Are Force Enhancement after Stretch and Muscle Fatigue Due to Effects of Elevated Inorganic Phosphate and Low Calcium on Cross Bridge Kinetics?

Figures

• 

  Figure 1. The impact of (A) different concentrations of inorganic phosphate (Pi) in pooled type I and IIa fibres and (B) low calcium (pCa 5.5), 15 mM inorganic phosphate (Pi 15) and 1 mM 2,3-Butanedione monoxime (BDM) in type I fibres only, on maximal isometric force (P0). Data are mean ± SD (panel A) and SEM (panel B) of 14–36 fibres. *: different from pCa 4.5 at p < 0.001.

• 

  Figure 2. The effects of 15 mM inorganic phosphate (Pi) and 1 mM 2,3-butanedione monoxime (BDM) on the force–velocity (FL·s−1: Fibre lengths·s−1) relationship of (A) type I and (B) type IIa fibres, (C) an example of data of a type I fibre in which the force–velocity relationship was completed in BDM, Pi and twice in pCa 4.5 and (D) the results of the model. Data are shown in Table 1.

• 

  Figure 3. The effect of (A) pCa5.5, 15 mM inorganic phosphate (Pi) and 1 mM 2,3-Butanedione monoxime (BDM) on the Ktr. (B) Ktr pCa4.5 and (C) pCa4.5 + 15 mmPi in type I fibres, where ‘C’ indicates the force at the end of the stretch and (D) the force ‘C’ at the end of the stretch (solid symbols) and initial isometric force (open symbols). Data are mean ± SD of 5–12 fibres.

• 

  Figure 4. The increase in force after a 1% length-step stretch. (A) Blue: pCa 4.5; Grey: pCa 5.5; Brown: 15 mM inorganic phosphate (Pi); Yellow: 1 mM 2,3-Butanedione monoxime (BDM) (B) average force increments after a 1% step stretch. (C) Force per cross-bridge (P0/Xbr). Blue bars: Type I fibres; Orange bars: Type IIa fibres. Data are mean ± SEM of 14–36 fibres. c: different from pCa 4.5 at p ≤ 0.002; b: different from 1 mM BDM at p < 0.001; p: different from 15 mM Pi at p < 0.001.

• 

  Figure 5. The force enhancement during a 5% stretch at 0.0625 lo·s-1 without or with 2.5 mM (orange), 5 mM (grey), or 15 mM (yellow) inorganic phosphate (Pi) in (A) absolute stress and (B) as a proportion of the isometric force before the stretch (insert: rise in force at start of stretch in expanded scale). Each curve is the average of 13–14 fibres. (C) Stretch relaxation fit with a three-exponential model (the open circles) to the observed relaxation (red line) with the contribution of each component (A1 fast, A2 slow and A3 very slow) illustrated by the dashed lines. (D) The contribution of A3 to the force increment during stretch and subsequent stretch relaxation.

• 

  Figure 6. The contribution of the slow exponential component (A3) of stretch relaxation to force during a (A) 5% stretch at 0.0625 l lo·s−1 at different concentrations of inorganic phosphate (Pi) and (B) at different stretch lengths. (C) shows an example of a 5% stretch at 0.0625 lo·s−1 in pCa4.5 and (D) a similar stretch in relaxed state.

• 

  Figure 7. (A) The cross-bridge scheme used for mathematical modelling with (B) the rate constants in s−1 used and (C) the calculated distribution of cross bridges in pCa4.5, 15 mM inorganic phosphate (Pi), 1 mM 2,3-Butanedione monoxime (BDM) and pCa5.5. The ‘a’, ‘b’, ‘c’ and ‘d’ refer to the cross-bridge states in panel A with the same letters. (D) The model predicts, in correspondence with the observations (Figure 1B), a 50% reduction in force in each condition and (E) reduced stiffness in pCa 5.5 only (corresponding with the observations in Figure 4B).