FDN2_SM_ReceptorFamilies&Signaling Flashcards
Ion channels, transport proteins, membrane potential, action potential, receptor families and signaling, drug transport, absorption and distribution, includes week 3 signal transduction
What kind of antagonist binds reversibly to the agonist-binding site?
Competitive antagonist
Note: when both ligands are present, partial agonists act as competitive inhibitors of the full agonist
Where might a noncompetitive antagonist bind?
Irreversibly to an orthosteric site or irreversibly to an allosteric site. This prevents receptor activation.
Where does an uncompetitive inhibitor bind?
The agonist-receptor complex
How does a competitive inhibitor affect potency and efficacy?
A competitive inhibitor decreases potency (increases EC50), and has no effect on efficacy
How does a noncompetitive inhibitor affect potency and efficacy?
A noncompetitive inhibitor lowers efficacy and has no effect on EC50
How does an uncompetitive inhibitor affect potency and efficacy?
An uncompetitive inhibitor lowers efficacy, but actually increases potency (decreases EC50)
In this image, what evidence supports the conclusion that Zolpidam is a postive allosteric activator?
Zolpidam has no effect on its own, but increases the efficacy of GABA when they are applied together
What is the mechanism of action of P4S? How do you know?
Partial agonist: in the presence of GABA and P4S, there is a right shift; potency is decreased and efficacy is unaffected (remember, that partial agonists can act as competitive inhibitors).
P4S alone results in some receptor activity, indicating that it isn’t just a competitive antagonist
What is the mechanism of action of Penicillin? How do you know?
Uncompetitive inhibitor. There is little effect at low agonist concentration, the efficacy is lowered, and the potency is increased (EC50 is lowered)
Which ligand is a partial inverse agonist? How do you know?
Beta Carboline is a partial inverse agonist. When it is present, the response is lower than the basal activity without the agonist
What is the effect of an inverse agonist in constitutively active system?
The inverse agonist reduces contitutive signaling.
Note: in a constitutively active system, an antagonist would have no effect. Antagonists interfere with the “on switch” of an agonist, but there is no “switch” in a constitutive system
Which ligands bind to the orthosteric site of a receptor?
Full agonists, partial agonists, inverse agonists, competitive antagonists
What is the effect of an inverse agonist in a quiescent system?
The inverse agonist acts as a competitive antagonist
Under what conditions does constitutive receptor activity occur?
- Tumor growth
- Research experiments where researchers artificially overexpress receptors
In the image, which ligand might be a competitive antagonist?
Which might be a partial agonist?
How do you know?
Ligand A could be a partial agonist, because it is able to cause a response on its own, without the agonist present
Ligand B could be a competitive antagonist. Actually, it coudl be any kind of antagonist, because it doesn’t produce any response in the absence of an agonist
What kind of antagonist only binds to the ligand-receptor complex?
Uncompetitive
Is biological response in a cell proportional to the number of receptors bound to the agonist? Why or why not?
No! Maximal biologial response even when only a small % of receptors are bound to the full agonist. Most of the receptors are spares!
The maximal response is produced because there are many chemical intermediates that amplify the initial signal within the cell.
This image shows the effect on biological response to Drug A when Drug M is also applied in increasing doses. Why doesn’t efficacy fall after the first two increases in Drug M dose?
Initially, spare receptors compensate for the increased concentration of the antagonist. As the dose of Drug M increases, the spare receptors are “used up,” and efficacy begins to fall.
This image shows the effect on biological response to Drug A when Drug M is also applied in increasing doses. Is Drug M a competitive antagonist? How do you know?
Drug M is not a competitive antagonist. At increasing doses of Drug M, efficacy falls. Competitive antagonists do not affect efficacy.
Describe the constrained model of coopertivity
In the constrained model, all subunits must take the same acive conformation.
- In the “stressed” conformation, all of the subunits are inactive
- In the “unstressed” conformation, all of the subunits are active
Describe the sequential model of cooperativity
Subunits can be sequentially activated and exist in different conformations at the same time
For example, a channel opening may increase in size as each subsequent subunit is activated.
