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We investigated intracellular Ca(2+) ([Ca(2+)](i)) oscillations evoked by glucagon-like peptide 1 (GLP-1) in relation to the ryanodine receptor (RyR) and Ca(2+)-induced Ca(2+)release (CICR) mechanism in pancreatic B cell HIT. GLP-1 produced [Ca(2+)](i) oscillations in the cells, both in media with and without Ca(2+), an effect inhibited by ruthenium red and mimicked by 8-Br-cAMPS. In addition, the GLP-1-evoked [Ca(2+)](i) rise was initiated at the local intercellular peripheral cytoplasm, and a resultant expansion of the intercellular space was also observed. Caffeine induced [Ca(2+)](i) elevation in the medium with or without Ca(2+), an effect inhibited by ruthenium red. GLP-1-evoked [Ca(2+)](i) oscillations were also enhanced by IBMX, and eliminated by Rp-8-Br-cAMPS or 20 microM H-89 treatments whereas they were unaffected by 2 microM H-89 treatment. Forskolin caused a transient elevation in [Ca(2+)](i) that was reduced by Rp-8-Br-cAMPS, 2 microM or 20 microM H-89. Our results indicate that GLP-1 initially generated a local [Ca(2+)](i) elevation at the peripheral cytoplasm, subsequently producing [Ca(2+)](i) oscillations that were inhibited by ruthenium red, involving ryanodine-sensitive and cAMP-activated CICR mechanisms. The cytoplasmic levels of cAMP as well as local Ca(2+) might be responsible for [Ca(2+)](i) oscillations.

We investigated intracellular Ca2+ ([Ca2+]i) oscillations evoked by glucagon-like peptide 1 (GLP-1) in relation to the ryanodine receptor (RyR) and Ca2+-induced Ca2+ release (CICR) mechanism in pancreatic B cell HIT. GLP-1 produced [Ca2+]i oscillations in the cells, both in media with and without Ca2+, an effect inhibited by ruthenium red and mimicked by 8-Br-cAMPS. In addition, the GLP-1-evoked [Ca2+]i rise was initiated at the local intercellular peripheral cytoplasm, and a resultant expansion of the intercellular space was also observed. Caffeine induced [Ca2+]i elevation in the medium with or without Ca2+, an effect inhibited by ruthenium red. GLP-1-evoked [Ca2+]i oscillations were also enhanced by IBMX, and eliminated by Rp-8-Br-cAMPS or 20 µM H-89 treatments whereas they were unaffected by 2 µM H-89 treatment. Forskolin caused a transient elevation in [Ca2+]i that was reduced by Rp-8-Br-cAMPS, 2 µM or 20 µM H-89. Our results indicate that GLP-1 initially generated a local [Ca2+]i elevation at the peripheral cytoplasm, subsequently producing [Ca2+]i oscillations that were inhibited by ruthenium red, involving ryanodine-sensitive and cAMP-activated CICR mechanisms. The cytoplasmic levels of cAMP as well as local Ca2+ might be responsible for [Ca2+]i oscillations. [ABSTRACT FROM AUTHOR]

We have investigated intracellular Ca2+ mobilization in oscillations of cytoplasmic Ca2+ in response to glucagon-like peptide 1 (GLP-1) and glucose in clonal HIT insulinoma cells with a confocal laser-scanning microscope (CLSM). We also used electron probe X-ray microanalysis to determine the GLP-1- and glucose-induced changes in electrolyte levels in the cytoplasm and insulin granules of the cells. GLP-1 produced 10- to 35-s duration oscillations in cytoplasmic Ca2+ concentration ([Ca2+]i), both with and without Ca2+ in the extracellular solution, suggesting that Ca2+ is mobilized from intracellular Ca2+ stores, namely secretory granules. Glucose caused 1- to 3-min duration oscillatory increases in [Ca2+]i when the extracellular solution contained Ca2+. When the cells were cultured without Ca2+ (no Ca2+ added, 1 mM EGTA), an oscillatory [Ca2+]i increase of amplitude and short duration (12–35 s) was produced by 11 mM glucose, and the oscillation was inhibited by ruthenium red. X-ray microanalysis showed that stimulation with glucose increased the total Ca concentration in the cytoplasm and decreased it in the insulin granules with and without Ca2+ in the extracellular solution. The application of glucose significantly decreased K, and increased Na and Cl in the cytoplasm when the extracellular solution contained Ca2+. Our result also suggests that the [Ca2+]i oscillation induced by glucose is involved in the release of Ca2+ from intracellular Ca2+ stores through the ryanodine receptor, which is blocked by ruthenium red, and/or through the inositol trisphosphate receptor that may be present in the membrane of insulin granules. [ABSTRACT FROM AUTHOR]