Adrenal Gland: The Adrenal Medulla

Introduction

The adrenal medulla functions as the body’s rapid stress-response center, releasing catecholamines during sympathetic activation. These hormones regulate cardiovascular performance, metabolism, and emergency adaptation. Their actions differ by receptor type, while persistent overstimulation may produce harmful long-term physiological effects.

Adrenal Gland

General Features

• There are two adrenal glands, each situated on the superior pole of a kidney.
• Each gland contains two structurally and functionally distinct endocrine regions.
• The outer region is the adrenal cortex, and the inner region is the adrenal medulla.
• The adrenal cortex forms about 80–90% of the gland mass.
• The adrenal medulla contributes approximately 10–20% of the gland.
• The cortex is organized into three zones, each producing specific steroid hormones.
• Developmentally, the adrenal cortex arises from mesodermal tissue. It secretes corticosteroids and small amounts of sex steroids.
• The adrenal medulla develops from neuroectoderm, related to sympathetic ganglia. It secretes catecholamines, mainly adrenaline and noradrenaline.

Blood Supply

• Each adrenal gland weighs approximately 5–10 grams.
• The glands are highly vascular and receive a rich arterial supply.
• Blood reaches them through branches of the aorta, renal arteries, and inferior phrenic arteries.
• Arterial blood first enters sinusoidal capillaries of the adrenal cortex. It then drains into venous channels of the medulla. This vascular arrangement exposes the medulla to high concentrations of cortical steroids.
• Cortisol reaching the medulla supports conversion of norepinephrine to epinephrine.

General Importance

• The adrenal glands are essential for normal physiological stability and survival.
• They help maintain energy homeostasis during fasting, illness, and stress.
• They regulate sodium balance, potassium balance, extracellular fluid volume, and blood pressure.
• They support glucose production and provide substrates for adenosine triphosphate generation.
• They influence protein, fat, and carbohydrate metabolism.
• Adrenal hormones also modulate immune and inflammatory responses.

• They affect cardiovascular, renal, nervous, and reproductive functions.

Adrenal Medulla

• Adrenal medulla secretes catecholamines, mainly adrenaline and noradrenaline. These hormones are not essential for immediate survival, unlike adrenal cortical hormones.
• They help the body respond rapidly to stressful emergencies through fight-or-flight reactions. ·  They also support cardiovascular function and correct hypoglycemic states by mobilizing energy stores.

Cell Types

• The adrenal medulla is the inner part of the adrenal gland and forms about 20–30% of its mass.
• In adults, medullary tissue weighs approximately 1 gram. It functions as a modified sympathetic ganglion in which postganglionic neurons are transformed into endocrine cells.
• The tissue contains clusters and cords of chromaffin cells separated by venous sinusoids. These cells stain strongly with chromium salts because of stored catecholamines.
• Catecholamines are stored in membrane-bound secretory granules and released into blood during stimulation.

Types of Chromaffin Cells

• Epinephrine-secreting cells are the predominant type in humans. They constitute about 80–90% of chromaffin cells. Their granules are relatively larger and less electron-dense.
• Norepinephrine-secreting cells form the smaller population. They account for about 10–20% of medullary cells. Their granules are smaller and more electron-dense.
• A small number of cells may produce dopamine, but they are much less common.

Innervation and Function

• Chromaffin cells receive preganglionic sympathetic cholinergic fibers through the splanchnic nerves.
• Acetylcholine stimulation triggers rapid exocytosis of catecholamines into venous sinuses. This allows an immediate hormonal response during stress.

Paraganglia

• Small collections of chromaffin tissue outside the adrenal gland are called paraganglia.
• They are located near sympathetic ganglia and can also secrete catecholamines.

Adrenomedullary Hormones

• The adrenal medulla mainly secretes catecholamines.
• The principal catecholamines are epinephrine, norepinephrine, and small amounts of dopamine.
• In humans, epinephrine is the dominant secretion.
• Approximate output is 85% epinephrine, 10–12% norepinephrine, and 1–3% dopamine.
• The gland also releases peptides such as adrenomedullin, enkephalins, beta-endorphin, neuropeptide Y, and chromogranins.
• Many sympathetic nerve endings release mainly norepinephrine, unlike the adrenal medulla.
• These hormones support rapid stress responses and metabolic adaptation.

Effect of Sympathetic Stimulation

• Sympathetic activation stimulates the adrenal medulla and sympathetic nerve endings simultaneously.
• Norepinephrine is released mainly from nerve terminals, while epinephrine is rapidly secreted into blood from the adrenal medulla.

• Therefore, stress increases circulating levels of both catecholamines.

