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Molecular Physiology Section, Laboratory of Molecular Cardiology, National Heart Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892-1760
Correspondence: Address reprint requests to Julien S. Davis, Molecular Physiology Section, Laboratory of Molecular Cardiology, NHLBI, NIH, 10 Center Drive, MSC 1760, Building 10, Rm. 8N202, Bethesda, MD 20892-1760. Tel.: 301-435-5285; E-mail: davisjs{at}nhlbi.nih.gov.
The Huxley-Simmons phase 2 controls the kinetics of the first stages of tension recovery after a step-change in fiber length and is considered intimately associated with tension generation. It had been shown that phase 2 is comprised of two distinct unrelated phases. This is confirmed here by showing that the properties of phase 2fast are independent of fiber type, whereas those of phase 2slow are fiber type dependent. Phase 2fast has a rate of 1000–2000 s-1 and is temperature insensitive (Q10 
1.16) in fast, medium, and slow speed fibers. Regardless of fiber type and temperature, the amplitude of phase 2fast is half (0.46) that of phase 1 (fiber instantaneous stiffness). Consequently, fiber compliance (cross-bridge and thick/thin filament) appears to be the common source of both phase 1 elasticity and phase 2fast viscoelasticity. In fast fibers, stiffness increases in direct proportion to tension from an extrapolated positive origin at zero tension. The simplest explanation is that tension generation can be approximated by two-state transition from attached preforce generating (moderate stiffness) to attached force generating (high stiffness) states. Phase 2slow is quite different, progressively slowing in concert with fiber type. An interesting interpretation of the amplitude and rate data is that reverse coupling of phase 2slow back to Pi release and ATP hydrolysis appears absent in fast fibers, detectable in medium speed fibers, and marked in slow fibers contracting isometrically. Contracting slow and heart muscles stretched under load could employ this enhanced reversibility of the cross-bridge cycle as a mechanism to conserve energy.
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