1. The rate constant of tension redevelopment (ktr) following a rapid release and subsequent re‐extension of muscle length has been demonstrated to be Ca2+ sensitive and is thought to reflect the rate‐limiting step in the cross‐bridge cycle leading to the formation of the strongly bound, force‐bearing state. The kinetics of this cross‐bridge state transition were investigated at 15 degrees C over a wide range of Ca2+ concentrations while varying pH from 7.00 to 6.20 in rat slow‐twitch soleus, rat fast‐twitch superficial vastus lateralis (SVL) and rabbit fast‐twitch psoas skinned single fibres. 2. At maximal levels of Ca2+ activation, ktr was unaffected by changes in pH from 7.00 to 6.20 while isometric tension was depressed to 0.60 +/‐ 0.02 P0 (mean +/‐ S.E.M.) at the low pH in fast‐twitch fibres and to 0.78 +/‐ 0.01 P0 in slow‐twitch fibres (P0 is the maximum isometric tension obtained at pH 7.00). 3. At reduced levels of Ca2+ activation, corresponding to pCa (‐log [Ca2+]) greater than 5.0, ktr was markedly depressed in all fibre types when pH was lowered. The Ca2+ sensitivity of steady‐state isometric tension was also reduced in all fibres at pH 6.20 compared to pH 7.00. 4. The results suggest that pH has a modulatory effect upon an apparent rate constant which is rate limiting in terms of the formation of the strongly bound, force‐bearing cross‐bridge state. This effect of altered pH may in part account for the reduction in the Ca2+ sensitivity of isometric force at low pH as well as the depression of the rate of rise of tension in living fibres during fatiguing stimulation.
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