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Biophys J, July 2002, p. 359-370, Vol. 83, No. 1
Molecular Physiology Section, Laboratory of Molecular Cardiology, National Heart Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892-1760 USA
Kinetic analysis of contracting fast and slow rabbit
muscle fibers in the presence of the tension inhibitor 2,3-butanedione monoxime suggests that regulatory light chain (RLC) phosphorylation up-regulates the flux of weakly attached cross-bridges entering the
contractile cycle by increasing the actin-catalyzed release of
phosphate from myosin. This step appears to be separate from earlier
Ca2+ regulated steps. Small step-stretches of single
skinned fibers were used to study the effect of phosphorylation on
fiber mechanics. Subdivision of the resultant tension transients into
the Huxley-Simmons phases 1, 2fast, 2slow, 3, and 4 reveals that phosphorylation reduces the normalized amplitude of
the delayed rise in tension (stretch activation response) by decreasing
the amplitudes of phase 3 and, to a lesser extent, phase
2slow. In slow fibers, the RLC P1 isoform phosphorylates at
least 4-fold faster than the P2 isoform, complicating the role of RLC
phosphorylation in heart and slow muscle. We discuss the functional
relevance of the regulation of stretch activation by RLC
phosphorylation for cardiac and other oscillating muscles and speculate
how the interaction of the two heads of myosin could account for the
inverse effect of Ca2+ levels on isometric tension and rate
of force redevelopment (kTR).
Biophys J, July 2002, p. 359-370, Vol. 83, No. 1
© 2002 by the Biophysical Society 0006-3495/02/07/359/12 $2.00
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