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Although bone has long been recognized as a target for hormones, only recently have researchers recognized that the skeleton itself produces at least two hormones. Fibroblast growth factor 23 (FGF23) is produced by bone cells in response to increased blood levels of vitamin D 3 or phosphate. It triggers the kidneys to inhibit the formation of calcitriol from vitamin D 3 and to increase phosphorus excretion. Osteocalcin, produced by osteoblasts, stimulates the pancreatic beta cells to increase insulin production. It also acts on peripheral tissues to increase their sensitivity to insulin and their utilization of glucose.
Adipose tissue produces and secretes several hormones involved in lipid metabolism and storage. One important example is leptin , a protein manufactured by adipose cells that circulates in amounts directly proportional to levels of body fat. Leptin is released in response to food consumption and acts by binding to brain neurons involved in energy intake and expenditure. Binding of leptin produces a feeling of satiety after a meal, thereby reducing appetite. It also appears that the binding of leptin to brain receptors triggers the sympathetic nervous system to regulate bone metabolism, increasing deposition of cortical bone. Adiponectin—another hormone synthesized by adipose cells—appears to reduce cellular insulin resistance and to protect blood vessels from inflammation and atherosclerosis. Its levels are lower in people who are obese, and rise following weight loss.
The skin functions as an endocrine organ in the production of the inactive form of vitamin D 3 , cholecalciferol. When cholesterol present in the epidermis is exposed to ultraviolet radiation, it is converted to cholecalciferol, which then enters the blood. In the liver, cholecalciferol is converted to an intermediate that travels to the kidneys and is further converted to calcitriol, the active form of vitamin D 3 . Vitamin D is important in a variety of physiological processes, including intestinal calcium absorption and immune system function. In some studies, low levels of vitamin D have been associated with increased risks of cancer, severe asthma, and multiple sclerosis. Vitamin D deficiency in children causes rickets, and in adults, osteomalacia—both of which are characterized by bone deterioration.
The thymus is an organ of the immune system that is larger and more active during infancy and early childhood, and begins to atrophy as we age. Its endocrine function is the production of a group of hormones called thymosins that contribute to the development and differentiation of T lymphocytes, which are immune cells. Although the role of thymosins is not yet well understood, it is clear that they contribute to the immune response. Thymosins have been found in tissues other than the thymus and have a wide variety of functions, so the thymosins cannot be strictly categorized as thymic hormones.
The liver is responsible for secreting at least four important hormones or hormone precursors: insulin-like growth factor (somatomedin), angiotensinogen, thrombopoetin, and hepcidin. Insulin-like growth factor-1 is the immediate stimulus for growth in the body, especially of the bones. Angiotensinogen is the precursor to angiotensin, mentioned earlier, which increases blood pressure. Thrombopoetin stimulates the production of the blood’s platelets. Hepcidins block the release of iron from cells in the body, helping to regulate iron homeostasis in our body fluids. The major hormones of these other organs are summarized in [link] .
| Organs with Secondary Endocrine Functions and Their Major Hormones | ||
|---|---|---|
| Organ | Major hormones | Effects |
| Heart | Atrial natriuretic peptide (ANP) | Reduces blood volume, blood pressure, and Na + concentration |
| Gastrointestinal tract | Gastrin, secretin, and cholecystokinin | Aid digestion of food and buffering of stomach acids |
| Gastrointestinal tract | Glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 1 (GLP-1) | Stimulate beta cells of the pancreas to release insulin |
| Kidneys | Renin | Stimulates release of aldosterone |
| Kidneys | Calcitriol | Aids in the absorption of Ca 2+ |
| Kidneys | Erythropoietin | Triggers the formation of red blood cells in the bone marrow |
| Skeleton | FGF23 | Inhibits production of calcitriol and increases phosphate excretion |
| Skeleton | Osteocalcin | Increases insulin production |
| Adipose tissue | Leptin | Promotes satiety signals in the brain |
| Adipose tissue | Adiponectin | Reduces insulin resistance |
| Skin | Cholecalciferol | Modified to form vitamin D |
| Thymus (and other organs) | Thymosins | Among other things, aids in the development of T lymphocytes of the immune system |
| Liver | Insulin-like growth factor-1 | Stimulates bodily growth |
| Liver | Angiotensinogen | Raises blood pressure |
| Liver | Thrombopoetin | Causes increase in platelets |
| Liver | Hepcidin | Blocks release of iron into body fluids |
Some organs have a secondary endocrine function. For example, the walls of the atria of the heart produce the hormone atrial natriuretic peptide (ANP), the gastrointestinal tract produces the hormones gastrin, secretin, and cholecystokinin, which aid in digestion, and the kidneys produce erythropoietin (EPO), which stimulates the formation of red blood cells. Even bone, adipose tissue, and the skin have secondary endocrine functions.
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