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X-ray Diffraction Studies of the Thick Filament in Permeabilized Myocardium from Rabbit

Sengen Xu ^*, Donald Martyn , Jessica Zaman ^* and Leepo C. Yu ^*

^* National Institute of Arthritis, Musculoskeletal and Skin Diseases, National Institutes of Health, Department of Health and Human Services, Bethesda, Maryland; and Department of Bioengineering, University of Washington, Seattle, Washington

Correspondence: Address reprint requests to Leepo C. Yu, E-mail: yule{at}mail.nih.gov.

Low angle x-ray diffraction patterns from relaxed permeabilized rabbit cardiac trabeculae and psoas muscle fibers were compared. Temperature was varied from 25°C to 5°C at 200 mM and 50 mM ionic strengths (µ), respectively. Effects of temperature and µ on the intensities of the myosin layer lines (MLL), the equatorial intensity ratio I_1,1/I_1,0, and the spacing of the filament lattice are similar in both muscles. At 25°C, particularly at µ = 50 mM, the x-ray patterns exhibited up to six orders of MLL and sharp meridional reflections, signifying that myosin heads (cross-bridges) are distributed in a well-ordered helical array. Decreasing temperature reduced MLL intensities but increased I_1,1/I_1,0. Decreases in the MLL intensities indicate increasing disorder in the distribution of cross-bridges on the thick filaments surface. In the skeletal muscle, order/disorder is directly correlated with the hydrolysis equilibrium of ATP by myosin, [M.ADP.P_i]/[M.ATP]. Similar effects of temperature on MLL and similar biochemical ATP hydrolysis pathway found in both types of muscles suggest that the order/disorder states of cardiac cross-bridges may well be correlated with the same biochemical and structural states. This implies that in relaxed cardiac muscle under physiological conditions, the unattached cross-bridges are largely in the M.ADP.P_i state and with the lowering of the temperature, the equilibrium is increasingly in favor of [M.ATP] and [A.M.ATP]. There appear to be some differences in the diffraction patterns from the two muscles, however. Mainly, in the cardiac muscle, the MLL are weaker, the I_1,1/I_1,0 ratio tends to be higher, and the lattice spacing D₁₀, larger. These differences are consistent with the idea that under a wide range of conditions, a greater fraction of cross-bridges is weakly bound to actin in the myocardium.

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