doctors often use a combination of two kinds of drops, one con-
taining a parasympatholytic drug and the other containing a
sympathomimetic drug, in order to maximize the amount of pupil
dilation. A currently used combination is the parasympatholytic
drug tropicamide (an antagonist at metabotropic acetylcholine recep-
tors) and the sympathomimetic drug phenylephrine (an agonist at
metabotropic norepinephrine receptors).
A similar pharmacologic combination is used for respiratory
inhalers used to treat conditions like asthma, chronic obstructive
pulmonary disease, and emphysema. Inhalers deliver a vaporized
preparation of drug directly into the lungs, so that, as with eye drops
for effects on the pupil, the desired site of action is close by. Sympa-
thomimetics dilate the air passages in the lungs, allowing for easier
breathing. So also do parasympatholytics. There are inhalers that
contain sympathomimetic drugs—the drug albuterol, for example.
Albuterol is an agonist at metabotropic receptors for norepinephrine.
OH OH
HO NH. NHO or os HO
Norepinephrine AlbuterolOther inhalers contain parasympatholytic drugs, such as one called
ipratropium, an antagonist at metabotropic acetylcholine receptors.
There is also an inhaler that contains both albuterol and ipratropium,
using the combination to maximize lung airway dilation effects that
make breathing easier.
The neurotransmitter acetylcholine (ACh) was discovered following