Abstract
Clinical studies have shown that patients with early Type 2 diabetes often have elevated serum glucagon rather than insulin deficiency. Imbalance of insulin and glucagon in favouring the latter may contribute to impaired glucose tolerance, persistent hyperglycaemia, microalbuminuria and glomerular injury. In the present study, we tested the hypothesis that long-term glucagon infusion induces early metabolic and renal phenotypes of Type 2 diabetes in mice by activating glucagon receptors. Five groups of adult male C57BL/6J mice were treated with vehicle, glucagon alone (1 microg/h via an osmotic minipump, intraperitoneally), glucagon plus the glucagon receptor antagonist [Des-His(1)-Glu(9)]glucagon (5 microg/h via an osmotic minipump), [Des-His(1)-Glu(9)]glucagon alone or a high glucose load alone (2% glucose in the drinking water) for 4 weeks. Glucagon infusion increased serum glucagon by 129% (P<0.05), raised systolic BP (blood pressure) by 21 mmHg (P<0.01), elevated fasting blood glucose by 42% (P<0.01), impaired glucose tolerance (P<0.01), increased the kidney weight/body weight ratio (P<0.05) and 24 h urinary albumin excretion by 108% (P<0.01) and induced glomerular mesangial expansion and extracellular matrix deposition. These responses were associated with marked increases in phosphorylated ERK1/2 (extracellular-signal-regulated kinase 1/2) and Akt signalling proteins in the liver and kidney (P<0.01). Serum insulin did not increase proportionally. Concurrent administration of [Des-His(1)-Glu(9)]glucagon with glucagon significantly attenuated glucagon-increased BP, fasting blood glucose, kidney weight/body weight ratio and 24 h urinary albumin excretion. [Des-His(1)-Glu(9)]glucagon also improved glucagon-inpaired glucose tolerance, increased serum insulin by 56% (P<0.05) and attenuated glomerular injury. However, [Des-His(1)-Glu(9)]glucagon or high glucose administration alone did not elevate fasting blood glucose levels, impair glucose tolerance or induce renal injury. These results demonstrate for the first time that long-term hyperglucagonaemia in mice induces early metabolic and renal phenotypes of Type 2 diabetes by activating glucagon receptors. This supports the idea that glucagon receptor blockade may be beneficial in treating insulin resistance and Type 2 diabetic renal complications.