Synthesis, Secretion, and Metabolism of Catecholamines

Synthesis and Secretion

• Catecholamines are synthesized mainly from the amino acid tyrosine, which can be derived from phenylalanine.
• Tyrosine is converted to dihydroxyphenylalanine by tyrosine hydroxylase, the rate-limiting enzyme.
• Dihydroxyphenylalanine is converted to dopamine by aromatic amino acid decarboxylase.
• Dopamine is converted to norepinephrine inside storage vesicles by dopamine beta-hydroxylase.
• Norepinephrine is converted to epinephrine by phenylethanolamine N-methyltransferase.
• After synthesis, catecholamines are stored in granules of chromaffin cells until release.
• Sympathetic stimulation causes rapid exocytosis into venous blood.

Effects of Glucocorticoid

• High cortisol levels from the adrenal cortex reach the medulla through local blood flow.
• Cortisol induces phenylethanolamine N-methyltransferase and increases epinephrine synthesis.
• Adrenal cortical failure may therefore reduce epinephrine production.
• Glucocorticoids also support normal growth and function of the medulla.

Effects of 21b-hydroxylase

• During fetal life, normal adrenal cortical steroid production helps proper medullary development.
• 21-hydroxylase deficiency may impair medullary growth and lead to reduced catecholamine levels after birth if untreated.

Regulation of Secretion

• Catecholamine release from the adrenal medulla increases during exercise, trauma, pain, cold exposure, fear, anger, and anxiety.
• The major trigger is sympathetic nervous system activation.
• Preganglionic sympathetic fibers stimulate chromaffin cells to release mainly epinephrine and some norepinephrine.
• Hypoglycemia is a powerful stimulus for secretion.
• When blood glucose falls significantly, glucose-sensing centers in the central nervous system activate sympathetic outflow.
• Released catecholamines increase blood glucose by promoting glycogenolysis, gluconeogenesis, and lipolysis. They also help maintain blood pressure and cerebral perfusion during stress.

Metabolism

• Catecholamines have a very short half-life, usually only a few minutes.
• Many circulating molecules are conjugated, especially with sulfate, before further metabolism. They are metabolized mainly in the liver and kidneys.
• Two key enzymes are catechol-O-methyltransferase and monoamine oxidase.
• Epinephrine is converted to metanephrine, and norepinephrine to normetanephrine.
• Final metabolites include vanillylmandelic acid and methoxyhydroxyphenylglycol, which are excreted in urine.
• Measurement of urinary vanillylmandelic acid or plasma metanephrines helps diagnose catecholamine-secreting tumors.

Degradation

• Epinephrine and norepinephrine are rapidly metabolized after release.
• Only 2–3% of catecholamines are excreted unchanged in urine.
• About half are excreted as metanephrine and normetanephrine.
• A substantial proportion is converted to vanillylmandelic acid, a major urinary metabolite.
• Smaller amounts form methoxyhydroxyphenylglycol.
• Dopamine is metabolized through separate but related pathways.
• Measurement of 24-hour urinary catecholamines and metabolites helps detect catecholamine-secreting tumors.
• Elevated urinary metanephrines or vanillylmandelic acid strongly suggest excess catecholamine production.

Mechanism of Action

• Catecholamines act through membrane-bound adrenergic receptors. These receptors are divided into alpha and beta classes.
• Major alpha receptors are alpha-1 and alpha-2.
• Major beta receptors are beta-1, beta-2, and beta-3.
• Epinephrine and norepinephrine can bind to both alpha and beta receptors.

• In general, epinephrine has greater activity at beta receptors, whereas norepinephrine acts more strongly at alpha receptors.
• Beta-1, beta-2, and beta-3 receptors are linked to stimulatory G proteins. Their activation stimulates adenylyl cyclase and increases cyclic adenosine monophosphate inside cells. This pathway mediates effects such as increased heart activity, bronchodilation, and lipolysis.
• Alpha-2 receptors are linked to inhibitory G proteins.
• Activation decreases cyclic adenosine monophosphate production. This often reduces neurotransmitter release and modulates sympathetic output.
• Alpha-1 receptors are linked to the phospholipase C pathway.
• Activation generates inositol trisphosphate and diacylglycerol.
• These second messengers raise intracellular calcium and cause smooth muscle contraction, especially vasoconstriction.

Table 58.1: Mechanism of action at various catecholaminergic receptors.

