Diagrams & Tables: Second Messengers

The membrane receptors that induce second messengers are GPCRs of GPCR families. Stimulation of the guanine nucleotide-binding protein-coupled receptors leads to signal transduction of sensory, hormonal, or photic stimuli into regulation of effector enzymes and ion channels. Signal transduction is accomplished through the coupling of G-proteins, via second messengers, to various secondary pathways involving ion channels, adenylyl cyclases, and phospholipases. Further, GPCRs may also couple to other proteins, such as those containing PDZ domains. GPCRs are diverse and of ancient evolutionary lineage, and are found in fungi, plants, and animals.

Messenger	Stimulator	Enzyme	Acts on
cAMP	Adrenaline, glucagon, luteinizing hormone (LH), parathyroid hormone (PTH), adrenocorticotropic hormone (ACTH)	adenylyl (adenylate) cyclases	cAMP-dependent protein kinase, (PKA), gene transcription
cGMP	Photic response of retinal rods (rhodopsin), nitric oxide (NO), atrial natriuretic peptide (ANP),	guanyl cyclase	protein kinase G (PKG)
diacylglycerol (DAG)	growth factors	phospholipase C-gamma	protein kinase C (calcium dependent)
Inositol-1,4,5-trisphosphate (IP3)	growth factors	phospholipase C-gamma	protein kinase C (calcium dependent)
Ca2+ ions	Voltage-gated channels – after depolarization by an action potential; receptor-operated ion channels in post-synaptic membranes, which open upon nt-binding; GPCRs	muscle contraction, neurotransmitter release at neuronal synapses (LTP, LTD), secretion of hormones, apoptosis, cell adhesion to extracellular matrix, T/B cell activation	neurotransmitter release at neuronal synapses, secretion of hormones, apoptosis, cell adhesion to extracellular matrix, activation of T and B cells