Abstract
In order to test the hypothesis that excitation in Drosophila photoreceptors is mediated by Ca2+ released from internal stores, the Ca2+ buffers EGTA, BAPTA and di-bromo-BAPTA (DBB) were introduced into dissociated photoreceptors via whole-cell recording pipettes. All buffers were preloaded with Ca2+ to provide the same free Ca2+ concentration (250 nM). EGTA (up to 18 mM free buffer) had only weak effects upon voltage-clamped flash responses in normal Ringer's solution (1.5 mM Ca 2+0 ), and no effect in Ca2+-free solution. The maximum BAPTA concentration tested (14.4 mM free BAPTA) reduced the initial rate of rise by ca. 5000-fold in normal Ringer's solution; by ca. 500-fold in Ca2+free solution; and only ca. 60-fold in the absence of Mg2+, which preferentially blocks one component of the light-sensitive current. Although BAPTA delayed the time-to-peak in normal Ringer's solution, responses in Ca2+ free Ringer's solution were accelerated. These results support the role of Ca2+ influx in regulating sensitivity and response kinetics; however, in view of the high concentrations required to attenuate responses in Ca2+ free Ringer's solution, the role of Ca2+ release in excitation remains unclear. DBB was ca. 2–3 fold more potent than BAPTA, and at concentrations > 5 mM had a qualitatively different action, greatly delaying the time-to-peak. This suggests DBB may have distinct pharmacological actions or access to compartments inaccessible to BAPTA.
The only current activated by introducing 5–500 μM Ca2+ (buffered with nitrilo-triacetic acid) was electrogenic Na+/Ca2+ exchange. When this was blocked by removing Nao +0 , a novel cationic conductance was activated. However, its properties did not resemble those the light-activated conductance, and thus do not support the hypothesis that Ca2+ is sufficient for excitation.
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Abbreviations
- BAPTA :
-
bis-(o-aminophenoxy)-ethane-N,N,N′-tetracetic acid
- DBB :
-
Di-bromo-bapta
- NTA :
-
nitrilo-triacetic acid
- InsP 3 :
-
inositol 1,4,5-trisphosphate
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Hardie, R.C. Effects of intracellular Ca2+ chelation on the light response in Drosophila photoreceptors. J Comp Physiol A 177, 707–721 (1995). https://doi.org/10.1007/BF00187630
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DOI: https://doi.org/10.1007/BF00187630