Hormones | Adrenal Medulla | Adrenalin | Noradrenalin | Physiology | biology|

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    Adrenal medulla

 

Adrenal Medulla

Keywords: Catecholamines | receptors | alpha | beta | inotropic |chronotropic | bathmotropic |dromotropic | COMT| MAO | VMA|

Table of contents

1.	Introduction
2.	Structure Ø Types of location Ø Nerve supply
3.	Secretion Ø Hormones Ø Concentration Ø Synthesis Ø Metabolism
4.	Functions

 Location

Adrenal glands or suprarenal glands are present on the superior pole of the Kidneys. There are two adrenal glands, one on the right side and one on the left side. The adrenal gland is divided into the outer adrenal cortex and the inner adrenal medulla. The outer part is the adrenal cortex which consists of three layers

1. Zona glomerulosa

2. Zona fasciculata and

3. Zona reticularis.

While the inner part is known as the adrenal medulla, the adrenal cortex consists of 80% mass, while the adrenal medulla is only 20% of the adrenal gland.

The adrenal medulla

 The adrenal medulla develops from neuroectoderm and is a part of the sympathetic nervous system. It is considered a sympathetic ganglion where the postganglionic neurons have no axons and become secretory cells.  

The cells of the adrenal medulla secrete when estimated by preganglionic nerve fibers of the sympathetic nervous system. Therefore, it is a part of the sympathetic nervous system.

Nerve supply of the adrenal medulla

 Splanchnic nerves innervate the adrenal medulla. The myelinated type B  preganglionic nerve fibers surround the medullary cells. These fibers originate from the lower thoracic segments(T-5 to T9 ) of the ipsilateral intermediolateral grey columns of the spinal cord.

Structure

 The adrenal medulla consists of densely packed cells. The cells are known as chromaffin cells as they are stained brown by chrome salts, e.g., potassium chromate, due to the oxidation of catecholamines present in the granules of these cells. This is known as the chromaffin reaction. The cells are also known as phaeochromocytes.

Types of cells

There are two types of cells.

Ø  About 90% of epinephrine secreting large cells with few granules in the cytoplasm.

Ø And about 10%  nor-epinephrine secreting small cells with many dense granules in the cytoplasm.

 In the granules, adrenalin and noradrenalin are bound to ATP, and a binding protein is known as chromogranin. The content of the granules is released by exocytosis into the bloodstream.

 

Secretion of the adrenal medulla

 Secretion of adrenal medulla Is is known as catecholamines as these hormones are amine derivatives of catechol. The plasma half-life is about two minutes.

The secretions  are :

 1. Adrenaline,

 2. Noradrenaline, and

 3. Dopamine

  The concentration of these hormones

 Adrenaline  3 microgram per decilitre of blood

 Noradrenaline 30 microgram per deciliter of blood, and

 Dopamine 3.5 microgram per deciliter of blood.

 Biosynthesis of catecholamines

 Epinephrine - site of biosynthesis is mainly in the adrenal medulla, and a small amount is synthesized in the brain. Therefore, almost all the circulating epinephrine originates from the adrenal medulla.

 Noradrenalin is synthesized in peripheral and central adrenergic neurons and the adrenal medulla.

Noradrenaline is distributed in the adrenal medulla, sympathetic post ganglions, and CNS. However, in the  CNS, its concentration is very high in the hypothalamus. 

 

 

Steps of synthesis of catecholamines 

 Phenylalanine and tyrosine are present in our diet. Phenylalanine is converted into tyrosine by ‘phenylalanine hydroxylase’ present in the liver.

 The chromaffin cells of the adrenal medulla actively take tyrosine and convert it into DOPA (3,4 dihydroxyphenylalanine ) by tyrosine hydroxylase.

 DOPA (3,4 dihydroxyphenylalanine ) is converted into dopamine by decarboxylation reaction; the enzyme is dopa decarboxylase.

In the granules, hydroxylation of dopamine occurs by the ‘dopamine beta-hydroxylase ‘enzyme, and noradrenalin is formed.

 

Methylation of noradrenalin by phenylethanolamine-N methyltransferase(PNMT) produces adrenaline.

 The half-life of noradrenaline and adrenaline is only 2 minutes.

Metabolism of  catecholamines

Non-enzymatic inactivation:

Adrenergic presynaptic terminals take up about 85%  of catecholamines activity from the circulation and inactivate. And more effective for noradrenalin than adrenaline.

Enzymatic inactivation:

Two enzymes, ‘Mono-amine oxidase’  and ‘catechol O methyl transferase’ in the postsynaptic cells, inactivate about 15% of catecholamines.

 COMT (‘catechol O methyl transferase’)

 This is an extraneuronal or extracellular enzyme present all over the body. In the liver and Kidneys, it is more.

