Pharmacology for Anaesthesia and Intensive Care

P1: PSB Printer: Yet To Come
9780521704632c12 CUFX213A/Peck 9780521618168 December 28, 2007 12:15

12 Sympathomimetics

Table 12.1.Summary of transmitters within the autonomic nervous system.

Pre-ganglionic Post-ganglionic

PNS acetylcholine acetylcholine

SNS acetylcholine noradrenaline

Adrenal medulla acetylcholine —

Sweat glands acetylcholine acetylcholine

fibres then pass into the adjacent spinal nerve via the grey rami communicans. They

release noradrenaline, which acts via adrenoceptors.

The adrenal medulla receives presynaptic fibres that synapse directly with its chro-

maffin cells using acetylcholine as the transmitter. It releases adrenaline into the

circulation, which, therefore, acts as a hormone, not a transmitter.

Post-ganglionic sympathetic fibres release acetylcholine to innervate sweat

glands.

All pre-ganglionic ANS fibres are myelinated and release acetylcholine, which acts

via nicotinic receptors (Table12.1).

Sympathomimetics

Sympathomimetics exert their effects via adrenoceptors or dopamine receptors

either directly or indirectly.Direct-actingsympathomimetics attach to and act

directly via these receptors, whileindirect-actingsympathomimetics cause the

release of noradrenaline to produce their effects via these receptors.

The structure of sympathomimetics is based on a benzene ring with various

amine side chains attached at the C1 position. Where a hydroxyl group is present

at the C3 and C4 positions the agent is known as acatecholamine (because 3,4-

dihydroxybenzene is otherwise known as ‘catechol’).

Sympathomimetic and other inotropic agents will be discussed under the follow-

ing headings:


Naturally occurring catecholamines


Synthetic agents


Other inotropic agents

Naturally occurring catecholamines

Adrenaline, noradrenaline and dopamine are the naturally occurring cate-

cholamines and their synthesis is interrelated (Figure12.3). They act via adrenergic

and dopaminergic receptors, which are summarized in Table12.2.

Adrenaline

Presentation and uses

Adrenaline is presented as a clear solution containing 0.1–1 mg.ml−^1 for admin-

istration as a bolus in asystole or anaphylaxis or by infusion (dose range