Abstract
The photoreceptor cells of the blowfly, Calliphora erythrocephala , have been shown to contain a well-developed network of subrhabdomeric (=submicrovillar) smooth endoplasmic reticulum (SER) cisternae. When the cells are made permeable by saponin treatment and subsequently incubated in a loading medium containing ∼ 8.5 × 10 −7 M free Ca 2+ , MgATP, and oxalate 2+ , the subrhabdomeric SER as well as mitochondrion associated SER contain electron-opaque deposits, identified as Ca oxalate by electron-probe microanalysis. Precipitate formation requires the presence of ATP in the loading medium. These results indicate that the SER is capable of actively accumulating Ca 2+ and suggest that the SER participates in the control of cytosolic free Ca 2+ in the submicromolar concentration range. In particular the subrhabdomeric SER may be important for Ca 2+ buffering in the immediate vicinity of the photoreceptive membrane, and thereby for control of photoreceptor sensitivity.
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