Fig. 2. Simplified scheme of sympathetic and parasympathetic signaling cascades of GPCRs down to the level of cellular responses (modified from 17,). Note the intimate crosstalk between the various signaling pathways. Lines with blunted ends (=) indicate inhibition. Information taken from 21,22,25–27,29,30,56,58,. AC = adenylyl cyclase; ACh = acetylcholine; AR = adrenergic receptor; cAMP = cyclic adenosine monophosphate; cGMP = cyclic guanosine monophosphate; DAG = diacylglycerol; GC = guanylyl cyclase; Gαs, Gαi, Gαq, Gβγ= G-protein signaling components; IP3 = inositol trisphosphate; M2 = muscarinic acetylcholine receptor; MAPK = mitogen activated protein kinase; NOS = nitric oxide synthase; PDK1 = phosphoinositide-dependent kinase-1; PI3K = phosphatidylinositol 3-kinase; PKA, PKB, PKC, PKG = target-specific serine/threonine protein kinases; PLC = phospholipase-C; Ras, small monomeric GTPase; RNOS = reactive nitric oxide species. From Reference 17 , used with permission from Oxford University Press.