Viva M1(i)b

This chapter is relevant to Section M (i) of the 2017 CICM Primary Syllabus, which expects the exam candidates to "describe the autonomic nervous system, including anatomy, receptors, subtypes and transmitters (including their synthesis, release and fate)". To control size, this section is limited to synthesis, metabolism and receptor effects of catecholamines.

What is the pathway of synthesis of catecholamine neurotransmission?

• Occurs at the synapse
• Tyrosine hydroxylase, the rate limiting enzyme of catecholamine synthesis, is made locally at the nerve terminal
• When the adrenal medulla is stimulated, this tyrosine hydroxylase  is the enzyme that gets phosphorylated by protein kinase C and calmodulin.
• Thus, to get systemic release of adrenaline in a fight-or-flight situation, tyrosine must be hydroxylated at a sufficient rate.
• Then, you decarboxylate your L-dopa into dopamine.  Pyridoxine is an essential cofactor for the decarboxylation of L-dopa
• If dopamine enters the vesicles, it gets converted to noradrenaline by dopamine beta-hydroxylase (dβH)
• Noradrenaline is N-methylated to form adrenaline in the chromaffin cells of the adrenal medulla

Weirdly, the size of the adrenaline store is controlled by  glucocorticoids. Glucocorticoids travel via the intra-adrenal portal venous network into the medulla, to induce the synthesis of phenethanolamine N-methyltransferase.

What are the pharmacological properties of L-dopa?

• L-Dopa  is tyrosine plus an extra hydroxyl group.
• It crosses the blood brain barrier and acts as a precursor for dopamine.

Class	Antiparkinsonian agent
Chemistry	Amino acid (L-tyrosine derivative)
Routes of administration	Oral
Absorption	Poor oral bioavailability (10-20%)
Solubility	pKa = 2.32, sparingly soluble in water (66mg/L).
Distribution	VOD= 2-3L/kg, minimally protein-bound
Target receptor	Acts as the pro-drug, a substante for dopamine synthesis, and therefore affects dopamine receptors, D1-D5;
Metabolism	Crosses the blood-brain barrier and is metabolised to dopamine primarily via decarboxyation by aromatic amino acid decarboxylase; minor metabolic pathways include O-methylation, transamination and oxidation. Can be metabolised by bacterial enzymes in the gut; implications for bioavailability.
Elimination	Minimal free drug is eliminated in the urine
Time course of action	Half life is 1 hour (but the clinical response is more durable because of nigrostriatal neuronal dopamine synthesis and storage)

 

What is the process of adrenergic neurotransmission?

• The VMAT-2 protein (vesicular amine transporter) concentrates the catecholamines in the vesicles
•  It is promiscuous: i.e. it has equal affinity for dopamine, adrenaline, noradrenaline, serotonin and so forth.  
• VMAT-2 gets about 90% of the catecholamines into vesicles; the rest float about in the cytoplasm and get metabolized by mitochondrial MAO. 
•  The low  pH inside the vesicles causes the catecholamines to become trapped; in the acidic environment they exist in their ionised water-soluble form, and thus cannot diffuse out.
• Catecholamine exocytosis is mediated by  calcium influx through voltage-gated N-type calcium channels.
• SNAP-25, syntaxin and synaptobrevin mediate vesicle fusion
• Reuptake of catecholamines is mediated by DAT, NET and SERT

Can you point out where these steps can be subject to negative feedback regulation?

• The process of exocytosis is affected by:
  • Presynaptic alpha-2 receptors (inhibits the release; clonidine=antagonist)
  • Presynaptic beta-2 receptors (enhances the release; adrenaline=agonist)
  • Presynaptic adenosine A1 receptors (inhibits the release; caffeine=antagonist)

Which are the drug targets in the pathway of catecholamine neurotransmission?

• Tyrosine metabolism: α-Methyltyrosine blocks the conversion of tyrosine into L-DOPA, which is a rate-limiting step. This is one way of treating phaeochromocytoma.
• L-Dopa Metabolism: α-Methyldopa blocks  the conversion of L-DOPA into dopamine, and its active metabolite α-Methylnorepinephrine  is an alpha-2 receptor agonist, which is functionally similar to clonidine.
• Interference with storage: Reserpine blocks VMAT-2, and can thus result in the depletion of catecholamines from all your nerve endings
• Interference with the mechanisms of exocytosis: Botulinum and tetanus toxins proteolyse the fusion proteins, synaptobrevin specifically,  and thus prevent release of  catecholamines and acetylcholine.
• Inhibition of exocytosis: Clonidine and dexmedetomidine act here to inhibit release of catecholamines. Ditto α-Methylnorepinephrine as mentioned above.
• Stimulation of exocytosis Xanthines like caffeine act as adenosine antagonists; they inhibit the inhibition. The net result is an enhanced release of catecholamines.
• Interference with synaptic concentration: Indirect sympathomimetics act here by displacing catecholamines out of the synapse and into the extracellular fluid.
• Inhibition of reuptake: cocaine and the tricyclic antidepressants act at the NAT and DAT channels to decrease reuptake of noradrenaline, thus increasing its synaptic dwell-time.
• Reversal of reuptake channels: Amphetamines also cause DAT and NET dysfunction, by causing internalization of the DAT protein, and by causing it to malfunction and actually leak dopamine back into the synapse

What are the cellular mechanisms of catecholamine receptor activation?

• α receptors: G-protein coupled receptors
  • α-1 : Gq protein coupled – second messenger is IP3, causing an increase of intracellular calcium
  • α-2– Gi protein coupled – inhibit adenylyl cyclase, decrease cAMP
• β receptors: Gs protein coupled – activate adenylyl cyclase, increase cAMP levels

What classes of adrenergic receptors are there?

