放射光X線回折法のラット拍動心臓への応用による心筋収縮タンパク質分子動態の解析
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Synchrotron radiation has been employed to analyze cross-bridge dynamics chiefly in isolated papillary muscle using x-ray diffraction techniques. Here we showed that these techniques can detect regional changes in rat left ventricle (LV) contractility and myosin lattice spacing in in situ ejecting hearts in real time. Further, we examined the sensitivity of these indices to changes in intracellular Ca2+ concentration with dobutamine or ryanodine, regional ischemia, and muscular hypertrophy with hypertension. The LV free wall of spontaneously beating rat hearts were directly exposed to brief high-flux, low-emittance x-ray beams provided at SPring-8. Myosin mass transfer to actin filaments (the number of cross-bridges) was determined as the decrease in reflection intensity ratio (intensity of 1,0 plane over the 1,1 plane) between end-diastole and end-systole. The distance between 1,0 reflections was converted to a lattice spacing between myosin filaments. We found that LV end-systolic pressure changed in proportion to mass transfer (cross-bridge formation) in response to changes in Ca2+ transients. Left coronary occlusion eliminated increases in myosin lattice spacing and severely reduced mass transfer in the ischemic region. In the hypertrophic LV, the periodic changes in reflection intensity ratio corresponding with cardiac cycle were maintained at the posterior wall regions, but not at the anterior wall regions. Our results suggest that x-ray diffraction techniques permit real time in situ analysis of regional cross-bridge dynamics at the molecular and fiber levels that might also facilitate investigations of ventricular output regulation by the Frank-Starling mechanism.