Endogenous buffers limit the spread of free calcium in hair cells

Mobile Ca2+ buffers in hair cells have been postulated to play a dual role. On one hand, they carry incoming Ca2+ away from synaptic areas, allowing synapses to be rapidly reset. On the other hand, they limit the spread of free Ca2+ into the cell, preventing cross-talk between different pathways that employ Ca2+ as a second messenger. We have obtained evidence for such mobile Ca2+ buffers in hair cells by comparing the patterns of Ca2+-induced fluo-3 fluorescence under whole-cell and perforated-patch recording conditions. Fluorescent signals under perforated-patch conditions are relatively weak and are limited to the immediate vicinity of the membrane. These observations can be explained by a diffusion-reaction scheme that, in addition to Ca2+ and fluo-3, incorporates endogenous fixed and mobile Ca2+ buffers. Our experiments also suggest that the mobility of the endogenous buffer might be higher than previously thought. A high buffer mobility is expected to enhance the cell's ability to rapidly modulate transmitter release.