Table 3.
Presence and concentrations of calcium-binding proteins in hair cells. The table summarizes part of the available information on calcium binding obtained from various preparations. Hair cells vary in the type and amount of calcium-binding proteins, as determined by the various approaches. The number of binding sites per protein also varies (for additional discussion, see Hackney et al. 2005; Desai et al. 2005b), e.g., mammalian calbindin D-28k has four and calretinin five functional calcium-binding sites, while parvalbumin binds only two calcium ions. The Ca2+ affinity under physiological conditions is rather similar for all three proteins (in the range of 0.3–1.5 μM), while kinetic properties differ considerably: calbindin D-28k and calretinin are considered “fast” buffers, while parvalbumin has properties similar to EGTA (Schwaller et al. 2002) (LF low frequency, HF high frequency, IHC inner hair cell, OHC outer hair cell, + positive, − negative)
| Species/organ | Calbindin-D28k (μM) | Calretinin (μM) | Parvalbumin-α (μM) | Parvalbumin-β (μM) | Parvalbumin–3 (μM) | Ca-binding sites (mM) | Approach | References |
|---|---|---|---|---|---|---|---|---|
| Tall frog saccular | − | 1,200±400 | 6 | Western blot, patch-clamp | Edmonds et al. 2000 | |||
| 700–3000 | Western blot | Heller et al. 2002 | ||||||
| Turtle auditory | 627±151 (HF) | 9±3 (HF) | 256±72 (HF) | 3,0 (HF) | Calibrated immunogold | Hackney et al. 2003 | ||
| 129±95 (LF) | 11±2 (LF) | 223±51 (LF) | 1,0 (LF) | |||||
| 1 | Patch-clamp | Tucker and Fettiplace 1996 | ||||||
| 0.1–0.4 | Patch-clamp | Ricci et al. 1998 | ||||||
| Mature rat cochlear IHC | − | ~35 (HF) | ~160 (HF) | − | 0.49 (HF) | Calibrated immunogold | Hackney et al. 2005 | |
| ~40 (LF) | ~170 (LF) | 0.53 (LF) | ||||||
| Mature rat cochlear OHC | 15±11 (HF) | ~70 (HF) | ~100 (HF) | 2892±120 (HF) | 6.0 (HF) | Calibrated immunogold | Hackney et al. 2005 | |
| 230±38 (LF) | ~40 (LF) | ~290 (LF) | 1954±151 (LF) | 5.4 (LF) | ||||
| Mouse cochlear IHC | + | ++ | ++ | Immunofluorescence | Sendin et al., unpublished | |||
| Submillimolar | Patch-clamp | Moser and Beutner 2000 | ||||||
| Mature rat and mouse type II vestibular hair cells | ++ (70&–80% of type II hair cells) | Immunoperoxidase | Desai et al. 2005a,b |
medium staining
strong staining