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Unloaded shortening increases peak of Ca²⁺ transients but accelerates their decay in rat single cardiac myocytes

¹Department of Cardiovascular Medicine, Graduate School of Medicine, University of Tokyo, Tokyo 113-8655; ²Institute of Environmental Studies, Graduate School of Frontier Sciences, University of Tokyo, Tokyo 113-0033; ³Department of Physiology, Dental School, Tsurumi University, Yokohama 230-0063; ⁴Cardiovascular Division, Toranomon Hospital, Tokyo 105-0001; ⁵Neuroscience Research Institute, National Institute of Advanced Industrial Science and Technology, and ⁶Recognition and Formation, Precursory Research for Embryonic Science and Technology, Tsukuba 305-8568; and ⁷Department of Physiology, Teikyo University, Tokyo 173-8605, Japan

Submitted 7 January 2003 ; accepted in final form 17 April 2003

It is of paramount importance to investigate the relation between the time-dependent change in intracellular Ca²⁺ concentration ([Ca²⁺]_i) (Ca²⁺ transients) and the mechanical activity of isolated single myocytes 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 with multicellular cardiac preparations that give conflicting results concerning Ca²⁺ transients during isometric twitches and during twitches with unloaded shortening. In the present study, we coupled intracellular Ca²⁺ measurement optics with a 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 exhibited a higher peak and a higher rate of decay than 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 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.

calcium ion transients; load dependence; carbon fibers

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