Mechanism and role of furosemide-sensitive K transport in L cells: A genetic approach

As part of a genetic study of the mechanisms for cation transport in cultured mammalian cells, two mouse fibroblastic cell lines have been compared with respect to unidirectionalK influx. The cell lines are LM(TK) and LTK-5, a mutant selected from LM(TK) by the ability to grow in medium containing 0.2 mm K. In both cell lines, the overall influx can be resolved into three components: (i) a ouabain- and vanadate-sensitive component (M), presumably the Na/K pump, which is a saturable function of extracellular K with a K of 1.3 mm; (ii) a furosemide-sensitive component (Mx), also a saturable function of extracellular K, with a K of 6 mm; and (iii) a diffusional component (M); which is a linear function of extracellular K. By several independent criteria,M andM appear to be distinct transport processes. First, as indicated above, they can be separated with the use of inhibitors. In addition, they can be separated genetically, since the LTK-5 mutant shows a threefold elevation inM with no change inM. And finally, extracellular Na has no effect onM, but stimulatesM, a result consistent with the notion thatM influx occurs by Na-K cotransport. Further experiments were directed towards understanding the nature of the LTK-5 mutation and the physiological role ofM. LTK-5 differs from the parental cell line, not only in having an increasedM, but also in having a large cell volume, a slow maximal growth rate, and an ability to grow at 0.2 mm K. The most straightforward interpretation - that the increasedM is itself responsible-is unlikely since the addition of furosemide to the growth medium had no effect upon the growth rate or cell volume of the mutant at either normal or low extracellular K concentrations. It did, however, render the parent capable of growth at 0.2 mm K. Possible interpretations are discussed. © 1980 Springer-Verlag New York Inc.