A male client has recently undergone surgical removal of a pituitary tumor. Dr. Wong prescribes corticotropin (Acthar), 20 units I.M. q.i.d. as a replacement therapy. What is the mechanism of action of corticotropin?
A. It decreases cyclic adenosine monophosphate (cAMP) production and affects the metabolic rate of target organs.
B. It interacts with plasma membrane receptors to inhibit enzymatic actions.
C. It interacts with plasma membrane receptors to produce enzymatic actions that affect protein, fat, and carbohydrate metabolism.
D. It regulates the threshold for water resorption in the kidneys.
Correct Answer: C. It interacts with plasma membrane receptors to produce enzymatic actions that affect protein, fat, and carbohydrate metabolism.
Corticotropin interacts with plasma membrane receptors to produce enzymatic actions that affect protein, fat, and carbohydrate metabolism. CRH is released from the hypothalamus. CRH stimulates the anterior pituitary to release ACTH. ACTH acts on the adrenal cortex to release cortisol and androgens. The increase in cortisol provides a negative feedback system to then decrease the amount of CRH released from the hypothalamus.
Option A: It doesn’t decrease cAMP production. ACTH works on G protein-coupled receptors on extracellular membranes on zona fasciculata and zona reticularis of the adrenal cortex. cAMP is the secondary messenger system. Activation of the g-coupled receptor activates adenylyl cyclase, thus increasing cAMP production.
Option B: The adrenal cortex secretes glucocorticoids from the zona fasciculata and androgens from the zona reticularis. The secretion of glucocorticoids provides a negative feedback loop for inhibiting the release of CRH and ACTH from the hypothalamus and anterior pituitary, respectively. Stress stimulates the release of ACTH.
Option D: The posterior pituitary hormone, antidiuretic hormone, regulates the threshold for water resorption in the kidneys. The anterior pituitary produces ACTH. It is considered a tropic hormone. Tropic hormones indirectly affect target cells by first stimulating other endocrine glands. Corticotropin-releasing hormone (CRH) is released from the hypothalamus which stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH). ACTH then acts on its target organ, the adrenal cortex.
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