Regulation Of Myofibrillar Activity By Calcium

Actomyosin systems generally require Ca for contraction; without Ca, synaeresis and ATPase activity are inhibited, actomyosin gels are dissolved, and synaeresis reversed by ATP. Relevant to the nature of the Ca-actomyosin interaction are: (1) a steep rise of ATPase activity over 1 pCa unit, (2) synaeresis after removal of most bound Ca with very low ATP, and (3) the existence of Ca-insensitive actomyosin. Sarcoplasmic reticulum from rabbit skeletal muscle (which retains 0$\cdot $1 $\mu $mole Ca/(mg protein) at a pCa of 8$\cdot $7 in the absence of oxalate) causes relaxation in correlation with the extent to which it removed Ca from myofibrils. The degree of relaxation caused by cardiac and skeletal reticulum is also correlated with the reduction of the Ca$^{2+}$ level in the medium. However, not infrequently, myofibrillar activity is inhibited at lower Ca levels in the absence than in the presence of reticulum possibly because an unknown agent which may be identical with the soluble relaxing factor decreases the Ca-affinity of the myofibrils. A second fraction, presumably mitochondria, also causes relaxation by lowering the Ca level. The requirement of a Ca-release mechanism for the various Ca-accumulating systems is discussed in relation to general muscular activity and it is suggested that muscular fatigue may result from a redistribution of cellular Ca.