Impact of OM on percent force increases at various levels of Ca²⁺ activation in WT and SA myocardial preparations. (A–H) Baseline forces generated by the myocardial preparations were initially measured in Ca²⁺ solutions with pCa ranging from 6.2 to 5.9. Forces were subsequently measured on the same preparations using the same range of pCa solutions following a 2-min incubation with either 0.5 or 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 in WT and SA myocardial preparations as done in our previous studies (Mamidi et al., 2015, 2017b). Incubation with OM led to a dose-dependent increase in the myocardial force generation at all the Ca²⁺ levels tested in the WT but such dose-dependent force increases were absent in the SA myocardial preparations. Data from multiple preparations from each heart were averaged, and 12–15 myocardial preparations from four different hearts were used per group. Values are expressed as mean ± SEM. #, Significantly different compared with the 0.5 µM OM group. &, Significantly different compared with the corresponding WT group. P < 0.05.