Citation: Eaton, D.M.; Martin, T.G.; Kasa, M.; Djalinac, N.; Ljubojevic-Holzer, S.; Von Lewinski, D.; Pöttler, M.; Kampaengsri, T.; Krumphuber, A.; Scharer, K.; et al. HDAC Inhibition Regulates Cardiac Function by Increasing Myofilament Calcium Sensitivity and Decreasing Diastolic Tension. Pharmaceutics 2022, 14, 1509. https://doi.org/10.3390/ pharmaceutics14071509 Academic Editors: Lucio Barile, Ionut Tudorancea and Radu Iliescu Received: 16 June 2022 Accepted: 19 July 2022 Published: 21 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). pharmaceutics Article HDAC Inhibition Regulates Cardiac Function by Increasing Myofilament Calcium Sensitivity and Decreasing Diastolic Tension Deborah M. Eaton 1,2,† , Thomas G. Martin 3,† , Michael Kasa 4 , Natasa Djalinac 4 , Senka Ljubojevic-Holzer 4 , Dirk Von Lewinski 4 , Maria Pöttler 4 , Theerachat Kampaengsri 3 , Andreas Krumphuber 4 , Katharina Scharer 4 , Heinrich Maechler 5 , Andreas Zirlik 4 , Timothy A. McKinsey 6,7 , Jonathan A. Kirk 3 , Steven R. Houser 1 , Peter P. Rainer 4,8 and Markus Wallner 1,4, * 1 Cardiovascular Research Center, Lewis Katz School of Medicine,Temple University, Philadelphia, PA 19140, USA; deborah.eaton@pennmedicine.upenn.edu (D.M.E.); srhouser@temple.edu (S.R.H.) 2 Penn Cardiovascular Institute, University of Pennsylvania Perelman School of Medicine, Philadelphia, PA 19104, USA 3 Department of Cell and Molecular Physiology, Loyola University Chicago Stritch School of Medicine, Chicago, IL 60153, USA; thomas.martin-2@colorado.edu (T.G.M.); june.thk39@gmail.com (T.K.); jkirk2@luc.edu (J.A.K.) 4 Division of Cardiology, Medical University of Graz, 8036 Graz, Austria; michael.kasa@gmx.at (M.K.); natasa.djalinac@medunigraz.at (N.D.); senka.ljubojevic@medunigraz.at (S.L.-H.); dirk.von-lewinski@medunigraz.at (D.V.L.); maria.poettler@medunigraz.at (M.P.); andreas.krumphuber@stud.medunigraz.at (A.K.); katharina.scharer@stud.medunigraz.at (K.S.); andreas.zirlik@medunigraz.at (A.Z.); peter.rainer@medunigraz.at (P.P.R.) 5 Department of Cardiothoracic Surgery, Medical University of Graz, 8036 Graz, Austria; heinrich.maechler@medunigraz.at 6 Division of Cardiology, Department of Medicine, University of Colorado Anschutz Medical Campus, Aurora, CO 80045, USA; timothy.mckinsey@cuanschutz.edu 7 Consortium for Fibrosis Research & Translation, University of Colorado Anschutz Medical Campus, Aurora, CO 80045, USA 8 BioTechMed Graz, 8010 Graz, Austria * Correspondence: markus.wallner@medunigraz.at † These authors contributed equally to this work. Abstract: We recently established a large animal model that recapitulates key clinical features of heart failure with preserved ejection fraction (HFpEF) and tested the effects of the pan-HDAC inhibitor suberoylanilide hydroxamic acid (SAHA). SAHA reversed and prevented the development of car- diopulmonary impairment. This study evaluated the effects of SAHA at the level of cardiomyocyte and contractile protein function to understand how it modulates cardiac function. Both isolated adult feline ventricular cardiomyocytes (AFVM) and left ventricle (LV) trabeculae isolated from non-failing donors were treated with SAHA or vehicle before recording functional data. Skinned myocytes were isolated from AFVM and human trabeculae to assess myofilament function. SAHA-treated AFVM had increased contractility and improved relaxation kinetics but no difference in peak calcium transients, with increased calcium sensitivity and decreased passive stiffness of myofilaments. Mass spectrometry analysis revealed increased acetylation of the myosin regulatory light chain with SAHA treatment. SAHA-treated human trabeculae had decreased diastolic tension and increased developed force. Myofilaments isolated from human trabeculae had increased calcium sensitivity and decreased passive stiffness. These findings suggest that SAHA has an important role in the direct control of cardiac function at the level of the cardiomyocyte and myofilament by increasing myofilament calcium sensitivity and reducing diastolic tension. Keywords: heart failure; contractility; calcium; cardiomyocyte; myofilament; HDAC inhibitor Pharmaceutics 2022, 14, 1509. https://doi.org/10.3390/pharmaceutics14071509 https://www.mdpi.com/journal/pharmaceutics