The state of cardiac contractile proteins during the diastolic phase

The molecular events underlying contraction and relaxation of heart muscle were studied by the X-ray diffraction method. In quiescent heart muscle most myosin heads were in the vicinity of the thick filaments. When heart muscle contracted, myosin heads moved to the vicinity of the thin filaments to react with actin. On relaxation of muscle, myosin heads returned to the thick filaments. However, in cyclically contracting heart muscle a significant fraction of myosin heads remained in the vicinity of the thin filaments until the end of the diastolic phase. Therefore, the molecular state during the diastolic phase was considerably different from that in the quiescent state. Paired-pulse stimulation, which enhanced the tension development, increased the number of myosin heads near the thin filaments not only during the systolic phase but also during the diastolic phase. Thus the diastolic molecular state was modified by inotropic intervention. These findings suggest that the diastolic phase should be regarded as a dynamic phase rather than a static phase.