The effects of GTP, with or without polyethylene glycol (PEG), on the release and uptake of Ca 2+ were examined by using saponin-treated macrophages and sarcoplasmic reticulum isolated from skeletal muscles. The application of GTP in concentrations in the range 0.1-10 μM induced a gradual, small but sustained release of Ca 2+ from the saponin-treated macrophages. The addition of PEG to GTP markedly enhanced the GTP-mediated Ca 2+ release. GTP at the same concentration ranges used for Ca 2+ release decreased the amount of Ca 2+ -precipitating anion. The addition of PEG abolished the GTP-evoked stimulation of Ca 2+ accumulation in the presence of oxalate. The stimulating effect on the rate of Ca 2+ accumulation by GTP and its elimination by PEG were not due to changes in the permeability of oxalate by either GTP or PEG., or both. The Ca 2+ -releasing effect of GTP without PEG was enhanced by eliminating the uptake activity by decreasing the content of ATP. These results indicate that GTP has an inherent activity to release Ca 2+ from non-mitochondrial intracellular stores of saponin-treated macrophages, and PEG enhances the GTP-mediated Ca 2+ release, partly owing to its eliminating effect on GTP-stimulated Ca 2+ uptake activity. These effects of GTP observed with saponin-permeabilized macrophages were not apparent in the isolated skeletal-muscle sarcoplasmic reticulum.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology