Transmural heterogeneity of crossbridge dynamics and lattice spacing in isolated rat hearts

Abstract

To analyze the fundamental mechanism of the Frank-Starling's law, we studied the left ventricular (LV) transmural difference in the time course of crossbridge dynamics (CBD) and the end-diastolic myosin filament lattice spacing (EML) in the isolated crystalloid-perfused isovolumically contracting rat hearts (n=11) paced at 2 Hz using X-ray diffraction at SPring-8. We recorded x-ray diffraction patterns of the epicardial (EPI) and deeper (DEEP) myocardium region of the LV free wall along with LV pressure (LVP) at the end-diastolic pressure of either 0 or 20 mmHg by adjusting LV volume. We analyzed transmural CBD from the X-ray diffraction patterns according to the transmural variations of regional myofilament orientation. The developments of CBD of both EPI and DEEP and normalized LVP were synchronous during contraction, but not during relaxation. The CBD decay was significantly faster in DEEP than EPI. The LVV increase significantly potentiated LVP development and more prolonged the CBD decay in EPI than DEEP associated with more reduction of EML in EPI than DEEP. From studies on CBD and EML, we were able to propose a possible underlying mechanism for the Frank-Starling's law in the whole heart. [J Physiol Sci. 2006;56 Suppl:S57]