PPARα is a ligand-activated transcription factor belonging to the nuclear receptor superfamily. PPARα is involved in the regulation of in vivo triglyceride levels, presumably through its effects on fatty acid and lipoprotein metabolism. Some nuclear receptors have been involved in rapid effects mediated by non-genomic mechanisms. In this paper, we report the rapid non-genomic effects of PPARα ligands on the intracellular calcium concentration ([Ca^{2 +}]_i), mitochondrial function, reactive oxygen species (ROS) generation, and secretion of insulin in freshly isolated mouse islets of Langerhans. The hypolipidemic fibrate PPARα agonist WY-14 643 decreased the glucose-induced calcium oscillations in intact islets. This effect was mimicked by the synthetic agonist GW7647 and the endogenous agonist oleylethanolamide. The WY-14 643 action was rapid in onset (5 min) and was still produced in the presence of protein and mRNA synthesis inhibitors, cycloheximide, and actinomycin-d. This suggests that it is independent of gene transcription. In addition, WY-14 623 impaired mitochondrial function, increased ROS formation and decreased insulin release. PPARα is present in β-cells, mainly in the cytosol and nucleus, with a small subpopulation localized in the plasma membrane. However, the presence of the PPARα ligand effects in mice bearing a disrupted Pparα gene raises the possibility that the rapid effects of the agonists in pancreatic β-cells are independent of the receptor. We conclude that PPARα agonists produce a decrease in glucose-induced [Ca^{2 +}]_i signals and insulin secretion in β-cells through a rapid, non-genomic mechanism.

The cannabinoid CB1 receptor is a well-known player in energy homeostasis and its specific antagonism has been used in clinical practice for the treatment of obesity. The G protein-coupled receptor GPR55 has been recently proposed as a new cannabinoid receptor and, by contrast, its pharmacology is still enigmatic and its physiological role is largely unexplored, with no reports investigating its putative role in metabolism. Thus, we aim to investigate in rats the presence, distribution and putative physiological role of GPR55 in a key metabolic tissue, the endocrine pancreas. We found high Gpr55 mRNA content in pancreatic islets and considerable protein distribution in insulin-secreting β-cells. Activation of GPR55 by the agonist O-1602 increased calcium transients (P<0.01) and insulin secretion (P<0.001) stimulated by glucose. This latter effect was blunted in Gpr55 KO mice suggesting that O-1602 is acting, at least in part, through GPR55. Indeed, acute in vivo experiments showed that GPR55 activation increases glucose tolerance (P<0.05) and plasma insulin levels (P<0.05), suggesting an in vivo physiological relevance of GPR55 systemic stimulation. Taken together, these results reveal the expression of GPR55 receptors in the endocrine pancreas as well as its function at stimulus-secretion coupling of insulin secretion, suggesting a role in glucose homeostasis. In this context, it may also represent a new target for consideration in the management of type 2 diabetes and related diseases.