21.21: Hormones Regulating Blood Glucose
Insulin is released by beta cells of the pancreas when blood glucose levels are high. It facilitates glucose absorption and utilization in insulin-dependent cells with insulin receptors on their plasma membranes. Insulin promotes glucose uptake by increasing the number of glucose transport proteins in the cell membrane, allowing glucose to enter the cell. As a result, glucose utilization and ATP production are enhanced.
In addition to accelerating glucose uptake and utilization, insulin has other effects. It stimulates glycogen formation, a glucose storage polysaccharide, in skeletal muscle fibers and liver cells for later use. Insulin also promotes amino acid absorption and protein synthesis in all target cells, preventing the conversion of amino acids into glucose. Furthermore, insulin stimulates the formation of triglycerides in adipocytes, facilitating the absorption of fatty acids and glycerol.
However, it is important to note that not all cells in the body are insulin-dependent. Cells in the brain, kidneys, lining of the digestive tract, and red blood cells lack insulin receptors but can still absorb and use glucose independently of insulin stimulation.
When blood glucose levels drop below normal, the pancreas releases glucagon from alpha cells to mobilize energy reserves. Glucagon binds to its receptor on the target cell's plasma membrane, activating adenylate cyclase and producing cAMP as a second messenger. The primary effects of glucagon include stimulating the breakdown of glycogen in skeletal muscle fibers and liver cells releasing glucose molecules for energy metabolism or into the bloodstream. Glucagon also stimulates the breakdown of triglycerides in adipocytes, releasing fatty acids for use by other tissues. Additionally, it stimulates glucose production and release from liver cells through gluconeogenesis, contributing to an increase in blood glucose concentration.
The secretion of glucagon and insulin is regulated by pancreatic alpha and beta cells, respectively, in response to changes in blood glucose levels.
