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Adrenal Medulla Hormones

Medulla adrenalis

For medical students2 min readUpdated 2026-10-10

Glucocorticoids (primarily cortisol) are steroid hormones with receptors present in virtually all tissues of the body. They act as key metabolic regulators and ensure an adequate physiological response to various stressors.

Half-lifeAbout 2 hours in systemic circulation
Inactivation siteLiver
Blood transportMore than 90% bound to transcortin and albumin
Glucose utilizationSuppressed everywhere except CNS tissues

Neuroendocrine Regulation and Circadian Rhythms

Glucocorticoid secretion is tightly regulated by the hypothalamic-pituitary-adrenal (HPA) axis. The hypothalamus secretes corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary gland to produce adrenocorticotropic hormone (ACTH). In turn, ACTH triggers cortisol synthesis in the adrenal cortex.

This process follows a pronounced circadian rhythm. Under standard sleep-wake cycles, cortisol concentration begins to rise shortly after sleep onset. It peaks in the morning hours, approximately 1–2 hours after the peak ACTH surge.

In addition to the basal rhythm, stress serves as a potent stimulus for hormone release. Physical exertion, illness, or emotional stress trigger immediate activation of the hypothalamus and pituitary, leading to a sharp spike in cortisol levels.

The system is controlled via a negative feedback loop. As glucocorticoid concentration in the blood rises, they inhibit CRH secretion in the hypothalamus and ACTH secretion in the pituitary. This is a protective mechanism that prevents excessive hormone synthesis.

Transport, Metabolism, and Excretion

Upon entering the bloodstream, free hormones rapidly bind to specific carrier proteins. More than 90% of circulating glucocorticoids are in an inactive (bound) state. The primary carriers are:

The free fraction of the hormone exerts biological effects on tissues while being rapidly degraded. The half-life of cortisol in plasma is approximately two hours. Inactivation of its metabolites occurs in the liver. Excretion of metabolic waste products occurs primarily via the kidneys, with only a minor fraction leaving the body through the gastrointestinal tract.

Physiological Functions: Metabolic Effects

Because glucocorticoid receptors are found in almost every cell, their effects are widespread. The primary function of these hormones is the mobilization of energy resources, manifested through alterations in all types of metabolism.

Carbohydrate Metabolism These hormones purposefully raise blood glucose concentrations. This is achieved by activating gluconeogenesis enzymes in the liver—the process of forming glucose from non-carbohydrate precursors. Simultaneously, cortisol inhibits glucose utilization by most tissues (with the exception of the central nervous system, for which glucose is critically vital).

Protein Metabolism Their effect on proteins is predominantly catabolic and anti-anabolic. The hormones enhance protein degradation in skeletal muscle, bone, and connective tissue. As a result, large amounts of amino acids are released into the blood and transported to the liver to participate in gluconeogenesis. Prolonged hormone excess can provoke a negative nitrogen balance.

Lipid Metabolism Glucocorticoids stimulate lipolysis (fat breakdown) and block the synthesis of new lipids from carbohydrates. The resulting free fatty acids are also transported to the liver, serving as an additional substrate to maintain elevated blood glucose levels.

Frequently asked questions

Which hormones are synthesized in the adrenal medulla?

The adrenal medulla synthesizes catecholamines: predominantly epinephrine (adrenaline) and, in lesser amounts, norepinephrine (noradrenaline). ATP and enkephalins are also coreleased with them into the blood.

From which amino acid are medullary catecholamines synthesized?

Adrenal medullary catecholamines are synthesized from the amino acid tyrosine.

Which enzyme catalyzes the conversion of norepinephrine to epinephrine?

The conversion of norepinephrine to epinephrine is catalyzed by the enzyme phenylethanolamine N-methyltransferase (PNMT).

Which types of receptors do adrenal medullary hormones interact with?

Adrenal medullary hormones interact with alpha- and beta-adrenergic receptors.

  • Epinephrine has a higher affinity for $\beta$-adrenergic receptors (acts via $\alpha_2$ and $\beta$ receptors).
  • Norepinephrine has a higher affinity for $\alpha$-adrenergic receptors.
How does epinephrine affect carbohydrate metabolism?

Epinephrine increases blood glucose levels (inducing hyperglycemia). Mechanisms:

  • Enhancement of glycogen breakdown (glycogenolysis) and inhibition of its synthesis in the liver and muscles.
  • Reduction of glucose consumption by tissues.
  • Enhancement of hepatic gluconeogenesis.
How do adrenal medullary hormones affect the cardiovascular system?

Medullary hormones exert varying effects on the cardiovascular system.

HormoneEffect on Blood Vessels and BPEffect on the Heart
NorepinephrineGeneralized vasoconstrictor effect, increases both systolic and diastolic BP.Less pronounced stimulatory effect.
EpinephrineRaises systolic BP primarily. Constricts blood vessels of the abdominal organs, lungs, and skin; constricts skeletal muscle arterioles at high concentrations, dilates them at low concentrations.Strongly stimulates cardiac activity (increases heart rate and myocardial contractility).
How is the secretion of adrenal medullary hormones regulated?

Secretion of adrenal medullary hormones is under neural and humoral control.

  • Neural regulation: Mediated by the hypothalamus via preganglionic sympathetic fibers (splanchnic nerve) forming cholinergic synapses. Acetylcholine binds to nicotinic ($N_n$) receptors, stimulating catecholamine release.
  • Humoral (paracrine) regulation: Cortisol from the adrenal cortex stimulates catecholamine synthesis (specifically the methylation of norepinephrine to epinephrine).
At what time of day do cortisol levels peak?

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 inhibition of glucose uptake by peripheral tissues (except the CNS).

Where does cortisol inactivation take place?

Hormone inactivation occurs in the liver, after which metabolites are excreted primarily via the kidneys.

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