Spatial Distribution of Actin and Mechanical Cycle of Myosin are Different in Right and Left Ventricles of Healthy Mouse Hearts

The contraction of the right ventricle (RV) expels blood into the pulmonary circulation, and the contraction of the left ventricle (LV) pumps blood into the systemic circulation through the aorta. The respective afterloads imposed on the LV and RV by aortic and pulmonary artery pressures create very different mechanical requirements for the two ventricles. In spite of these functional differences, it is commonly believed that the right and left ventricular muscles are identical because there were no differences in stress development, twitch duration, work performance and power among the RV and LV. This report shows that the two ventricles in rigor differ in the degree of orientational disorder of actin within thin filaments, and during contraction they differ in the kinetics of the cross-bridge cycle. Mouse ventricle muscle is the source of sample for experiments in this project. Glycerinated muscle bundles were homogenized and myofibrils were extracted. Myofibrils were labeled with 1 nM rhodamine-phalloidin (RP) + 10 nM unlabeled-phalloidin (UP) in Ca2+-rigor solution in the ratio of 1:1000 fluorescent to non-fluorescent phalloidin to ensure 1 in ~105actin monomers carry a fluorophore. Labeled myofibrils were analyzed for error of the mean of polarized fluorescence to determine kinetic rate constants in the ATPase cycle and distribution of orientations emanating from myosin cross-bridges. Histograms were plotted from the polarized fluorescence data and the Full Width at Half Maximum (FWHM) of the mean was calculated. The mean polarization of a contracting WT LV myofibril power stroke 0.159±0.086 was higher as compared to 0.085±0.035 for RV. Similarly, dissociation of myosin from actin was significantly faster in LV compared to RV. The FWHM of actins of RVs were significantly narrower (better ordered) than those of LVs which shows that the LV and RV of the heart are different. The study suggest that the differences in the rate constants during contraction and orientation of cross bridges during rigor signify the functional differences between left and right ventricles of the healthy mouse heart.