Calcium uptake by smooth endoplasmic reticulum of peeled retinal photoreceptors of the crayfish

Abstract

The localization and basic properties of Ca2+-accumulating sites in crayfish photoreceptors were studied with a novel preparation of peeled retinula cells in suspension. Peeled photoreceptors were obtained by gentle mechanical disruption of the retina, and incubated in media based on a Ca2+-EGTA buffer with ATP and oxalate. Electron microscopy of photoreceptors so treated showed the appearance of peculiar dense deposits inside vesicles of smooth endoplasmic reticulum (SER). EGTA-extraction and energy-dispersive X-ray microanalysis identified Ca as a major constituent of such deposits.45Ca2+ uptake experiments with peeled photoreceptors or with the crude particulate fraction of retinal homogenates revealed a rapid binding of radioactivity over the first 8 min, followed by a slower continued accumulation, which did not occur in the absence of ATP.45Ca2+ uptake is stimulated by an increase in the concentration of free Ca over the range 4×10−7 to 5×10−6 M, and becomes inhibited at higher levels.45Ca2+ uptake is depressed when K+ is replaced by Na+ or Li+ as the main monovalent cation in the medium, but it is not affected by illumination nor by the presence of caffeine or ruthenium red. These findings attest that the SER has a Na+-sensitive capacity for regulating the intracellular concentration of Ca2+ in these photoreceptors, and support the hypothesis of its probable role in the control of pigment granule transport and other structural changes involved in light/dark adaptation.