Adrenocorticotropic hormone

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Adrenocorticotropic hormone

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Adrenocorticotropic hormone
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Adrenocorticotropic hormone

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Adrenocorticotropic hormone acts on the of the adrenal gland.

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A 62-year-old woman comes to the clinic because she has gained considerable weight in the past few months. She says that despite a healthy diet, her waistline continues to expand and none of her clothes fit anymore. Her medical history includes non-Hodgkin lymphoma at age 44 and invasive ductal carcinoma at age 28. Her temperature is 37.0°C (98.6°F), pulse is 86/min, respirations are 16/min, and blood pressure is 168/96 mm Hg. Physical examination shows truncal obesity with abdominal striae and significant facial fat stores. Laboratory studies show a glucose of 280 mg/dL. Serial cortisol measurements show a level at time: 0800 hours of 62 μg/dL and a level at time: 2000 hours of 64 μg/dL. Abdominal computed tomography (shown below) shows a mass involving the adrenal gland. Which of the following hormones most likely has decreased serum levels due to feedback suppression from the tumor product?

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Adrenocorticotropic hormone, also known as adrenocorticotropin, corticotropin, or simply ACTH, is a peptide hormone that helps regulate the release of hormones by the adrenal glands which sit above the kidneys.

Secretion of ACTH is dependent on the hypothalamic-pituitary axis.

The hypothalamus, which is at the base of the brain, secretes corticotropin releasing hormone, or CRH, into the hypophyseal portal system - which is a network of capillaries linking the hypothalamus to the anterior 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 corticotropin releasing hormone binds to a surface protein of one of these cell types, called corticotroph cells, and stimulates them to release ACTH.

Inside corticotroph cells, ACTH is synthesized from a large precursor molecule called pre- proopiomelanocortin, or pre- POMC.

Pre- proopiomelanocortin has a short tail called a leader or signal peptide which is cleaved off to form proopiomelanocortin, or POMC, and POMC is then split into multiple peptide hormones, and one of them is ACTH.

ACTH is then stored inside granules within the corticotroph cells, where it waits until it’s released into the blood.

Normally, ACTH is released in a pulsatile manner throughout the day and peaks in the morning around 6am but it is also secreted in response to various forms of stressful stimuli.

For example, the hypothalamus senses when there’s hypoglycemia or low blood sugar, and in response it secretes more corticotropin releasing hormone.

Another example, is during an infection, where pro-inflammatory cytokines, act on the hypothalamus and anterior pituitary to cause ACTH secretion.

Now, when ACTH is released it travels to the adrenal glands, which sit above each kidney and binds to the ACTH receptor, also called melanocortin receptor 2, located in the membrane of their target cells which are adrenocortical cells.

Each gland is made up of an inner medulla, which secretes catecholamines, and an outer cortex.

The adrenal cortex itself is divided into three zones, each one secreting a different steroid hormone.

The outermost zone is the zona glomerulosa, which secretes mineralocorticoids.

Next, there’s the zona fasciculata, which secretes glucocorticoids, of which, cortisol is the most important one.

And finally, there’s the zona reticularis that secretes androgens.

Zooming back into the adrenocortical cells, the ACTH receptor is a seven pass transmembrane receptor, meaning they are really long proteins that have one end that sits outside the cell and binds ACTH, then the snake- like protein dips in and out of the cell membrane seven times, and finally ends on the inside of the cell.

The end of the protein within the cell activates intracellular proteins.

When ACTH binds to the ACTH receptor, it causes the adrenocortical cells to release corticosteroid hormones - mainly glucocorticoids, which have anti- inflammatory and metabolic effects, but also mineralocorticoids to some extent, which influence electrolyte and fluid balance.

Glucocorticoids are a class of steroid hormones, and the most important glucocorticoid in humans is cortisol.

Once it’s made, cortisol travels via the blood and binds to the glucocorticoid receptors, which are generally intracellular receptors within nearly every cell in the body.

Cortisol helps to regulate both the immune response as well as cellular metabolism.

Sources
  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "ACTH protects against glucocorticoid-induced osteonecrosis of bone" Proceedings of the National Academy of Sciences (2010)
  6. "ACTH Receptor (MC2R) Specificity: What Do We Know About Underlying Molecular Mechanisms?" Frontiers in Endocrinology (2017)
  7. "Glucocorticoids inhibit prostaglandin synthesis not only at the level of phospholipase A2 but also at the level of cyclo-oxygenase/PGE isomerase" British Journal of Pharmacology (1989)