Glucagon-like peptide-1 mobilizes intracellular Ca²⁺ and stimulates mitochondrial ATP synthesis in pancreatic MIN6 β-cells

Glucagon-like peptide-1 (GLP-1) is a potent regulator of glucose-stimulated insulin secretion whose mechanisms of action are only partly understood. In the present paper, we show that at low (3 mM) glucose concentrations, GLP-1 increases the free intra-mitochondrial concentrations of both Ca²⁺ ([Ca²⁺]_m), and ATP ([ATP]_m) in clonal MIN6 β-cells. Suggesting that cAMP-mediated release of Ca^2- from intracellular stores is responsible for these effects, increases in [ATP]_m that were induced by GLP-1 were completely blocked by the Rp isomer of adenosine-3′,5′-cyclic monophosphothioate (Rp-cAMPS), or by chelation of intracellular Ca²⁺. Furthermore, inhibition of Ins(1,4,5)P₃ (IP₃) receptors with xestospongin C, or application of ryanodine, partially inhibited GLP-1-induced [ATP]_m increases, and the simultaneous blockade of both IP₃ and ryanodine receptors (RyR) completely eliminated the rise in [ATP]_m. GLP-1 appeared to prompt Ca²⁺-induced Ca²⁺ release through IP₃ receptors via a protein kinase A (PKA)-mediated phosphorylation event, since ryanodine-insensitive [ATP]_m increases were abrogated with the PKA inhibitor, H89. In contrast, the effects of GLP-1 on RyR-mediated [ATP]_m increases were apparently mediated by the cAMP-regulated guanine nucleotide exchange factor cAMP-GEFII, since xestospongin C-insensitive [ATP]_m increases were blocked by a dominant-negative form of cAMP-GEFII (G114E,G422D). Taken together, these results demonstrate that GLP-1 potentiates glucose-stimulated insulin release in part via the mobilization of intracellular Ca²⁺, and the stimulation of mitochondrial ATP synthesis.