Functional studies of fSUR1 R74X and E128X K_ATP channels. (A) Representative traces from inside-out voltage clamp experiments performed in COSm6 cells transfected with WT Kir6.2 and WT, R74X (top), or E128X (bottom) SUR1. ATP concentrations are indicated above each trace (black line, 1 mM; dotted line, 0.1 mM; white bar, 0.01 mM), and zero current is indicated by the dotted line. Bars: horizontal, 10 s; vertical, 1,000 pA for WT and E128C, and 200 pA for the rest. (B and C) ATP sensitivity expressed as half-maximal inhibitory concentration (IC₅₀) for each R74X (B) and E128X (C) mutant. IC₅₀ values are from best fits using the Hill equation (Irel = 1/(1 + ([ATP]/ IC₅₀)^H)). Five ATP concentrations were tested (0.01, 0.1, 0.3, 1, and 3 mM). n = 3–11 patches for each ATP concentration tested. Note that the axes have different ranges for each mutagenesis set: a dotted line denotes the 0.1-mM value on each.