Transverse stiffness of myofibrils of skeletal and cardiac muscles studied by atomic force microscopy

Abstract

The transverse stiffness of single myofibrils of skeletal and cardiac muscles was examined by applying an atomic force microscope (AFM). The contribution of various sarcomere components to the transverse stiffness of myofibrils was estimated by treating myofibrils with proteolytic enzymes and examining resulting changes in the transverse stiffness of the myofibrils. Single myofibrils were prepared by homogenizing glycerinated muscle fibers of the left ventricle of rat heart and the psoas muscle of rabbit. The transverse stiffness of myofibrils was determined as previously based on indentation of myofibrils by pressing a cantilever of AFM to the surface of myofibrils. The overall transverse stiffness of cardiac myofibrils was 3-4 times greater than that of skeletal myofibrils. The calpain treatments selectively digested α-actinin, and clarified that Z-bands of cardiac myofibrils were 3-4 times stiffer than those in skeletal myofibrils. The trypsin treatments selectively digested connectin/titin, and suggested that the connectin/titin filaments contribute to the transverse stiffness more in cardiac myofibrils than in skeletal myofibrils. The results obtained suggest that the transverse stiffness difference between skeletal and cardiac myofibrils is due to the difference in the content and the arrangement of specific protein components in their sarcomere structures. [J Physiol Sci. 2007;57 Suppl:S35]