Spatial calcium buffering in saccular hair cells.
The potential importance of intracellular calcium-binding proteins in rapid and highly localized Ca2+ signalling is poorly understood. During fast synaptic transmission, which occurs at specialized active zones where Ca2+ diffuses only a few tens of nanometers from channels to neurotransmitter release sites, a cytoplasmic Ca2+ buffer would have to be extremely fast or present in millimolar concentrations to intercept a significant fraction of the calcium ions en route to their targets. Therefore, Ca2+ buffers have been presumed to be unimportant in fast exocytosis and another fast calcium-mediated process, electrical resonance in hair cells. Here I present evidence to the contrary by showing that hair cells in the frog sacculus contain millimolar concentrations of a mobile cytoplasmic calcium buffer that captures Ca2+ within a few microseconds after it enters through presynaptic Ca2+ channels and carries it away from the point of entry. This spatial buffering reduces the presynaptic free Ca2+ by up to 60 per cent and probably restricts the region in which the internal calcium ion concentration exceeds 1 microM to within < 250 nm of each synaptic site. The buffer can thus influence both electrical resonance and synaptic transmission. Calbindin-D28K or a related protein may serve as the mobile calcium buffer, an action similar to its function in transporting Ca2+ across intestinal epithelial cells.
Roberts
W M
WM
Institute of Neuroscience, University of Oregon, Eugene 97403.
D000818:Animals; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D006198:Hair Cells, Auditory / Q000378:metabolism*; D008564:Membrane Potentials; D011894:Rana pipiens; D012444:Saccule and Utricle / Q000378:metabolism*; D013570:Synaptic Membranes / Q000378:metabolism*
D002021:Buffers; D002135:Calcium-Binding Proteins; D002118:Calcium