If all four subunits of hemoglobin must exist in either the T-state (stressed) or R-state (unstressed), what type of coopertivity does it exhibit?
Constrained aka concerted coopertivity
If each oxygen molecule binding to hemoglobin further increases the protein’s affinity for oxygen, what type of coopertivity does hemoglobin exhibit?
Sequential coopertivity
How do you calcualte the Therapeutic Index of a drug?
Toxic ED50/Beneficial ED50
OR
LD50/Beneficial ED50
(Use the latter if the adverse event measured is death)
What does the LD50 Represent?
The LD50 is the median lethal dose; the effective dose at which 50% of subjects have died
What does the ED50 represent in a quantal dose-response curve?
In a quantal dose-response curve, the ED50 is the median effective dose at which 50% of subjects have achieved a therapeutic effect
What is the difference between toxic ED50 and LD50?
Toxic ED50 is used when the adverse event in question is toxicity, rather than death
LD50 is used when the adverse event in question in death
What does an Na/K ATPase do?
The Na/K ATPase hydrolyzes 1 ATP molecule to move…
3 NA+ ions out of the cell
2K+ ions into the cell
Which ions have a higher concentration inside of the cell than out?
K+ only
Which ions have a higher concentration outside of the cell than in?
Na+, Cl-, Ca2+
What is the resting membrane potential of most cells in the body?
About -70 mEV
What is the Nernst equation for a cation?
(Note: [ion]out is in the denominator and [ion]in is in the numerator for anions)
What does the GHK equation caluclate? What is the equation?
The GHK equation calculates the resting membrane potential, taking into account the contribution of Na+, K+, and Cl-
Which ions determine the resting membrane potential of a cell?
Na+, K+, Cl-
Describe the determinants of resting membrane potential
The Na/K ATPase pump keeps the contentration of Na+ high outside of the cell, and the concentration of K+ high inside of the cell
Na+ and K+ leak channels oppose the action of the ATPase, pulling the membrane potential toward the equilibrium potential for each ion. The membrane is more permeable to K+, so the resting membrane potential (-70mV) is closer to the Nernst potential of K+ (-95mV)
Which ions have a negative equilibrium potential?
K+ and Cl-
Which ions have a positive equilibrium potential?
Na+, Ca2+
What will happen to extracellular Na+ concentration if the membrane becomes more permeable to Na+
Nothing! Intracellular and extracellular bulk ion concentrations remain the same
What will happen to membrane potential if the membrane becomes more permeable to Na+
Na+ will flow into the cell, depolarizing the membrane.
This is typically considered excitatory
What will happen to membrane potential if the membrane becomes more permeable to Ca+
Ca+ will flow into the cell, depolarizing the membrane
What will happen to membrane potential if the membrane becomes more permeable to Cl-
Cl- will flow into the cell, hyperpolarizing the membrane
This is typically considered inhibitory. This is important in skeletal muscle
What will happen to membrane potential if the membrane becomes more permeable to K+
K+ will flow out of the cell, hyperpolarizing the membrane
This is typically considered inhibitory
What kinds of stimuli, applied at step 2, might prevent a membrane from reaching the threshold potential?
IPSPs (Inhibitory Graded Potentials) due to transient Cl- channel opening, leading to hyperpolarization. May be caused by Glycine or GABA binding to its receptor
How is the hyperpolarization that occurs after an aciton potential rectified?
K+ channels close, and Na+/K+ ATPase re-establishes resting membrane potential
Describe what is happening at the apex, between stages 3 and 4
Voltage-gated Na+ channels inactivate (flow of ions stops via an inactivation mechanism)
Voltage-gated K+ Channels open. K+ begins to flow out of the cell, causing repolarization
Which channels are open at stage 3?
Voltage-gated Na+ channels. Na+ flows, in, causing depolarization.
What kinds of stimuli, applied at step 2, might cause a membrane to reach the threshold potential?
EPSPs: Excitatory Graded Potentials due to transient Na+ or Ca2+ channel opening. May be caused by ACh or Glutamate binding to its receptor.
