Calcium homeostasis in photoreceptor cells ofDrosophilamutantsinaCandtrpstudied with the pupil mechanism

Abstract

AbstractThe light-driven pupil mechanism, consisting of an assembly of mobile pigment granules inside the photoreceptor cells, has been investigated byin vivoreflection microspectrophotometry in wild type (WT)Drosophilaand in the photoreceptor mutantsinaCandtrp. The pupillary response of a dark-adapted WT eye to a step in light is a monophasic reflectance increase reaching a plateau afterca. 15-s light adaptation. This reflectance change is due to photoreceptor pigment granules that accumulate near the tips of the rhabdomeres under light adaptation and that are withdrawn towards the periphery in the dark (Franceschini & Kirschfeld, 1976). The step response of the pupil mechanism ofinaCis triphasic. Strikingly, the reflectance level at light onset is distinctly higher than that in WT, due to a partly aggregated state of the photoreceptor pigment granules near the rhabdomere tips that persists in the dark-adapted state, in line with direct calcium measurements of Peretz et al. (1994b). The step response of the pupil mechanism ofinaCis slightly elevated compared to that of WT. The step response intrpis a transient, biphasic reflectance change, approximating a log normal function. This function is also a good approximation of the pulse response in WT andinaC. The intensity range of pupillary sensitivity is about 4 log unit. The range ofinaCcompared to that of WT is slightly (≈0.5 log unit) shifted towards lower intensities, but that intrpis strongly shifted to higher intensities (≈2.5 log unit). The results can be interpreted with the present knowledge of the primary steps in fly phototransduction and the hypothesis that the local intracellular calcium concentration determines the position of the pigment granules, and hence are in line with the notion that the pupil can be used as a qualitative Ca2+probe.