Ca²⁺ stores in Purkinje neurons: Endoplasmic reticulum subcompartments demonstrated by the heterogeneous distribution of the InsP₃ receptor, Ca²⁺-ATPase, and calsequestrin

The nature of second messenger-responsive intracellular Ca²⁺ stores in neurons remains open for discussion. Here, we demonstrate the existence in Purkinje cells (PCs) of endoplasmic reticulum (ER) subcompartments characterized by an uneven distribution of three proteins involved in Ca²⁺ storage and release: the inositol 1,4,5-trisphosphate (InsP₃) receptor, Ca²⁺-ATPase, and calsequestrin. Ca²⁺-ATPase and the InsP₃ receptor have a widespread, although not identical, distribution throughout the ER. Calsequestrin is localized throughout the smooth ER and is particularly concentrated in pleiomorphic vesicles with a moderately electron-dense core, which appear to represent a subcompartment of the smooth ER. In double- labeling experiments many of these vesicles were unlabeled by InsP₃ receptor antibodies. These results suggest a key role of the ER as an intracellular Ca²⁺ store and demonstrate a possible structural basis for distinct intracellular Ca²⁺ pools regulated by different second messengers.