Effect of brimonidine on rabbit trabecular meshwork hyaluronidase activity

PURPOSE

To study the presence of hyaluronidase activity in the rabbit trabecular meshwork and its regulation by brimonidine.

METHODS

A spectrophotometric assay that consists of the assessment of N-acetylhexosamine groups released from hyaluronic acid was used to examine hyaluronidase activity. Cyclic adenosine monophosphate (cAMP) levels were assessed by radioimmunoassay.

RESULTS

Hyaluronidase activity was detected in the rabbit trabecular meshwork. Its optimal activity was in the acid range of pH 3.8. Brimonidine significantly increased trabecular hyaluronidase-specific activity and decreased cAMP accumulation. Yohimbine significantly inhibited the effect of brimonidine on both hyaluronidase activity and cAMP accumulation.

CONCLUSIONS

The finding of endogenous hyaluronidase activity in rabbit trabecular meshwork supports the hypothesis that this tissue can metabolize its own glycosaminoglycan (GAG) products. The present results suggest, however, that the hypotensive effect of brimonidine could be mediated, at least in part, by its ability to increase GAG catabolism, probably through a cAMP-independent mechanism.

[Morphometric study of the sclerocorneal trabeculum of rabbits after treatment with corticosteroids or hyaluronidase]

In eyes treated with corticosteroids the thickness of the trabecula is increased, the width of the intertrabecular spaces reduced and the number of trabecula per surface unit greater. In eyes perfused with hyaluronidase the opposite is seen, i.e., the thickness of the trabecular is reduced, the width of the intertrabecular spaces increased and the number of trabecula per surface unit diminished. The number of glycoproteins in the ground substance increases after treatment with corticosteroids while perfusion with hyaluronidase has the opposite effect. We have treated 27 rabbits, of which 17 with local administration of Dexamethasone 0,1%, 5 with subconjunctival injections and 5 as controls.

[The sclerocorneal trabeculum following cortisone treatment and perfusion of the anterior chamber with hyaluronidase (author's transl)]

Following perfusion of the anterior chamber with hyaluronidase, a great reduction in the basic substance of the sclero corneal trabeculum as well as increased activity of the goniocytes were observed with the electron microscope. The opening of the trabecular meshes thus produced constitutes a kind of chemical trabeculectomy, attributable to the depolymerization of the hyaluronates of the basic substance by the hyaluronidase of the goniocytes.