Whole-cell voltage clamp recordings were made from photoreceptors of dissociated Drosophila ommatidia under conditions when the light-sensitive channels activate spontaneously, generating a "rundown current" (RDC). The Ca2+ and voltage dependence of the RDC was investigated by applying voltage steps (+80 to -100 mV) at a variety of extracellular Ca2+ concentrations (0-10 mM). In Ca(2+)-free Ringer large currents are maintained tonically throughout 50-ms-long voltage steps. In the presence of external Ca2+, hyperpolarizing steps elicit transient currents which inactivate increasingly rapidly as Ca2+ is raised. On depolarization inactivation is removed with a time constant of approximately 10 ms at +80 mV. The Ca(2+)-dependent inactivation is suppressed by 10 mM internal BAPTA, suggesting it requires Ca2+ influx. The inactivation is absent in the trp mutant, which lacks one class of Ca(2+)-selective, light-sensitive channel, but appears unaffected by the inaC mutant which lacks an eye-specific protein kinase C. Hyperpolarizing voltage steps applied during light responses in wild-type (WT) flies before rundown induce a rapid transient facilitation followed by slower inhibition. Both processes accelerate as Ca2+ is raised, but the time constant of inhibition (12 ms with 1.5 mM external Ca2+ at -60 mV) is approximately 10 times slower than that of the RDC inactivation. The Ca(2+)-mediated inhibition of the light response recovers in approximately 50-100 ms on depolarization, recovery being accelerated with higher external Ca2+. The Ca2+ and voltage dependence of the light-induced current is virtually eliminated in the trp mutant. In inaC, hyperpolarizing voltage steps induced transient currents which appeared similar to those in WT during early phases of the light response. However, 200 ms after the onset of light, the currents induced by voltage steps inactivated more rapidly with time constants similar to those of the RDC. It is suggested that the Ca(2+)-dependent inactivation of the light-sensitive channels first occurs at some concentration of Ca2+ not normally reached during the moderate illumination regimes used, but that the defect in inaC allows this level to be reached.

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