Single cell mechanics of rat cardiomyocytes under isometric, unloaded, and physiologically loaded conditions

1 Department of Cardiovascular Medicine, Graduate School of Medicine, and 2 Biomechanics Division, Institute of Environmental Studies, Graduate School of Frontier Sciences, University of Tokyo, Tokyo 113-0033; 3 Department of Physiology, School of Dental Medicine, Tsurumi University, Kanagawa 230-5801; and 4 Department of Cardiovascular Dynamics, National Cardiovascular Center Research Institute, Osaka 565-8565, Japan Submitted 10 October 2003 ; accepted in final form 2 March 2004 One of the most salient characteristics of the heart is its ability to adjust work output to external load. To examine whether a single cardiomyocyte preparation retains this property, we measured the contractile function of a single rat cardiomyocyte under a wide range of loading conditions using a force-length measurement system implemented with adaptive control. A pair of carbon fibers was used to clamp the cardiomyocyte, attached to each end under a microscope. One fiber was stiff, serving as a mechanical anchor, while the bending motion of the compliant fiber was monitored for force-length measurement. Furthermore, by controlling the position of the compliant fiber using a piezoelectric translator based on adaptive control, we could change load dynamically during contractions. Under unloaded conditions, maximal shortening velocity was 106 ± 8.9 µm/s ( n = 13 cells), and, under isometric conditions, peak developed force reached 5,720 nN (41.6 ± 5.6 mN/mm 2 ; n = 17 cells). When we simulated physiological working conditions consisting of an isometric contraction, followed by shortening and relaxation, the average work output was 828 ± 123 J/m 3 ( n = 20 cells). The top left corners of tension-length loops obtained under all of these conditions approximate a line, analogous to the end-systolic pressure-volume relation of the ventricle. All of the functional characteristics described were analogous to those established by studies using papillary muscle or trabeculae preparations. In conclusion, the present results confirmed the fact that each myocyte forms the functional basis for ventricular function and that single cell mechanics can be a link between subcellular events and ventricular mechanics. cardiomyocyte; mechanics; carbon fiber Address for reprint requests and other correspondence: S. Sugiura, Biomechanics Div., Institute of Environmental Studies, Graduate School of Frontier Sciences, Univ. of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan (E-mail: sugiura{at}k.u-tokyo.ac.jp )