Amino acid movements across the wall of anuran small intestine perfused through the vascular bed.

1. L-leucine transfer across the wall of the small intestine has been studied in a vascularly perfused preparation from four species of frog. Some properties of the preparation are described. 2. A description is given of the endogenous amino acids appearing in the vascular bed and of the kinetic properties of this washout of endogenous L-leucine. 3. In the steady state of absorption, the transfer function relating the net flux of exogenous L-leucine into the vascular bed to the concentration in the lumen exhibits saturation. Under the conditions of the experiments the apparent concentration in the lumen for half-maximum transfer of L-leucine is found to be 2-1 +/- 0-4 (5) mM. 4. When Na ions are removed from the lumen the transfer of L-leucine into the vascular bed is inhibited. However, the additional removal of Na ions from the fluid in the vascular bed is further inhibitory to the transfer of the amino acid. 5. L-leucine previously absorbed from the lumen appears in the vascular bed in a biphasic fashion. Estimates are deduced of the size of the pool of L-leucine within the tissue which drains into the vascular bed. 6. These results are discussed in relation to previous work on amino acid transport undertaken with various sorts of preparation of small intestine.