Amino acids move into and out of the amphibian cornea across its inner aqueous, side only. Most alpha-aminoisobutyric acid (AIB) accumulation takes place in the corneal epithelium; the endothelium and stroma do not limit exchanges with these cells. The apical, or tear, surface of the epithelium is an impermeable barrier to the transport of amino acids. There are several sites or mechanisms by which AIB enters and leaves the cornea. 2. The entry of AIB is not Na-dependent; however, the exit site is very sensitive to changes in internal Na concentration. Any factor, such as ouabain or metabolic inhibitors, that increases internal Na, markedly stimulates AIB efflux. 3. Site are also present for the exchange of internal for external amino acids, and this process is Na-dependent. There was no measurable movement of Na into or out of the cells with these amino acids. Exchange efflux was more specific than uptake, since both alanine and leucine inhibit AIB uptake, but only alanine stimulates AIB efflux. 4. Although the largest amount of AIB accumulated by the cornea was present in the epithelium, evidence is presented that the endothelium and stromal keratocytes may also concentrate and retain amino acids.
