Dependence upon high-energy phosphates of the effects of inorganic phosphate on contractile properties in chemically skinned rat cardiac fibres

The effects of inorganic phosphate (P_i) on mechanical properties of Triton X100 treated ventricular fibres have been studied in different substrate conditions. In the presence of both MgATP and phosphacreatine, increasing concentrations of P_i progressively decreased maximal active force, up to 50–60% at 20 mM P_i. The reduction in stiffness was slightly less. These effects appeared nearly independent of the diameter of the preparations. 20 mM P_i decreased Ca sensitivity of the myofilaments and increased the Hill coefficient of the tension/pCa relationship; furthermore, the time constant of tension recovery was decreased from 12.9 to 8.9 ms suggesting that the cycling rate of cross-bridges was increased in the presence of P_i. When MgATP was regenerated by the myofilament bound creatine kinase in the presence of phosphocreatine, P_i was less efficient in decreasing the maximal tension and it weakened the relaxing effect of MgATP upon rigor tension. These effects are related to the inhibition of creatine kinase by P_i. The effects of P_i on maximal force and kinetics of contraction were antagonized by the effects of a decrease in phosphocreatine. The results are discussed in terms of the antagonistic role of P_i increase and phosphocreatine decrease upon contractile properties of myofilaments during hypoxia in heart muscle.