Fig. 1. (A) Representative scan of tetraethylammonium (TEA) flux at locations along the secretory segment of the Malpighian tubule (MT) and the lower MT (LMT). Tubules were bathed in saline containing 100 µmol l^–1 TEA. The common ureter and part of the other LMT of the pair are shown. The tip of the TEA-SeR microelectrode is located just above the asterisk. The image is a collage formed from two images. At each site, indicated by arrowheads, ASET software calculated the TEA-specific signal differences (V; µV) between the two limits of microelectrode excursion by subtracting the voltage at the outer limit of the excursion from that measured at the inner limit. The length of each arrow corresponds to the magnitude of TEA influx. (B) TEA influx as a function of distance from the ureter along lower, main and distal segments of the MT. An influx of TEA reduces TEA concentration in the unstirred layer adjacent to the surface of the tissue, and the corresponding voltage difference is therefore negative. Distance 0 on the abscissa corresponds to the junction of the ureter and the LMT. Both the differential signal recorded by the TEA-SeR microelectrode (right ordinate) and the calculated TEA influx (left ordinate) are shown (N=4).