Nonspecific adaptation of jejunal amino acid uptake in the rat

Luminal nutrients are a major effector of intestinal adaptation. Amino acids are trophic to the intestine, but their role in regulating amino acid transport is not well documented. The presence of several distinct amino acid transport systems raises the question of whether adaptation is class-specific. Studies were carried out in parenterally nourished rats receiving a 7-day jejunal infusion of a 3% solution of either aminoisobutyric acid, aspartic acid, glutamine, histidine, lysine, or valine. While all amino acids were trophic to the intestine, their effects on the in vitro uptake of 0.1, 1.0 and 10.0 mM aspartic acid, lysine, and valine (representative acid, basic, and neutral amino acids) were variable and nonspecific. Compared to controls receiving either total parenteral nutrition alone or total parenteral nutrition plus luminal saline, prior lysine and aspartic acid infusion significantly increased in vitro uptake of all three amino acids tested, whereas valine had little effect on transport. No effect on transport was seen with glutamine (actively metabolized by the intestine as is aspartic acid), aminoisobutyric acid (a nonmetabolizable amino acid congener), or histidine (the most trophic amino acid). In conclusion, while individual amino acids cause an adaptation of amino acid uptake, the effects are nonspecific and independent of their metabolic or trophic potential.