In myocardium of mammals there are two isoforms of myosin heavy chains, alpha and beta. In ventricle, together with ventricular isoforms of light chains they form two isomyosins: V1 and V3, homodimers of alpha- and beta-heavy chains, respectively. In atria, alpha- and beta-heavy chains together with atrial light chains form A1 (alpha alpha) and A2 (beta beta) isomyosins. Besides in myocardium two isoforms of alpha-actin, skeletal and cardiac, are expressed. We assume that the differences in the amino acid sequence of cardiac and skeletal actin may affect its interaction with myosin. To test this hypothesis, we investigated characteristics of actin-myosin interactions of cardiac and skeletal isoforms of alpha-actin with the isoforms of cardiac myosin using an optical trap technique and an in vitro motility assay. It was found that the mechanical and kinetic characteristics of the interactions of the isoforms of cardiac myosin with actin depend on the isoforms of myosin not alpha-actin. (C) 2016 Elsevier Inc. All rights reserved.