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 beta-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 microM), 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 microM), 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.
This study examines the calcium store-regulated (capacitative) calcium influx pathway in the endocrine pancreatic cell line RINmSF, utilizing thapsigargin. After preincubation of the cells with the phorbol ester TPA, thapsigargin induced a sustained elevation of cytosolic calcium as well as a sustained stimulation of manganese entry, the latter being used to assess calcium influx. Thapsigargin given alone provoked a smaller and only transient elevation of cytosolic calcium and stimulation of manganese entry. The protein kinase C inhibitor staurosporine antagonized the effect of the phorbol ester. Verapamil, nifedipine, or measures to hyperpolarize the cells exerted no inhibitory actlon against this effect, which excludes an involvement of voltage-dependent calcium channels. In conclusion, our data shows for the first time that protein kinase C stimulation activates the capacitative calcium influx pathway of endocrine pancreatic insulin-producing cells.
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