In previous studies, glucagon receptor knockout mice (Gcgr ؊/؊ ) display reduced blood glucose and increased glucose tolerance, with hyperglucagonemia and increased levels of glucagon-like peptide (GLP)-1. However, the role of glucagon receptor signaling for the regulation of islet function and insulin sensitivity is unknown. We therefore explored ␤-cell function and insulin sensitivity in Gcgr G lucagon is a 29 -amino acid peptide secreted from pancreatic ␣-cells. In response to hypoglycemia, glucagon acts as a counterregulatory hormone to insulin, stimulating hepatic glucose production (HGP) and thus increasing blood glucose (1,2). In impaired glucose tolerance, suppression of glucagon secretion is impaired, leading to inappropriate glucagon levels in relation to the blood glucose level (3-5). As glucagon stimulates HGP, elevated glucagon levels may contribute to hyperglycemia in type 2 diabetes (6,7). In fact, several studies have demonstrated a correlation between elevated glucagon levels and increased HGP in type 2 diabetes (8), and animal studies have shown that inhibition of glucagon signaling has glucose-lowering effects (9 -12). Increased glucagon secretion has also been shown to be a strong predictor of the development of type 2 diabetes, suggesting that augmenting glucagon action is of pathophysiologic importance for type 2 diabetes (13). However, although glucagon has profound actions on HGP and also might be involved in diabetes pathophysiology, the role of glucagon for normal glucose homeostasis, insulin sensitivity, and islet function is not known.Mice with a targeted deletion in the glucagon receptor gene (Gcgr Ϫ/Ϫ ) have been developed to enable insights in the physiological actions of glucagon in glucose metabolism (14). Gcgr Ϫ/Ϫ mice have been found to exhibit improved glucose tolerance and reduced blood glucose levels in the presence of normal plasma insulin levels. In contrast, glucagon levels were dramatically increased, probably in compensation to the reduced glucagon receptor signaling. Interestingly, circulating levels of glucagonlike peptide (GLP)-1 of pancreatic origin were markedly increased in Gcgr Ϫ/Ϫ mice, while GLP-1 levels in intestinal extracts were similar to controls. Although islet function was not studied, it was found that the Gcgr Ϫ/Ϫ mice had altered islet architecture with marked islet and ␣-cell hyperplasia but without changes in ␤-cell morphology or pancreatic insulin content (14).The improved glucose tolerance and lowered blood glucose levels in Gcgr Ϫ/Ϫ mice point to a physiological role of glucagon signaling in glucose homeostasis. Here, we have therefore examined ␤-cell function and insulin sensitivity in Gcgr Ϫ/Ϫ mice. Hyperinsulinemic-euglycemic clamp studies were carried out to evaluate the effects of the glucagon receptor knockout on insulin sensitivity. In addition, the effect of glucagon receptor gene disruption on insulin secretion was investigated both in vivo and in vitro in isolated islets. RESEARCH DESIGN AND METHODS GcgrϪ/Ϫ mice were previously generate...

The human insulin receptor is expressed as two isoforms that are generated by alternate splicing of its mRNA; the B isoform has 12 additional amino acids

In type 2 diabetes, glucagon levels are elevated in relation to the prevailing insulin and glucose levels. The relative hyperglucagonemia is linked to increased hepatic glucose output (HGO) and hyperglycemia. Antagonizing the effects of glucagon is therefore considered an attractive target for treatment of type 2 diabetes. In the current study, effects of eliminating glucagon signaling with a glucagon monoclonal antibody (mAb) were investigated in the diabetic ob/ob mouse. Acute effects of inhibiting glucagon action were studied by an oral glucose tolerance test (OGTT) and by measurement of HGO. In addition, the effects of subchronic (5 and 14 days) glucagon mAb treatment on plasma glucose, insulin, triglycerides, and HbA 1c (A1C) levels were investigated. Glucagon mAb treatment reduced the area under the curve for glucose after an OGTT, reduced HGO, and increased the rate of hepatic glycogen synthesis. Glucagon mAb treatment for 5 days lowered plasma glucose and triglyceride levels, whereas 14 days of glucagon mAb treatment reduced A1C. In conclusion, acute and subchronic neutralization of endogenous glucagon improves glycemic control, thus supporting the contention that glucagon antagonism may represent a beneficial treatment of diabetes. Diabetes 55:2843-2848, 2006 I n type 2 diabetes, glucose-induced suppression of glucagon secretion is impaired, causing inappropriately elevated glucagon levels relative to the elevated glucose levels (1). Furthermore, endogenous glucose production is inappropriately increased in type 2 diabetes (2-4). Several studies have demonstrated a correlation between elevated glucagon levels and increased hepatic glucose production in both healthy subjects infused with exogenous glucagon (5-7) and type 2 diabetic patients (8 -10). Thus, reduction of glucagon levels is considered an interesting target for the treatment of type 2 diabetes, and therefore it is relevant to further characterize the relationship between hyperglucagonemia and the metabolic disturbances associated with type 2 diabetes.Immunoneutralization of endogenous glucagon with glucagon monoclonal antibodies (mAbs) provides a selective and complete suppression of glucagon action without directly affecting the secretion of other hormones. Studies using these antibodies have demonstrated significant blood glucose-lowering effects in diabetic animal models (11-13).In this study, monoclonal glucagon antibodies were used to provide isolated elimination of glucagon signaling in the ob/ob mouse, a commonly used model of type 2 diabetes displaying hyperglycemia, hyperinsulinemia, and insulin resistance (14 -16). With this approach, the aim was to gain further insight into the effects of glucagon on the regulation of hepatic glucose output (HGO) in an animal model of type 2 diabetes, both in the basal state and during the handling of a glucose load. Furthermore, the effects of prolonged elimination of glucagon signaling were studied. The results support the view that glucagon plays a prominent role in the hyperglycemia associated ...

MJ. Pancreatic ␤-cell overexpression of the glucagon receptor gene results in enhanced ␤-cell function and mass.

The relative expression patterns of the two IR (insulin receptor) isoforms, +/- exon 11 (IR-B/IR-A respectively), are tissue-dependent. Therefore we have developed insulin analogues with different binding affinities for the two isoforms to test whether tissue-preferential biological effects can be attained. In rats and mice, IR-B is the most prominent isoform in the liver (> 95%) and fat (> 90%), whereas in muscles IR-A is the dominant isoform (> 95%). As a consequence, the insulin analogue INS-A, which has a higher relative affinity for human IR-A, had a higher relative potency [compared with HI (human insulin)] for glycogen synthesis in rat muscle strips (26%) than for glycogen accumulation in rat hepatocytes (5%) and for lipogenesis in rat adipocytes (4%). In contrast, the INS-B analogue, which has an increased affinity for human IR-B, had higher relative potencies (compared with HI) for inducing glycogen accumulation (75%) and lipogenesis (130%) than for affecting muscle (45%). For the same blood-glucose-lowering effect upon acute intravenous dosing of mice, INS-B gave a significantly higher degree of IR phosphorylation in liver than HI. These in vitro and in vivo results indicate that insulin analogues with IR-isoform-preferential binding affinity are able to elicit tissue-selective biological responses, depending on IR-A/IR-B expression.