Glucagon-Like Peptide 1 Elevates Cytosolic Calcium in Pancreaticβ -Cells Independently of Protein Kinase A

Glucagon-like peptide 1 (7–36)amide (GLP-1) is an insulinotropic intestinal peptide hormone with a potential role as antidiabetogenic therapeutic agent. It mediates a potentiation of glucose-induced insulin secretion, by activation of adenylate cyclase and subsequent elevation of cytosolic free calcium,[ Ca2+]cyt. We investigated the role of protein kinase A (PKA) in GLP-1 signal transduction, using isolated mouse islets as well as the differentiated β-cell line INS-1. Two specific inhibitors of PKA, (Rp)-adenosine cyclic 3′,5′-phosporothioate (Rp-cAMPS, up to 3 mm) and KT5720 (up to 10 μm), did not inhibit the GLP-1-induced[ Ca2+]cyt elevation. Another PKA inhibitor, H-89, reduced the [Ca2+]cyt elevation only when applied at high concentrations (10–40 μm), higher than sufficient for PKA inhibition in many cell types. Furthermore, at these concentrations, H-89 also inhibited presumably PKA-independent processes such as glucose-induced [Ca2+]cyt elevations and intracellular calcium storage. This suggests a PKA-independent action of H-89. Similarly to H-89, the potent but unselective protein kinase inhibitor staurosporine inhibited the GLP-1-induced [Ca2+]cyt elevation only at high concentrations, at which it also inhibited glucose-induced [Ca2+]cyt elevations. The same observations as with GLP-1 were made when adenylate cyclase was stimulated with forskolin, for selective examination of signal transduction downstream of receptor and G protein. Our results suggest that the GLP-1-induced[ Ca2+]cyt elevation is mediated independently of PKA and thus belongs to the yet-little-characterized ensemble of effects that are mediated by binding of cAMP to other target proteins.