Compare and contrast the pharmacology of noradrenaline, vasopressin and phenylephrine

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College Answer

Noradrenaline is the catecholamine released by postganglionic adrenergic nerves. Direct agonist acting on alpha (vasoconstrictor: arterial and venous) and beta-1 (contractility, pro-arrhythmic) adrenergic receptors. Not absorbed enterally. Rapidly metabolised by COMT and MAO, resulting short (minutes) duration of effect (usually administered as intravenous infusion into central vein at rate of 0.5 to 100 mcg/min). Used clinically to increase blood pressure (usually in the setting of vasodilatory shock).

Vasopressin is a hormone/neurotranmitter with a complex series of effects. Direct action on a number of receptors (V1 (vascular: vasoconstriction), V2 (renal: anti-diuresis), V3 (pituitary), OTR (oxytocin receptor subtypes) and P2 (purinergic). Not absorbed enterally. Rapidly inactivated by trypsin and peptidases, resulting in short (minutes) duration of effect (longer on kidneys as very low concentration  are  required).  Used  clinically  as  treatment  for  diabetes  insipidus  (IM,  IV  or intranasal), and more recently by intravenous infusion (via central vein at rates of 0.01 to 0.1
U/min) to increase blood pressure (usually in the setting of vasodilatory shock) or as a large intravenous bolus providing potent vasoconstriction during cardiac arrest (40 units). Potentiates the action of other vasoconstrictor agents.

Phenylephrine is a synthetic alpha-1 adrenoreceptor agonist, similar in structure to adrenaline. Not administered enterally, biotransformation not well described but duration of action longer than naturally  occurring  catecholamines  (still  minutes).  Used  clinically  for  vasoconstrictor  effects, usually administered intravenously either in small bolus doses or occasionally as an intravenous infusion (via a central vein at rates of 40 to 180 mcg/min). Refractory hypotension may respond to agents with combined alpha-1 & alpha-2 activity (e.g. noradrenaline). Can be administered topically for alpha-adrenergic effect.

Discussion

Of these "compare and contrast" questions, vasopressin noradrenaline adrenaline phenylephrine dobutamine levosimendan and dopamine have all been asked about.

In fact, even this current mixture has cropped up in Question 18 of the second paper from 2005.