Effects of mavacamten on Ca2+-activated isometric contraction
Steady-state isometric stress (force normalized to cross-sectional area) was measured as activating [Ca2+] increased from pCa 8 (relaxed) to 4.5 (maximally activated) in myocardial strips isolated from donor hearts at 1.9 or 2.3 μm sarcomere length (Fig. 2). For both sarcomere lengths, mavacamten decreased Ca2+-activated force and reduced Ca2+-sensitivity of the force-pCa relationship. As sarcomere length increased, Ca2+-activated force and Ca2+-sensitivity of the force-pCa relationship increased for control and mavacamten-treated myocardial strips. Therefore, the typical length-dependent activation response was maintained in the presence of mavacamten. Parameter values for 4-parameter Hill fits (Eq. 1) to each stress-pCa relationship are summarized in Fig. 3 (Fact and Fpas) and Fig. 4 (pCa50 and nH).
At 1.9 and 2.3 μm sarcomere length, 0.5 μM mavacamten reduced Ca2+-activated force by ~25-30%, compared to control strips (p<0.001 for main effect of mavacamten; Fig. 3A). Ca2+-activated force also increased as sarcomere length increased (p=0.005 for main effect of length). Similar findings occurred for passive force, with 0.5 μM mavacamten reducing passive force at both sarcomere lengths (p<0.001 for main effect of mavacamten; Fig. 3B) and passive force increasing as sarcomere length increased (p<0.001 for main effect of length). These mavacamten-dependent decreases in passive force suggest that a portion of the passive force measured at pCa 8.0 arises from bound cross-bridges, and that mavacamten stabilization of the myosin OFF state reduces this myosin-based contribution to passive force.
pCa50 values of the force-pCa relationship represent the Ca2+ concentration required to produce half-maximal Ca2+-activated force and describe Ca2+-sensitivity of myocardial force development (Fig. 4A). For both sarcomere lengths, 0.5 μM mavacamten reduced the Ca2+-sensitivity of force by more than 0.1 pCa units compared to control (p<0.001 for main effect of mavacamten; average ΔpCa50=0.17 for 1.9 μm strips and ΔpCa50=0.12 for 2.3 μm strips). The Ca2+-sensitivity of force also increased as sarcomere length increased (p=0.002 for main effect of length), exhibiting a length-dependent activation response for both control and mavacamten-treated strips. Hill coefficients (nH) represent the degree of cooperativity in the force-pCa relationship (Fig. 4B), wherein mavacamten increased nH(p<0.001 for main effect of mavacamten) and sarcomere length did not significantly affect nH.