J. Gen. Physiol., Volume 112, Number 2, August 1, 1998 113-124

Light Dependence of Calcium and Membrane Potential Measured in Blowfly Photoreceptors In Vivo

Johannes Oberwinkler, and Doekele G. Stavenga

From the Department of Neurobiophysics, University of Groningen, 9747 AG Groningen, The Netherlands

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 µM. 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 µM, buffering becomes minimal.

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