Ca²⁺ release from endoplasmic reticulum is mediated by a guanine nucleotide regulatory mechanism

Ca²⁺ accumulation and release from intracellular organelles is important for Ca2+ -signalling events within cells^1,2. In a variety of cell types, the active Ca²⁺-pumping properties of endoplasmic reticulum (ER) have been directly studied using chemically per-meabilized cells^3-6. 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 fractions^2,11,13-15 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.