Intracameral muramyl dipeptide-induced paracellular permeability associated with decreased glutamate transporter and gamma-glutamyltranspeptidase activities

Muramyl dipeptide (MDP) (N-acetylmuramyl-L-alanyl-D-isoglutamine was inject ed intracamerally to test if MDP applied to the aqueous side of the blood-a queous barrier would increase paracellular permeability in association with diminished uptake of glutamate. The symptoms of anterior uveitis, i.e., in crease in vascular dilatation, could be detected as early as 30 min post MD P injection while aqueous protein concentration did not increase at this ti me suggesting an initial dissociation between the circulatory and epithelia l barrier responses. However, at 45 min, the aqueous protein concentration increased 10-fold (201+/-174 to 2094+/-1835 mu g ml(-1); P < 0.001) rising progressively to 20-fold above the control eye at 60 min post injection (25 4+/-194 vs. 5038+/-2514 mu g ml(-1); P < 0.001). Epithelial cell barrier pa racellular permeability increased at 45 min as evidenced by the enhanced ef flux of radiolabelled L-glucose out of the aqueous (8% and 13% faster than control at 45 and 60 min post MDP injection, respectively), coinciding with the accelerated protein influx. A near 50% reduction in efflux of both rad iolabelled glutamate and D-aspartate was consistent with reduced glutamate uptake by the transport system X-AG(-). In addition, a 24% decline in aqueo us glutamate, but not aspartate, was detected in the aqueous of the MDP-tre ated eyes in association with a 54% decrease in iris/ciliary body gamma-glu tamyltranspeptidase activity consistent with reduced de novo glutamate form ation from glutamine. The aqueous of MDP injected eyes also had 6-fold and 34-fold higher prostaglandin E-2 and F-2 alpha concentrations, respectively (P less than or equal to 0.03) as well as reduced AH bicarbonate concentra tion. These results suggest that increased paracellular permeability is ass ociated with diminished gamma-glutamyltranspepidase-mediated glutamate prod uction, X-AG(-) transport activity, and cellular acidosis in the MDP-induce d prostaglandin-mediated inflammation. (C) 1999 Academic Press.