Growth hormone and somatostatin

Definitions & Key takeaways

Growth hormone (GH) is a peptide hormone that stimulates growth, cell reproduction, and regeneration. It is also known as somatotropin and is released by the anterior pituitary gland. Somatostatin is a peptide hormone produced by the hypothalamus. It inhibits the release of GH from the anterior pituitary gland and the secretion of insulin and glucagon from the pancreas, and also reduces gastrointestinal motility.

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Introduction0:00–0:50

Growth hormone, also known as somatotropin, is a hormone which helps regulate the rate of growth in the body. Secretion of growth hormone is dependent on the hypothalamic-pituitary axis.
The hypothalamus, which is a part of the brain, secretes growth hormone-releasing hormone into the hypophyseal portal system - which is a network of capillaries linking the hypothalamus to the anterior, or front part of the pituitary gland.
In the anterior pituitary, there are many different types of cells, each responsible for producing a type of hormone. The growth hormone-releasing hormone binds to a surface protein on one of these cells, called somatotroph cells, and stimulates them to release of growth hormone.
Normally, growth hormone releasing hormone is released in a pulsatile manner, throughout the day and peaks one hour after you fall asleep, but it is also secreted in response to various forms of internal and external stimuli.

Control0:50–3:02

For example, the hypothalamus senses when there’s hypoglycemia, or low blood sugar, and in response it secretes growth hormone releasing hormone.
Exercise causes the adrenal glands to secrete epinephrine and that stimulates the hypothalamus to secrete growth hormone releasing hormone as well.
Also, during puberty, increased levels of estrogen and testosterone stimulate the hypothalamus to release growth hormone releasing hormone, which is responsible for the growth spurt.
Now there are a few negative feedback loops that generally control the release of growth hormone. First, increased levels of growth-hormone-releasing hormone in the blood signals the hypothalamus to stop making more.
Second, when growth hormone reaches tissues like the liver, muscles, and bones, they make somatomedins, which are hormones that signal the anterior pituitary to stop producing growth hormone.
Third, growth hormone and somatomedins together signal to the hypothalamus to produce somatostatin, which is also called the growth hormone inhibiting hormone.
Somatostatin acts like a lawyer that tells other endocrine organs to “cease and desist” in producing more hormones. In the pituitary, it blocks growth hormone releasing hormone from acting on the somatotroph cells, so that secretion of growth hormone is inhibited.
In addition to the hypothalamus, somatostatin is also secreted in the gastrointestinal tract by the delta (δ) cells of the pancreas, where it inhibits the release of other hormones, like insulin, glucagon, and vasoactive intestinal peptides, as well as gastrin, from the stomach and small intestine.
Now, somatostatin is released in small amounts into the blood - just enough to reach local target organs, so somatostatin released in the gastrointestinal system and the hypothalamic-pituitary axis work independently.

Direct effects3:02–4:04

Growth hormone travels via the blood to various target tissues in the body to stimulate growth in two ways - directly and indirectly.
Direct effects of growth hormone is due to stimulation of cellular metabolism which leads to tissue growth. In adipose tissue, it causes lipolysis, which is the breakdown of fats for energy, and this fuels the increased metabolism of other cells in the body.
In the liver, it promotes gluconeogenesis, which is the production of glucose, and glycogenolysis, which is the breakdown of glycogen to give glucose.
Another direct effect of growth hormone is that it increases insulin resistance in tissues, which means that insulin is now less effective in moving glucose into the cells, leading to an increase in blood glucose levels.
This increased glucose level also stimulates the release of more insulin. And, because this is similar to what happens in people with diabetes, this effect of growth hormone is called diabetogenic.

Indirect effects4:04–5:05

Growth hormone also has indirect effects, which are seen in certain tissues like the liver, skeletal muscles, bones, and kidneys, where growth hormone stimulates the steady release of a type of somatomedin called somatomedin C, which is also called insulin-like growth factor 1, or IGF-1.
This IGF-1 then binds to insulin-like growth factor 1 receptors as well as insulin receptors, and promotes cellular metabolism, prevents cell death, and increase the rate of cell division and differentiation throughout the body.
In the muscles, it stimulates amino acid uptake into the muscle cells, which helps with protein production and muscle growth.
It’s also the key hormone that stimulates the growth in length of long bones. IGF-1 acts on the epiphyseal cartilage, or growth plates, of the bones, and stimulates the activity of osteoblasts in the bones and chondrocytes in the cartilage.
This helps boost growth, particularly during the growth spurts of puberty. Alright, as a quick recap… growth hormone is released by the anterior pituitary and is mainly regulated by two hormones from the hypothalamus; growth hormone releasing hormone which increases its release, and somatostatin which inhibits its release.

Review5:05–5:31

It promotes overall growth of the body through direct effects, like lipolysis, gluconeogenesis, and glycogenolysis, as well as indirect effects, mostly in the muscles and bones.