An impurity in phenol red opens an ion channel in cultured human cells.

Human fibroblast, bladder carcinoma, and breast carcinoma cells in commercial serum-free media or balanced salt solutions rapidly lose K+ and gain Na+. This rapid K+ loss is caused by one or more impurities in phenol red. Adding serum or albumin to media or to balanced salts prevents K+ loss. Quinine also prevents part of this loss in fibroblasts and breast carcinoma cells, suggesting that the impurity acts on an ion channel.