How is Cl- potential different in development vs. adult life?
In development, [Cl-] is increased inside of neurons due to transporter action. When GABA binds, to open a channel, Cl- flows out, causing depolarization.
In all other stages of life, Cl- flows in, causing hyperpolarization upon GABA binding
Which channels are open in phase 0?
Fast, voltage-gated Na+ channels open, allowing Na+ to rush into the cell
Which channels are open in phase 1?
Fast acting K+ channels open to generate ITO (Trasient outward current). Na+ channels are closed and inactivated
Which channels are open in phase 3?
Voltage-gated L-type Ca2+ channels close and become inactivated.
K+ channels remain open. Repolarization is completed by IKi, the rectifying current
Which channels are open in phase 2?
Voltage-gated L-type Ca2+ channels allow Ca2+ into the cell, slowing the rate of repolarization
K+ channels open, contriubuting to repolarization through IKr and IKs
What is long QT syndrome, and what causes it?
Long QT is when the interval between the beginning of ventricular depolarization and the end of ventricular repolarization is too long.
Theoretically this may be caused by overactive Ca2+ channels or impaired K+ channels. Most often it is caused by impaired K+ channels
What is Diazepam (Valium) and how does it work?
Diazepam (as well as all benzodiazepines) is an Allosteric Activator of GABAA
Increasing GABAA activity results in increased Cl- flowing into the cell, which enhances inhibition
This helps to reduce anxiety, reduce muscle spasms, and promote sleep
How would inhibiting cardiac Ca+ channels affect the cardiac action potential?
Inhibiting cardiac Ca+ channels would prevent Ca2+ from entering the cell. There would be less Ca2+ to oppose the K+ currents, and repolarization would occur more quickly
How would inhibiting cardiac K+ channels affect the cardiac action potential?
Inhibiting cardiac K+ channels would lengthen the cardiac action potential, because it woud take longer for K+ to leave the cell and repolarize the membrane
What mechanism lengthens cardiac action potentials in comparison to neuronal action potentials?
Cardiac muscle cells conduct Ca2+, which opposes the re-polarizing K+ current.
It takes longer for the membrane to repolarize, thus lengthening the action potential
What mechanism causes the absolute refractory period?
Inactivation of Na+ channels
What mechanism causes the relative refractory period?
Hyperpolarization of the membrane due to open K+ channels. Due to hyperpolarization, the membrane is less likely to reach the threshold potential.
Describe active transport
Substances are transported across the cell membrane against thier concentration gradient in an energy-dependent manner.
Usually mediated by the action of ATPase pumps
(ex: Na/K ATPase)
What is the difference between passive transport and facilitated diffusion?
Both processes move substances across the cell membrane down their concentration gradient.
Passive transport invovles diffusion through a membrane protein. It is controlled by the opening and closing of channels (ex: Na+ and K+ leak channels)
Facilitated Diffusion is a type of passive transport that is mediated by carriers and uniporters that move across the membrane, rather than channels
Describe the three types of facilitated diffusion and their associated proteins
All three processes involve carrying at least one substrate across the cell membrane, along its concentration gradient
Symporters: A carrier protein that transports two substrates across the cell membrane in the same direction
Antiporters: A carrier protein that transports two substrates across the cell membrane in opposite directions
Uniporter: Carries one substrate across the membrane
Which ion channels are most important in setting resting membrane potential?
Na+ and K+ leak channels
What is the purpose of a K+ inward-rectifying channel? Why is their existence somewhat of a paradox?
K+ inward-rectifying channels allow K+ to flow across the cell membrane.
The paradox: These channels are designed to be slightly better at letting K+ into the cell than out. However, [K+] is normally much higher inside of the cell than out, so when the channel is open, K+ will flow out of the cell, under normal physiological conditions.
How are KATP channels regulated?
KATP channels are inhibited by ATP
Under high energy conditions, KATP is inhibited and K+ is less likely to flow out and hyperpolarize the cell. This increases the probability of an action potential
Under low-energy conditions, KATP is active and K+ is more likely to flow out of the cell, hyperpolarizing it. This decreases the probability of an action potential, reducing the work that a Na/A ATPase would have to do to restore membrane potential after the AP.
