Characteristics of Lysine Transport Across the Serosal Pole of the Anuran Small Intestine

The transport of the dibasic amino acid L-lysine across the serosal pole of the intestinal epithelium has been studied using the vascularly perfused anuran small intestine. The exit of pre-loaded lysine into the vascular bed was inhibited by L-ornithine (2 mM) and L-arginine (10 mM) when pulsed through the lumen during the wash-out, while 2-aminoisobutyric acid (AIB), L-histidine, L-citrulline and L-cystine had no effect. Luminal L-leucine and L-alanine at a concentration of 10 mM markedly stimulated the unloading of lysine into the vascular bed and sarcosine, L-proline and beta-alanine also did so to a lesser extent. The instantaneous rate constant for lysine exit into the vascular bed was increased by the presence of L-arginine, L-ornithine, L-citrulline, L-histidine, AIB, L-leucine and L-alanine at a concentration of 10 mM in the vascular bed. L-proline had no effect. The simultaneously measured efflux of lysine into the lumen was unaffected by the presence of the other amino acids in the vascular bed. The uptake of lysine into the epithelium from the vascular bed was accelerated by L-ornithine and slightly by L-arginine when they were present in the lumen, while L-leucine, L-alanine, beta-alanine, L-proline, L-citrulline, sarcosine, L-histidine and AIB had no effect. The instantaneous rate constant for lysine wash-out into the vascular bed was transiently increased by the presence of L-leucine in the vascular bed at concentrations of 10, 0.10 and 0.01 mM. The steady-state transfer of lysine from the lumen to the vascular bed was stimulated in a biphasic manner by 5 mM-leucine in the lumen and by 0.5 mM-leucine in the vascular bed. The mechanisms for these interactions between lysine transport across the basolateral membrane of the enterocyte and other amino acids are discussed and a possible role for neutral amino acid stimulation of lysine exit is proposed.