,
Eq. (1)
where Fpas corresponds to passive stress under relaxed
conditions; Fact corresponds to maximal
Ca2+-activated stress; pCa50represents the free Ca2+ concentration required to
develop half the maximum Ca2+-activated stress, and
nH is the Hill coefficient.
Sinusoidal length-perturbations of 0.125% myocardial strip length
(clip-to-clip) were applied at 41 discreet frequencies from 0.125-100 Hz
to measure the complex modulus as a function of angular frequency (Kawai
and Brandt, 1980; Mulieri et al., 2002; Palmer et al., 2007). The
complex modulus represents viscoelastic myocardial stiffness, which
arises from the change in stress divided by the change in muscle length
that is in-phase (elastic modulus) and out-of-phase (viscous modulus)
with the oscillatory length change at each frequency.
Characteristics of the elastic and viscous moduli responses over the
measured frequency range provide a signature of cross-bridge binding and
cycling kinetics. Shifts in the elastic modulus are useful for assessing
changes in the number of bound cross-bridges between experimental
conditions. Shifts in the viscous modulus are useful for assessing
changes in the work-producing and work-absorbing characteristics of the
myocardium that arise from force-generating cross-bridges. Frequencies
producing negative viscous moduli represent regions of work-producing
muscle function. The “dip frequency” or frequency of the minimum
viscous modulus describes force-generating events and cross-bridge
recruitment rate (Mulieri et al., 2002; Campbell et al., 2004).
Frequencies producing positive viscous moduli represent regions of
work-absorbing muscle function. The “peak frequency” or frequency of
the maximum viscous modulus describes cross-bridge distortion events and
cross-bridge detachment rate (Campbell et al., 2004; Palmer et al.,
2007, 2011). These characteristic
regions of minima and maxima in
the viscous modulus vs. frequency relationship were used to assess
effects of mavacamten on cross-bridge kinetics under maximal
Ca2+-activated conditions. Given that viscous moduli
were only measured at discrete frequencies, these regions of minima
viscous modulus (using 0.125-4 Hz data) and maxima viscous modulus
(using 3-40 Hz data) were fit to a polynomial using MATLAB to create
fitted curves at 0.05 Hz resolution. From these interpolated curves we
extracted the frequency of minimum viscous modulus and frequency of
maximum viscous modulus.