Radial Motion of Myosin Heads in Isolated Intact Rat Myocardium in Diastole

The main cellular mechanism that underlies the so-called “Frank -Starling Law of the Heart” is an increase in the responsiveness of cardiac myofilaments to activating Ca2+ ions at longer sarcomere lengths (SL). The fundamental mechanism responsible for this increase in responsiveness has been elusive, despite considerable experimental scrutiny. Here we tested the hypothesis that the increase in calcium sensitivity upon increasing SL is correlated with a radially outward movement of the myosin heads during diastole. 2D x-ray diffraction patterns were obtained from electrically stimulated intact, twitching papillary muscle isolated from rat hearts during a 10 ms time window in diastole just prior to electrical stimulation. A range of sarcomere lengths was compared either at Lmax (SL= ∼2.3 µm) or following a quick release to slack length (SL=∼1.9µm). The relative position of myosin heads was first assessed by the I11/I10 equatorial intensity ratio. To our surprise, I 11/I10 was negatively correlated with SL, i.e. I11/I10 was less at Lmax vs. slack length. A more direct measure of the radial position of the myosin heads can be estimated from the position of the first maxima on the unsampled myosin layer lines, which are prominent in diastole. The intensity maxima, when examined pair-wise, moved outwards to a maximum of 5-6% for a 0.4µm change in SL indicating that the heads must be moving radially outward at slack length. Our data suggest that myofilament length dependent activation does not derive from a radial extension of the myosin heads at the long SL and must, therefore, involve some other mechanism. Supported by NIH HL75494 and RR08630.