Wayne Green asks if there is any evidence that nigericin, used for intracellular pH calibration, could contaminate sample lines. We have a great deal of experience with pHi measurement by flow cytometry using nigericin for the calibration procedure. I have not encountered sample line contamination as a problem, or seen any reference to this. Presumably if it was a major problem you should see a difference in the pHi value for the test sample if this was run before and then after the calibration run using nigericin. This could easilly be tested if you were concerned. As an alternative to nigericin, we recently developed a calibration procedure based on the partitioning of weak acids and bases across the cell membrane. These shift pHi to an extent predicted by the Henderson-Hasselbalch equation, and do not require the use of ionophores such as nigericin. Sue Chow published this in Cytometry last year (1996;24:360-367). Intellectually it is the most satisfying flow method ever to come out of my lab. David Hedley Ontario Cancer Institute/Princess Margaret Hospital Toronto
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