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

Summary 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.