Catecholamines Flashcards
The three endogenous catecholamines
- Epinephrine
- Norepinephrine
- Dopamine
How do cells generate tyrosine if the tyrosine supply is low?
Make tyrosine from phenylalanine with the enzyme phenylalanine hydroxylase
Step by step process of dopamine synthesis in dopaminergic cells
- Tyrosine is converted to L-DOPA by the enzyme tyrosine hydroxylase
- L-DOPA is converted to dopamine by the enzyme aromatic L-amino acid decarboxylase
- Dopamine is transported into vesicles
The rate limiting step to catecholamine synthesis
Tyrosine conversion to L-DOPA by the enzyme tyrosine hydroxylase
How is tyrosine hydroxylase kept in check?
Feedback inhibition by dopamine and norepinephrine
Step by step process of norepinephrine synthesis in noradrenergic neurons
- Tyrosine is converted to L-DOPA by the enzyme tyrosine hydroxylase
- L-DOPA is converted to dopamine by the enzyme aromatic L-amino acid decarboxylase
- Dopamine is transported into vesicles
- Dopamine is converted to norepinephrine by the enzyme dopamine- beta- hydroxylase
Step by step process of epinephrine synthesis in adrenergic neurons and chromaffin cells
- Tyrosine is converted to L-DOPA by the enzyme tyrosine hydroxylase
- L-DOPA is converted to dopamine by the enzyme aromatic L-amino acid decarboxylase
- Dopamine is transported into vesicles
- Dopamine is converted to norepinephrine by the enzyme dopamine- beta- hydroxylase
- Norepinephrine is converted to epinephrine by the enzyme pheylethanolamine N-methyltransferase in the cytoplasm of adrenergic cells
How are catecholamines released?
Action potential depolarizes the nerve terminal > Voltage-gated Ca++ channels open > Ca++ diffuses into the nerve terminal > Increased neuronal Ca++ = vesicles fuse with the nerve terminal and release the catecholamine into the synaptic cleft
How is norepinephrine release inhibited?
By stimulating presynaptic alpha-2 receptor
How can norepinephrine augment its own release?
By stimulating presynaptic beta-2 receptors
In what three ways are catecholamines removed from the synaptic cleft?
- Reuptake into the presynaptic nerve
- Diffusion away from the synaptic cleft with metabolism by MAO and COMT
- Reuptake by extraneural tissue
What is the primary mechanism that removes catecholamines from the synaptic cleft?
Reuptake into the presynaptic neuron
What are the features of reuptake into the presynaptic neuron?
Takes place by active transport- energy-consuming and temperature-dependent process
Most recycled and repackaged into vesicles
Some metabolized by MAO in the nerve terminal
What happens after norepinephrine diffuses away from the synaptic cleft?
Enters the circulation > metabolized by MAO and COMT > metabolites excreted in the urine
What happens to norepinephrine if it enters a extraneural tissues?
MAO and COMT in the extraneural cells metabolize it
