Inorganic phosphate affects the pCa-force relationship more than the pCa-ATPase by increasing the rate of dissociation of force generating cross-bridges in skinned fibers from both EDL and soleus muscles of the rat

The effect of inorganic phosphate (Pi) on Ca ²⁺-activation of actomyosin ATPase activity and force in permeabilized (skinned) single extensor digitorum longus (EDL) and soleus muscle fibers of the rat were investigated. Increasing concentrations of Pi decreased force more than ATPase rate at all Ca ²⁺ concentrations and this effect was more pronounced at submaximal Ca ²⁺-activation. Increasing Pi caused both the normalized pCa-ATPase and pCa-force relationship to be shifted to a higher Ca ²⁺ concentration. At all Ca ²⁺ concentrations ATPase was activated at a lower concentration of Ca ²⁺ than force and this difference in Ca ²⁺ concentration required for the activation of ATPase and force was greater in fast-twitch (EDL) than in slow twitch (soleus) muscle. Soleus muscle pCa-ATPase and pCa-force curves were more sensitive to Ca ²⁺ (pCa ₅₀ = 5.97 and 5.89, respectively) than EDL (pCa ₅₀ = 5.68 and 5.54, respectively). Finally the shape of the pCa-ATPase and pCa-force curves was similar and not affected by Pi. Analysis shows that Pi increases the rate of dissociation of force generating myosin cross-bridges (ratio of ATPase/force (g _app)) at all Ca ²⁺ concentration, especially at submaximal Ca ²⁺-activation levels. Pi effects on g _app are discussed in terms Pi interacting with the isomerization high force AM*ADP states to form high force transitional AM*ADP*Pi* states which facilitate the dissociation of ADP from AM*ADP. Increasing Ca ²⁺ during Ca ²⁺-activation of the fibers is associated with a progressive decrease in rate of dissociation of force generating myosin cross-bridges g _app.