Fig. 7. Sample recording showing the change in electrical potential of a tetraethylammonium (TEA)-selective microelectrode positioned in droplets of secreted fluid or calibration solutions. Microelectrode voltage was sampled at 3 Hz by the data acquisition system. The labels 10, 1 and 0.1 refer to calibration solutions containing 10, 1 or 0.1 mmol l^–1 TEA chloride, respectively, in Drosophila saline. BG refers to the background voltage recorded due to endogenously secreted compounds in the absence of TEA. W1 and W2 refer to secreted droplets collected from a pair of whole tubules after the addition of TEA. M1 and M2 refer to secreted droplets collected from the main segment of one of the same pair of tubules after the addition of TEA. All tubules were exposed to 100 µmol l^–1 TEA in the bathing saline.