X-ray Diffraction Studies of the Thick Filament
In Permeabilized Myocardium from Rabbit
Sengen Xu 1, Don Martyn 2, Jessica Zaman 1 and Leepo C. Yu 1*
1 NIH
2 Univ Washington
* To whom correspondence should be addressed. E-mail: lcyu{at}helix.nih.gov.
Submitted on May 25, 2006
Revised on June 19, 2006
Accepted on 8 August 2006
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Abstract |
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Low angle X-ray diffraction patterns from relaxed permeabilized rabbit cardiac trabeculae and psoas muscle fibers were compared. Temperature was varied from 25° 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, the equatorial intensity ratio I1,1/I1,0 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 myosin layer lines (MLL) and sharp meridional reflections, signifying that myosin heads (crossbridges) are distributed in a well ordered helical array. Decreasing temperature reduced MLL intensities but increased I1,1/I1,0. Decreasing myosin layer line intensities indicates increasing disorder in the distribution of crossbridges on the thick filaments surface. In the skeletal muscle, order/disorder is directly correlated with the hydrolysis equilibrium of ATP by myosin, [M.ADP.Pi]/[M.ATP] (1). Similar effects of temperature on MLL and similar biochemical pathways (2) found in both types of muscles suggest that the order/disorder states of cardiac crossbridges may well be correlated with the same biochemical and structural states. This implies that in relaxed cardiac muscle under physiological conditions, the unattached crossbridges are largely in the M.ADP.Pi 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 I1,1/I1,0 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 crossbridges is weakly bound to actin in the myocardium.
Key Words:
affinity of cardiac myosin for actin, order-disorder transition in cardiac thick filaments, temperature and ionic strength effects on cardiac myosin filaments, trabeculae of rabbit, weakly bound crossbridges in cardiac muscle
