In the human body, the most significant use of carbohydrates is for energy production. Carbs are ingested in many forms, the majority of which are made up of the sugar units glucose and fructose. Use of these sugars by cells is governed by several hormones, one of which is called epinephrine, or adrenaline, which is produced and released by the adrenal gland.
Function
The adrenal glands play a role in carbohydrate metabolism through secreting adrenaline into the bloodstream. This hormone, notes Dr. Lauralee Sherwood in her book, "Human Physiology," plays an important role in stress response. Once the adrenal glands release adrenaline into the bloodstream, it travels throughout the body and binds to receptors on many cells. Most relevant to carbohydrate metabolism are the receptors on liver and muscle cells. Once adrenaline binds to these receptors, cells respond by producing a molecule called cyclic AMP, which signals the cells to modify their metabolism of carbs.
Significance
The purpose of adrenaline playing a role in modifying carbohydrate metabolism is that under stressful conditions, human bodies need more energy available to them. Both the muscles and the liver store glycogen, which is a storage form of the sugar glucose. Adrenaline, through causing production of cyclic AMP, signals liver and muscle cells to break down glycogen into glucose. The liver releases this glucose into the bloodstream, explain Drs. Reginald Garrett and Charles Grisham in their book, "Biochemistry," while the muscles use the glucose for their own purposes.
Benefits
While other hormones can cause the liver and muscles to break down glycogen and increase available glucose, adrenaline from the adrenal glands provides an additional benefit--it activates a pathway whereby the cells can make glucose out of other molecules, including lactic acid, glycerol and various amino acids, which are the building blocks of protein. Note Drs. Mary Campbell and Shawn Farrell in their book, "Biochemistry," this process, called gluconeogenesis, is very important under stressful circumstances because it ensures that even if glycogen levels become depleted, cells will have adequate glucose.
Considerations
The adrenal glands also help direct cells--specifically muscle cells--toward the most efficient possible mechanism for burning glucose. Adrenaline, when it binds to a muscle cell receptor, causes a muscle cell organelle called the sarcoplasmic reticulum to release calcium ions, which increase the rate of glucose metabolism. In particular, note Garrett and Grisham, calcium release signals the muscles to burn glucose aerobically, meaning using oxygen as a reactant, which increases the efficiency of glucose.
Expert Insight
While the adrenal glands have tremendous influence over carbohydrate metabolism, they don't act autonomously, explains Sherwood. Instead, the glands are under the control of the anterior pituitary gland, which is found in the brain. The anterior pituitary produces and releases a hormone called adrenocorticotropic hormone, or ACTH, which binds to receptors in the adrenal glands. With ACTH bound, adrenal cells begin to convert the amino acid tyrosine into epinephrine, or adrenaline, ultimately leading to a store of epinephrine in the adrenal gland that can be released when needed.
References
- "Human Physiology"; Lauralee Sherwood; 2004
- "Biochemistry"; Reginald Garrett and Charles Grisham; 2007
- "Biochemistry"; Mary Campbell and Shawn Farrell; 2005
