ABSTRACT
AIMS/HYPOTHESIS: To investigate the pathways by which cyclic AMP (cAMP) stimulates glucagon-like peptide-1 (GLP-1) secretion, using the GLUTag enteroendocrine cell line. MATERIALS AND METHODS: GLP-1 release from GLUTag cells was measured in response to agents that increase cAMP, and single cells were studied by fluorescence calcium imaging and electrophysiology to evaluate the underlying pathways. RESULTS: Pituitary adenylate cyclase-activating polypeptide increased cAMP levels and stimulated GLP-1 release from GLUTag cells. Agents that increase cAMP levels, including forskolin plus 3-isobutyl-1-methylxanthine (fsk/IBMX), triggered a rise in the intracellular calcium concentration and enhanced the response to glucose by increasing both the number of cells responding to glucose and the magnitude of calcium responses in individual cells. Importantly, fsk/IBMX also stimulated GLP-1 release and intracellular calcium elevation even in the absence of nutrients. fsk/IBMX triggered membrane depolarisation and the firing of action potentials, associated with a +14 mV shift in the voltage-dependence of activation of hyperpolarisation-activated currents and the closure of a background potassium conductance. CONCLUSIONS/INTERPRETATION: We show here that cAMP elevation directly triggers GLP-1 release and enhances the secretory response to other stimuli like glucose, by modulating hyperpolarisation-activated currents and the background potassium current. cAMP-elevating pathways and the cAMP-modulated conductances in L cells present important targets for the development of therapeutic GLP-1 secretagogues.
