Kinetic effects of fiber type on the two subcomponents of the Huxley-Simmons phase 2 in muscle.

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 2(fast) are independent of fiber type, whereas those of phase 2(slow) are fiber type dependent. Phase 2(fast) has a rate of 1000-2000 s(-1) and is temperature insensitive (Q(10) approximately 1.16) in fast, medium, and slow speed fibers. Regardless of fiber type and temperature, the amplitude of phase 2(fast) is half (approximately 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 2(fast) 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 2(slow) 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 2(slow) back to P(i) 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.