Fluorescence imaging of individual bullfrog saccular stereocilia quantifies free [Ca2+] dynamics under MET-channel gating. Four mechanisms collectively account for observed Ca2+ clearance: free diffusion, PMCA pumping, fixed buffer binding, and mobile buffer binding. This is the primary source for apical compartment geometry and resting [Ca2+] used in h05 ODE models.
Key finding
Resting free [Ca2+] in bullfrog saccular stereocilia is 20–310 nM depending on condition (Figure 9A reports ~0.31 µM at saturating deflection; baseline is lower). The stereociliary compartment is geometrically small (~0.05 pL = 50 fL for a single stereocilium), justifying the lumped V_apex parameter. All four Ca2+ regulatory mechanisms are required: no single mechanism accounts for observed clearance kinetics.
Numbers that matter
| Parameter | Value | Units | Conditions | Notes |
|---|---|---|---|---|
| Resting free [Ca2+] (bullfrog) | 20–310 | nM | Saccular stereocilia, fluorescence imaging | ~0.31 µM at saturating deflection; baseline ~60 nM |
| Single stereocilium volume | ~50 | fL (0.05 pL) | Estimate from geometry (0.5 µm diameter × ~3 µm) | Not directly stated; derivable from geometric parameters in paper |
| Ca2+ clearance time constant | 10–100 | ms | Short vs. prolonged stimuli | Faster component from PMCA + diffusion |
| PMCA involvement | confirmed essential | — | Vanadate/carboxyeosin block raises Ca2+ | Quantitative pump rate not given; see Yamoah 1998 |
Species caveat: Bullfrog (Rana catesbeiana) sacculus. OHC values may differ. Ca2+ fraction parameters require mammalian cross-referencing.
Limitations
- Bullfrog sacculus, not mammalian OHC. The model transfers qualitatively but resting [Ca2+] in OHC stereocilia may be different.
- Compartment volume estimated from geometry, not directly measured.
- Does not report absolute PMCA pump rate; only establishes pump necessity.
Connections
[source]calcium-oscillation — primary parameter source for V_apex and Ca_rest[applies]STRC Calcium Oscillation Acoustic Therapy — h05 ODE parameter origin[see-also]1998-yamoah-pmca-stereocilia-extrusion — companion paper with PMCA quantitation