What kind of channel do Sulfonylurea drugs target? Why?
Sulfonylureas inhibit KATP channels (a type of inward-rectifying* K+ channel) to stimulate insulin release.
If the channel is inhibited, K+ cannot flow out of the cell. The increased voltage in the cell opens Ca2+ channels, allowing Ca2+ to flow in and trigger insulin release
*Rembember: although inward-rectifiers are better at allowing ions into a cell, K+ will flow out of an open channel due to its concentration gradient
Is the opening of K+ channels considered excitatory or inhibitory?
Inhibitory
when the channel is open, K+ will flow out and hyperpolarize the cell. This reduces the probability that it will reach the threshold potential.
What is hERG? What does it do?
hERG (human Ether a Go Go) is the protein that creates K+ channels responsible for establishing IKR in cardiac muscle.
hERG can be thought of like goldilocks (Go Go = Goldilocks): If hERG is overactive, the QT interval is too short, causing arrhythmia. If hERG is impaired, the QT interval is too long, causing arrhythmia.
What kind of drugs target hERG? Why?
Class III anti-arrhythmatic drugs target hERG.
If hERG is blocked, IKR is impaired; K+ cannot flows out of the cell less effectively, slowing the rate of repolarization. This can suppress arrhythmia caused by re-entry, if the QT interval is too short
Why are drugs with off-target hERG effects blocked by the FDA?
Off-target interactions with hERG can be pro-arrhythmic.
Inhibiting hERG inhibits IKR and slows repolarization, resulting in arrhythmia.
In general, would K+ channel blockers have an inhibitory or excitatory effect?
Excitatory
Blocking K+ channels stops (+) charges from flowing out of the cell. This increases the probability of depolarization and action potential firing
In general, would K+ channel activators have an inhibitory or excitatory effect?
Inhibitory
Activating K+ channels alows (+) current to flow out of the cell. This reduces the probability of depolarization and action potential firing
In general, would Ca2+ channel blockers have an inhibitory or excitatory effect?
Inhibitory
Blocking Ca2+ channels reduces the calcium influx into the neuon end plate, inhibiting the release of neurotransmitters
In general, would Na+ channel blockers have an inhibitory or excitatory effect?
Inhibitory
Blocking Na+ channels reduces the probability of depolarization and action potential firing
What condition can be treated by a drug that activates K+ channels in the brain?
Epilepsy
Drugs that activate K+ channels in the brain activate M-current that repolarizes neurons and dampens excitability.
What conditions can be treated by neuronal Na+ channel blockers?
Epilepsy, migrane, chronic pain
Blocking Na+ channels inhibits action potential firing; this can decrease convulsions and pain signals
What conditions can be treated by drugs that block T-type Ca2+ channels?
Epilespy
By inhibiting Ca2+ channels during an action potential, they inhibit Ca2+ influx into the cell. This inhibits neurotransmitter release and muscle contraction.
What conditions can be treated by drugs that block L-type Ca2+ channels?
Angina pectoris, hypertension, arrhythmia, cardiac eschemia
These drugs are important in…
- Shortening the QT interval (If Ca2+ cannot get into the cell, it will re-polarize more quickly)
- Excitation-contraction coupling in smooth muscle
What conditions can be treated by cardiac Na+ channel blockers?
Arrythmia
Blocking Na+ channels inhibits action potential firing
What conditions can be treated by nerve-cell Na+ channel blockers?
Acute, localized pain (the drug is a local anesthetic)
Blocking Na+ channels inhibits the action potential that carries the pain signal.
These drugs may have use-dependent properties!
What conditions can be treated by cardiac K+ channel blockers?
Arrhythmia
Blocking K+ channels slows repolarization, preventing arrhythima caused by re-entry
These drugs act on hERG!
What conditions can be treated by nerve-cell Na+ channel blockers?
