[Mode of membrane transport of amino acids--the kidney]

In the kidney, more than 99% of filtered free amino acids are reabsorbed by the proximal tubule. The brush border membrane of this segment is known to possess at least 5 independent amino acid carriers; neutral alpha-amino, basic, acidic, neutral beta-amino and imino acid carriers. Ionic interactions have been shown to be markedly different among these carriers. Characteristics of the interaction were outlined for each carrier. Basolateral membrane carriers have not been investigated extensively, but existence of neutral amino acid carrier (s) devoid of ionic interaction has been established. In addition, a Na+ plus K+ gradient-dependent transport has been identified for acidic amino acids and a Na+ exchange carrier has been suggested for basic amino acids. Also, asymmetric ionic interactions are suggested for luminal and basolateral membrane carriers for beta-amino acids. Proton-dependent carrier (s) for di- and tri-peptides has also been identified in the proximal tubule.