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This Article Full Text Full Text (PDF) All Versions of this Article: 291/4/H1847    most recent 01237.2005v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (7) Citing Articles via Google Scholar Google Scholar Articles by Farman, G. P. Articles by Irving, T. C. Search for Related Content PubMed PubMed Citation Articles by Farman, G. P. Articles by Irving, T. C.

Impact of osmotic compression on sarcomere structure and myofilament calcium sensitivity of isolated rat myocardium

¹Department of Biological, Chemical and Physical Sciences, Illinois Institute of Technology, and ²Center for Cardiovascular Research, Department of Physiology and Biophysics, University of Illinois at Chicago, College of Medicine, Chicago, Illinois

Submitted 21 November 2005 ; accepted in final form 18 May 2006

Changes in interfilament lattice spacing have been proposed as the mechanism underlying myofilament length-dependent activation. Much of the evidence to support this theory has come from experiments in which high-molecular-weight compounds, such as dextran, were used to osmotically shrink the myofilament lattice. However, whether interfilament spacing directly affects myofilament calcium sensitivity (EC₅₀) has not been established. In this study, skinned isolated rat myocardium was osmotically compressed over a wide range (Dextran T500; 0–6%), and EC₅₀ was correlated to both interfilament spacing and I_1,1/I_1,0 intensity ratio. The latter two parameters were determined by X-ray diffraction in a separate group of skinned muscles. Osmotic compression induced a marked reduction in myofilament lattice spacing, concomitant with increases in both EC₅₀ and I_1,1/I_1,0 intensity ratio. However, interfilament spacing was not well correlated with EC₅₀ (r² = 0.78). A much better and deterministic relationship was observed between EC₅₀ and the I_1,1/I_1,0 intensity ratio (r² = 0.99), albeit with a marked discontinuity at low levels of dextran compression; that is, a small amount of external osmotic compression (0.38 kPa, corresponding to 1% Dextran T500) produced a stepwise increase in the I_1,1/I_1,0 ratio concomitant with a stepwise decrease in EC₅₀. These parameters then remained stable over a wide range of further applied osmotic compression (up to 6% dextran). These findings provide support for a "switch-like" activation mechanism within the cardiac sarcomere that is highly sensitive to changes in external osmotic pressure.

skinned muscle; trabeculae; X-ray diffraction; dextran; osmotic pressure; myofilament length-dependent activation; regulation

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