Sodium-dependent and -independent choline uptake by type II epithelial cells from rat lung.
The uptake of 3H-labeled choline by a suspension of isolated type II epithelial cells from rat lung has been studied in a Ringer medium. Uptake was linear for 4 min at both 0.1 microM and 5.0 microM medium choline; at 5 microM, only 10% of the label was recovered in a lipid fraction. Further experiments were conducted at the low concentration (0.1 microM), permitting characterization of the properties of high-affinity systems. Three fractions of choline uptake were detected: (i) a sodium-dependent system that was totally inhibited by hemicholinium-3 (HC-3); (ii) a sodium-independent uptake, when Na+ was replaced by Li+, K+ or Mg2+, inhibited by HC-3; (iii) a residual portion persisting in the absence of Na+ and unaffected by HC-3. Choline uptake was sigmoidally related to the medium Na+ concentration. Kinetic properties of the uptake of 0.1 microM 3H-choline in the presence and absence of medium Na+ were examined in two ways. (a) Inhibition by increasing concentrations of unlabeled choline (0.5-100 microM) was consistent with the presence of two Michaelis-Menten-type systems in the presence of Na+; a Na(+)-dependent portion (a mean of 0.52 of the total) had a K(m) for choline of 1.5 microM while K(m) in the absence of Na+ (Li+ substituting) was 18.6 microM. (b) Inhibition by HC-3 (0.3-300 microM) gave Ki values of 1.7 microM and 5.0 microM HC-3 for the Na(+)-dependent and -independent fractions. The apparent K(m) of the Na(+)-dependent uptake is lower than that reported previously for lung-derived cells and is in the range of the K(m) values reported for high-affinity, Na(+)-dependent choline uptake by neuronal cells.
van Rossum
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GD
Department of Pharmacology, Temple University School of Medicine, Philadelphia, Pennsylvania 19140, USA.
Boyd
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CA
D000818:Animals; D002478:Cells, Cultured; D002794:Choline / Q000378:metabolism / Q000493:pharmacokinetics*; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism*; D006426:Hemicholinium 3 / Q000494:pharmacology; D007553:Isotope Labeling; D007700:Kinetics; D008094:Lithium; D008168:Lung / Q000378:metabolism; D008274:Magnesium; D008297:Male; D011188:Potassium; D051381:Rats; D012964:Sodium / Q000494:pharmacology*; D013997:Time Factors; D014316:Tritium
D014316:Tritium; D006426:Hemicholinium 3; D008094:Lithium; D012964:Sodium; D008274:Magnesium; D002794:Choline; D011188:Potassium