The properties of Na +-dependent and Na +-independent lysine uptake by isolated intestinal epithelial cells

Abstract

1. 1. The 5-min uptake of 1 mM leucine by epithelial cells isolated from rat intestine was reduced 75% in the absence of extracellular Na + and leucine did not attain intracellular concentrations greater than unity. In contrast, the uptake of 1 mM lysine was reduced only 32% and lysine reached an intracellular concentration of 1.54 mM, showing that a large component of the concentrative uptake of lysine is Na + independent. 2. 2. The Na +-independent component of lysine uptake attained levels significantly greater than unity after 5 min, but the total contribution of Na +-independent lysine uptake decreased from 82 to 62% as the incubation times was increased from 1 to 10 min. 3. 3. The Na +-independent component of lysine uptake was significantly inhibited by dinitrophenol after 5 and 10 min, but not after 2 min. Ouabain was non-inhibitory. 4. 4. At pH 9.0, Na +-dependent lysine uptake increased while Na +-independent lysine uptake decreased. At pH 3.5 lysine uptake in the presence and absence of Na + was reduced to the same level. 5. 5. Dinitrophenol, anaerobiosis and ouabain specifically decreased the Na +-dependent component of lysine uptake after 2 min. Ethacrynic acid, N-ethylmaleimide ATP and arginine inhibited both the Na +-dependent and Na +-independent components of lysine uptake. 6. 6. The isotonic replacement of K +, Ca 2+ and Mg 2+ by choline produced similar changes in the 2-min uptake of lysine in the presence or absence of Na +.