Guanine nucleotide-induced Ca2+ release in permeabilized murine thymocytes

GTP and IP3 induced Ca2+ release from an internal store in permeabilized murine thymocytes loaded with Ca2+ by ATP. Ca2+ release was dependent on the concentration of GTP: half-maximal release with 0.5 μM and maximal release with 10 μM. The GTP effect was completely abolished by 100 μM GTPγS, GMPPNP and UTP. None of the other nucleotides used except ITP induced Ca2+ release. When GTP was added after the effect of IP3 had virtually subsided, and vice versa, further Ca2+ release occurred, which led to the conclusion that the mechanism of GTP-mediated Ca2+ release may be different from that of IP3-mediated release.