Effect of Sodium Ions on the Transport of Neutral Amino Acids by Calf Lens

¹ Department of Ophthalmology, Albert Einstein College of Medicine of Yeshiva University, New York, N.Y.

Replacement of Na⁺ in the incubation medium depressed uptake of glycine and L-hydroxyproline and increased net loss of glycine from the calf lens. Passive, nonmediated diffusion was not altered markedly when Na⁺ was removed from the medium. A mediated component of influx was demonstrable for Na⁺, and the capacity of this system was comparable with that concerned with transport of sarcosine. The findings suggest that external Na⁺ may activate the system capable of accumulating amino acids by combining with a carrier associated with both systems of transport. Sodium ions and amino acids are considered to combine at different sites on the mutual carrier. Dependence on Na⁺ was noted for uptake of representative neutral amino acids at the A-site and the X-site (a special site for amino acids of low molecular weight). In contrast, uptake of L-leucine at the L-site did not depend significantly on the level of external Na⁺. These experimental results indicate that there are both Na⁺-dependent and Na⁺-independent systems concerned with the transport of neutral amino acids in the lens.