Aims/hypothesis. This study examined the biological effects of the GIP receptor antagonist, (Pro 3 )GIP and the GLP-1 receptor antagonist, exendin(9-39)amide. Methods Cyclic AMP production was assessed in Chinese hamster lung fibroblasts transfected with human GIP or GLP-1 receptors, respectively. In vitro insulin release studies were assessed in BRIN-BD11 cells while in vivo insulinotropic and glycaemic responses were measured in obese diabetic (ob/ob) mice. Results. In GIP receptor-transfected fibroblasts, (Pro 3 )-GIP or exendin(9-39)amide inhibited GIP-stimulated cyclic AMP production with maximal inhibition of 70.0±3.5% and 73.5±3.2% at 10 −6 mol/l, respectively. In GLP-1 receptor-transfected fibroblasts, exendin-(9-39)amide inhibited GLP-1-stimulated cyclic AMP production with maximal inhibition of 60±0.7% at 10 −6 mol/l, whereas (Pro 3 )GIP had no effect. (Pro 3 )-GIP specifically inhibited GIP-stimulated insulin release (86%; p<0.001) from clonal BRIN-BD11 cells, but had no effect on GLP-1-stimulated insulin release. In contrast, exendin(9-39)amide inhibited both GIP and GLP-1-stimulated insulin release (57% and 44%, respectively; p<0.001). Administration of (Pro 3 )GIP, exendin(9-39)amide or a combination of both peptides (25 nmol/kg body weight, i.p.) to fasted (ob/ob) mice decreased the plasma insulin responses by 42%, 54% and 49%, respectively (p<0.01 to p<0.001). The hyperinsulinaemia of non-fasted (ob/ob) mice was decreased by 19%, 27% and 18% (p<0.05 to p<0.01) by injection of (Pro 3 )GIP, exendin(9-39)amide or combined peptides but accompanying changes of plasma glucose were small. Conclusions/interpretation. These data show that (Pro 3 )GIP is a specific GIP receptor antagonist. Furthermore, feeding studies in one commonly used animal model of obesity and diabetes, (ob/ob) mice, suggest that GIP is the major physiological component of the enteroinsular axis, contributing approximately 80% to incretin-induced insulin release. [Diabetologia (2003) 46:222-230] Keywords Enteroinsular axis, GIP receptor antagonist, glucagon-like peptide-1, glucose-dependent insulinotropic polypeptide, insulin release. The insulin secretory activity of pancreatic beta cells can be modulated in both a positive or negative manner by several peptide hormones and neurotransmitters [1,2,3]. The two intestinal hormones, glucosedependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1-(7-36)amide (GLP-1) have long been recognised as potent stimulators of insulin secretion under physiological conditions, and for that reason they are considered important incretin hormones [4,5,6]. The effects of both GIP and GLP-1 are glucose-dependent, providing a safeguard against potentially hazardous hypoglycaemic episodes, and as a

Although the incretin hormone glucagon-like peptide-1 (GLP-1) is a potent stimulator of insulin release, its rapid degradation in vivo by the enzyme dipeptidyl peptidase IV (DPP IV) greatly limits its potential for treatment of type 2 diabetes. Here, we report two novel

Glucagon-like peptide-1(7-36)amide (GLP-1) possesses several unique and beneficial effects for the potential treatment of type 2 diabetes. However, the rapid inactivation of GLP-1 by dipeptidyl peptidase IV (DPP IV) results in a short half-life in vivo (less than 2 min) hindering therapeutic development. In the present study, a novel His 7 -modified analogue of GLP-1, N-pyroglutamyl-GLP-1, as well as N-acetyl-GLP-1 were synthesised and tested for DPP IV stability and biological activity. Incubation of GLP-1 with either DPP IV or human plasma resulted in rapid degradation of native GLP-1 to GLP-1(9-36)amide, while N-acetyl-GLP-1 and N-pyroglutamyl-GLP-1 were completely resistant to degradation. N-acetyl-GLP-1 and N-pyroglutamyl-GLP-1 bound to the GLP-1 receptor but had reduced affinities (IC 50 values 32·9 and 6·7 nM, respectively) compared with native GLP-1 (IC 50 0·37 nM). Similarly, both analogues stimulated cAMP production with EC 50 values of 16·3 and 27 nM respectively compared with GLP-1 (EC 50 4·7 nM). However, N-acetyl-GLP-1 and Npyroglutamyl-GLP-1 exhibited potent insulinotropic activity in vitro at 5·6 mM glucose (P,0·05 to P,0·001) similar to native GLP-1. Both analogues (25 nM/kg body weight) lowered plasma glucose and increased plasma insulin levels when administered in conjunction with glucose (18 nM/kg body weight) to adult obese diabetic (ob/ob) mice. N-pyroglutamyl-GLP-1 was substantially better at lowering plasma glucose compared with the native peptide, while N-acetyl-GLP-1 was significantly more potent at stimulating insulin secretion. These studies indicate that N-terminal modification of GLP-1 results in DPP IV-resistant and biologically potent forms of GLP-1. The particularly powerful antihyperglycaemic action of N-pyroglutamyl-GLP-1 shows potential for the treatment of type 2 diabetes.

Age-related macular degeneration (AMD) leads to irreversible visual loss, therefore, early intervention is desirable, but due to its multifactorial nature, diagnosis of early disease might be challenging. Identification of early markers for disease development and progression is key for disease diagnosis. Suitable biomarkers can potentially provide opportunities for clinical intervention at a stage of the disease when irreversible changes are yet to take place. One of the most metabolically active tissues in the human body is the retina, making the use of hypothesis-free techniques, like metabolomics, to measure molecular changes in AMD appealing. Indeed, there is increasing evidence that metabolic dysfunction has an important role in the development and progression of AMD. Therefore, metabolomics appears to be an appropriate platform to investigate disease-associated biomarkers. In this review, we explored what is known about metabolic changes in the retina, in conjunction with the emerging literature in AMD metabolomics research. Methods for metabolic biomarker identification in the eye have also been discussed, including the use of tears, vitreous, and aqueous humor, as well as imaging methods, like fluorescence lifetime imaging, that could be translated into a clinical diagnostic tool with molecular level resolution.

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