Review papers by their claims

Concise critical takeaway

Pluteanu et al. provide a focused, up-to-date review arguing that intracellular acidosis (pH i <6.8) depresses striated muscle contractility via multi-site proton actions that (a) reduce myofilament Ca2+ sensitivity, (b) inhibit SR Ca2+ release (RyRs) and SR reuptake (SERCA), and (c) alter plasmalemmal Ca2+ fluxes (NCX, SOCE), with measurement caveats because Ca2+ and pH probes are pH sensitive and require calibration Biomolecules review summary

Detailed evidence‑based review and critique

This analysis critically examines the claims, strengths, limitations, and experimental implications of the review Ca2+ Signaling in Striated Muscle Cells During Intracellular Acidosis (DOI 10.3390/biom15091244). Every claim below is supported by verbatim, tightly-targeted excerpts from the paper and annotated commentary.

1) Central claims and supporting excerpts

• Definition and physiological scope — The authors define intracellular acidosis and its relevance: Definition of intracellular acidosis
• Mechanistic summary — The review lists multiple proton targets that combine to depress contractility: "protonation reduces Ca 2+ sensitivity of contractile proteins (troponin/C), inhibits RyR-mediated Ca 2+ release (especially RyR1), slows cross-bridge cycling via ADP/Pi accumulation, depresses SERCA Ca 2+ uptake, and modulates NCX and SOCE" with organelle and buffer effects noted Mechanistic summary list

2) Detailed strengths

• Comprehensive synthesis — The review integrates biochemical, electrophysiological and imaging literature across skeletal and cardiac muscle and highlights organelle cross-talk and methodological caveats Review scope and imaging caveats
• Quantitative points highlighted — The authors provide usable numbers: resting pH i ~7.2, acidosis threshold pH i <6.8, resting cytosolic Ca2+ ~100 nM and contraction transients 1–2 µM, plus SERCA Vmax reduction ~50% between pH 7.0→6.0; these are directly extracted from the review and provide anchors for experiment design Quantitative parameter list

3) Key technical caveats (emphasized and well-stated)

• Probe pH sensitivity — The review correctly warns that chemical and genetically encoded Ca2+ indicators change properties with pH (example: fura-2 Kd shifts ~2x between pH 7.2 and 6.6), so uncalibrated fluorescence can misreport Ca2+ during acidosis Indicator pH dependence (fura-2 Kd shift)
• Model dependence — The authors explicitly state that single-fiber and skinned preparations yield parameter values that may not translate to intact muscle or in vivo ischemia/fatigue because temperature, storage, and tissue context alter the responses Limitations of isolated fiber data

4) Points that need stronger evidence or are underexplored

• In vivo pH microdomains — The review notes local pH heterogeneity and organelle pH changes but admits direct, high-resolution in vivo mapping is limited. The review correctly leaves open whether microdomain protonation patterns (near T-tubules, SR junctions, mitochondria) are causal vs correlative for altered ECC in intact tissue Need for compartmental pH mapping
• Molecular mechanism details — Protonation sites on key proteins (RyR, SERCA, NCX, troponin) are sketched functionally, but structural/biophysical atomic-resolution evidence tying specific protonation to functional changes is sparse in the review and would strengthen mechanistic claims (this is a knowledge gap the authors acknowledge) Acknowledged molecular blindspots

5) Practical experimental recommendations implied by the review

• When measuring Ca2+ during acidosis, co-measure pH and explicitly calibrate indicators at the studied pH range (example: perform in situ Kd calibration for fura-2 across pH 7.4→6.4) Recommendation pH calibration
• Prefer ratiometric indicators or organelle-targeted genetically encoded sensors for compartmental resolution; report temperature and storage conditions because these change buffering and contractile properties Probe types and organelle targeting

6) Where the review changes or confirms practice

• Confirms — That low pH reduces myofilament Ca2+ sensitivity and that RyR1 is inhibited at low pH are well supported and summarized (useful for interpreting force‑pCa shifts) RyR1 pH sensitivity and consequence
• Clarifies — That acidosis can both decrease Ca2+ release and, via transporter inhibition (NCX), increase diastolic Ca2+ in cardiac cells during prolonged acidosis — explaining paradoxical observations of reduced contractility despite higher systolic Ca2+ in some intact myocyte experiments Paradoxical Ca2+ observations explained

7) Critical appraisal and limitations

• Scope limitation — It is a literature review, not a meta-analysis; there is no systematic extraction of effect sizes across studies, nor raw data deposition. The authors correctly acknowledge this and the paper therefore serves as a conceptual synthesis rather than a quantitative adjudication Review nature and data availability
• Comparability of models — The review draws from many species and preparations; extrapolation to human in vivo physiology (especially whole-organ ischemia) remains tentative. Authors explicitly state temperature, storage and preparative artifacts may bias conclusions Model extrapolation caveat

8) Concrete suggestions to improve the field (and future papers)

1. Publish standardized in situ calibration datasets for commonly used Ca2+ and pH probes across pH 7.4→6.0 and temperatures 22C vs 37C; share calibration curves and raw fluorescence traces.
2. Use paired organelle-targeted sensors (ER/SR, mitochondria, cytosol) in the same cell to directly test whether local proton rises precede, coincide, or follow local Ca2+ changes.
3. Where possible, combine electrophysiology (voltage clamp) with simultaneous ratiometric imaging to parse membrane flux changes (LTCC, NCX, SOCE) from altered SR release or buffering.

9) Short actionable checklist for experimentalists

• Always co-measure pH and Ca2+; calibrate sensors across expected pH range Calibration imperative
• Report temperature and storage (glycerol freezing affects stiffness) because protocol-dependent artifacts change contractile readouts Temperature and storage artifact note
• When reporting Ca2+ transients during acidosis, show raw ratio traces plus pH traces and include Kd or Hill-fit parameters used for conversion to [Ca2+].

10) Short conclusion and confidence note

Overall, the review is a high-quality, well‑referenced synthesis highlighting that intracellular acidosis has multiple, often-opposing effects on Ca2+ handling and contractility; the core mechanistic claims are well grounded in the literature cited, but the field needs standardized calibrations, better organelle-resolved in vivo measurements, and quantitative meta-analytic effort to place effect sizes on firmer footing Final conclusion excerpt