FIGURE 15-17 In heart muscle, the muscarinic acetylcholine receptor activates its effector K⁺ channel via the G_βγ subunit of a G_i protein. Binding of acetylcholine triggers activation of the G_αi subunit and its dissociation from the G_βγ subunit in the usual way (see Figure 15-14). In this case, however, the released G_βγ subunit (rather than G_αi·GTP) binds to and opens the associated effector protein, a K⁺ channel. The increase in K⁺ permeability hyperpolarizes the membrane, which reduces the frequency of heart muscle contraction. Though not shown here, activation is terminated when the GTP bound to G_αi is hydrolyzed (by a GAP enzyme that is an intrinsic part of the G_αi subunit) to GDP and G_αi·GDP recombines with G_βγ. See K. Ho et al., 1993, Nature 362:31, and Y. Kubo et al., 1993, Nature 362:127.