Sympathomimetic drugs

Sympathomimetics, also known as adrenergic
agonists, act by stimulation of alpha, beta or both
the receptors.
Classification
Depending upon the mode of action, these can be
classified as follows:
1. Both alpha and beta-receptor stimulators e.g.,
epinephrine.
2. Direct alpha-adrenergic stimulators e.g.,
norepinephrine and clonidine hydrochloride.
3. Indirect alpha-adrenergic stimulators e.g.,
pargyline.
4. Beta-adrenergic stimulator e.g., isoproterenol.
Mechanisms of action
1. Increased aqueous outflow results by virtue of
both alpha and beta-receptor stimulation.
2. Decreased aqueous humour production occurs
due to stimulation of alpha-receptors in the ciliary
body.
Side-effects
1. Systemic side-effects include hypertension,
tachycardia, headache, palpitation, tremors,
nervousness and anxiety.
2. Local side-effects are burning sensation, reactive
hyperaemia of conjunctiva, conjunctival
pigmentation, allergic blepharo conjunctivitis,
mydriasis and cystoid macular oedema (in
aphakics).
Preparations
1. Epinephrine. This direct-acting sympathomimetic
drug stimulates both alpha and beta- adrenergic
receptors. Indications: (i) It is one of the standard
drugs used for the management of POAG. (ii) It is also
useful in most of the secondary glaucomas.
Preparations: It is available as 0.5 percent, 1 percent
and 2 percent eyedrops. Dosage: The action starts
within 1 hour and lasts up to 12-24 hours. Therefore,
it is instilled twice daily.
2. Dipivefrine(Propine or dipivalylepinephrine). It is a
prodrug which is converted into epinephrine after its
absorption into the eye. It is more lipophilic than
epinephrine and thus its corneal penetration is
increased by 17 times. Preparations: It is available as
0.1 percent eyedrops. Dosage: Action and efficacy is
similar to 1 percent epinephrine. It is instilled twice daily.
3. Clonidine hydrochloride. It is a centrally-acting
systemic antihypertensive agent, which has been
shown to lower the IOP by decreasing aqueous
humour production by stimulation of alpha-receptors
in the ciliary body. Preparations and dosage. It is
used as 0.125 percent and 0.25 percent eye drops,
twice daily.
4. Brimonidine (0.2%). Mechanism of action. It is a
selective alpha-2 adrenergic agonist and lowers IOP
by decreasing aqueous production and enhancing
uveoscleral outflow. It has an additive effect to betablockers.
Dosage: It has a peak effect of 2 hours and
action lasts for 12 hours; so it is administered twice
daily.
5. Apraclonidine (0.5%, 1%). It is also alpha-2
adrenergic agonist like brimonidine. It is an extremely
potent ocular hypotensive drug and is commonly used
prophylactically for prevention of IOP elevation
following laser trabeculoplasty, YAG laser iridotomy
and posterior capsulotomy. It is of limited use for
long-term administration because of the high rate of
ocular side-effects.