Letter to the editor: The use of extracellular, ion-selective microelectrodes to study the function of heterologously expressed transporters in Xenopus oocytes

to the editor: In a recent issue of American Journal of Physiology-Cell Physiology, Blanchard et al. (1) published an elegant article in which they report that they have “developed the ion-trap technique” that “uses a large ISE [ion-selective electrode] … gently pushed against the surface of an oocyte … [to isolate] a small volume of extracellular solution (the trap) in which transport-dependent changes in ionic concentrations can be detected.”

The authors cite others' use of extracellular ISEs, including work from our own laboratory published by Endeward et al. (4), in which we used a large-tipped H⁺-selective electrode to monitor the depletion of H⁺ on the surface of a Xenopus oocyte during the influx of CO₂ through aquaporin 1, due to the reaction HCO₃⁻ + H⁺ → CO₂ + H₂O. Blanchard et al. (1) might also have noted the work of Parker et al. (5), in which we used a large-tipped Cl⁻-selective electrode to measure transporter-mediated efflux of Cl⁻ from oocytes. In both sets of experiments from our laboratory, Dr. Musa-Aziz positioned a 5–15 μM ISE tip at the oocyte surface and then advanced it ∼40–60 μm further to create a dimple. She subsequently monitored the appearance of ions in (or disappearance from) the unstirred layer trapped between the oocyte surface and the abutting electrode tip. This approach is essentially the same as that first described by de Hemptinne and Huguenin (3) in 1984 to measure pH transients (on the surface of muscle) caused by CO₂/HCO₃⁻ fluxes and then implemented somewhat differently by Chesler (2) in 1986 to monitor pH transients (in the restricted extracellular space near neurons) caused by NH₃/NH₄⁺ and CO₂/HCO₃⁻ fluxes.

Blanchard et al. (1) have coined the phrase “ion-trap” and are unique in applying this technology to study transporters under voltage clamp. They attempted to distinguish their work from ours by stating that Endeward et al. (4) used a “simple static [i.e., immobile] extracellular ISE,” which is, in fact, the approach used by all of the aforementioned groups. To our knowledge, the “ion-trap” technique itself was first developed by de Hemptinne and Huguenin (3) and first exploited to study the molecular function of heterologously expressed transporters by our laboratory (4, 5).