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
- Adrenaline: 3 µg/dL
- Noradrenaline: 30 µg/dL
- 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:
- Adrenaline-secreting cells (90%)
- 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
- Adrenaline or epinephrine
- 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
- Formation of tyrosine from phenylalanine in the presence of enzyme phenylalanine hydroxylase
- Uptake of tyrosine from blood into the chromaffin cells of adrenal medulla by active transport
- Conversion of tyrosine into dihydroxyphenylalanine (DOPA) by hydroxylation in the presence of tyrosine hydroxylase.
- Decarboxylation of DOPA into dopamine by DOPA decarboxylase
- Entry of dopamine into granules of chromaffin cells
- Hydroxylation of dopamine into noradrenaline by the enzyme dopamine beta-hydroxylase
- Release of noradrenaline from granules into the cytoplasm
- 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
- 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
- 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).