blood glucose control

Meaning: Rising glucose after a meal stimulates β cells of the pancreatic islets to release insulin. Insulin increases glucose uptake into muscle and adipose tissue (via GLUT4) and promotes glycogen and fat synthesis, so blood glucose falls. Between meals, falling glucose stimulates α cells to release glucagon, which acts mainly on the liver to promote glycogenolysis and gluconeogenesis, so blood glucose rises. Epinephrine and cortisol also raise blood glucose during stress.

On the MCAT: Passages show a graph of glucose, insulin and glucagon after a meal or a fast, or an experiment that removes a hormone, cell type or receptor. Predict the direction: high glucose gives more insulin and less glucagon, and fasting gives the reverse. Diabetes is used as an illustration. In type 1, β cells are lost and insulin is low. In type 2, target cells respond poorly to insulin (insulin resistance). Both can leave blood glucose high.

Common confusion: Glucagon is not glycogen. Glucagon is a peptide hormone, and glycogen is the stored polymer of glucose. Students also assume any glycogen store can refill blood glucose. Liver cells have glucose-6-phosphatase and can release free glucose into blood. Muscle lacks it, so muscle glycogen fuels the muscle itself.