Origin and renewal of the intrinsic glycoproteins of the aqueous humor

Abstract

Rabbits were injected either intravitreally or intra-aqueously with l-[³H]-fucose and killed at several intervals after the administration of this marker for glycoproteins. The aqueous humor, the vitreous body and the ciliary body were processed for radiometry (liquid scintillation counting), sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and fluorography. Light microscopic autoradiography was carried out on semi-thin sections of the ciliary body and revealed intense activity in terms of the synthesis, migration and renewal of glycoproteins in the ciliary epithelium. The amount of unbound [³H]-fucose in the aqueous humor decreased sharply by 4 h, and the labeled glycoproteins were present only in very small quantities at 1 day after the intra-aqueous injection. When [³H]-fucose was injected intravitreally, unbound radiolabel could be detected in the aqueous humor for >1 day and the labeled glycoproteins, for up to 21 days after injection. The amount of unbound or bound [³H]-fucose was higher in the vitreous than in the aqueous at any interval after the intravitreal injection. Following the intra-aqueous injection, the amount of label that reached the vitreous body was practically insignificant. The levels of radioactivity in the serum were extremely low, as they were in the contralateral eye when tritiated fucose was injected into one eye only. Most of the Coomassie blue-stained bands detected in SDS-PAGE contained labeled glycoproteins as revealed by fluorography of gels simultaneously containing aqueous and vitreous samples. One notable exception was the most prominent band in the stained gels; this unlabeled band had an apparent mol. wt. of 69 kDa and was assumed to represent serum albumin. The labeled aqueous bands detected by fluorography were practically the same ones identified in the vitreous body, except that the bands in the latter were more radioactive and appeared earlier than those in the aqueous humor. In conclusion, most of the aqueous proteins are intrinsic glycoproteins of the vitreous body, which, in turn, are secretory products of the inner epithelial layer of the ciliary body.