Receptor	G Protein Pathway	Main Second Messenger Effect
Alpha-1	Gq → Phospholipase C	↑ IP3, DAG, Ca²⁺
Alpha-2	Gi → Adenylyl cyclase inhibition	↓ cAMP
Beta-1 / Beta-2 / Beta-3	Gs → Adenylyl cyclase stimulation	↑ cAMP

Table 58.2: Various major actions of catecholamines.

Receptor Group	Major Physiological Actions
Beta receptors (epinephrine > norepinephrine)	↑ Glycogenolysis, gluconeogenesis, lipolysis, insulin and glucagon release, cardiac rate/force, conduction, skeletal muscle potassium uptake, bronchodilation, smooth muscle relaxation, arteriolar dilation.
Alpha receptors (norepinephrine > epinephrine)	↑ Vasoconstriction, blood pressure, hepatic glycogenolysis, gluconeogenesis, sphincter contraction, pupillary dilation, reduced insulin secretion, and some increase in cardiac contractility.

Physiological Actions of Catecholamines

• Catecholamines rapidly prepare the body for stress, exercise, and emergency situations.
• They increase energy availability and regulate many autonomic functions.

Effects on Intermediary Metabolism

On Carbohydrate Metabolism

• Catecholamines increase blood glucose, mainly through epinephrine.
• They stimulate hepatic glycogenolysis, causing breakdown of stored glycogen to glucose.
• They also enhance glycogen breakdown in skeletal muscle for local energy use.
• They promote gluconeogenesis in the liver from lactate, glycerol, and amino acids.
• They inhibit glycogen synthesis by reducing glycogen synthase activity.
• Epinephrine decreases insulin-mediated glucose uptake in skeletal muscle and adipose tissue.
• They stimulate glucagon secretion and suppress insulin release, especially through alpha-2 receptors.
• These combined actions raise plasma glucose during stress.

On Fat Metabolism

• Catecholamines strongly promote lipolysis in adipose tissue.
• They activate hormone-sensitive lipase, breaking triglycerides into free fatty acids and glycerol.
• Free fatty acids serve as alternative fuel for muscle and liver.
• Increased fatty acid oxidation may enhance ketone body production during fasting.
• Because they oppose insulin action, catecholamines are considered diabetogenic hormones when chronically elevated.

On Thermogenesis

• Epinephrine increases basal metabolic rate and oxygen consumption.
• It stimulates non-shivering thermogenesis, especially in brown adipose tissue.
• It also contributes to diet-induced thermogenesis after meals.
• These actions help maintain body temperature during cold exposure.

During Hypoglycemia

• Catecholamine secretion rises markedly during hypoglycemia. This occurs during prolonged fasting, insulin excess, or strenuous exercise. They restore glucose levels by stimulating glycogenolysis and gluconeogenesis.
• Lipolysis provides free fatty acids as an alternative energy source.
• Simultaneously, glucagon secretion increases and insulin secretion decreases.

Effects of Epinephrine

• Epinephrine increases heart rate and myocardial contractility mainly through beta-1 receptors. This raises cardiac output and increases systolic blood pressure.
• It causes vasoconstriction in cutaneous, renal, and splanchnic vessels through alpha-1 receptors. It also produces vasodilation in skeletal muscle and hepatic vessels through beta-2 receptors.
• In usual physiological doses, vasodilation in muscle may predominate.
• Total peripheral resistance therefore falls or changes only slightly.
• Diastolic blood pressure may decrease mildly.

• As systolic pressure rises and diastolic pressure falls, pulse pressure widens. These changes are prominent during exercise and acute stress.
• Blood flow is redirected toward active muscles while coronary and cerebral perfusion is preserved. This supports oxygen delivery and rapid energy use during the fight-or-flight response.

Effects of Norepinephrine

• Norepinephrine strongly stimulates alpha-1 receptors and causes generalized vasoconstriction.
• Peripheral resistance rises markedly.
• Both systolic and diastolic blood pressures increase.
• Norepinephrine can also increase myocardial contractility through beta-1 receptors.
• However, the sharp rise in arterial pressure activates baroreceptors in the carotid sinus and aortic arch. This triggers reflex vagal stimulation and slows the heart rate.

• Reflex bradycardia may outweigh its direct cardiac stimulatory effect.
• As a result, heart rate often falls despite increased blood pressure.
• Cardiac output may remain unchanged or decrease depending on reflex intensity.

Effects on Other Systems

GI System

• In the gastrointestinal system, catecholamines reduce gastric secretion and decrease intestinal motility.
• They also increase sphincter tone, slowing digestive transit during stress.

Respiratory System

• In the gastrointestinal system, catecholamines reduce gastric secretion and decrease intestinal motility.
• They also increase sphincter tone, slowing digestive transit during stress.