Acute pain, localized pain (the drug is a local anesthetic)
Blocking Na+ channels inhibits the action potential that carries the pain signal.
These drugs may have use-dependent properties!
What is a use-dependent block?
A use-dependent block is more active when its target is firing at a high frequency
For example, high-frequency stimulation of pain nerves enhances exposure of the drug to its binding site within the targeted ion channel
If a patient is suffering from a QT interval that is too short, what kind of drug might you prescribe?
K+ channel blocker;
Decreaseing K+ current into the cell slows the rate or repolarization.
(Theoretically, a Ca2+ channel activator would also work but I don’t think any of these currently exist)
If a patient is suffering from a QT interval that is too long, what kind of drug might you prescribe?
Ca2+ channel blocker;
Decreaseing Ca2+ current into the cell allows it to repolarize more quickly.
(Theoretically, a cardiac K+ channel activator would also work but I don’t think any of these currently exist)
If the upstroke of a cardiac action potential was too strong, what kind of drug would you prescribe?
Na+ channel blocker.
Decreasing Na+ current into the cell will attenuate the upstroke
Tetrodoxin is a toxin that is found in pufferfish that blocks Na+ channels. Why might this be dangerous?
Blocking Na+ channels can prevent action potential firing. This can cause paralysis and interfere with heart beat
Sea anemones and some scorpions can activate Na+ channels. Why might this be dangerous?
Activating Na+ channels can cause erratic action potential firing. This can lead to convulsions and muscle spasms.
What are the 5 excitatory ionotropic (neurotransmitter) receptors?
- Nicotinic: Receptor for Ach
- 5HT-3: Receptor for Serotonin
- NMDA: Receptor for Glutamate
- AMPA: Receptor for Glutamate
P2X: Receptor for ATP
What are the 2 inhibitory ionotropic (neurotransmitter) recptors?
- GABAA: Receptor for GABA
- Glycine: Receptor for Glycine
Describe the mechanism of action of excitatory neurotransmitter receptors
Excitatory neurotransmitter receptors incorporate a nonspecific cation channel.
When the ligand binds, the channel opens, and a cation (always Na+) flows into the cell
Describe the mechanism of action of inhibitory neurotransmitter receptors
Inhibitory neurotransmitter recetors are selective Cl- chanels
If Cl- flows in, the cell is hyperpolarized and less likely to fire an action potential
What ligand-gated receptor family does Acetylcholine bind to?
Nicotinic
What receptor family does serotonin bind to?
5HT-3
What receptor families does Glutamate bind to?
NMDA and non-NMDA (AMPA and Kainate)
What receptor family does ATP bind to when it acts as a neurotransmitter?
P2X
What kind of drugs treat nausea and vomiting?
What receptor do they act on? How?
Drugs with a setron suffix (ondansetron, granisetron) treat nausea and vomiting, especially in chemotherapy patients
They are competitive inhibitors of Serotonin5 HT-3 receptors
What condition do -setron drugs treat? How?
Drugs with -setron suffixes treat vomiting and nausea by competitively inhibiting serotonin 5HT-3 receptors
Which receptors are active in mediating fast, excitatory CNS transmission?
What is their mechanism of action?
Glutamate-AMPA receptors. They are non-selective, ligand-gated Na+ channels.
*They desensitize very rapidly
What is the mechanism of action of Memantine?
What condition does it treat?
Memantine is an open-channel blocker (uncompetitive inhibitor) for Glutamate-NMDA receptors
This drug can treat Alzheimer’s by reducing Ca2+ influx to lower Ca2+ excitotoxicity.
Describe the mechanism of action fo Glutamate-NMDA receptors.
In what ways can their action be good? Bad?
Glutamate-NMDA receptors are excitatory Glutamate-gated Ca2+ channels. They require 2 waves of depolarization to allow Ca2+ in; the first washes out Mg2+, and the second allows Ca2+ in
They are associated with slower proccesses, such as learning and memory.
Good: Learning and memory
Bad: Excess Ca2+ is associated with excitotoxicity and Alzheimer’s disease
What is the mechanism of action fo Benzodiazepines?
