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 Ca2+-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 Ca2+ concentrations and this effect was more pronounced at submaximal Ca2+-activation. Increasing Pi caused both the normalized pCa-ATPase and pCa-force relationship to be shifted to a higher Ca2+ concentration. At all Ca2+ concentrations ATPase was activated at a lower concentration of Ca2+ than force and this difference in Ca2+ 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 Ca2+ (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 Ca2+ concentration, especially at submaximal Ca2+-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 Ca2+ during Ca2+-activation of the fibers is associated with a progressive decrease in rate of dissociation of force generating myosin cross-bridges g(app).