Sodium-amino acid cotransport by type II alveolar epithelial cells.
Type II alveolar epithelial cell monolayers have been shown to actively transport sodium (Na+). Coupling to amino acid uptake could be an important mechanism for Na+ entry into these cells. This study demonstrates the presence of such a coupled cotransport mechanism in the plasma membrane of isolated type II cells by use of the nonmetabolizable amino acid analogue alpha-methylaminoisobutyric acid (MeAIB). Transport of MeAIB in 137 mM Na+ is saturable, with the uptake constant (Vmax) equaling 13.9 pmol X mg prot-1 X s-1 and the Michaelis-Menten constant (Km) equaling 0.13 mM. In the presence of Na+, MeAIB is accumulated against a concentration gradient. MeAIB uptake in the absence of Na+ is linear with MeAIB concentration, as expected for simple diffusion. The Hill coefficient for Na+-MeAIB cotransport is 1.11, suggesting a 1:1 stoichiometry. Proline inhibits Na+-MeAIB cotransport, with Ki equaling 0.5 mM. These findings suggest that Na+-amino acid cotransport may be an important pathway for Na+ (and/or amino acid) uptake into type II alveolar epithelial cells.
D000621:Aminoisobutyric Acids / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D004848:Epithelium / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D011650:Pulmonary Alveoli / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism*
D000621:Aminoisobutyric Acids; C017911:2-(methylamino)isobutyric acid; D012964:Sodium