Glucagon is a hormone produced by the alpha cells of the pancreas. Its effect is the opposite to insulin  -- it causes the liver to release stored glucose into the blood, raising blood glucose levels. This process is called glycogenolysis. Glucagon is the main counterregulatory hormone to insulin. 
Glucagon and insulin, in a healthy animal, form an equilibrium  that regulates blood sugar levels.  In a diabetic, the equilibrium mechanism is often seriously skewed or completely broken. 
The hormone is used at times for hypoglycemia treatment,  since it causes the liver to release its stores of glycogen, which the body turns into glucose.  Treating hypoglycemia with glucagon will be unsuccessful if the liver's glycogen stores are depleted, because there would be nothing available for release. 
This 1954 Lilly study  was an early one with regard to the insulin/hypoglycemia countering hormone glucagon. Cats were selected because of their sensitivity to high blood sugar and their well-known responses to it. It should be remembered that back in 1954, most of the work which had been done with regard to improving insulins had dealt with various ways to extend their activity. At the time this study was done, highly purified insulin was still a long way off, so it was possible to have insulin which might contain some glucagon via the extraction process.
Glucagon release at low, or fast-dropping blood sugar levels is known as Somogyi rebound.
- ↑ Glucose Metabolism. Intervet UK.
- ↑ Dean L, McEntyre J. (2004). The Genetic Landscape of Diabetes. National Institutes of Health.
- ↑ Physiologic Effects of Glucagon. Colorado State University School of Veterinary Medicine.
- ↑ Insulin & Glucagon--Glucose Metabolism. Intervet UK.
- ↑ Diagram of Glucose Metabolism. Intervet UK.
- ↑ Pathophysiology of Diabetes Mellitus-What Happens With Insufficient Insulin. Intervet UK.
- ↑ Pathophysiology of diabetes. Intervet UK.
- ↑ Glucagon Infusions. Southpaws.com.
- ↑ Greco, Deborah. FAQ's About Diabetic Dogs. BD Diabetes.
- ↑ Schaer, Michael (2003). Ineffectivity of Glucagon When Liver Glycogen Stores Are Depleted. District of Columbia Academy of Veterinary Medicine.
- ↑ Staub, A., Behrens, Otto K. (1954). Glucagon Content of Insulin Preparations. Journal of Clinical Investigation.
- Article on Glucagon/Insulin equilibrium Colorado State University School of Veterinary Medicine
- Illustration of the actions of insulin and glucagon in the bodyNussey S, Whitehead S., 2001, Endocrinology: An Integrated Approach
- Diagram of Glucagon Metabolism Intervet
- Hormones Which Raise of Lower Blood Glucose Clinical Chemistry
- Insulin and Glucagon Tutorial and Animation