Effect of guanosine triphosphate on the release of Ca²⁺ from intracellular store sites of saponin-treated human peripheral lymphocytes

The effects of guanosine triphosphate (GTP) on the release and uptake of Ca²⁺ in nonmitochondrial intracellular store sites of human peripheral lymphocytes were examined. GTP in the presence of 3% polyethylene glycol released Ca²⁺ from the intracellular store sites of lymphocytes in a dose-dependent manner, and the maximal release was obtained at 10 μm GTP. GDP and 5′-GMP also enhanced the release of Ca²⁺. On the other hand, Ca²⁺ uptake in the presence of oxalate by saponin-treated lymphocytes was stimulated by GTP and this stimulation was abolished when polyethylene glycol was concomitantly present. The dose dependence of the stimulated Ca²⁺ uptake by GTP was much the same as that of the Ca²⁺ released by GTP. These results indicate that GTP has an inherent activity to release Ca²⁺ as well as to stimulate the uptake of Ca²⁺ in nonmitochondrial intracellular store sites of saponin-treated lymphocytes. The stimulatory effect of polyethylene glycol on GTP-mediated Ca²⁺ release may occur by inhibiting functions of the Ca²⁺ pump.