$|$ Second Messengers Phospholipase C Not all inositol$-$containing second messengers remain in the lipid bilayer of a membrane. When acetylcholine binds to a smooth muscle cell, the bound receptor activates a heterotrimeric G protein, which activates the effector phosphatidylinositol$-$specific phospholipase C$-$ $($PLC$).$ PLC is situated at the inner surface of the membrane, bound there by the interaction between its PH domain and a phosphoinositide embedded in the bilayer. PLC catalyzes a reaction that splits PI$(4,5)$P$2$ into two molecules, inositol $1,4,5-$trisphosphate $($IP$3)$ and diacylglycerol $($DAG$),$ both of which play important roles as second messengers in cell signaling. $2017$ John Wiley & Sons, Inc. All rights reserved. $4|$ Second Messengers Diacylglycerol DAG, a plasma membrane lipid molecule, recruits and activates effector proteins protein kinase C $($PKC$).$ Protein kinase C isoforms have a number of important roles in cellular growth and differentiation, cellular metabolism, cell death, and immune responses. A group of powerful plant compounds that resemble DAG, called phorbol esters, activate protein kinase C isoforms in a variety of cultured cells, causing them to lose growth control and behave temporarily as malignant cells. Cells genetically engineered to constitutively express protein kinase C exhibit a permanent malignant phenotype in cell culture and can cause tumors in susceptible mice. $2017$ John Wiley & Sons, Inc. All rights reserved. $4|$ Second Messengers Inositol $1,4,5-$Triphosphate $($IP$3)$ Inositol $1,4,5-$ trisphosphate $($IP$3)$ is a sugar phosphate, a small, water$-$soluble molecule capable of rapid diffusion throughout the cell. IP$3$ molecules formed at the membrane diffuse into the cytosol and bind to a specific IP$3$ receptor located at the smooth endoplasmic reticulum. The IP$3$ receptor is a tetrameric Ca$2+$ channel. Binding of IP$3$ opens the channel, allowing Ca$2+$ ions to diffuse into the cytoplasm. Free calcium concentration changes after vasopressin stimulation: liver cell injected with aequorin $2017$ John Wiley & Sons, Inc. All rights reserved. $4|$ Second Messengers Inositol $1,4,5-$Triphosphate $($IP$3)$ Ca$2+$ ions are second messenger because they bind to various target molecules, triggering specific responses. Smooth muscle cell contraction is triggered by elevated Ca$2+$ levels. Vasopressin binds to its receptor at Ca$2+$ the liver cell surface and causes a series of IP$3-$mediated oscillation bursts of Ca$2+$ release. Several types of cell responses are mediated by IP$3.2017$ John Wiley & Sons, Inc. All rights reserved. $5|$ The Specificity of G Protein$-$Coupled Responses As a group, GPCRs are capable of binding a diverse array of ligands. Also, the receptor for a given ligand can exist in several different versions $($isoforms$)$; there are $15$ different isoforms of the receptor for serotonin. Different isoforms can have different affinities for the ligand or may interact with different types of G proteins. Different isoforms of a receptor may coexist in the same plasma membrane, or they may occur in the membranes of different types of target cells. Heterotrimeric G proteins and effectors can have different isoforms. The human genome encodes at least $16$ G$$ subunits, $5$ G$$ subunits, and $11$ G$$ subunits, and $9$ isoforms of adenylyl cyclase. Different combinations of specific subunits construct G proteins having different capabilities of reacting with specific isoforms of both receptors and effectors. $2017$ John Wiley & Sons, Inc. All rights reserved. $5|$ The Specificity of G Protein$-$Coupled Responses Some G proteins act by inhibiting their effectors. The same stimulus can activate a stimulatory G protein $($G$$s$)$ in one cell and an inhibitory G protein $($G$$i$)$ in a different cell. When epinephrine binds to a $-$adrenergic receptor on a cardiac muscle cell, a G protein with a G$$s subunit is activated, which stimulates cAMP production, leading to an increase in the rate and force of contraction. When epinephrine binds to an $-$adrenergic receptor on a smooth muscle cell in the intestine, a G protein with a G$$i subunit is activated, which inhibits cAMP production, producing muscle relaxation. Finally, some adrenergic receptors turn on G proteins with G$$q subunits, leading to activation of PLC$.$