Myocardium of mammals contains a wide range of isoforms of proteins that provides contractile function of the heart. These are two isoforms of ventricular and two of atrial myosin, alpha- and beta-tropomyosin, and two isoforms of alpha-actin: cardiac and skeletal. We believe that the difference in the amino acid sequence of alpha-actin can affect the calcium regulation of the actin-myosin interaction. To test this hypothesis, we investigated effects of the isoforms of alpha-actin, cardiac and skeletal, and the isoforms of cardiac myosin on the calcium regulation of the actin-myosin interaction in an in vitro motility assay using reconstructed regulated thin filaments. The results show that isoforms of alpha-actin and the ratio of alpha/beta-chains of Tpm differently affect the calcium regulation of the actin-myosin interaction in myocardium in dependence on cardiac myosin isoforms. (C) 2017 Elsevier Inc. All rights reserved.