Transport of circulating reduced glutathione at the basolateral side of the anterior lens epithelium: physiologic importance and manipulations.

Transport of circulating reduced glutathione (GSH) was studied at the basolateral side of the lens epithelium by using an in situ vascular eye perfusion technique in guinea-pigs with rapid sampling to ensure detecting initial uptakes. The unidirectional transport rates of [35S]-GSH (4 nM) from plasma and aqueous into the epithelial cytosol were 0.046 +/- 0.003 and 6.88 +/- 0.39 min-1, respectively. HPLC analysis indicated that over 94% of [35S]-GSH remained intact in the epithelium and cortex in the presence or absence of gamma-glutamyl transpeptidase inhibitor, serine borate. Simultaneous infusion of [35S-cysteine]-GSH and [3H-glycine]-GSH confirmed the non-involvement of gamma-glutamyl transpeptidase in GSH transport across the lenticular membranes by showing that 35S/3H ratio in the epithelium and cortex was the same as in the aqueous and plasma. GSH epithelial influx was reduced by 53% (P < 0.01) by 0.3 mM sulphobromophtalein-GSH, a GSH conjugate that does not inhibit the facilitative GSH transporter, RcGshT, recently found in the lens. At physiologic concentration of circulating GSH at 30 microM, GSH epithelial influx was 0.77 nmol min-1 g-1; a t1/2 of 85.4 hr was estimated if endogenous epithelial GSH had to be replaced exclusively by plasma-derived GSH. The level of GSH in the epithelium was increased by 38% (P < 0.05) by 1 hr arterial infusion of GSH at 20 mM. The aqueous concentration of GSH under these conditions was 1.2 mM so that accumulation in the epithelium occurred a greater than six-fold concentration gradient. It is concluded that: (a) transport of GSH at the basolateral side of the epithelium is mediated by a concentrative mechanism distinct from RcGshT: (b) circulating GSH may represent a major source for epithelial GSH under physiologic conditions; and (c) the level of GSH in the epithelium can be manipulated by exogenous GSH.