FabReads Digest with Clinical Sciences – 003 (Adrenal Medulla)

Medulla is the inner part of  adrenal gland and it  forms 20% of the mass of adrenal gland. It is made up of interlacing cords of cells known as chromaffin cells. These cells contain fine granules which are stained brown by potassium dichromate.

PLASMA LEVEL OF CATECHOLAMINES

1. Adrenaline: 3 µg/dL
2. Noradrenaline: 30 µg/dL
3. Dopamine: 3.5 µg/dL

HALF-LIFE OF CATECHOLAMINES

The Half-life of catecholamines is about 2 minutes.

Types of chromaffin cells

Adrenal medulla is formed by two types of chromaffin cells:

1. Adrenaline-secreting cells (90%)
2. Noradrenaline-secreting cells (10%).

HORMONES OF ADRENAL MEDULLA

Adrenal medullary hormones are the amines derived from catechol and so these hormones are called catecholamines.

Catecholamines secreted by adrenal medulla

1. Adrenaline or epinephrine
2. Noradrenaline or norepinephrine

SYNTHESIS OF CATECHOLAMINES

Catecholamines are synthesized from the amino acid tyrosine in the chromaffin cells of adrenal medulla. These hormones are formed from phenylalanine also. But phenylalanine has to be converted into tyrosine.

Stages of Synthesis of Catecholamines

1. Formation  of  tyrosine  from  phenylalanine  in  the presence of enzyme phenylalanine hydroxylase
2. Uptake  of  tyrosine  from  blood  into  the  chromaffin cells of adrenal medulla by active transport
3. Conversion  of tyrosine  into dihydroxyphenylalanine (DOPA) by hydroxylation in the presence  of  tyrosine hydroxylase.
4. Decarboxylation  of DOPA  into dopamine  by  DOPA decarboxylase
5. Entry of dopamine into granules of chromaffin cells
6. Hydroxylation  of dopamine  into noradrenaline  by the enzyme dopamine  beta-hydroxylase
7. Release  of noradrenaline from granules  into the cytoplasm
8. Methylation  of noradrenaline  into  adrenaline  by the most important enzyme called phenylethanolamineN-methyltransferase  (PNMT).  PNMT  is present  in chromaffin cells.

METABOLISM OF CATECHOLAMINES

Eighty five percent of noradrenaline is taken up by the sympathetic adrenergic neurons. Remaining  15% of noradrenaline and adrenaline are degraded:

Stages of Metabolism of Catecholamines

1. Methoxylation of adrenaline into meta-adrenaline and noradrenaline into metanoradrenaline  in the pres ence of  ‘catechol-O-methyltransferase’ (COMT).  Meta-adrenaline  and  meta-noradrenaline are toget her called metanephrines
2. Oxidation  of metanephrines  into  vanillylmandelic acid (VMA)  by  monoamine oxidase (MAO)
   • 15% as free adrenaline and free noradrenaline
   • 50%  as free or conjugated  meta-adrenaline  and meta-noradrenaline
   • 35% as vanillylmandelic acid (VMA).