Adrenal Medulla Flashcards
What is the first step in catecholamine synthesis?
Tyrosine is converted into DOPA by tyrosine hydroxylase.
What is the second step of catecholamine synthesis?
DOPA is converted into dopamine by DOPA decarboxylase.
What is the third step of catecholamine synthesis?
Dopamine is converted into norepinephrine by Dopamine beta-hydroxylase.
What is the fourth and final step of catecholamine synthesis?
The conversion of norepinephrine into epinephrine by phenylethanolamine-N-methyltransferase (PNMT).
What is the rate limiting step of catecholamine synthesis?
The first step (tyrosine hydroxylase).
What inhibits tyrosine hydroxylase?
Norepinephrine.
What does DOPA decarboxylase use as a co-factor?
Pyridoxal phosphate.
What happens to the end product of DOPA decarboxylase (dopamine)?
The dopamine is packaged in secretory vesicles.
Where is dopamine beta-hydroxylase located?
It is membrane bound in the vesicles dopamine is packaged in.
What does dopamine beta-hydroxylase use as a co-factor?
Vitamin C.
What is the signal for catecholamine synthesis?
Acetylcholine released from pre-ganglionic sympathetic fibers (nicotinic receptors)
What is another name for the chromaffin cells of the adrenal medulla?
Pheochromocytes.
Where is phenylethanolamine-N-methyltransferase located?
In the chromaffin cell cytosol.
How does norepinephrine leave the secretory vesicle? (remember that PNMT is in the cytosol).
It passively diffuses out of the secretory vesicle.
How does epinephrine (converted from NE by PNMT in the cytosol) get back into the secretory vesicle?
Epinephrine re-enters the secretory vesicle by ATP-dependent active transport (monoamine transporter or VMAT-1).
What effect does cortisol have on PNMT?
An increase in stress hormones (such as glucocorticoids) or nerve impulses due to stress can cause PNMT to convert more norepinephrine into epinephrine.
How does COMT degrade epinephrine and norepinephrine?
COMT converts epinephrine into metanephrine and norepinephrine into normetanephrine.
How does monoamine oxidase (MOA) metabolize catecholamines?
It converts intracellular (sympathetic neuron) norepinephrine into DHPG. DHPG then diffuses fropm the neuron.
What is the fate of DHPG?
It is metabolized by aldehyde dehydrogenase (AD) or COMT into VMA.
What else can MAO metabolize?
MAO can also convert metanephrine and normetanephrine into VMA.
What is another route of metabolism for catecholamines and metanephrines?
They can be conjugated by the liver (sulfate or glucuronide).
How are catecholamines excreted?
In the urine (renal).
What GPCR subtype are α1 adrenoceptors?
α1 adrenoceptors are Gq receptors.
What is the general effect of α1 adrenoceptor stimulation?
α1 adrenoceptors generally cause smooth muscle contraction.
