Peer review of: Orthophosphate increases the efficiency of slow muscle-myosin isoform in the presence of (2020)

Reviewers' comments:
Reviewer #1 (Remarks to the Author):
The authors have resubmitted a greatly improved version of their manuscript. I only have a few additional comments on the rebuttal responses and associated manuscript updates.
Major comments:
1. The authors justify their choice to use permeabilized rabbit soleus muscle preparations over a permeabilized cardiac muscle system with (i) the lab’s previous experiences with skinned rabbit soleus and (ii) the want to have a homogeneous ensemble of beta-MHC. In response to the first point, what prevents the authors from using skinned rat trabeculae? There are many examples of other studies successfully using skinned rat trabeculae to study OM.
The reasoning behind the second argument is understandable as human cardiac MHC, as the authors mention, is predominantly of the beta isoform. However, it seems that it would still be preferable to study the joint effects of [OM] and [Pi] in the rat cardiac system. This data would be more readily compared to previous OM studies and more translatable to other cardiac systems despite the differences in MHC isoform expression. Additionally, the work in (Pinzauti et al. 2018) shows that a similar model of sarcomere compliance and strain can be applied to rat trabeculae; comparing table 2 in (Pinzauti et al. 2018 ) to table 2 in (Percario et al. 2018) further reveals that the necessary parameters for skinned rat trabeculae could be determined. Were there other more substantial limitations to using skinned rat cardiac trabeculae that were not mentioned in the manuscript and rebuttal?
2. The addition of Table 3 greatly improved the clarity and presentation of the proposed kinetic model. It would further help if the authors would add uncertainties to the parameters that were “adjusted” or “set” to indicate how sensitive the model fit is.
3. The authors should include some goodness-of-fit metric to quantify how well the model fits the experimental data.
Minor Comments:
1. In the system of equations variables for [OM], [ATP], and [ADP] lack the square brackets. This makes them difficult to distinguish from the other variables in the equations.
References:
Pinzauti, F., I. Pertici, M. Reconditi, T. Narayanan, G.J.M. Stienen, G. Piazzesi, V. Lombardi, M. Linari, and M. Caremani. 2018. The force and stiffness of myosin motors in the isometric twitch of a cardiac trabecula and the effect of the extracellular calcium concentration. J. Physiol. 596:2581–2596. doi:10.1113/JP275579.
Percario, V., S. Boncompagni, F. Protasi, I. Pertici, F. Pinzauti, and M. Caremani. 2018. Mechanical parameters of the molecular motor myosin II determined in permeabilised fibres from slow and fast skeletal muscles of the rabbit. J. Physiol. 596:1243–1257. doi:10.1113/JP275404.
Reviewer #2 (Remarks to the Author):
The authors have addressed my comments in their revisions and I thank them for their consideration of these comments. The paper is strengthened by their revisions.
I have only the following minor suggestions for the Introduction:
1) After the first sentence in the introduction, I would include a sentence about heart failure with reduced ejection fraction. While this is an acquired cardiomyopathy, increasing cardiac contractility may also play an important role in treatment.
2) In the third sentence of the introduction - omecamtiv should not be capitalized.
Reviewer #3 (Remarks to the Author):
The authors have provided a satisfactory answer to my questions. The paper is adequate for publication.