Exam IV: Pharmacodynamics Flashcards

Question

Mechanisms of Membrane Signaling

Answer 1

1. Lipid soluble ligand and intracellular receptors 2. Transmembrane receptor with intrinsic enzyme activity 3. Transmembrane receptor without intrinsic enzyme activity 4. Ligand-gated ion channels 5. G-protein coupled receptors

Answer 2

There are receptors located in the cytosol and nucleus Ligands have to be lipid soluble (lipophilic) to cross the phospholipid bilayer Steroids (estrogen) and gases (nitric oxide) Upon ligand binding, cytosolic or nuclear receptors are activated and cytosolic receptors translocate to the nucleus The activated receptors bind to very specific DNA sequences determined by the response element, which is in the promoter region of many genes This turns on transcription, translation, and protein synthesis Once in the cytoplasm, some ligands will bind to cytoplasmic receptors or nuclear receptors, but no matter what must get to the nucleus to cause an effect to activate transcription Need to be tightly regulated because you don’t want just anything getting in to change proteins/synthesize proteins you don’t want

Answer 3

Intracellular receptors contain: 1. Ligand-binding domain 2. DNA-binding domain 3. Transcription-activating domain Receptors are kept inactive by chaperone proteins (heat shock protein 90 [hsp90]) Upon ligand binding, hsp90 dissociates DNA-binding domain binds to DNA Transcription-activating domain activates transcription

Answer 4

Effect is very slow (30 min~12 hours to see an effect) Steroids are usually bound to a carrier protein in circulation Upon binding to receptor, chaperone dissociates Dimerization of receptors is required for activation Upon binding to DNA, process leads to protein synthesis- response persists for hours to days after [agonist]=0 No simple relationship between plasma [drug] and effect

Answer 5

Nitric oxide (NO) is a bioactive gas that can rapidly cross the membrane NO stimulates soluble guanylate cyclase, generates cyclic GMP and leads to vasodilation cGMP is inactivated by phosphodiesterases/PDE5 (sildenafil blocks this enzyme) Sildenafil: vasodilator Viagra which increases cGMP levels via inhibition of cGMP breakdown Never give this drug with nitrates because causes too much of an increase of vasodilation = dangerous hypotension and die Nitrates and NO effect two pathways (one inhibits breakdown and one increases cGMP)

Answer 6

These receptors contain intrinsic enzymatic activity Ligands for these receptors are trophic hormones (act on another endocrine gland): EGF, TGFβ, insulin, PDGF, ANP Span membrane once Consist of extracellular ligand-binding domain and cytoplasmic enzyme domain Enzyme domain may be tyrosine kinase, serine kinase, or guanylyl cyclase

Answer 7

1. The receptor is inactive in its monomeric form 2. Upon ligand binding, receptors dimerize, bringing the enzymes (kinases) to close proximity 3. Close proximity of enzymes lead to phosphorylation of the same receptor (auto-phosphorylation) or adjacent receptor 4. Phosphorylated enzymes in turn phosphorylate and activate adjacent protein targets

Answer 8

The phosphorylation can last 10-20 seconds Since receptors can phosphorylate themselves, receptor remains active after ligand has disappeared Activation of downstream pathways lead to amplification of signal Termination of signal occurs in two ways: 1. Ligand dissociation and degradation 2. Receptor endocytosis for proteolytic degradation- leads to receptor downregulation (reduced receptor number due to digestion, need to produce new receptors for cell reactivation)

Answer 9

These receptors do not contain intrinsic enzymatic activity Receptors respond to peptide ligands that include growth hormone, cytokines, interferons Span membrane once Consist of extracellular ligand-binding domain A separate protein kinase, Janus-kinase (JAK), binds non-covalently to the receptor Upon ligand binding, receptor dimerizes and brings JAK to close proximity This activates JAK and JAK phosphorylates the receptors

Answer 10

Phosphorylation of the dimerized receptors recruit a second signaling molecule called signal transducer activator of transcription (STAT) When STAT binds to JAK, JAK phosphorylates STAT After JAK phosphorylates STAT, STAT dissociates from the receptor as a dimer STAT translocates to the nucleus, functions as a transcriptional activator, and causes transcriptional activation of a variety of genes

Answer 11

Effects usually take minutes to hours to see JAK-STAT phosphorylation amplifies signal Termination of signal occurs in two ways: 1. Ligand dissociation and degradation 2. Receptor endocytosis for proteolytic degradation- leads to receptor downregulation and receptors need to be resynthesized since old ones were broken down

Answer 12

Channels are important for synaptic transmission Ligand binding domain can be extracellular, inside the channel, or intracellular Natural ligands are acetylcholine, serotonin, GABA, glutamate Different ligands increase transmembrane conductance of different (and relevant) ions to alter membrane potential across the membrane Brain requires fast transmission of signals for catching a ball or exercise or responding to an immediate stimulus Depends on ligand and receptor to cause a function

