A. This is a recording from a typical experiment to measure short-circuit current (ΔIsc) across the confluent epithelial monolayers grown in Millicell inserts. Short circuit current (ΔIsc) is measured under voltage-clamp that maintains the potential difference across the epithelium at 0 mV. 10 μM amiloride was added in all experiments to the lumen side of the inserts to block absorption, which caused a decrease in the basal current as shown in Fig. 1A. In this recording, five minutes prior to cumulative addition of ACh, 3 inserts were exposed serosally to 10-9, 3 × 10-9, or 10-8 M atropine (red, green, and blue traces respectively). The response to ACh reaches a peak and then declines. The difference of peak response to baseline line at each concentration of agonist was measured as the response to each concentration of ACh (n = 9) B. Concentration-response relationships were generated as the peak response to each concentration of ACh (normalized to the maximal response of the epithelium to ACh) plotted versus the ACh concentration. Atropine causes parallel shifts to the right in the concentration-response relationships with increases in atropine concentration. Average data from 9 experiments. Data were then used to generate Schild plots shown in the next figure.