Test 1- DIRECT acting sympatholytics

Question 1

Phenoxybenzamine

Answer 1

Nonspecific alpha antagonist that binds irreversibly to the receptor and so will last the lifetime of
the receptor (3-4 days).
- Available as an oral (human) 10mg capsule (so often needs to be compounded.
- Usually takes a few days to see a clinical response.

Question 2

What are the most common uses for phenoxybenzamine?

Answer 2

Most commonly used in small animal medicine for two purposes:
o Used to treat urinary retention by relaxing the internal urethral sphincter (smooth muscle), and can be combined with other agents (eg. benzodiazepines like diazepam or alprazolam) that will relax the skeletal muscle.
o Used to manage pheochromocytoma (perioperatively) – manage hypertension and blood pressure spikes caused by catecholamine release.

Question 3

What are the cautions of Phenoxybenzamine?

Answer 3

Caution:
o Can cause excessive alpha-blockade (esp. at higher doses) leading to hypotension and
tachycardia (caution with cardiac disease or risk factors). May cause miosis, changes in intraocular pressure, or GI signs in some patients.

Question 4

Prazosin

Answer 4

• Alpha-1 antagonist

- Available as oral (human) capsules in variable concentrations

Question 5

What is prazosin commonly used to treat?

Answer 5

currently, more commonly used to treat urinary retention (functional) by relaxing the smooth muscle sphincter (similar to phenoxybenzamine).
o Compared to phenoxybenzamine it may have a slightly greater effect and faster onset of action, but potentially greater cardiovascular effects.
- Has been looked at in humans for management of pheochromocytoma, but phenoxybenzamine was found to be superior.