4-phenyl-2-propionamidotetraline and Diabetes-Mellitus--Type-2

Melatonin has been shown to modulate glucose metabolism by influencing insulin secretion. Recent investigations have also indicated a regulatory function of melatonin on the pancreatic α-cells. The present in vitro and in vivo studies evaluated whether melatonin mediates its effects via melatonin receptors and which signaling cascade is involved. Incubation experiments using the glucagon-producing mouse pancreatic α-cell line αTC1 clone 9 (αTC1.9) as well as isolated pancreatic islets of rats and mice revealed that melatonin increases glucagon secretion. Preincubation of αTC1.9 cells with the melatonin receptor antagonists luzindole and 4P-PDOT abolished the glucagon-stimulatory effect of melatonin. In addition, glucagon secretion was lower in the pancreatic islets of melatonin receptor knockout mice than in the islets of the wild-type (WT) control animals. Investigations of melatonin receptor knockout mice revealed decreased plasma glucagon concentrations and elevated mRNA expression levels of the hepatic glucagon receptor when compared to WT mice. Furthermore, studies using pertussis toxin, as well as measurements of cAMP concentrations, ruled out the involvement of Gαi- and Gαs-coupled signaling cascades in mediating the glucagon increase induced by melatonin. In contrast, inhibition of phospholipase C in αTC1.9 cells prevented the melatonin-induced effect, indicating the physiological relevance of the Gαq-coupled pathway. Our data point to the involvement of the phosphatidylinositol 3-kinase signaling cascade in mediating melatonin effects in pancreatic α-cells. In conclusion, these findings provide evidence that the glucagon-stimulatory effect of melatonin in pancreatic α-cells is melatonin receptor mediated, thus supporting the concept of melatonin-modulated and diurnal glucagon release.

Topics: Animals; Cell Line; Cyclic AMP; Diabetes Mellitus, Type 2; Disease Models, Animal; Dose-Response Relationship, Drug; Gene Expression Regulation; Glucagon; Glucagon-Secreting Cells; GTP-Binding Protein alpha Subunits, Gq-G11; Liver; Male; Melanins; Mice; Mice, Knockout; Pertussis Toxin; Phosphatidylinositol 3-Kinase; Rats; Rats, Wistar; Receptor, Melatonin, MT1; Receptor, Melatonin, MT2; Receptors, Glucagon; RNA, Messenger; Signal Transduction; Tetrahydronaphthalenes; Tissue Culture Techniques; Tryptamines; Type C Phospholipases

Recent investigations have demonstrated an influence of melatonin on insulin secretion in pancreatic beta-cells. The effects are receptor-mediated via two parallel signaling pathways. The aim of this study was to examine the relevance of a second melatonin receptor (MT2) as well as the involvement of a third signaling cascade in mediating melatonin effects, i.e. the cyclic guanosine monophosphate (cGMP) pathway. Our results demonstrate that the insulin-inhibiting effect of melatonin could be partly reversed by preincubation with the unspecific melatonin receptor antagonist luzindole as well as by the MT2-receptor-specific antagonist 4P-PDOT (4-phenyl-2-propionamidotetraline). As melatonin is known to modulate cGMP concentration via the MT2 receptor, these data indicate transmission of the melatonin effects via the cGMP transduction cascade. Molecular investigations established the presence of different types of guanylate cyclases, cGMP-specific phosphodiesterases and cyclic nucleotide-gated channels in rat insulinoma beta-cells (INS1). Moreover, variations in mRNA expression were found when comparing day and night values as well as different states of glucose metabolism. Incubation experiments provided evidence that 3-isobutyl-1-methylxanthine (IBMX)-stimulated cGMP concentrations were significantly decreased in INS1 cells exposed to melatonin for 1 hr in a dose- and time-dependent manner. This effect could also be reversed by application of luzindole and 4P-PDOT. Stimulation with 8-Br-cGMP resulted in significantly increased insulin production. In conclusion, it could be demonstrated that the melatonin receptor subtype MT2 as well as the cGMP signaling pathway are involved in mediating the insulin-inhibiting effect of melatonin.

Topics: 1-Methyl-3-isobutylxanthine; Animals; Brain; Cell Line, Tumor; Colforsin; Cyclic GMP; Cyclic Nucleotide Phosphodiesterases, Type 2; Cyclic Nucleotide-Gated Cation Channels; Diabetes Mellitus, Type 2; Dose-Response Relationship, Drug; Glucose; Guanylate Cyclase; Insulin; Insulin Secretion; Insulin-Secreting Cells; Insulinoma; Melatonin; Pineal Gland; Rats; Rats, Wistar; Receptor, Melatonin, MT2; Signal Transduction; Tetrahydronaphthalenes; Tryptamines