Fig. 1. Representative recording from a Malpighian tubule showing transepithelial potential (TEP) oscillations. At the beginning of the trace, the electrode is in the bath. The negative deflection reflects the potential across the basolateral membrane of a principal cell as the electrode enters the cell; as the electrode is advanced into the lumen of the tubule, the recorded potential becomes positive. At the end of the trace, the electrode is withdrawn into the bath again.