Glucagon is a key hormonal agent that, in cooperation with insulin, mediates homeostatic regulation of the amount of glucose in the blood. Glucagon primarily acts by stimulating certain cells (important among these are liver cells) to release glucose when blood glucose levels fall. The action of glucagon is opposite to that of insulin, which stimulates cells to take up and store glucose whenever blood glucose levels rise. Both glucagon and insulin are peptide hormones. Glucagon is produced in the alpha islet cells of the pancreas and insulin is produced in the beta islet cells. Glucagon exerts its action by binding to and activating its receptor, which is a member of the Glucagon-Secretin branch of the 7-transmembrane G-protein coupled receptor family. The receptor functions by activating the adenylyl cyclase second messenger system resulting in an increase in cAMP levels. The glucagon receptor, or naturally occurring variants of the receptor, may possess intrinsic constitutive activity, in vitro, as well as in vivo (i.e. activity in the absence of an agonist). Compounds acting as inverse agonists can inhibit this activity.
Diabetes mellitus is a common disorder of glucose metabolism. The disease is characterized by hyperglycemia and may be classified as type 1 diabetes, the insulin-dependent form, or type 2 diabetes, which is non-insulin-dependent in character. Subjects with type 1 diabetes are hyperglycemic and hypoinsulinemic, and the conventional treatment for this form of the disease is to provide insulin. However, in some patients with type 1 or type 2 diabetes, absolute or relative elevated glucagon levels have been shown to contribute to the hyperglycemic state. Both in healthy control animals as well as in animal models of type 1 and type 2 diabetes, removal of circulating glucagon with selective and specific antibodies has resulted in reduction of the glycemic level. Mice with a homozygous deletion of the glucagon receptor exhibit increased glucose tolerance. Also, inhibition of glucagon receptor expression using antisense oligonucleotides ameliorates diabetic syndrome in db/db mice. These studies suggest that glucagon suppression or an action that antagonizes glucagon could be a useful adjunct to conventional treatment of hyperglycemia in diabetic patients. The action of glucagon can be suppressed by providing an antagonist or an inverse agonist, i.e. substances that inhibit or prevent constituitive, or glucagon-induced, glucagon receptor mediated responses.