Calcium imaging demonstrates colocalization of calcium influx and extrusion in fly photoreceptors.
During illumination, Ca(2+) enters fly photoreceptor cells through light-activated channels that are located in the rhabdomere, the compartment specialized for phototransduction. From the rhabdomere, Ca(2+) diffuses into the cell body. We visualize this process by rapidly imaging the fluorescence in a cross section of a photoreceptor cell injected with a fluorescent Ca(2+) indicator in vivo. The free Ca(2+) concentration in the rhabdomere shows a very fast and large transient shortly after light onset. The free Ca(2+) concentration in the cell body rises more slowly and displays a much smaller transient. After approximately 400 ms of light stimulation, the Ca(2+) concentration in both compartments reaches a steady state, indicating that thereafter an amount of Ca(2+), equivalent to the amount of Ca(2+) flowing into the cell, is extruded. Quantitative analysis demonstrates that during the steady state, the free Ca(2+) concentration in the rhabdomere and throughout the cell body is the same. This shows that Ca(2+) extrusion takes place very close to the location of Ca(2+) influx, the rhabdomere, because otherwise gradients in the steady-state distribution of Ca(2+) should be measured. The close colocalization of Ca(2+) influx and Ca(2+) extrusion ensures that, after turning off the light, Ca(2+) removal from the rhabdomere is faster than from the cell body. This is functionally significant because it ensures rapid dark adaptation.
Oberwinkler
J
J
Department of Neurobiophysics, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands. j.oberwinkler@phys.rug.nl
Stavenga
D G
DG
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004175:Diptera / Q000378:metabolism*; D005260:Female; D005453:Fluorescence; D005456:Fluorescent Dyes; D019476:Insect Proteins / Q000378:metabolism; D017956:Photoreceptor Cells, Invertebrate / Q000378:metabolism*
D005456:Fluorescent Dyes; D019476:Insect Proteins; D002118:Calcium