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

Abstract

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.

Scheme 1.

FIGURE 1

L-jump tension transients typical of fast, medium speed, and slow-fiber types. The Rabbit type IIb (fast) and mouse type IIa (medium speed) and I (slow fibers) fibers were subjected to small −1.5 nm/half-sarcomere step releases in ∼180 μs at a temperature of 6°C. Ten tension and sarcomere length transients were recorded and averaged. Contributions of the Huxley-Simmons phases 1, 2_fast, 2_slow, and 4 to the tension transient were determined by nonlinear least squares analysis and the resultant fit to these data drawn as a solid line through each data set. The simulated time-course of phase 2_fast shows its rate to be independent of fiber type. The rate of phase 2_slow, on the other hand, correlates with fiber type at low to moderate temperatures, slowing progressively from type IIb to IIa to I. Averaged tensions normalized to cross-sectional area presented as the mean ± SE were 117.4 ± 4.6 kN m⁻², n = 9 at 6°C for type IIb fibers; 91.3 ± 7.6 kN m⁻², n = 8 at 5°C for type IIa fibers and 97.6 ± 9.9 kN m⁻², n = 10 at 5°C for type I fibers. The arrow indicates the point at which the step-release is applied.

FIGURE 2

Temperature dependencies of the rates of phases 2_fast and 2_slow in fast, medium speed and slow fibers. The distinctly different properties of phases 2_fast and 2_slow are apparent in these Arrhenius plots. Phase 2_fast shows a typical, minimal dependence of rate on temperature in all fiber types. Q₁₀ values of 1.22, 1.16, and 1.10 and associated apparent rate constants at 10°C of 1198 s⁻¹, 1408 s⁻¹, and 2207 s⁻¹ for type IIb, IIa, and I fibers, respectively were computed from the fit to the Arrhenius equation. Phase 2_slow, on the other hand, shows a complex dependence of rate on increasing temperature with a typical rate decline to a minimum value (centered between 10°C and 15°C) followed by a monotonic rate rise at higher temperatures with Q₁₀ values of 1.89, 11.41, and 21.92 for type IIb, IIa, and I fibers, respectively computed from the fit of the reduced data set to the Arrhenius equation. The very high Q₁₀ values indicate increased coupling to adjacent steps in the cross-bridge cycle. Overall, the rate of phase 2_slow declines in concert with fiber type from IIb to IIa to I with measured rates of 227 ± 19 s⁻¹, 109 ± 13 s⁻¹, and 20.4 ± 0.2 s⁻¹, respectively at 1°C. Data from between three and eight individual fibers were averaged at each temperature point. Error bars are SE.

FIGURE 3

Temperature dependencies of the normalized amplitudes of phases 1, 2_fast, 2_slow in fast, medium speed, and slow fibers. The amplitude of phase 1, normalized (relative) to isometric tension at the temperature of the experiment, declines with increasing temperature to show that isometric tension increases more than stiffness as temperature is raised. The form (shape) and magnitude is similar in each panel irrespective of fiber type. Phase 2_fast versus temperature data resembles phase 1 save that the amplitudes are reduced ∼twofold, a relationship explored in Fig. 4. Phase 2_slow amplitude versus temperature data is different. In fast, but also in medium speed fibers, normalized amplitudes are constant at 5°C and higher temperatures. However, in slow fibers the response is different and normalized tension does not plateau, but declines with increasing temperature. Data from between three and eight individual fibers were averaged at each temperature. Error bars are SE.

FIGURE 4

Linear interdependence of the amplitudes of phases 1 and 2_fast in fast, medium speed and slow fibers. Normalized phases 1 and 2_fast amplitude data from Fig. 3 are replotted. The slopes of the linear fits to these fast, medium speed and slow fibers data are a similar 0.47, 0.42, and 0.49, respectively. This indicates a fixed relationship between the amplitudes of phase 1 and 2_fast that is independent of both fiber type and temperature. Error bars are SE.

FIGURE 5

Dependence of y₀ on fiber temperature. These data were obtained by extrapolating the Huxley-Simmons T1 (1 phase 1 amplitude) vs. the −1.5 nm/half-sarcomere length change data to zero tension and recording the value of y₀ at the intercept. Symbols for the three different fiber types are the same as used elsewhere.

FIGURE 6

Linear dependence of phase1 stiffness on isometric tension. The plot shows that stiffness increases with temperature. Extrapolation to a limit stiffness at zero tension provides a stiffness of 4.1 kN m⁻² nm⁻¹ compared to a limit stiffness at maximum tension (233 kN m⁻²) of 11.7 kN m⁻² nm⁻¹. The temperature dependence of the normalized stiffness of fast rabbit psoas type IIb fibers determined with a −1.5 nm L-jump and the tension versus temperature data obtained from a three-parameter fit to the van't Hoff equation (see Fig. 1 of Davis, 1998) for the fit to the data of Ranatunga, 1994). Tension and stiffness were both determined under identical activating conditions employing the temperature insensitive buffer, glycerol 2-phosphate.