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Titin isoform-dependent effect of calcium on passive myocardial tension

¹Department of Veterinary and Comparative Anatomy, Pharmacology, and Physiology, Washington State University, Pullman, Washington 99164; ²Institut für Anästhesiologie und Operative Intensivmedizin, Universitätsklinikum Mannheim, Mannheim 68167, and ³Max-Delbrueck Center for Molecular Medicine, Berlin 13092, Germany

Submitted 8 June 2004 ; accepted in final form 28 July 2004

We studied the effects of Ca²⁺ on titin (connectin)-based passive tension in skinned myocardium expressing either predominantly N2B titin (rat right ventricle, RRV) or predominantly N2BA titin (bovine left atrium, BLA). Actomyosin-based tension was abolished to undetectably low levels by selectively removing the thin filaments with a Ca²⁺-insensitive gelsolin fragment (FX-45). Myocardium was stretched in the presence and absence of Ca²⁺, and passive tension was measured. Ca²⁺ significantly increased passive tension during and after stretch in the BLA. The increase was insensitive to the actomyosin inhibitor 2,3-butanedione 2-monoxime, supporting the conclusion that the effect is titin based. Passive tension did not respond to calcium in the RRV, indicating that passive tension developed by N2B titin is calcium insensitive. Western blot analysis and immunofluorescence studies indicated that N2BA titin expresses E-rich PEVK motifs, whereas they are absent from N2B titin, supporting earlier single molecule studies that reported that E-rich motifs are required for calcium sensitivity. We conclude that calcium affects passive myocardial tension in a titin isoform-dependent manner.

connectin; diastole; passive stiffness; proline-glutamate-valine-lysine residue

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