Transport of Amino acids into the Posterior Chamber of the Rabbit Eye

¹ Kresge Eye Institute, Detroit, Mich.

The rate of entrance of C-14-labeled alpha aminoisobutyric acid (-AIB) into the posterior chamber of rabbit eyes decreases asymptotically with increasing concentrations of nonlabeled compound in the blood to one fifth that found when tracer compound alone is given. The results indicate that approximately 80 per cent of the tracer -AIB enters by active transport and the remainder by passive diffusion. The agreement between experimental results and the hypothesis underlying Michaelis-Menten kinetics is in accord with the idea that a carrier is involved in the active transport mechanism. The following parameters of the transport mechanism have been determined or calculated: The coefficient of diffusion (k_d.ph) is 0.63 per cent per minute. When the active transport system is saturated, i.e., when it is operating at maximum capacity, 2.4 x 10^-6 mmoles per minute enter the posterior chamber, and, the concentration in the fluid believed to flow from the ciliary epithelium into the posterior chamber (C₈) is 0.72 mmoles per liter. The transport system for -AIB can be saturated to varying degrees by the administration of naturally occurring neutral amino acids but is not affected appreciably by either basic or acidic amino acids or by y-aminobutyric acid. The D and DL amino acids are less effective than the L form in saturating the transport system. The observations are compatible with the concept that at least three carrier systems must be involved in the active transport of naturally occurring amino acids across the ciliary epithelium.