Tissue distribution of exogenous amino acids during transport across the vascularly perfused anuran small intestine.

Microdensitometric analysis of autoradiographs has been used to measure the distribution and intracellular concentration of L-leucine and L-lysine during transport across the vascularly perfused small intestine of Rana pipiens. L-leucine was not accumulated in the mucosal epithelium to a concentration higher than that in the lumen under steady-state conditions, whereas L-lysine was concentrated on average three- to four-fold. 2. At the end of 30 min loading, the majority of both amino acids were found in the mucosal epithelium and the villous core, although significant amounts were also present in the muscle. Lysine showed a gradient of accumulation within the mucosal epithelium along the length of the villous folds, the highest concentrations being achieved in the cells near the tip. Leucine showed no such gradient under steady-state conditions. 3. Superfusion of the mucosal surface of the tissue with leucine for only 3 min did reveal a gradient for uptake into the mucosal epithelium, although it was still not as steep as that seen for lysine. 4. The presence of leucine in the vascular bed while lysine was perfused through the lumen significantly lowered the concentration of lysine in the mucosal epithelium and villous core and eliminated the concentration gradient in the mucosal epithelium seen along the villous fold. 5. When leucine was perfused on its own through the vascular bed, the uptake into the muscle was greatly increased compared to when the amino acid was presented from the lumen. At the same time, the uptake into the mucosal epithelium was reduced by 45%. 6. Analysis of the tissue content of leucine after loading for 30 min from the lumen and then washing out the amino acid for 15 min showed that the mucosal epithelium, villous core and muscle had contributed 74%, 17% and 9% respectively to the total amino acid lost from the tissue. 7. These results are discussed with regard to the significance of the exit mechanisms for these amino acids and the validity of previous compartmental analyses made under similar conditions.