CA-2+ LIMITS THE DEVELOPMENT OF THE LIGHT RESPONSE IN DROSOPHILA PHOTORECEPTORS

The development of the light response was followed in Drosophila photo receptors at 25-degrees-C. In whole-cell recordings from dissociated o mmatidia, responses to light were first detected at 82 h post-puparium formation; over the next 8 h sensitivity to light increased exponenti ally by 5 or 6 orders of magnitude. The end of this phase coincided wi th the maturation of the rhabdomere as measured by whole-cell capacita nce. There was a modest 5-10fold further increase in sensitivity over the final 10 h of pupal development (90-100 h). During a narrow develo pmental time window (82-87 h) no responses could be detected using non -invasive recording techniques (electroretinogram or suction electrode ), and responses to light could only be elicited in whole-cell recordi ngs when micromolar concentrations of Ca2+ are included in the pipette . It seems unlikely that cytosolic Ca2+ per se is the limiting factor, and we suggest instead that the failure to respond to light is due to the lack of Ca2+ in the InsP3-sensitive intracellular stores and that the presence of Ca2+ in these stores is an absolute requirement for p hototransduction in Drosophila.