Ranganathan R, Bacskai B J, Tsien R Y, Zuker C S
Department of Biology, University of California at San Diego, La Jolla 92093-0649.
Neuron. 1994 Oct;13(4):837-48. doi: 10.1016/0896-6273(94)90250-x.
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 Ca2+ ([Ca2+]i) in this process, we developed novel techniques for the measurement of [Ca2+]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 [Ca2+]i changes while minimally exciting the photoreceptor cells. We show that activation with UV generates [Ca2+]i increases that are spatially localized to the rhabdomeres and that are entirely dependent on the influx of extracellular Ca2+. Application of intracellular Ca2+ chelators of varying affinities demonstrates that the Ca2+ influx initially generates a large-amplitude transient that is crucial for negative regulation. Internal Ca2+ stores were revealed by discharging them with thapsigargin. But, in contrast to proposals that IP3-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 [Ca2+]i recovery.
