Fig. S3.

Low concentration of ATP preferentially activates “P2Y-like” ramp currents. A voltage-ramp protocol (as in Fig. 3) was used to examine membrane currents evoked in type II fibers by lower concentrations of ATP (1, 5, and 10 µM) that might differentially activate lower (P2X)- and higher-affinity (P2Y) receptors. As ATP concentration was decreased, the evoked ramp current changed from “P2X-like” to P2Y-like. With the lowest concentration tested (1 µM), the ATP-dependent current was inward at positive voltages (i.e., reduced outward current), suggesting that P2Y receptors may be preferentially activated. With increasing concentration, the ATP-evoked current became more inward at negative voltages, and less inward at positive voltages, suggesting the P2X response (increased cation conductance) overwhelms the P2Y response in higher concentrations of ATP (two experiments in two cells).