Light dependence of calcium and membrane potential measured in blowfly photoreceptors in vivo.
Light adaptation in insect photoreceptors is caused by an increase in the cytosolic Ca2+ concentration. To better understand this process, we measured the cytosolic Ca2+ concentration in vivo as a function of adapting light intensity in the white-eyed blowfly mutant chalky. We developed a technique to measure the cytosolic Ca2+ concentration under conditions as natural as possible. The calcium indicator dyes Oregon Green 1, 2, or 5N (Molecular Probes, Inc., Eugene, OR) were iontophoretically injected via an intracellular electrode into a photoreceptor cell in the intact eye; the same electrode was also used to measure the membrane potential. The blue-induced green fluorescence of these dyes could be monitored by making use of the optics of the facet lens and the rhabdomere waveguide. The use of the different Ca2+-sensitive dyes that possess different affinities for Ca2+ allowed the quantitative determination of the cytosolic Ca2+ concentration in the steady state. Determining the cytosolic Ca2+ concentration as a function of the adapting light intensity shows that the Ca2+ concentration is regulated in a graded fashion over the whole dynamic range where a photoreceptor cell can respond to light. When a photoreceptor is adapted to bright light, the cytosolic Ca2+ concentration reaches stable values higher than 10 microM. The data are consistent with the hypothesis that the logarithm of the increase in cytosolic Ca2+ concentration is linear with the logarithm of the light intensity. From the estimated values of the cytosolic Ca2+ concentration, we conclude that the Ca2+-buffering capacity is limited. The percentage of the Ca2+ influx that is buffered gradually decreases with increasing Ca2+ concentrations; at cytosolic Ca2+ concentration levels above 10 microM, buffering becomes minimal.
Oberwinkler
J
J
Department of Neurobiophysics, University of Groningen, 9747 AG Groningen, The Netherlands. j.oberwinkler@bcn.rug.nl
Stavenga
D G
DG
D000221:Adaptation, Ocular / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004175:Diptera / Q000502:physiology*; D004594:Electrophysiology; D005260:Female; D005456:Fluorescent Dyes; D006706:Homeostasis / Q000502:physiology; D008564:Membrane Potentials / Q000502:physiology; D008856:Microscopy, Fluorescence / Q000379:methods; D017956:Photoreceptor Cells, Invertebrate / Q000502:physiology*; D014785:Vision, Ocular / Q000502:physiology*
D005456:Fluorescent Dyes; D002118:Calcium