Impact of OM on the magnitude of force generation at various levels of Ca²⁺ activation in PKA-treated WT and SA myocardial preparations. (A–D) Myocardial preparations were incubated for 1 h in PKA, and then baseline forces generated by the PKA-treated preparations were first measured in Ca²⁺ solutions with pCa ranging from 6.2 to 5.9. Forces were subsequently measured on the same myocardial preparations using the same range of pCa solutions following a 2-min incubation with 1.0 µM OM. The net increase in force generation following OM incubation at each pCa was calculated and expressed as percent increase in force from the preincubation baseline force measured in Ca²⁺ solutions with pCa 6.2 (A), 6.1 (B), 6.0 (C), and 5.9 (D). Force enhancements following OM incubation were less pronounced in the PKA-treated SA myocardial preparations when compared with the PKA-treated WT myocardial preparations, especially at low Ca²⁺ activations, where OM has more potent effects on enhancing the force generation (Mamidi et al., 2015, 2017b). Data from multiple preparations from each heart were averaged, and six to seven myocardial preparations from three different hearts were used per group. Values are expressed as mean ± SEM. *, Significantly different compared with the corresponding 1.0 µM OM-treated WT group; P < 0.05.