Behaviour of myosin projections during the staircase phenomenon of heart muscle.

Abstract

PEAK tension of an excised heart muscle increases with the frequency of contraction1. This ‘staircase’ phenomenon has been attributed to changes in the distribution of Ca ions; it has been proposed that the sarcoplasmic Ca2+ concentration increases with the frequency of contraction2–4. The increase in the sarcoplasmic Ca2+ is supposed to raise the peak tension by changing the behaviour of contractile proteins; however, no such molecular change has yet been clearly demonstrated. In the present study, we have investigated the behaviour of myosin projections during the staircase phenomenon by the X-ray diffraction method, in an attempt to clarify the underlying molecular events. The results have indicated that an increase in the contraction frequency causes an increase in the number of myosin projections transferred to the vicinity of the thin filaments during contraction.