FIGURE 6. Effect of OM on the rate of XB detachment (k_rel) and the rate of XB recruitment (k_df).

Isometrically-activated ventricular preparations were subjected to a sudden 2% stretch in their muscle length and the elicited force responses were used to estimate (A) k_rel and (B) k_df in WT and KO preparations before (white bars) and following (grey bars) incubation with OM at pCa 6.1 [26, 35]. As reported earlier [53], KO preparations exhibited a significantly increased k_rel when compared to the WT preparations before incubation with OM. However, such a trend was abolished after incubation with OM because the k_rel in the KO decreased to the level observed in the WT preparations following incubation with OM. Furthermore, k_rel was significantly decreased in WT and KO groups when compared to their respective pre-OM groups–indicating that the rate of XB detachment from actin was significantly slowed upon incubation with OM. As reported earlier [53], KO preparations exhibited a significant acceleration of k_df when compared to the WT preparations before incubation with OM, a trend that persisted even after incubation with OM. Furthermore, k_df was significantly decreased in both WT and KO groups when compared to their respective pre-OM groups–indicating that the rate of XB recruitment into the force-generating state was significantly slowed upon OM incubation. Values are expressed as mean ± S.E.M. Independent t-tests were used to compare the data between WT and KO groups and paired t-tests were used to compare the data between pre- and post-OM treatment with in the same group. 14–16 preparations and 15–18 preparations from 5 and 7 hearts were analyzed for WT and KO groups, respectively with multiple preparations from each heart. * P < 0.05.