Epinephrine ----COMT-----→produces metanephrine.

Nor-epinephrine ----COMT-----→produces nor-metanephrine.

 

MAO (monoamine oxidase)

 Monoamine oxidase is present in the adrenergic nerve endings, liver, kidneys, stomach, and intestine.

 Epinephrine and Nor-epinephrine------ MAO------- → VMA (vanillyl mandelic acid)

 

Regulation of catecholamine secretion

The myelinated type B secretomotor preganglionic splanchnic nerves originating from the lower thoracic segments (T3 to T9 )of the spinal cord control the adrenal medullary secretion,

Acetylcholine is the neurotransmitter.

Higher centers present in the reticular formation of the medulla and hypothalamus control the splanchnic nerves activities. The activities of these centers are modified by afferent impulses from many parts of the body.

 The glucocorticoids promote the conversion of  noradrenaline to adrenaline 

In addition, angiotensin II, bradykinin, and histamine stimulate catecholamine secretion.

 Functions of catecholamines

There are receptors for adrenaline and noradrenaline on the cell membrane. The adrenergic receptors are divided into

Alpha:

α- 1 and  α- 2 

 Beta

 Β-1 and β-2, recently β-3 and β-  4 are discovered, but their functions are not precise.

Adrenergic receptors have different sensitivities for catecholamines and produce different responses via G –protein.

There is a reciprocal relationship between catecholamine concentration and the number of adrenergic receptors. A prolonged decrease level of catecholamines increases the number of their receptors along with their increased responsiveness.

 Nor-adrenaline and adrenaline stimulate the nervous system.

 1. Adrenaline has a significant role in metabolic functions.

 2. Adrenaline and noradrenaline have effects on the cardiovascular system.

Adrenal medullary secretions have the same actions as sympathetic nerve stimulation, and both are involved in the immediate response in an emergency- Fight or flight reactions. In addition, the sympathetic system is activated in fear, anxiety, pain trauma, blood loss, excess fluid loss, hypoxia, asphyxia exposure to extreme temperatures, hypoglycemia, and hypertension.

They may work independently.

 The effects of adrenaline and noradrenaline from the adrenal medulla are more prolonged than sympathetic nerve stimulation.

Adrenaline acts on both α and β receptors.

Noradrenaline acts on α and β1 receptors but does not affect β2.

Mode of action

Alpha receptors respond to adrenaline and noradrenaline. The receptors have excitatory actions except for the inhibitory activity on gastric motility. In addition, alpha-receptors lead to an increase in [Ca++].

 

α -1 activates Ip3 true phosphorylase. The receptors are present on the postsynaptic membrane and excitatory.

 α -2  are present on presynaptic nerve terminals and inhibitory. It inhibits adenyl cyclase, and

Beta receptors respond to adrenaline and usually do not respond to noradrenaline. The receptors have inhibitory actions except for the excitation of the myocardium. Beta receptors cause decrease in [Ca++].

 β1  receptors in the heart increase rate, force of contractions, excitability, and conductivity. Therefore it may cause ectopics or fibrillation.

β-2 receptors cause relaxation of smooth muscles in GIT, skeletal blood vessels bronchioles.

 Noradrenaline secretion increases when one is familiar with the adverse situation.

 Adrenaline secretion increases in an unexpected adverse situation.

 

Functions of catecholamines

 Catecholamine has ‘calorigenic action.’ It increases the basal metabolic rate only in the presence of T4 and hormones of the adrenal cortex.

 It has a stimulatory effect on CNS and produces hyperventilation.

 It may cause urine retention.

 

 In the skin, catecholamines cause piloerection of hair and adrenergic sweating.

Catecholamines improve skeletal muscle blood supply and increase the force of contraction of muscles.

 Β-2 receptors relax bronchial muscles producing bronchodilations.

 Catecholamines cause glycogenolysis and neoglucogenesis.

 Catecholamines cause fat metabolism releasing free fatty acids.

 RBC number increases due to splenic and other blood vessels contraction and reduces clotting time.

Catecholamines have positive inotropic, chronotropic, bathmotropic, and dromotropic actions.

Catecholamines cause an increase in the systolic blood pressure by one mechanism and diastolic blood pressure, so pulse pressure increases.

 Hashtag:# Catecholamines#receptors #alpha#beta# inotropic # chronotropic # bathmotropic # dromotropic

Internal link: https://blog.totalphysiology.com/2022/01/endocrinology.adrenal.html

https://blog.totalphysiology.com/2022/01/adrenalanatomy.html

https://blog.totalphysiology.com/2021/03/2021-google-ductlessglands-we-must-know.html

https://blog.totalphysiology.com/2021/11/autonomic-nervous-system -what-.html

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