Does Ca2+ reach millimolar concentrations after single photon absorption in Drosophila photoreceptor microvilli?
The quantum bump, the elementary event of fly phototransduction induced by the absorption of a single photon, is a small, transient current due to the opening of cation-channels permeable to Ca2+. These channels are located in small, tube-like protrusions of the cell membrane, the microvilli. Using a modeling approach, we calculate the changes of free Ca2+ concentration inside the microvilli, taking into account influx and diffusion of Ca2+. Independent of permeability ratios and Ca2+ buffering, we find that the free Ca2+ concentrations rise to millimolar values, as long as we assume that all activated channels are located in a single microvillus. When we assume that as much as 25 microvilli participate in a single bump, the free Ca2+ concentration still reaches values higher than 80 microM. These very high concentrations show that the microvilli of fly photoreceptors are unique structures in which the Ca2+ signaling is even more extreme than in calcium concentration microdomains very close to Ca2+ channels.
Postma
M
M
Department of Neurobiophysics, University of Groningen, Nijenborgh 4, NL-9747 AG Groningen, The Netherlands. postma@bcn.rug.nl
Oberwinkler
J
J
Stavenga
D G
DG
D000818:Animals; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D020013:Calcium Signaling; D002147:Calmodulin / Q000378:metabolism; D002463:Cell Membrane Permeability; D004058:Diffusion; D004331:Drosophila melanogaster / Q000166:cytology*; D004553:Electric Conductivity; D008564:Membrane Potentials; D008871:Microvilli / Q000378:metabolism*; D017785:Photons; D017956:Photoreceptor Cells, Invertebrate / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure; D013997:Time Factors; D014785:Vision, Ocular
D002021:Buffers; D015220:Calcium Channels; D002147:Calmodulin; D002118:Calcium