The effect of acidic pH on the ATPase activity and troponin Ca2+ binding of rabbit skeletal myofilaments.

We compared the effect of reductions in pH from 7.0 to 6.5 and to 6.2 on Mg-ATPase activity and Ca2+ binding of rabbit psoas myofilaments. The amounts of Ca2+ bound attributable to myofilament whole troponin complexes (Tn) were determined from studies on Tn free (desensitized) myofibrils and from measurements of the myofilament Ca2+ binding subunit of Tn content (0.55 +/- 0.10 nmol/mg of myofilament protein). The myofibrillar Mg-ATPase activity was half-maximal at the -log molar free Ca2+ concentration (pCa) 5.83 at pH 7.0, 5.54 at pH 6.5, and 5.43 at pH 6.2 Maximal ATPase activity was the same at the three pH values as was the ATPase activity of desensitized myofibrils. The amounts of Ca2+ bound to myofibrils or chemically skinned fibers as a function of pCa decreased as pH was lowered from 7.0 to 6.5 and to 6.2 in a manner predicted from the pH-induced shifts in Mg-ATPase activity as a function of pCa. The alterations in myofibrillar ATPase activity and Ca2+ binding associated with the acidic pH values occurred whether or not EGTA was used to buffer the free Ca2+ concentration. Moreover, lowering the pH from 7.0 to 6.2 reduced Ca2+ binding to myofilament Tn whether or not the fibers and myofibrils were in rigor. Our data indicate that the effects of acidic pH on myofilament ATPase activity and Ca2+-binding are related to a reduction in the affinity of Ca2+-binding sites on Tn.