Inositol 1,4,5-trisphosphate and guanine nucleotides activate calcium release from endoplasmic reticulum via distinct mechanisms

A sensitive and specific guanine nucleotide regulatory process has recently been shown to rapidly mediate a substantial release of Ca²⁺ from endoplasmic reticulum within the N1E-115 neuronal cell line (Gill, D.L., Ueda, T., Chueh, S.H., and Noel, M.W. (1986) Nature 320, 461-464). The relationship between this mechanism and Ca²⁺ efflux mediated by the intracellular regulator inositol 1,4,5-trisphosphate (IP₃) has been investigated. Using saponin-permeabilized N1E-115 cells, studies reveal a number of distinctions between the activation of Ca²⁺ release mediated by GTP and IP₃. Thus, the GTP-mediated Ca²⁺ release process is specifically activated by polyethylene glycol which increases both GTP sensitivity and the extent of GTP-activated Ca²⁺ release; in contrast, IP₃-dependent Ca²⁺ release is unaffected by polyethylene glycol. The nonhydrolyzable GTP analogue guanosine 5'-O-(3-thio)triphosphate, which completely inhibits GTP-mediated Ca²⁺ release, does not alter release mediated by IP₃. Decreasing the release temperature from 37 to 4°C decreases IP₃-activated Ca²⁺ release by only 20%, whereas the action of GTP on Ca²⁺ release is abolished at 4° C. Activation of Ca²⁺ release by IP₃ is completely inhibited by increasing free Ca²⁺ from 0.1 to 10 μM, whereas the fraction of GTP-dependent Ca²⁺ release (approximately 50% of ionophore-releasable Ca²⁺) remains unaltered with increasing free Ca²⁺. These distinctions between IP₃- and GTP-mediated Ca²⁺ release indicate that the two effectors function via distinct mechanisms to activate Ca²⁺ release; however, they do not preclude the possibility that coupling between the two mechanisms can occur or that a common Ca²⁺-translocating pathway activated by both effectors exists.