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1 Department of Cardiovascular Medicine, The University of Tokyo, Bunkyo-ku, Tokyo, Japan
2 Institute of Environmental Studies, The University of Tokyo, Bunkyo-ku, Tokyo, Japan
3 Department of Physiology, Tsurumi University, Yokohama, Kanagawa, Japan
4 Department of Cardiovascular Division, Toranomon Hospital, Minato-ku, Tokyo, Japan
5 Neuroscience Research Institute, National Institute of Advanced Industrial Sciences and Technology, Tsukuba, Ibaragi, Japan; Recognition and Formation, Precursory Research for Embryonic Science and Technology, Tsukuba, Ibaragi, Japan
6 Department of Physiology, Teikyo University, Itabashi-ku, Tokyo, Japan
* To whom correspondence should be addressed. E-mail: sugiura{at}k.u-tokyo.ac.jp.
It is of paramount importance to investigate the relation between the time-dependent change in [Ca2+]i (Ca2+ transients), and the mechanical activity of isolated single myocytes in order to understand the regulatory mechanisms of heart function. However, because of technical difficulties in performing mechanical measurements with single myocytes, the simultaneous recording of Ca2+ transients and mechanical activity has mainly been performed using multicellular cardiac preparations that give conflicting results concerning the Ca2+ transients during isometric twitches and during twitches with unloaded shortening. In the present study, we coupled the intracellular Ca2+ measurement optics with the force measurement system using carbon fibers to examine the relation between Ca2+ transients and the mechanical activity of rat single ventricular myocytes over a wide range of load. To minimize the possible load-dependence of sarcoplasmic reticulum Ca2+ loading, contraction mode was switched at every twitch from unloaded shortening to isometric contraction. During a twitch with unloaded shortening, the Ca2+ transients exhibit a higher peak and a higher rate of decay than in the transients during an isometric twitch. Similarly, when we changed the contraction mode in every pair of twitches, Ca2+ transients were dependent only on the mode of contraction. Mechanical uncoupling with 2,3-butanedione monoxime abolished this dependence on the mode of contraction. Our results suggest that the Ca2+ transients reflect the affinity of troponin-C for Ca2+, which is influenced by the change in strain on the thin filament but not by the length change per se.
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