GPCR Ligands Differentially Modulate Islet Metabolism and Ca 2+ Signaling

Abstract

Glucose homeostasis is a tightly regulated process coordinated by insulin and glucagon secretion. In α- and β-cells, G-protein coupled receptor (GPCR) activation facilitates the optimization of insulin and glucagon secretion. Glucagon-like peptide-1 (GLP-1), a ligand activating the Gs pathway, increases insulin secretion, but inhibits glucagon secretion. Neuropeptide-Y (NPY) and somatostatin (SST), whose receptors are coupled to Gi, inhibit insulin secretion; however, NPY stimulates glucagon release, while SST inhibits glucagon secretion. Here, we question whether and how these GPCR ligands impact cellular metabolism and oscillations of intracellular Ca2+ activity ([Ca2+]i) to alter secretion in the pancreatic islet. We utilized two-photon excitation microscopy to measure the combined autofluorescence of NADH and NADPH (NAD(P)H, a cellular redox state indicator) upon application of NPY, SST, or GLP-1. Under untreated conditions, an increase in extracellular glucose results in elevated NAD(P)H fluorescence intensities. Treatment with NPY and SST produces a supplementary increase in NAD(P)H autofluorescence at glucose concentrations above 5 mM compared to untreated control. NAD(P)H autofluorescence is not impacted significantly (relative to untreated control) by GLP-1 treatment, which is consistent with a previous report that found that GLP-1 does not alter β-cell metabolism. At glucose concentrations greater than ∼7 mM, pancreatic islets display synchronous oscillations in [Ca2+]i, leading to the pulsatile release of insulin. The [Ca2+]i oscillation frequency increases significantly in the presence of NPY. SST treatment decreases the oscillation frequency in β-cells, but does not affect α-cells. Addition of GLP-1 does not significantly alter the [Ca2+]i oscillation frequency. Thus, GLP-1 likely modulates insulin and glucagon secretion downstream of Ca2+ signaling. NPY may inhibit insulin secretion downstream of Ca2+ signaling, but stimulates glucagon secretion upstream of Ca2+ activity. Islet hormone secretion may be inhibited by SST through alteration of pathways up- and downstream of Ca2+ signaling.