Light dependence of calcium and membrane potential measured in blowfly photoreceptors in vivo

J Gen Physiol. 1998 Aug;112(2):113-24. doi: 10.1085/jgp.112.2.113.

Abstract

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.

MeSH terms

  • Adaptation, Ocular / physiology*
  • Animals
  • Calcium / metabolism*
  • Cytosol / metabolism
  • Diptera / physiology*
  • Electrophysiology
  • Female
  • Fluorescent Dyes
  • Homeostasis / physiology
  • Membrane Potentials / physiology
  • Microscopy, Fluorescence / methods
  • Photoreceptor Cells, Invertebrate / physiology*
  • Vision, Ocular / physiology*

Substances

  • Fluorescent Dyes
  • Calcium