Rapamycin Treatment Promotes Myofibril Relaxation Kinetics and Reduces Myocardial Stiffness to Improve Diastolic Function in Old Murine Hearts
Authors: Chiao Ying Ann , Moussavi‐Harami Farid , Cheng Yuanhua , Powers Joe , Razumova Maria , Regnier Michael , Rabinovitch Peter S. 
Published: June 2021

At least half of heart failure patients have normal systolic function (heart failure with preserved ejection fraction, HFpEF) and there is no current approved therapy that improves mortality in this patient population. Aging is associated with a decline in diastolic function and is a very strong risk factor for HFpEF, which is especially prevalent in aged women. We previously showed that late‐life 10‐week rapamycin treatment can reverse established cardiac aging, including a reversal of diastolic dysfunction, in old mice. Rapamycin induces mitochondrial remodeling within first 2 weeks of treatment to rejuvenate energy metabolism in old hearts, but the time course of improved diastolic function is much slower. This indicates that additional undefined mechanisms are responsible for the reversal of diastolic dysfunction induced by rapamycin. The objective of this study is to determine the mechanisms of rapamycin‐induced reversal of diastolic dysfunction in old mice.Proteomic analysis revealed that 10‐week rapamycin treatment can partially rescue the age‐related changes in abundance of proteins in actin cytoskeleton and calcium handling pathways. The effects of rapamycin on protein expression of these key pathways regulating contraction and relaxation kinetics suggest that rapamycin may promote relaxation kinetics of the aged myocardium. To determine the effects of aging and rapamycin treatment on biomechanical properties of the myocardium, we measured contractile and relaxation properties of multicellular ventricular trabeculae preparations and isolated myofibrils. Analysis of the passive length‐tension relationship of demembranated trabeculae showed a substantial increase in the slope of the length‐tension curve with aging, indicating an age‐related increase in myocardial stiffness. This age‐related increase in myocardial stiffness is significantly reduced by rapamycin treatment. Trichrome staining revealed similar myocardial collagen contents in old control and rapamycin‐treated mice, suggesting the reduced stiffness is not due to reduced fibrosis. Old myocardial tissues also exhibit increased Ca2+ sensitivity, as denoted by a left‐shift of the isometric force‐pCa curve, which is partially restored by rapamycin treatment. In particular, the Ca2+ concentration required to elicit half maximum force (pCa50) decreased from pCa 5.59±0.08 for young controls to 5.75±0.14 for old controls, and rapamycin treatment restored it to 5.65±0.14. For myofibril relaxation kinetics, myofibrils from rapamycin treated mice displayed increased rate of the fast phase of relaxation (kREL, fast) compared to old control at both maximal (pCa 4.0) and submaximal (pCa 5.6) Ca2+ levels.In summary, these results suggest that rapamycin treatment alters the kinetics of myofibril relaxation and reverses the age‐related increase in myocardial stiffness and Ca2+ sensitivity. These biomechanical changes likely contribute to the reversal of diastolic dysfunction in old murine hearts.Support or Funding InformationNIH K99 AG051735 and Glenn/AFAR Postdoctoral Fellowship for Translational Research on Aging to Y.A.C., NIH K08HL128826 to F.M‐H., and NIH R01 AG038550 to P.S.R.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.