Hiroko Toyota

X-ray Diffraction from a Left Ventricular Wall of Rat Heart

We studied x-ray diffraction from the left ventricular wall of an excised, perfused whole heart of a rat using x rays from the third-generation synchrotron radiation facility, SPring-8. With the beam at right angles to the long axis of the left ventricle, well-oriented, strong equatorial reflections were observed from the epicardium surface. The reflections became vertically split arcs when the beam passed through myocardium deeper in the wall, and rings were observed when the beam passed into the inner myocardium of the wall. These diffraction patterns were explained by employing a layered-spiral model of the arrangement of muscle fibers in the heart. In a quiescent heart with an expanded left ventricle, the muscle fibers at the epicardium surface were found to have a (1,0) lattice spacing smaller than in the rest of the wall. The intensity ratio of the (1,0) and (1,1) equatorial reflections decreased on contraction with a similar time course in all parts of the wall. The results show that it is possible to assign the origin of reflections in a diffraction diagram from a whole heart. This study offers a basis for interpretation of x-ray diffraction from a beating heart under physiologically and pathologically different conditions.

An X-ray diffraction study on contraction of rat papillary muscle with different afterloads.

In the crossbridge theory of muscle contraction (Huxley, 1957), muscle shortening is caused by crossbridges rapidly attaching and detaching from actin. At a given moment the number of crossbridges formed during shortening is smaller than in isometric contraction because they cannot keep attached to allow filament sliding. In skeletal muscles, this prediction has been tested by the x-ray diffraction technique (Podolsky et al., 1976; Huxley, 1979; Amemiya et al., 1980; Yagi & Takemori, 1995). Generally, the intensities of the equatorial (1,0) and (1,1) reflections, which are related to the number of myosin heads in the vicinity of the thin filament (Haselgrove & Huxley, 1973) are affected only by shortening with a small load (0–30%). This observation has been explained by the presence of same myosin heads that are weakly attached to actin during shortening (Yagi & Takemori, 1995). They are not actively producing force but remain in the vicinity of the thin filament, making the equatorial intensities close to those during isometric contraction.

Ramp-Rate Dependent External Work During Ramp-Load Release in Cardiac Muscle

When the load of frog cardiac muscle was decreased linearly from the maximum force to the resting force (ramp-load release), the muscle shortened with various velocities calculated from the instantaneous lengths, which showed the instantaneous force-velocity relationship. These velocities increased with the increase of the ramp rate. To study these dependency of ramp rate, the parameters obtained from Huxley model in 1957 during quick load-release were fixed throughout the calculations, but even if the shortening of the series elastic component (SEC) was taken into account, the model did not seem to explain these velocities. If the ramp rate was reduced, velocities were more decreased as expected, especially at smaller load, which suggested that the cross-bridges should bear some extra load. Thus the internal load was added in calculation. The velocities in the modified model well explained those measured even the ramp rate varied widely, especially in higher force. At lower force, the calculated velocities increased rapidly than those measured, mainly due to the shortening of SEC which were calculated from the initial shortening of quick release. On the force-sarcomere length plane, the calculated sum-force (internal-plus external-load) was almost linearly decreased versus length with the initial slope of 2.66 (mean). Adding the original dependency (= 1), the value of 3.66 was similar to 3.5 in skeletal muscle. The external work during the ramp-load release was also calculated and decreased with the increase of ramp rate, and showed the peak or plateau, which suggested the existence of optimal work.