Role of GLP-1 induced glucagon suppression in type 2 diabetes mellitus

This project consisted of two parts: a biochemical part and clinical studies. The overall aim was to elucidate the defective regulation of glucagon secretion in type 2 diabetes (T2DM). The aim in the biochemical part was to develop a glucagon ELISA by using C- and N-terminal antibodies generated in the laboratory. Much effort was put into this attempt; however, we were unsuccessful and had to use an alternative method in our attempt to characterize the paradoxical diabetic glucagon response further. By using Sep-Pac and HPLC separation methods, plasma from patients with T2DM known to have a defective suppression of glucagon was analyzed using three antibodies and RIA. In this way the hyperglucagonaemia was found to consist mainly of authentic glucagon, rather than abnormally processed forms. The first clinical study included ten healthy controls matched to ten patients with T2DM. The aim was to investigate if GLP-1 induced glucagon inhibition was dose dependent and if suppression was equally potent in healthy controls and T2DM patients. Further, we investigated if the potency of the inhibition depended on the prevailing plasma glucose (PG) level. All participants were investigated with increasing doses of GLP-1 administered as iv-infusions and saline (control) during a glycaemic clamp at fasting plasma glucose (FPG) levels. Patients were investigated on a third occasion with GLP-1 infusions after an over-night normalisation of PG using adjustable insulin infusions. From these experiments we were able to conclude that GLP-1-induced glucagon inhibition is dose-dependent, but surprisingly GLP-1 suppressed the alpha cell equally potently in patients and controls - and the suppression was independent of PG level. Therefore we concluded that the paradoxical glucagon response to orally ingested glucose is not caused by decreased potency of GLP-1 with respect to glucagon suppression. It may be due to the decreased secretion of this hormone reported in earlier studies. My second protocol aimed towards quantifying the glucose-lowering effect of GLP-1-induced glucagon inhibition seen in patients with T2DM. The glucose-lowering effect of GLP-1 is due to both insulin stimulation leading to peripheral glucose disposal and glucagon inhibition resulting in decreased stimulation of hepatic glucose production. With a five-day protocol including both glycaemic and pancreatic clamps in ten patients with T2DM we were able to isolate the contribution of glucagon suppression to the increased glucose turn-over seen during a GLP-1-glycaemic clamp, and interestingly it was equal to the known insulinotropic effect of GLP-1. Finally, we investigated patients with type 1 diabetes (T1DM) and no residual beta cell function with oral glucose tolerance test (OGTT) and isoglycaemic intravenous glucose infusion (IIGI) in order to evaluate any differences in glucagon response to glucose +/- gastri-intestinal (GI)-stimulation. Here we found that despite a perfectly normal inhibition of glucagon during the IIGI in the T1DM, they had a defective glucagon suppression in response to orally ingested glucose and a paradoxical secretion of glucagon was seen as in T2DM. Hereby, we proved that glucagon suppression in response to hyperglycaemia does not entirely depend on intra-islet insulin effects as has been suggested. Therefore we conclude that GI-tract factors rather than intraislet dysregulation explain the paradoxical glucagon response in patients with diabetes.