2000A12 Outline the pharmacology of oxytocin.

 

List:

·      PC

·      PK

·      PD

 

Physicochemical:

Structure

·  Nonapeptide

Endogenous source

·  Produced by supra-optic and paraventricular hypothalamic nuclei

·  Released by posterior pituitary

 

Pharmacokinetics:

Administration

·  IV bolus e.g. 5 units

·  IV infusion e.g. 5-10 units/h

Time course

·  Onset <1 min

·  Duration 20 mins

Absorption

·  Digested by the gut

Distribution

·  30% plasma protein bound

Metabolism

·  Oxytocinase in liver and plasma

·  t1/2β 5 mins (note carbetocin t1/2β 40 mins)

Excretion

·  Oxidation and recycling for protein synthesis

 

 

Pharmacodynamics:

Mechanism

·  Bind to oxytocin receptor, a Gq G-protein coupled receptor

·  ↑IP3 -> ↑ICF [Ca2+], (also ↑DAG)

Uterine effects

·  Smooth muscle contraction

·  Intermittent at low dose

·  Tonic at high dose

·  Receptors expressed from 36/40

·  Ante-partum: expedites labour, risk of foetal distress in excess

·  Post-partum: ↓risk PPH

Other effects

·  Mammary gland: contraction

·  Brain: nausea/vomiting at high dose

·  Vascular smooth muscle: vasodilatation (not vasoconstriction like ADH)

·  Kidney: some ADH effect via V2 (↑H2O reabsorption)

 

 

 

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