Why do CHO cells require the use of probenecid in calcium assays?

In general, CHO cells express many organic anion transporters which mediate the transport of organic anions across the cell membrane. During dye-loading, these transporters can actively extrude the dyes back into the medium, resulting in significant background fluorescence. To eliminate this noise and reduce the activity of these transporters, use an organic anion transporter inhibitor such as Probenecid.

Probenecid (PBC) should be included in the dye-loading solution and buffer following loading whenever using CHO cells. This prevents the release of the dye from the cells back into to the medium.