Selectivity properties of a Na-dependent amino acid cotransport system in adult alveolar epithelial cells.
We investigated the amino acid specificity of a Na-dependent amino acid cotransport system that contributes to transepithelial Na absorption in the apical membrane of cultured adult rat alveolar epithelial cell monolayers. Short-circuit current was increased by basic, uncharged polar, and nonpolar amino acids but not by L-aspartic acid or L-proline. EC(50) values for L-lysine and L-histidine were 0.16 and 0.058 mM, respectively. The L-lysine-stimulated short-circuit current was Na dependent, with a concentration causing a half-maximal stimulation by Na of 44.24 mM. L-Serine, L-glutamine, and L-cysteine had EC(50) values of 0.095, 0.25, and 0.12 mM, respectively. L-Alanine had the highest affinity, with an EC(50) of 0.027 mM. We conclude that monolayer cultures of adult rat alveolar epithelial cells possess a broad-specificity Na-dependent amino acid cotransport system with properties consistent with system B(0,+). We suggest that this cotransport system plays a critical role in recycling of constituent amino acids that make up glutathione, thus ensuring efficient replenishment of this important antioxidant within the alveolar fluid.
Jiang
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Department of Physiology, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Ingbar
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O'Grady
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D000584:Amiloride / Q000494:pharmacology; D000596:Amino Acids / Q000378:metabolism* / Q000494:pharmacology*; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D008239:Lysine / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D011650:Pulmonary Alveoli / Q000166:cytology / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D020545:Respiratory Mucosa / Q000166:cytology / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*; D013379:Substrate Specificity
D000596:Amino Acids; D002352:Carrier Proteins; D000584:Amiloride; D012964:Sodium; D008239:Lysine