Figure 3.

Proposed pathways responsible for M₂ muscarinic receptor stimulation of cAMP-dependent ion channel responses. Muscarinic stimulatory responses may be mediated by direct activation of AC via the βγ subunits of a PTX-sensitive G protein G_i or G_o or by inhibition of PDE3 via production of nitric oxide (NO) and cGMP. Direct stimulation of AC by βγ subunits is only observed under conditions, such as the presence of a β₁-adrenergic receptor agonist, that can activate AC via direct interaction of the α subunit (α_s) of the stimulatory G protein G_s. See text for details; sGC, soluble guanylyl cyclase.