Impact of OM on myofilament Ca²⁺ sensitivity (pCa₅₀) in WT and SA myocardial preparations. (A and B) Force–pCa relationships were constructed by plotting normalized forces generated against a range of pCa with and without OM incubations in WT (A) and SA (B) myocardial preparations. OM incubation caused a significant leftward shift in the force–pCa relationships in WT myocardial preparations, indicating that OM caused a significant increase in pCa₅₀. However, pCa₅₀ was unaltered in the SA myocardial preparations following OM incubation, presumably because 1.0 µM OM-mediated force enhancements in SA myocardial preparations were mainly confined to <50% of maximal force. Data from multiple preparations from each heart were averaged, and 9–13 myocardial preparations from three or four hearts were used per group.