Abstract
Drosophila phototransduction is a phosphoinositide-mediated and Ca 2+-regulated signaling cascade ideal for the dissection of feedback regulatory mechanisms. To study the roles of intracellular Ca 2+ ([Ca 2+] i) in this process, we developed novel techniques for the measurement of [Ca 2+] i in intact photoreceptors. We genetically engineered flies that express a UV-specific rhodopsin in place of the normal rhodopsin, so that long wavelength light can be used to image [Ca 2+] i changes while minimally exciting the photoreceptor cells. We show that activation with UV generates [Ca 2+] i increases that are spatially localized to the rhabdomeres and that are entirely dependent on the influx of extracellular Ca 2+. Application of intracellular Ca 2+ chelators of varying affinities demonstrates that the Ca 2+ influx initially generates a large-amplitude transient that is crucial for negative regulation. Internal Ca 2+ stores were revealed by discharging them with thapsigargin. But, in contrast to proposals that IP 3-sensitive stores mediate phototransduction, thapsigargin does not mimic or acutely interfere with photoexcitation. Finally, we identify a photoreceptor-specific PKC as essential for normal kinetics of [Ca 2+] i recovery.
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