SUBCELLULAR CALCIUM LOCALIZATION AND ATP-DEPENDENT CA2+-UPTAKE BY SMOOTH ENDOPLASMIC-RETICULUM IN AN INVERTEBRATE PHOTORECEPTOR CELL - ULTRASTRUCTURAL, CYTOCHEMICAL AND X-RAY MICROANALYTICAL STUDY

Abstract

In Hirudo medicinalis an extensive and highly elaborate 3 dimensional network of smooth endoplasmic reticulum cisternae is found in very close structural relationship to the receptive (microvillar) membrane, as reported for many other invertebrates. A variant of the potassium pyroantimonate technique showed that these submicrovillar endoplasmic reticulum cisternae (SMC) and mitochondria are major intracellular Ca stores. Using saponine-skinned photoreceptors for an in situ accumulation experiment, calcium oxalate precipitates in SMC demonstrate that this organelle accumulates Ca2+ from a concentration of 2 .times. 10-5 M, when ATP, Mg2+ and oxalate ions are present in the accumulation medium. SMC may play a particularly important role in the regulation of intracellular ionized Ca in invertebrate photoreceptor cells. Morphological evidence supports this view.