Involvement of the ryanodine-sensitive Ca²⁺ store in GLP-1-induced Ca²⁺ oscillations in insulin-secreting HIT cells

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.