A study of the transfer of amino acids across the endothelium of the rabbit cornea

Abstract

The purpose of this study was to determine whether the endothelium of the cornea is capable of transporting amino acids from the aqueous humor to the extracellular fluid of the stroma against a concentration gradient. Isolated rabbit corneas, free of epithelium, were placed in a chamber and perfused independently on both surfaces with an artificial aqueous humor. The flux of 14C-labelled amino acids across the tissue was then determined both from the aqueous to tear side and from tear side to aqueous. The permeability of the endothelium to each amino acid was found to be equal in the two directions. It is concluded that movement against a concentration gradient from aqueous to stroma is, therefore, not possible. These findings are consistent with measured levels of amino acids in the cornea in vivo, and indicate that stromal concentrations higher than those in the aqueous humor are the result of accumulation in the keratocytes rather than of active transport across the endothelium. The boundary cell layer of the cornea, unlike that of the lens, appears to have no active nutritive role in the supply of amino acids from the aqueous humor.