Abstract
Background: Heart rate (HR) is one of the determinant factors of cardiac performance. However, there is no direct evidence for the effect of heart rate on the actin-myosin interaction (AMI). Purpose: To test the effect of HR on AMI in hearts with X-ray diffraction (XRD) analysis using SPring-8. Methods: Seven isolated isovolumically contracting rat hearts were mounted so that the X-ray beam (15.0 keV) passed the subepicardial region. We recorded XRD images and LV pressure at HRs of 120 and 300bpm under 0 mmHg end-diastolic pressure. Between two different HRs, we compared 1) the amount and duration of AMI from the intensity ratio reduction of inner (1,0) and outer (1,1) XRD, and 2) myosin-filament lattice spacing (MLS) from the position of inner XRD. Results: Increasing HR from 120 to 300bpm significantly decreased the intensity ratio reduction at the end-systole (1.05±0.15 vs. 1.70±0.31 unitless, p<0.01) and shortened duration of AMI (128±12 vs. 91±18ms, p<0.01), suggesting a smaller amount of AMI, but did not change MLS (35.4±0.5 vs. 35.3±0.5nm, NS). Conclusion: Increasing HR reduces the AMI without causing incomplete relaxation, indicating intact intracellular Ca2+handling. These results may derive from shortening of Ca2+-myofilament interaction. [J Physiol Sci. 2007;57 Suppl:S82]
