Skinned cardiac muscle preparations exhibit spontaneous tension oscillations (spontaneous oscillatory contractions; SPOCs) in the absence of Ca2+, and in the presence of MgATP, MgADP and inorganic phosphate (Pi; ADP- SPOC). Similar oscillations occur in the presence of sub-micromolar concentrations of Ca2+ under normal activating conditions without MgADP and Pi (Ca-SPOC). In the study presented here, we investigated the effects of pH on both types of SPOC in skinned bovine cardiac ventricular muscle. First, a decrease in pH increased the MgADP concentration required to induce the half- maximal isometric tension that is obtained in the absence of Ca2+ and in the presence of MgATP (ADP-contraction). The inhibitory effect of Pi on ADP- contractions was not affected by pH. Second, ADP-SPOCs occurred upon the addition of Pi to the solution that resulted in ADP-contraction, and the relative amplitude and the period of the tension oscillation in the presence of 2 mM MgATP, 10 mM MgADP and 10 mM Pi were unchanged under all pH conditions examined (6.6, 7.0, 7.4). On the contrary, the relative amplitude and the period of the Ca-SPOCs were markedly diminished at pH 6.6. Finally, we constructed state diagrams showing the effects of pH on SPOC conditions. The state diagram shows that SPOCs occur less frequently under acidic conditions than at neutral pH. We suggest that the intermediate state of crossbridges that is required for SPOCs is more difficult to attain at a low pH.
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