Ca2+ accumulation and release from intracellular organelles is important for Ca2+ -signalling events within cells1,2. In a variety of cell types, the active Ca2+-pumping properties of endoplasmic reticulum (ER) have been directly studied using chemically per-meabilized cells3-6. The same preparations have been extensively used to study Ca2+ release from ER, in particular, release mediated by the intracellular messenger inositol 1,4,5-trisphosphate (InsP3) 1,2,5,7-12. So far, these studies and others using microsomal membrane fractions2,11,13-15 have revealed few mechanistic details of Ca2+ release from ER, although a recent report16 indicated that InsP3-mediated Ca22+ release from liver microsomes may be dependent on GTP. In contrast to the latter report, we describe here the direct activation of a specific and sensitive guanine nucleotide regulatory mechanism mediating a substantial release of Ca 2+ from the ER of cells of the neuronal cell line N1E-115. These data indicate the operation of a major new Ca2+ gating mechanism in ER which is specifically activated by GTP, deactivated by GDP, and which appears to involve a GTP hydrolytic cycle.
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