Biphasic intraocular pressure response to Q-switched Nd:YAG laser irradiation of the iris and the apparent mediatory role of prostaglandins

Biphasic intraocular pressure response to calcitonin gene-related peptide

PURPOSE

To examine the effects of calcitonin gene-related peptide (CGRP) on intraocular pressure (IOP).

METHODS

IOP was periodically measured in rabbits treated with intravitreal injection (20 ael) of either: 1) CGRP (10(-4) approximately 10(-7) M) into one eye; 2) CGRP (10(-4) or 10(-6) M) into both eyes 30 min after intravitreal administration of CGRP-(8-37) (10( -3) M), a CGRP1 receptor antagonist, into one eye; 3) CGRP (10(-4 ) or 10(-6) M) into one eye 30 min after intravenous administration (200 mg/kg) of either Nw-nitro-L-arginine methyl ester (L-NAME), a nonselective inhibitor of nitric oxide synthase (NOS), or aminoguanidine (AG), a selective inhibitor of inducible NOS; or 4) CGRP (10(-4) or 10(-6) M) into one eye along with intraperitoneal indomethacin (50 mg/ kg at -1 and 4 h).

RESULTS

CGRP (10(-4) and 10(-5) M) produced a biphasic IOP response, which consisted of an early ocular hypertensive phase and a subsequent sustained hypotensive phase. While CGRP (10(-6) M) yielded only a profound IOP reduction. CGRP-(8-37) significantly inhibited the IOP elevation induced by CGRP (10(-4) M) and completely abolished the IOP reduction induced by CGRP (10(-6) M). The IOP increase induced by CGRP (10(-4) M) was completely abolished by L-NAME, but not affected by AG. The IOP reduction induced by CGRP (10(-6) M) was not affected by L-NAME. Indomethacin did not significantly affect the IOP responses to CGRP.

CONCLUSIONS

CGRP dose-dependently produces a biphasic IOP response, which is mediated by CGRP1 receptors. It is suggested that constitutive NOS is involved in the early ocular hypertensive response.