Fig. 5. Concentration-dependent effect of halothane, sevoflurane, and isoflurane on acetylcholine (ACh)–promoted exchange of the radioactive, nonhydrolyzable form of guanosine-5′-triphosphate (GTP), [³⁵S]GTPγS, for guanosine-5′-diphosphate (GDP) ([³⁵S]GTPγS–GDP exchange) at the α subunit of the G^qheterotrimeric G protein (Gα^q). Assays were performed using crude membranes prepared from COS-7 cells expressing the human M³muscarinic receptor and human Gα^q. The ACh-promoted increase in Gα^q[³⁵S]GTPγS–GDP exchange was expressed as the percentage of the difference between the values measured in the absence of acetylcholine or halothane, and the values measured in the presence of ACh (% ACh-promoted). The  dashed lines identify the values measured at anesthetic concentrations equivalent to 2 minimum alveolar concentration. Data are mean ± SD; n = 9.