Fig. 4. Effect of 0.55 mm halothane (  A ) and 10 mm hexanol (  B ) on acetylcholine-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. Measurements were made in the absence and presence of acetylcholine. Data are mean ± SD; n = 6. * Significant difference from control.