Calcium stores in drosophila retina revisited: An electron-cytochemical localization of calcium

According to the commonly accepted model of phototransduction in insects, the endoplasmic submicrovillar cisternae (SMC) is the main element of Ca²⁺ homeostasis in the photoreceptor cell. It is generally believed that the light-induced inositol 1,4,5-trisphosphate-mediated Ca²⁺ release from these putative Ca²⁺ stores is an obligatory intermediate event in the cascade of phototransduction resulting in activation of the light-sensitive channels. However, it appears that this model fits well mainly the insects with a fused rhabdom and large SMC. In this study it has been found electron-cytochemically that in Drosophila that has an open rhabdom and miniature SMC, the reaction product for Ca²⁺ (calcium oxalate) accumulates not in SMC, but in the specialized extracellular compartment formed by the distended lacunae at the bases of the rhabdomeral microvilli, which sometimes deeply invaginate into the submicrovillar cytoplasm. It is suggested that in Drosophila and probably in other insects with an open rhabdom, it is this extracellular compartment, not SMC, that together with the calphotin area of the cytoplasm functions as a key element of the Ca²⁺ homeostasis in the photoreceptor cell.