers exhibited distinct tension recovery Pr following quick release. The value of Pr relative to the maximum Ca-activated isometric tension Po (Pr/Po) was about 0.4 at 10s after the transfer of the fiber into rigor solution, and decreased with time to completely disappear in 10-20min. In the presence of 5-10mM EDTA, chelating Mg, the amplitude of Pr was markedly reduced, and disappeared in 5-10min. After disappearance of Pr, the fiber only showed tension drop coincident with quick release. These results suggest that AM-ADP myosin heads are responsible for the tension recovery foolowing quick release, If, on the other hand, the ionic strength of rigor solution was reduced from 170 to 50mM by totally removing KCl, the amplitude of Pr was increased appreciably, and did not disappear for 20-30min. Concerning the cyclic actin-myosin interaction, it seems possible that, after the end of powerstroke, myosin heads take the form of AM-ADP having a long average lifetime.