Pre- and postsynaptic effects of brimonidine on isolated rabbit iris dilator muscles.
OBJECTIVE
Brimonidine is an imidazoline compound used for the treatment of glaucoma, but having very little effect on pupil diameter. Like para-aminoclonidine, most imidazoline compounds interact with postsynaptic α-adrenoceptors and cause pupil dilatation. Therefore, as part of an investigation of the mechanism of action of brimonidine on pupil diameter, the present study was initiated to measure, in vitro, the relative potency of brimonidine on the pre- and postsynaptic α-adrenoceptors of rabbit iris dilator muscle.


METHODS
The contractile activity of brimonidine and its effect on twitch contraction evoked by electrical field stimulation were studied in isolated rabbit iris dilator muscles by isometric tension recording.


RESULTS
Brimonidine significantly inhibited the twitch contraction of the dilator muscle caused by field stimulation, without affecting the response to exogenously applied phenylephrine. Compared to phenylephrine, brimonidine caused only a small contractile response with % maximum contraction values of <10%.


CONCLUSIONS
These results suggest that brimonidine may act on nerve endings to inhibit adrenergic neurotransmission with very little effect on postsynaptic α-adrenoceptors. This may indicate that brimonidine reduced the pupil diameter just a little, thus improving night vision.
Department of Ophthalmology, School of Medicine, Kitasato University, Sagamihara, Japan.
Sonoko
S
Tatsui
Department of Orthoptics and Visual Sciences, School of Allied Health Sciences, Kitasato University, Sagamihara, Japan.
Hitoshi
H
Ishikawa
Department of Ophthalmology, School of Medicine, Kitasato University, Sagamihara, Japan.
Kimiya
K
Shimizu
Department of Ophthalmology, School of Medicine, Kitasato University, Sagamihara, Japan.
Kimiyo
K
Mashimo
brimonidine; electrical field stimulation; imidazolin; postsynaptic α1-adrenoceptor; presynaptic α2-adrenoceptor; rabbit iris dilator