Effect of globular and ribbon isomers of α-conotoxin BuIA at nAChR subtypes expressed in Xenopus oocytes. (A)Representative ACh-evoked currents mediated by α3β2 and α3β4 nAChR subtypes obtained in the absence (control) and presence of 10 nM and 100 nM BuIA-globular, respectively. Complete recovery was observed after 10 min washout (broken line). (B)Concentration-response curves for the inhibition of α3β2 and α3β4 nAChRs by globular (filled symbols) and ribbon (open symbols) isomers of BuIA. Best fit of the data gave IC50 values of 4.8 ± 0.4 nM (nH = 1.3) and 59.1 ± 2.3 nM (nH = 1.2) for α3β2 and α3β4 nAChRs, respectively, whereas the ribbon isomer of BuIA exhibited no inhibition at 1 μM. Responses are shown as a percentage of ACh (100 μM)-induced peak current amplitude after a 5 min incubation of the BuIA isomers with respect to control (ACh alone). Error bars are SEM with n = 4–7 for each data point.