Pharmacology for Anaesthesia and Intensive Care

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12 Sympathomimetics

Table 12.2.Actions and mechanisms of adrenoceptors.

Receptor Subtype Location

Actions when

stimulated Mechanism

α 1vascular smooth

muscle

vasoconstriction Gq-coupled phospholipase

Cactivated→↑IP 3 →↑

Ca^2 +

2 widespread

throughout the

nervous system

sedation, analgesia,

attenuation of

sympathetically

mediated

responses

Gi-coupled adenylate

cyclase inhibited→↑

cAMP

β 1 platelets platelet aggregation

heart +ve inotropic and

chronotropic

effect

Gs-coupled adenylate

cyclase activated

→↑cAMP

2bronchi, vascular

smooth muscle,

uterus (and heart)

relaxation of smooth

muscle

Gs-coupled adenylate

cyclase activated

→↑cAMP→↑Na+/K+

ATPase activity and

hyperpolarization

3 adipose tissue lipolysis Gs-coupled adenylate

cyclase activated

→↑cAMP

D1within the central

nervous system

modulates

extrapyramidal

activity

Gs-coupled adenylate

cyclase activated

→↑cAMP

peripherally vasodilatation of

renal and

mesenteric

vasculature

2 within the central

nervous system

reduced pituitary

hormone output

Gi-coupled adenylate

cyclase inhibited→↑

peripherally inhibit further

noradrenaline

release

cAMP

0.01–0.5μg.kg−^1 .min−^1 )inthe critically ill with circulatory failure. It may also be

nebulized into the upper airway where its vasoconstrictor properties will temporarily

reduce the swelling associated with acute upper airway obstruction. A 1% ophthalmic

solution is used in open-angle glaucoma, and a metered dose inhaler delivering 280

μgfor treatment of anaphylaxis associated with insect stings or drugs. In addition,

it is presented in combination with local anaesthetic solutions at a strength of 1 in

80 000–200 000.