On Eye

• In the eye, catecholamines produce pupillary dilation through alpha-1 receptors.
• This improves distant vision and visual awareness in emergencies.

On Endocrine Glands

• In endocrine regulation, catecholamines stimulate release of antidiuretic hormone and increase renin secretion from juxtaglomerular cells.
• They can modestly increase thyroid hormone release and peripheral conversion of thyroxine to triiodothyronine.

On Kidney

• In the kidney, catecholamines increase sodium reabsorption and alter renal blood flow through vasoconstriction.
• Activation of the renin-angiotensin-aldosterone system promotes sodium and water retention.

On Electrolyte Balance

For electrolyte balance, beta-2 stimulation shifts potassium into skeletal muscle cells. This lowers plasma potassium and may help prevent hyperkalemia.

Effects of Dopamine

• Dopamine dilates renal and mesenteric blood vessels at lower doses, improving regional perfusion. It can cause vasoconstriction elsewhere at higher doses.
• Through beta-1 receptors, it increases myocardial contractility and cardiac output.
• Systolic pressure rises more than diastolic pressure.
• It also promotes natriuresis.

Applied Physiology

Clinical Uses of Catecholamines

• Adrenergic agonists are widely used in medicine. They are used as nasal decongestants, bronchodilators, cardiac stimulants, and emergency vasopressors.
• Some agents reduce appetite, although many are now limited because of adverse effects.
• Alpha- and beta-blockers are commonly used to treat hypertension.
• Beta-blockers are also useful in hyperthyroidism to control tremor, palpitations, and tachycardia before definitive treatment.
• Catecholamine drugs may be used in selected forms of shock to support circulation.
• Dopamine has been used in cardiogenic shock, though current use is more selective.

Pheochromocytoma

• Pheochromocytoma is a catecholamine-secreting tumor arising from adrenal medullary chromaffin cells.
• Similar tumors outside the adrenal gland are called paragangliomas.
• Many tumors secrete norepinephrine, epinephrine, or both.
• Excess catecholamine release causes marked sympathetic overactivity.

Features

• Sustained or episodic hypertension is a common feature.
• Patients may develop headaches, palpitations, sweating, tremor, anxiety, and tachycardia.
• Hyperglycemia, weight loss, and heat intolerance may occur.
• Attacks can be triggered by posture change, stress, exercise, surgery, or abdominal pressure.
• Episodes may cause pallor or flushing and a sense of impending doom.

Diagnosis

• Diagnosis is based on elevated plasma free metanephrines or increased urinary fractionated metanephrines.
• Urinary catecholamines and vanillylmandelic acid may also be increased.
• Imaging is then used to locate the tumor.

Treatment

• Definitive treatment is surgical removal after careful preparation.
• Preoperative alpha-blockade is essential to control blood pressure and prevent operative crises.
• Beta-blockers may be added only after adequate alpha blockade if tachycardia persists.

Role of Catecholamines in Stress

• Stress activates hypothalamic neurons that release corticotropin-releasing hormone and antidiuretic hormone.
• Corticotropin-releasing hormone increases sympathetic outflow and raises plasma catecholamine levels.
• Catecholamines rapidly increase blood glucose by stimulating glycogenolysis.
• Cortisol supports this response by promoting gluconeogenesis.
• Together, these hormones prioritize glucose delivery to the brain and other vital organs.
• Epinephrine increases free fatty acid release, providing fuel for the heart and muscles.
• Catecholamines increase heart rate, cardiac output, and blood pressure. These actions improve oxygen and nutrient delivery during emergencies.

• Acute stress produces alertness, vigilance, and rapid reaction.
• Chronic stress may suppress feeding, reproductive, and sexual functions through neuroendocrine adaptation.

Other Adrenomedullary Peptides

Adrenomedullin

• Adrenomedullin is a peptide hormone formed from proadrenomedullin. It is produced in the adrenal medulla and also in tissues such as the brain, kidneys, and vascular endothelium.
• It lowers blood pressure mainly by causing vasodilation, partly through increased nitric oxide release.
• It can reduce aldosterone secretion and thereby decrease extracellular fluid volume.
• It may also suppress excessive sympathetic activity.

Chromogranin

• Chromogranins are proteins stored within secretory granules of chromaffin cells. They help package catecholamines inside dense-core vesicles.
• Chromogranins are released together with catecholamines during secretion.
• Plasma levels may reflect chromaffin cell activity and catecholamine release.

Other Hormones  

The adrenal medulla also contains enkephalins, endorphins, and neuropeptide Y. These peptides modulate stress, pain, and autonomic responses.