Neuroendocrine regulation and circadian rhythms
Glucocorticoid secretion is under the strict control of the hypothalamic-pituitary system. The hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the adenohypophysis to produce adrenocorticotropic hormone (ACTH). In turn, ACTH triggers the synthesis of cortisol in the adrenal glands.
This process follows a pronounced circadian (daily) rhythm. Under a standard sleep-wake cycle, cortisol concentration begins to rise shortly after falling asleep. It peaks in the morning hours, approximately 1–2 hours after the maximum ACTH surge.
In addition to the baseline rhythm, a powerful stimulus for hormone release is stress. Physical activity, illness, or emotional tension cause immediate activation of the hypothalamus and pituitary gland, leading to a sharp spike in cortisol levels.
The system is controlled via negative feedback. As soon as the concentration of glucocorticoids in the blood increases, they begin to inhibit the secretion of CRH in the hypothalamus and ACTH in the pituitary gland. This is a protective mechanism that prevents excessive hormone synthesis.
Transport, metabolism, and excretion
Upon entering the bloodstream, free hormones quickly bind to specific proteins. More than 90% of circulating glucocorticoids are in an inactive (bound) state. The main carriers are:
- Cortisol-binding globulin (transcortin);
- Albumin.
The free hormone fraction acts on tissues but is rapidly degraded in the process. The half-life of cortisol in plasma is about two hours. The inactivation of metabolites takes place in the liver. Waste products are excreted predominantly by the kidneys, and only a minor part leaves the body via the gastrointestinal tract.
Physiological functions: effects on metabolism
Since glucocorticoid receptors are present in almost every cell, their effects are systemic. The main task of these hormones is the mobilization of energy resources, which manifests as changes in all types of metabolism.
Carbohydrate metabolism The hormones purposefully increase blood glucose concentration. This is achieved by activating gluconeogenesis enzymes in the liver—the process of forming glucose from non-carbohydrate precursors. At the same time, cortisol inhibits glucose utilization in most tissues (the central nervous system is the only exception, for which glucose is critically important).
Protein metabolism The effect on proteins is distinctly catabolic and anti-anabolic. The hormones enhance protein breakdown in skeletal muscles, bone, and connective tissue. As a result, a large amount of amino acids is released into the blood and sent to the liver to participate in gluconeogenesis. Prolonged hormone excess can provoke a negative nitrogen balance.
Fat metabolism Glucocorticoids stimulate lipolysis (fat breakdown) and block the synthesis of new lipids from carbohydrates. The free fatty acids released in the process are also transported to the liver, becoming an additional substrate to maintain high glucose levels.
Briefly: answers to topical questions
Which hormones are synthesized in the adrenal medulla?
The adrenal medulla synthesizes catecholamines: predominantly adrenaline and, in smaller amounts, noradrenaline. ATP and enkephalins are also released into the blood along with them.
From which amino acid are medullary catecholamines synthesized?
Medullary catecholamines of the adrenal glands are synthesized from the amino acid tyrosine.
Which enzyme catalyzes the conversion of noradrenaline to adrenaline?
The conversion of noradrenaline to adrenaline is catalyzed by the enzyme phenylethanolamine N-methyltransferase.
Which types of receptors do adrenal medullary hormones interact with?
Adrenal medullary hormones interact with alpha- and beta-adrenergic receptors.
- Adrenaline has a higher affinity for $\beta$-adrenergic receptors (acts via $\alpha_2$- and $\beta$-receptors).
- Noradrenaline has a higher affinity for $\alpha$-adrenergic receptors.
How does adrenaline affect carbohydrate metabolism?
Adrenaline increases blood glucose levels (development of hyperglycemia). Mechanisms:
- Enhancement of glycogen breakdown (glycogenolysis) and suppression of its synthesis in the liver and muscles.
- Decreased glucose uptake by tissues.
- Enhancement of hepatic gluconeogenesis.
How do adrenal medullary hormones affect the cardiovascular system?
Medullary hormones have diverse effects on the cardiovascular system.
| Hormone | Effect on blood vessels and blood pressure | Effect on the heart |
|---|---|---|
| Norepinephrine | Generalized vasoconstrictive effect, increases systolic and diastolic blood pressure. | Less pronounced stimulating effect. |
| Adrenaline | Causes an increase in systolic blood pressure only. Constricts blood vessels of the abdominal cavity, lungs, and skin; constricts skeletal muscle arterioles at high concentrations, dilates them at low concentrations. | More strongly stimulates cardiac activity (increases heart rate and contractile force). |
How is the secretion of adrenal medullary hormones regulated?
The secretion of adrenal medullary hormones is under neural and humoral control.
- Neural regulation: carried out by the hypothalamus via preganglionic sympathetic fibers (splenic nerve), which form cholinergic synapses. Acetylcholine interacts with nicotinic (Nn) receptors, stimulating catecholamine release.
- Humoral (paracrine) regulation: cortisol from the adrenal cortex stimulates catecholamine synthesis (methylation of noradrenaline to adrenaline).
Class and exam questions
At what time of day does the cortisol level reach its maximum?
Maximum cortisol concentration is observed in the morning hours, 1–2 hours after the peak of ACTH secretion, assuming a normal sleep-wake cycle.
How do glucocorticoids affect blood glucose levels?
They cause hyperglycemia. This occurs due to the stimulation of hepatic gluconeogenesis and the suppression of glucose uptake by tissues (except the CNS).
Where does cortisol inactivation take place?
Hormone inactivation occurs in the liver, after which metabolites are excreted mainly via the kidneys.
What else to study on this topic
- Detailed mechanism of negative feedback in the hypothalamic-pituitary-adrenal axis
- Binding specificity of transcortin and albumin to cortisol
- Molecular mechanisms of gluconeogenesis enzyme activation
- Link between hyperglycemia and subsequent hormonal shifts (mentioned in the text note)
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