A Reappraisal of the Oil-Gap Technique for the Measurement of Transtubular Potentials in Insect Epithelia

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Abstract
Transtubular potentials of insect Malpighian tubules and salivary glands are commonly measured in unperfused tubules in which the two ends are separated by means of an insulating oil gap. The validity of this method has been examined, using Malpighian tubules of a tenebrionid beetle, Onymacris plana. The measured interelectrode potential was found to depend not only on the transtubular potential but also on potentials within the oil gap and at the open end of the tubule, and upon the core resistance, the length constant and a small but finite electrical leak beneath the insulating oil. In perfused tubules of O. plana, the apparent transtubular potential recorded at the collecting end (analogous to the oil gap) was indistinguishable from that previously measured with the oil-gap technique. However, it was invariably lower than that recorded by an intraluminal pipette. The results demonstrate that the interelectrode potential observed with the oil-gap technique is a poor measure of the true transtubular potential.