21.21:
Hormones Regulating Blood Glucose
The pancreatic hormones glucagon and insulin regulate blood glucose levels.
Between meals or during fasting, the blood glucose levels drop, triggering the pancreatic alpha cells to release glucagon.
Glucagon's primary function is to promote the synthesis and release of glucose, consequently increasing the amount of blood glucose.
In the liver and skeletal muscles, glucagon stimulates glycogenolysis, which converts glycogen to glucose.
It also stimulates the conversion of carbon-containing compounds, such as lactic acid and amino acids, into glucose through gluconeogenesis.
Right after meals, the blood glucose levels surge, triggering the pancreatic beta cells to release insulin.
Insulin binds its receptors on the target cell membrane and promotes glucose uptake and metabolism.
After glucose enters the cells, insulin further stimulates glycolysis, accelerating the rate of glucose breakdown and ATP production.
It also promotes protein synthesis, glycogen synthesis, and conversion of glucose to fat to prevent excess blood glucose levels.
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.