Magnesium-dependent block of the light-activated and trp-dependent conductance in Drosophila photoreceptors.
1. The effect of Mg2+ on the light-sensitive conductance in Drosophila photoreceptors was examined with the use of whole cell voltage-clamp recordings from dissociated ommatidia. In wild type (WT) photoreceptors, at resting potential (-70 mV). Mgo2+ reduces response amplitude by up to approximately 4-fold in the presence of normal (1.5 mM) Cao2+ and by up to 20-fold in the absence of Cao2+. The Mg2+ concentration required for 50% maximum block (K1/2) was approximately 1 mM with 1.5 mM Cao2+ and approximately 280 microM in Ca(2+)-free Ringer. 2. The Mg2+ block was largely relieved in photoreceptors of the transient receptor potential mutant (trp): the maximum block being only approximately twofold with a K1/2 of approximately 4 mM in both Ca(2+)-free and 1.5 mM Cao2+. 3. The Mg2+ block in WT, but not in trp, was strongly voltage dependent, being relieved by both hyperpolarization and depolarization. The Mg2+ block in WT also resulted in slower response kinetics because of the associated decrease in Ca2+ influx. 4. Noise analysis indicates that, with normal Ca(zero)2+, the Mg2+ block in WT is associated with a approximately 10-fold reduction in effective single-channel conductance al resting potential. 5. The results support the hypothesis that the trp gene encodes a subunit of a light-sensitive channel, which is required for sensitivity to block by Mg2+. The concentration and voltage dependence of the Mg2+ block suggest it plays an important physiological role in determining the gain, kinetics, and signal-to-noise of transduction.
Hardie
R C
RC
Department of Zoology, Cambridge University, United Kingdom.
Mojet
M H
MH
D000818:Animals; D002118:Calcium / Q000502:physiology; D004330:Drosophila; D004594:Electrophysiology; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D007700:Kinetics; D008274:Magnesium / Q000502:physiology*; D008564:Membrane Potentials / Q000502:physiology; D009154:Mutation; D009431:Neural Conduction / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010775:Photic Stimulation; D017956:Photoreceptor Cells, Invertebrate / Q000187:drug effects / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology
D007473:Ion Channels; D008274:Magnesium; D002118:Calcium