INHIBITION OF GLUTAMATE UPTAKE CAUSES AN ACUTE INCREASE IN AQUEOUS-HUMOR PROTEIN

Inhibition of glutamate transport has been shown to increase paracellu lar permeability of epithelial cell monolayers in vitro. To determine if blocking glutamate transport would affect tissue permeability in vi vo, D-aspartate (D-Asp; 300 nmol 30 mu l(-1)) (a non-toxic competitive inhibitor of glutamate transport) or a placebo was injected into the anterior chambers of the fellow eyes of 15 adult rabbits. [C-14]-L-glu cose and/or [I-125]-rabbit albumin were included in the injection vehi cle as aqueous humor (AH) outflow markers. The specific inhibition of glutamate uptake by D-Asp was indicated by a 15% increase in AH glutam ate (174 +/- 9 nmol ml(-1) to 205 +/- 13 nmol ml(-1); P = 0.03) at 1-1 .5 hr post injection. Also, the efflux of [C-14]-L-glucose and [I-125] -rabbit albumin from the AH of D-Asp injected eyes was increased 22% o ver the placebo-injected control eyes (P less than or equal to 0.02). Concomitantly, the total protein concentration in the AH from D-Asp in jected eyes (517 +/- 35 mu g ml(-1)) was 19% greater (P < 0.02) than t he protein concentration in AH from placebo-injected control eyes (420 +/- 36 mu g ml(-1)). In additional studies, an irreversible inhibitor of glutamate transport, threo-beta-hydroxyaspartate (THA; 30 nmol 30 mu l(-1)), was shown to increase the efflux of [C-14]-L-glucose (22%; P < 0.05) from the anterior chamber and increase AH protein concentrat ions by 29% (484 +/- 112 mu g ml(-1) in control AH versus 686 +/- 117 mu g ml(-1) in THA AH, P = 0.08) at 1 hr post intracameral injection. SDS-PAGE analysis of the AH associated the protein increase in the D-A sp and THA injected eyes but not placebo-injected control eyes with a detectable increase in a 66 kDa protein (aligns with serum albumin) an d several lower molecular weight (23-35 kDa) AH proteins. The results found suggest that inhibition of glutamate transport from the AH acute ly increases intraocular epithelial/endothelial paracellular permeabil ity. (C) 1997 Academic Press Limited.