Fig. 7. Concentration-dependent effect of halothane on the exchange of the radioactive, nonhydrolyzable form of gua-nosine-5′-triphosphate, [35S]GTPγS, for guanosine-5′-diphosphate ([35S]GTPγS–GDP exchange) at the α isoform-3 G-protein subunit (Gαi-3). Assays were performed using crude membranes prepared from COS-7 cells cotransfected with the complementary DNA constructs encoding for the human M2muscarinic receptor and human Gαi-3. Measurements were made in the absence (basal ex-change, A ) or presence ( B ) of 10 μm acetylcholine (ACh) in the assay, and the reactions were terminated 10 min after initiation with [35S]GTPγS. Basal Gαi-3[35S]GTPγS–GDP exchange values are expressed as the percentage of the values measured in the absence of halothane (basal control exchange), whereas the acetylcholine-promoted exchange values are expressed as a percentage of the difference between the basal control values and those measured in the presence of the 10 μm acetylcholine but in the absence of halothane in the assay ( i.e. , percentage acetylcholine-promoted). Data are presented as mean ± SE of 9–42 assays. * Indicates significant difference from the values obtained in the absence of halothane.