Fig. 1. The arrangements for recording salicylate flux using salicylate-selective microelectrodes based on the ion exchanger tridodecylmethylammonium chloride. (A) The tip of a salicylate self-referencing microelectrode (Sal-SeR) microelectrode is moved from a position 10 µm away from the surface of the Malpighian tubule epithelium to a position 100 µm farther away by computer-controlled stepper motors. The differential signal between the two positions is amplified 10x in the head stage and a further 100x in the connected amplifier for a total voltage amplification of 1000x. An associated PC-based data acquisition system running ASET software records the voltages and controls the stepper motors for microelectrode positioning and movement. The relative locations of the three segments of an anterior pair of Malpighian tubules and the connecting common ureter are indicated. Further details in text.