Guanethidine

Guanethidine is an antihypertensive drug that reduces the release of catecholamines, such as noradrenaline. Its mechanism is inhibition of the Na+ATPase dependent pump.

Side effects
Side effects include orthostatic and exercise hypotension, sexual dysfunction (delayed or retrograde ejaculation), and diarrhea.

Pharmacology
Guanethidine is transported by uptake 1 into the presynaptic terminal. (In this it competes with noradrenaline so can potentiate exogenously applied noradrenaline). Once inside the terminal it blocks the release of noradrenaline in response to arrival of an action potential. Spontaneous release is not affected.

Uses
Guanethidine was once a mainstay for hypertension resistant to other agents, but it is now rarely used in the US due to side effects and because substantially better drugs are available. It is still licensed in some countries, e.g. UK, for the rapid control of blood pressure in a hypertensive emergency.

Intravenous nerve block (Bier block) using guanethidine has been used to treat chronic pain caused by complex regional pain syndrome.