INTRACELLULAR CA2- HETEROGENEITY WITHIN AND DISTINCTION FROM ENDOPLASMIC-RETICULUM( STORES OF RAT CEREBELLUM )

Rat cerebellum microsomes were subfractionated on isopycnic linear suc rose (20-42 %)-density gradients. The distribution of endoplasmic reti culum (ER) markers (RNA, signal-sequence receptor alpha, calnexin, cal reticulin, the immunoglobulin-binding protein Bip) and markers of intr acellular rapidly exchanging Ca2+ stores [Ca2+ channels sensitive to e ither Ins(1,4,5)P3 or ryanodine) was investigated biochemically and im munologically. The comparison indicates that: (a) vesicles bearing the InsP3 receptor were separated from those bearing the ryanodine recept or; (b) ER markers, i.e. Bip, calnexin, signal-sequence receptor a, RN A, did not sediment as either InsP3 or ryanodine receptors did; (c) ca lreticulin, an intralumenal low-affinity high-capacity Ca2+-binding pr otein, had a widespread distribution, similar to that of Bip and calne xin, and was present in Purkinje, granule, Golgi and stellate neurons, as indicated by immunofluorescent labelling of cerebellum cortex cryo sections. The present results show that the ER is not a homogeneous en tity, and that Ca2+ stores are heterogeneous insofar as InsP3 receptor s and ryanodine receptors are segregated, either to discrete intracell ular organelles or to specialized ER subcompartments.