SODIUM-DEPENDENT AND SODIUM-INDEPENDENT CHOLINE UPTAKE BY TYPE-II EPITHELIAL-CELLS FROM RAT LUNG

The uptake of H-3-labeled choline by a suspension of isolated type II epithelial cells from rat lung has been studied in a Ringer medium. Up take was linear for 4 min at both 0.1 mu M and 5.0 mu M medium choline ; at 5 mu M, only 10% of the label was recovered in a lipid fraction. Further experiments were conducted at the low concentration (0.1 mu M) , permitting characterization of the properties of high-affinity syste ms. Three fractions of choline uptake were detected: (i) a sodium-depe ndent 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 absen ce of Na+ and unaffected by HC-3. Choline uptake was sigmoidally relat ed to the medium Na+ concentration. Kinetic properties of the uptake o f 0.1 mu M H-3-choline in the presence and absence of medium Na+ were examined in two ways. (a) Inhibition by increasing concentrations of u nlabeled choline (0.5-100 mu M) was consistent with the presence of tw o Michaelis-Menten-type systems in the presence of Na+; a Na+-dependen t portion (a mean of 0.52 of the total) had a K-m for choline of 1.5 m u M while K-m in the absence of Na+ (Li+ substituting) was 18.6 mu M. (b) Inhibition by HC-3 (0.3-300 mu M) gave K-i values of 1.7 mu M and 5.0 mu M HC-3 for the Na+-dependent and -independent fractions. The ap parent K-m of the Na+-dependent uptake is lower than that reported pre viously for lung-derived cells and is in the range of the K-m values r eported for high-affinity, Na+-dependent choline uptake by neuronal ce lls.