Hypothalamic-pituitary-adrenal axis
The physiological cascade of cortisol production is initiated in the central nervous system and completed in the periphery—the adrenal cortex. This process can be divided into several strictly sequential stages:
- In response to provoking stimuli, such as acute infection, trauma, or a critical drop in blood glucose concentration, hypothalamic structures release a specific releasing factor into the bloodstream—corticotropin-releasing hormone (CRH).
- Corticotropin-releasing hormone reaches the anterior pituitary gland, stimulating its cells to synthesize and secrete adrenocorticotropic hormone (ACTH).
- In turn, circulating ACTH acts on receptors in the adrenal cortex, directly activating the synthesis and subsequent secretion of cortisol.
It is important to note that cortisol is one of the counter-regulatory hormones. In the module of hormonal metabolic regulation, it is considered a critical physiological antagonist of insulin alongside epinephrine, growth hormone (somatotropin), and iodothyronines (thyroid hormones).
Negative feedback mechanism
To prevent excessive depletion of the body's energy resources and the toxic effects of high glucocorticoid concentrations, the regulatory system is equipped with a strict self-monitoring mechanism. As soon as systemic blood cortisol levels rise significantly, it begins to directly inhibit the higher endocrine glands.
High cortisol inhibits:
- The synthesis and release of corticotropin-releasing hormone in hypothalamic structures.
- The secretion of ACTH by pituitary cells.
Due to this biological "brake," the hormonal background smoothly returns to normal values after the initial stress stimulus is eliminated.
Catabolism and excretion of corticosteroids
Used corticosteroid hormones undergo chemical transformations for subsequent elimination from the body. The main products of corticosteroid catabolism (with the exception of compounds such as aldosterone and corticosterone) are transformed into 17-ketosteroids.
To ensure water solubility and successful urinary excretion, these metabolites form stable conjugates. The excretion forms are complexes with glucuronic and sulfuric acids.
Assessment of 17-ketosteroids in 24-hour urine has immense clinical significance, although the interpretation of results depends on the patient's sex:
- In men, normal excretion ranges from 12 to 17 mg per day. About two-thirds (2/3) of this volume are derived from adrenal corticosteroid metabolites, and the remaining third (1/3) comes from testosterone derivatives synthesized by the testes.
- In women, reference values are notably lower: from 7 to 12 mg per day, and these excreted compounds are predominantly of adrenal origin.
Briefly: answers to topical questions
Which enzymes are involved in the biosynthesis of cortisol in the adrenal cortex?
Hydroxylation enzymes are involved in the biosynthesis of cortisol in the adrenal cortex.
Key enzymes of the synthesis pathway:
- 21-hydroxylase — the main enzyme whose genetic defect most commonly causes a block in the cortisol synthesis pathway.
- 11-beta-hydroxylase — an enzyme whose deficiency is less common but also leads to a block in the formation of cortisol and aldosterone.
From which molecular precursor is cortisol synthesized?
The molecular precursor for cortisol synthesis is cholesterol.
Sources of this starting material include intracellular stored esters as well as cholesterol esters delivered via low-density lipoproteins (LDLs). The conversion of the precursor involves several stages:
- Pregnenolone — formed in the mitochondria by the cleavage of a 6-carbon fragment.
- Progesterone — formed in the endoplasmic reticulum.
- 11-deoxycortisol — formed from progesterone via 17-OH-progesterone and returned to the mitochondrion for final conversion into cortisol.
Class and exam questions
What factors stimulate cortisol production?
The physiological secretion cascade is powerfully triggered by a drop in blood glucose levels, as well as by acute conditions: severe trauma and infectious processes.
What is the essence of negative feedback in this system?
An elevated concentration of cortisol in the bloodstream acts as an inhibitory signal: it suppresses the secretion of corticotropin-releasing hormone in the hypothalamus and ACTH in the pituitary, halting further stimulation of the adrenal glands.
In what form are corticosteroid metabolites eliminated from the body?
They are converted into 17-ketosteroids and excreted in the urine in a water-soluble form—as conjugates with sulfuric and glucuronic acids.
What else to study on this topic
- Functions of insulin and glucagon as key metabolic energy hormones
- Mechanism of action of counter-regulatory hormones (epinephrine, somatotropin, iodothyronines)
- Catabolism of aldosterone and corticosterone
- Hormonal regulation of metabolism during a normal feeding rhythm
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