title: Caffeine- and ryanodine-sensitive Ca(2+)-induced Ca2+ release from the endoplasmic reticulum in honeybee photoreceptors.
abstract: Light stimulation of invertebrate microvillar photoreceptors causes a large rapid elevation in Cai, shown previously to modulate the adaptational state of the cells. Cai rises, at least in part, as a result of Ins(1,4,5)P3-induced Ca2+ release from the submicrovillar endoplasmic reticulum (ER). Here, we provide evidence for Ca(2+)-induced Ca2+ release (CICR) in an insect photoreceptor. In situ microphotometric measurements of Ca2+ fluxes across the ER membrane in permeabilized slices of drone bee retina show that (a) caffeine induces Ca2+ release from the ER; (b) caffeine and Ins(1,4,5)P3 open distinct Ca2+ release pathways because only caffeine-induced Ca2+ release is ryanodine sensitive and heparin insensitive, and because caffeine and Ins(1,4,5)P3 have additive effects on the rate of Ca2+ release; (c) Ca2+ itself stimulates release of Ca2+ via a ryanodine-sensitive pathway; and (d) cADPR is ineffective in releasing Ca2+. Microfluorometric intracellular Ca2+ measurements with fluo-3 indicate that caffeine induces a persistent elevation in Cai. Electrophysiological recordings demonstrate that caffeine mimics all aspects of Ca(2+)-mediated facilitation and adaptation in drone photoreceptors. We conclude that the ER in drone photoreceptors contains, in addition to the Ins(1,4,5)P3-sensitive release pathway, a CICR pathway that meets key pharmacological criteria for a ryanodine receptor. Coexpression of both release mechanisms could be required for the production of rapid light-induced Ca2+ elevations, because Ca2+ amplifies its own release through both pathways by a positive feedback. CICR may also mediate the spatial spread of Ca2+ release from the submicrovillar ER toward more remote ER subregions, thereby activating Ca(2+)-sensitive cell processes that are not directly involved in phototransduction.
authors.affiliation: Institut für Zoologie, Universität Regensburg, Germany.
authors.forename: B
authors.initials: B
authors.lastname: Walz
authors.forename: O
authors.initials: O
authors.lastname: Baumann
authors.forename: B
authors.initials: B
authors.lastname: Zimmermann
authors.forename: E V
authors.initials: EV
authors.lastname: Ciriacy-Wantrup
mesh_terms: D000246:Adenosine Diphosphate Ribose / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001516:Bees / Q000378:metabolism*; D001718:Birefringence; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology*; D002129:Calcium Oxalate / Q000378:metabolism; D036563:Cyclic ADP-Ribose; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D008839:Microelectrodes; D010775:Photic Stimulation; D017956:Photoreceptor Cells, Invertebrate / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D012433:Ryanodine / Q000494:pharmacology*
chemical_list: D036563:Cyclic ADP-Ribose; D012433:Ryanodine; D000246:Adenosine Diphosphate Ribose; D002129:Calcium Oxalate; D002110:Caffeine; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium