EMS evaluation: criteria for standards and research designs

Transepithelial potential difference (p.d.) was measured in the proximal tubule of Necturus kidney in vivo, by means of microelectrodes filled either with a 3M KClion or with a Ringer's solution for amphibians. The average transepithelial p.d., measured with KCl-tips, was: -1.4 +/- 2.4 mV (early convolutions), -0.1 +/- 2.0 mV (middle convolutions) and +0.1 +/- 2.4 mV (straight segment). The corresponding values obtained with Ringer's-filled microelectrodes were -2.3 +/- 1.8 mV, -1.3 +/- 1.1 mV and +0.1 +/- 1.2 mV, respectively. Tip localization into the lumen was ascertained by luminal injection of either oil (KCl electrode measurements) or artificial solutions which produced a measurable shift of transepithelial p.d. (determinations obtained with Ringer's-tips). Transepithelial p.d. in split-drops (mean reabsorptive half time 27.1 +/- 2.5 min) was -1.8 +/- 1.1 mV. The magnitude of transepithelial p.d. is discussed with respect to an equivalent electrical circuit; it is shown that high transepithelial p.d.'s are inconsistent with the known values of relative conductances of cell membranes in series and shunt pathway, respectively.