Dose-Dependent Effects of the Myosin Activator Omecamtiv Mecarbil on Cross-Bridge Behavior and Force Generation in Failing Human Myocardium

Abstract

Background Omecamtiv mecarbil (OM) enhances systolic function in vivo by directly binding the myosin cross-bridges (XBs) in the sarcomere. However, the mechanistic details governing OM-induced modulation of XB behavior in failing human myocardium are unclear. Methods and results The effects of OM on steady-state and dynamic XB behavior were measured in chemically-skinned myocardial preparations isolated from human donor and heart failure (HF) left ventricle. HF myocardium exhibited impaired contractile function as evidenced by reduced maximal force, magnitude of XB recruitment (Pdf), and a slowed rate of XB detachment (krel) at submaximal Ca2+-activations. Ca2+-sensitivity of force generation (pCa50) was higher in HF myocardium when compared to donor myocardium, both prior to and following OM incubations. OM incubation (0.5μM and 1.0μM) enhanced force generation at sub-maximal Ca2+-activations in a dose-dependent manner. Notably, OM-induced a slowing in krel with 1.0μM OM but not with 0.5μM OM in HF myocardium. Additionally, OM exerted other differential effects on XB behavior in HF myocardium as evidenced by a greater enhancement in Pdf and slowing in the time-course of cooperative XB recruitment (Trec), which collectively prolonged achievement of peak force development (Tpk), compared to donor myocardium. Conclusions Our findings demonstrate that OM augments force generation but also prolongs the time course of XB transitions to force-bearing states in remodeled HF myocardium which may extend the systolic ejection time (SET) in vivo. Optimal OM dosing is critical for eliciting enhanced systolic function without excessive prolongation of SET which may compromise diastolic filling.

Impact of the Myosin Modulator Mavacamten on Force Generation and Cross‐Bridge Behavior in a Murine Model of Hypercontractility

“…Force‐ pC a relationships were constructed by plotting normalized forces generated against a range of pC a to assess pC a 50 in WT and KO myocardial preparations 37, 38, 40. Detergent‐skinned myocardial preparations were exposed to a 2‐minute incubation with either 0.5 or 1.0 μmol/L Myk461 to assess the impact of Myk461 on force‐ pC a relationships in (A) WT and (B) KO myocardium.…”

“…Forces were subsequently measured on the same preparations using the same range of pC a solutions following a 2‐minute incubation with Myk461. The net decrease in force generation following Myk461 incubation at each pC a was calculated and expressed as % decrease in force from the pre‐incubation baseline force in (A) WT and (B) KO preparations as done earlier 37, 40. Incubation with Myk461 significantly decreased the myocardial force generation at all the concentrations of Myk461 tested and at all the pC a's tested in the WT and KO groups, but the Myk461‐induced force depression is attenuated as the level of Ca 2+ activation increases.…”

“…Detergent‐skinned multicellular ventricular preparations were attached between a motor arm (312C; Aurora Scientific Inc., Aurora, Ontario, Canada) and a force transducer (403A; Aurora Scientific Inc.), as described previously 37, 38, 39, 40. Changes in the motor arm position and force transducer signals were sampled at 2000 Hz using a custom‐built sarcomere length control software 41.…”

“…Changes in the motor arm position and force transducer signals were sampled at 2000 Hz using a custom‐built sarcomere length control software 41. For all mechanical measurements, the sarcomere length of the ventricular preparations was set to 2.1 μm 33, 37, 38, 40. Force‐pCa relationships were determined by measuring the forces generated by the skinned myocardial preparations in a range of pCa solutions that yield submaximal to maximal forces.…”

“…Stretch activation amplitudes were normalized to prestretch steady‐state Ca 2+ ‐activated force as done before 37, 38, 39, 40. The magnitude of new steady‐state force (P3) was measured from prestretch steady‐state force to the peak force value attained in Phase 3, and P2 was measured from prestretch steady‐state force to the nadir of the force response in Phase 2, whereas P df was measured as the difference between P3 and P2 values 37, 38, 39, 40…”