Answer 13

Nicotinic acetylcholine (ACh) receptor is one of the best characterized Consists of 5 subunits α subunits contain ligand binding domain When two ACh binds, conformational change occurs Channel opens and allows sodium to cross the membrane and enter the cell Channel opens for 2.4 milliseconds, effect is very rapid Contains two alpha subunits which contain the ligand binding site so you need two ACh to bind to the subunits to activate the channel… NEED 2 ACh

Answer 14

Regulated by phosphorylation and endocytosis Although ACh receptors only assume two states (open and close), other ion channels can assume other states and become refractory or inactivated During refractory period, the channel cannot be reactivated for a number of milliseconds Some drugs utilize this state-depend binding to regulate channel action Eg. Local anesthetic drugs

Answer 15

Most abundant membrane protein All G-protein coupled receptors (GPCR) cross the membrane 7 times, also called serpentine receptors Amino terminus is extracellular and carboxy terminus is intracellular These receptors couple to a trimeric G-protein composed of α (binds GDP), β, and γ subunit The G proteins are usually bound to the GDP, and when ligand binds and elicits activation, GDP leaves and GTP binds so the subunits separate into the 1. alpha and the 2. beta/gamma complexes

Answer 16

One receptor can activate ~100 G-proteins Kd < EC50, spare receptors! G-protein activation is longer than ligand on-off rate GPCR regulation: desensitization A lot of signal amplification because 1 receptor activates 100 G proteins, which activate downstream proteins so the effect lasts longer and amplification of the signals

Answer 17

G proteins go through desensitization G protein activation causes release of subunits so G protein can bind to GRK causing phosphorylation of intracellular portion of the receptor Then arrestin: 1. Binds to G protein itself so it cannot go back to the G protein coupled receptor to limit effect and not allow anymore activation 2. Causes activation of endocytosis of G protein receptors so the ligands can no longer activate the receptors Receptors are endocytosed into the cells and go through lysosome, which breaks them down Proteolytic degradation occurs, so you need to re-synethesize the G protein receptors Aka desensitization causes recycling of the receptors

Answer 18

Desensitization: Normally, GPCR mediates ligand response After receptor activation and dissociation of G-protein, GPCR undergoes a conformational change in the 3rd intracellular loop with a new higher affinity Allow association of GPCR kinase (GRK), which phosphorylates specific residues on the cytoplasmic tail Phosphorylation recruits β-arrestin protein Two consequences: 1. β-arrestin prevents binding of G-protein 2. Internalization of GPCR for recycling

Answer 19

There are four types of G-proteins and response depends on which G-proteins get activated Gs, Gi, and Gq are the important ones

Answer 20

Ligands bind to selective cell-surface receptors The receptor triggers activation of a G-protein G-protein changes activity of an effector element (enzyme or ion channel) 1. Adenylate cyclase activate cAMP (Gs) 2. Guanylate cyclase activate cGMP (NO) 3. Phospholipase C activate DAG, IP3 (Gq) Effector changes 2nd messenger concentration and can amplify or dampen response Effector molecules (adenylate cyclase, guanylate cyclase, and phospholipase c) are activated causing activation of second messagers (cAMP, cGMP, DAG, and IP3)

Answer 21

cAMP: Activated by β adrenergics, glucagon, histamine (H2) on Gs Inhibited by α2 adrenergics, muscarinics (M2, 4) on Gi Gs: cAMP is the second messenger and activates protein kinase A PKA contains two regulatory, so requires 2 cAMP to activate them= activate PKA which goes downstream to cause cellular responses Gi: Gi proteins inhibit adenylate cyclase, reduced activity/formation of cAMP Drugs that activate them: functional antagonists

Answer 22

Gq proteins: ligands binds to receptor to activate the G proteins, then activate phospholipase C, which causes breakdown of PIP2 into DAG and IP3 DAG activates protein kinase c (PKC) IP3 goes into endoplasmic reticulum to release Ca2+ (second messenger) causing contraction of blood vessels Alpha 1 adrenergics are on blood vessels Calcium signaling and phophoinositides- activated by α1 adrenergics, muscarinics (M1, 3, 5), histamine (H1) on G

Answer 23

Enzymes terminate 2nd messenger responses by breaking them down into inactive or less active form cGMP is degraded by phosphodiesterases (inhibited by sildenafil) cAMP is degraded by phosphodiesterases (inhibited by caffeine, theophylline) IP3 is inactivated by dephosphorylation DAG is phosphorylated to a less active messenger called phosphatidic acid by DAG kinase Calcium is actively removed from the cytoplasm by calcium pumps

Answer 24

Drugs do not necessarily have to bind to receptors to exert their actions: Antimicrobials Osmotic diuretics Antacids- no receptors because just neutralize acid in the stomach