α-1 receptors α-1A α-1B α-1D

α-2 receptors α-2A α-2B α-2C

β-receptors β-1 β-2 β-3

What are the effects of activating  α-1 receptors?

• Airway and respiratory effects:
  • Decreased secretions in the bronchial glands
• Circulatory effects
  • Vasoconstriction of arteries in:
    • Skin and mucosa
    • Skeletal muscle
    • Splanchnic circulation
  • Slight vasoconstriction of some critically important arteries:
    • Coronary arteries (local effects still result in vasodilation)
    • Cerebral arteries (very minor influence)
    • Pulmonary erteries
  • Significant vasoconstriction of the arteries of the abdominal viscera
  • Significant vasoconstriction of the renal arteries
  • Significant vasoconstriction of the arteries of the salivary glands
  • Vasoconstricts the veins
• Neurological effects
  • Contraction of the radial muscle of the iris, dilating the pupil
  • In the CNS, mediates learning and memory, and mostly have a presynaptic inhibitory function
• Exocrine gland effects
  • Slightly increases lacrimation (weak effect, its mainly a parasympathetic thing)
  • Localised sweating
  • Increased salivation (again, not a very strong effect - this is mostly a parasympathetic function)
• Endocrine gland and metabolic effects
  • Increased glycogenolysis and gluconeogenesis in the liver
  • Decreased pancreatic secretions (digestive enzymes)
  • Increased lipolysis and thermogenesis at the adipocyte
• Gastrointestinal effects
  • Decreased motility and tone of the stomach
  • Contraction of the gastric sphincters
  • Decreased motility and tone of the intestine
  • Contraction the intestinal sphincters
  • Causes contraction of the splenic capsule (in some animals, this causes an autotransfusion of a significant volume of blood)
• Renal and genitourinary effects
  • Increased  renin secretion by the kidney
  • Contraction the trigone of the bladder, and the urinary sphincter
  • Increased the motility and tone of the ureter
  • Contraction a pregnant uterus
  • Ejaculation
• Thermoregulatory effects
  • Piloerection in the skin

What are the effects of activating  α-2 receptors?

• Circulatory effects
  • Vasoconstriction of arteries and veins much the same as α-1 receptors, except with different selectivity, and they are present at non-synaptic sites
• Neurological effects
  • In the CNS, analgesic and sedating effects, and sympatholytic by a  presynaptic inhibitory function
• Exocrine gland effects
  • Slightly increases lacrimation (weak effect, its mainly a parasympathetic thing)
  • Localised sweating
  • Increased salivation (again, not a very strong effect - this is mostly a parasympathetic function)
• Endocrine gland and metabolic effects (α-2A)
  • Significantly decreases the secretion of insulin
  • Inhibits lipolysis at the adipocyte (opposite of what α-1 receptors do)
• Gastrointestinal effects (α-2A)
  • Depress peristalsis and increase sphincter tone, much as α-1 receptors

What are the effects of activating  β-1 receptors?

• Circulatory effects
  • Increased sinoatrial node firing rate, increasing the heart rate
  • Increased atrial contractility and conduction velocity
  • Increased AV node automaticity and conduction velocity
  • Increased His-Purkinje system automaticity and conduction velocity
  • Increased ventricular contractility, conduction velocity, as well as automaticity and rate of any random idioventricular pacemakers
• Renal effects
  • Increased renin secretion by the kidney
• Gastrointestinal effects
  • Decreased motility and tone of the stomach

What are the effects of activating  β-2 receptors?

• Airway and bronchi:
  • Bronchodilation
  • Increased secretion of the bronchial glands
• Neurological effects:
  • Relaxation of the ciliary muscle for far vision
• Modest cardiac effects: β-2 receptors make up about 25% of the total cardiac β-receptor population (Brodde et al, 2006); but whereas β-1 receptors are expressed on myocytes, β-2 receptors seem to be hanging out on fibroblasts like weirdos.
  • Increased sinoatrial node firing rate, increasing the heart rate
  • Increased atrial contractility and conduction velocity
  • Increased AV node automaticity and conduction velocity
  • Increased His-Purkinje system automaticity and conduction velocity
  • Increased ventricular contractility, conduction velocity, as well as automaticity and rate of any random idioventricular pacemakers
  • Vasodilation of vascular beds:
    • coronary arteries
    • arteries of the abdominal viscera
    • renal arteries
    • capacitance veins veins
• Gastrointestinal effects
  • Decreased motility and tone of the stomach
  • Decreased motility and tone of the intestine
  • Relaxation of the gallbladder
  • Relaxation of the splenic capsule
• Renal and genitourinary effects
  • Relaxation of the urinary bladder
  • Relaxation of the uterus
• Skeletal muscle effects
  • Increased contractility of the skeletal muscle
  • Increased glycogenolysis in skeletal muscle, leading to hyperlactataemia
  • Increased potassium uptake into skeletal muscle (this is why salbutamol is useful in the treatment of hyperkalemia)
  • Increased skeletal muscle protein synthesis; a potent myocyte/differentiation signal
• Metabolic and endocrine effects
  • Increased glycogenolysis and gluconeogenesis in the liver
  • Increased the secretion of insulin
  • Increased lipolysis and thermogenesis at the adipocyte
  • Increased melatonin synthesis at the pineal gland

What are the effects of activating  β-3 receptors?

• Bladder (detrusor) relaxation
• Increased water and solute resorption at the kidney
• Increased lipolysis and thermogenesis at the adipocyte
• Modulation of cardiac contractility
• Relaxation of uterine contractions

What are the adverse effects of an unselective β-1 and  β-2 agonist? What is an example of one?

Isoprenaline. 

• Tachycardia
• Arrhythmias
• Low diastolic pressure
• Lactic acidosis
• Hyperglycaemia