Mutation or Lanthanum Inhibits Light - induced C a 2 + Influx into Fly Photoreceptors

Ion-selective calcium microelectrodes were inserted into the compound eyes of the wild-type sheep blowfly Lucilia or into the retina of the no steady state (ms) mutant of Lucilia. These electrodes monitored light-induced changes in the extracellular concentration of calcium (A[Ca2+]o) together with the extracellularly recorded receptor potential. Prolonged dim lights induced a steady reduction in [Ca2+]o during light in the retina of normal Lucilia, while relatively litde change in [Ca2+]o was observed in the retina of the nss mutant. Prolonged intense light induced a multiphasic change in [Ca~+]o: the [Ca~+]o signal became transient, reaching a minimum within 6 s after light onset, and then rose to a nearly steady-state phase below the dark concentration. When lights were turned off, a rapid increase in [Ca2+]o was observed, reaching a peak above the dark level and then declining again to the dark level within 1 rain. In analogy to similar studies conduced in the honeybee drone, we suggest that the reduction in [Ca2÷]o reflects light-induced Ca 2÷ influx into the photoreceptors, while the subsequent increase in [Ca~÷]o reflects the activation of the Na-Ca exchange which extrudes Ca ~+ from the cells. In the ms mutant in response to intense prolonged light, the receptor potential declines to baseline during light while the Ca 2+ signal is almost abolished, revealing only a short transient reduction in [Ca~+]o. Application of lanthanum (LaSt), but not nickel (Ni2+), into the retinal extracellular space of normal Lucilia mimicked the effect of the ms mutation on the receptor potential, while complete elimination of the Ca ~+ signal in a reversible manner was observed. The results suggest that La 3+ and the ms mutation inhibit light-induced Ca ~+ influx into the photoreceptor in a manner similar to the action of the trp mutation in Drosophila, which has been shown to block specifically a light-activated Ca 2+ channel necessary to maintain light excitation. I N T R O D U C T I O N T he c o m p o u n d eye o f the fly has been an impor tant model system for studies of invertebrate phototransduct ion, particularly because of the available phototransducAddress reprint requests to Dr. Baruch Minke, Department of Physiology, The Hebrew UniversityHadassah Medical School, Jerusalem 91010, Israel. J. GEN. PHYSIOL. © The Rockefeller University Press 0022-1295/92/I 1/0767/15 $2.00 Volume 100 November 1992 767-781 767 on Jne 5, 2017 D ow nladed fom Published November 1, 1992