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.