What condiitons can they treat?
Benzodiazepines are positive allosteric modulators of GABAA
They enhance inhibition to reduce anxiety, decrease muscle spasm, and promote sleep
What is the mechanism of action of Zolpidem?
Zolpidem (like all benzodiazepines) is a positive allosteric modulator of GABAA receptors, known as Ambien.
This drug promotes sleep.
What is the mechanism of action of Diazepam?
Diazepam (like all benzodiazepines) is a positive allosteric modulator of GABAA receptors known as Valium.
It is used to reduce anxiety and muscle spasms
What is the mechanism of penicillin’s off-target effect on GABAA receptors?
Penicllin is an open-channel blocker (uncompetitive inhibitor) of GABAA receptors.
It interferes with inhibition; Cl- influx is reduced, resulting in inappropriate action potentials and seizures.
What is the mechanism of action of Strychnine?
Strychnine is a competitive inhibitor of glycine receptors.
This interfers with normal inhibition, resulting in increased spasticity
What is the mechanism of action of Tetanus Toxin?
Tetanus Toxin interferes with glycine release; it is a presynapitc inhibitor
Tetanus toxin interferes with normal inhibition, resulting in increased spasticity
What is the mechanism of action of hyperekplexia (familial startle disease)?
Hyperekplexia results in a mutation in the alpha-subunit of glycine receptors.
This intereferes with the normal inhibitory action of the receptors, resulting in increased spasticity and an exacerbated startle response.
List the 4 receptor families in order from fastest to slowest
- Ligand-gated ion channels
- G-Protein Coupled Receptors
- Receptor Tyrosine Kinases and Related
- Cytoplasmic and Nuclear Receptors
What are the main types of signalling used by GPCRs, from fastest to slowest?
- Inhibitory signalling mediated by the beta/gama subunits (milliseconds)
- Smooth muscle contraction mediated by the PLC water-soluble and lipid-soluble pathways (seconds)
- cAMP/PKA signaling; cardiac muscle contraction and smooth muscle relaxation (minutes)
What signaling process do growth factors use?
Growth factors activate a receptor tyrosine kinase that activates the RAS/MAP Kinase pathway that goes on to alter gene transcription to promote growth
- Increased proliferation, adhesion, migration
Note: Receptor dimerizes after ligand binding
The scaffold protein is GRB2
What signaling process does insulin use?
Insulin activates a receptor tyrosine kinase that activates the PI3/AKT/mTOR pathway that goes on to alter gene transcription to promote fuel storage
- Increases gulcose uptake
- Increases glycogen synthesis
- Increases fat storage
Note: The insulin receptor is pre-dimerized prior to insulin binding
What are the three main types of signaling used by GPCRs, in order from fastest to slowest?
Fastest (milliseconds): Inhibitory action of GBeta/Gamma subunit activity. Inhibits ion channels
Medium (seconds): Smooth muscle contraction mediated by GAlpha. Water-soluble and non-water soluble
Slowest (minutes): Downstream phosphorylation mediated by GAlpha/cAMP pathway. B1 cardiac muscle contraction and B2 smooth muscle relaxation
What signaling process do cytokines use?
Cytokines activate the JAK/STAT pathway that is similar to the RTK pathway. Dimerizes STATs go on to alter gene transcription to mount a coordinated immune system response
Note: JAK is NOT an RTK; the tyrosine kinase associates with the receptor, but is not part of it
What signaling process do gulcocorticoids use?
Glucocorticoids must cross the cell membrane on their own and activate cytoplasmic receptors.
- Upon binding to glucocorticoid, inhibitory HSP90 dissociates from the receptor
- The receptors dimerize and translocate to the nucleus, where they alter gene transcription, often to inhibit inflammatory responses
What kind of signaling would a cell use to prevent a coordinated cytokine response?
Glucocorticoid signaling;
Glucocorticoids cross the cell membrane on their own and bind to their cytoplasmic receptors, then go on to alter gene transcription. The changes in transcription can prevent cytokines from aggregating to mount a coordinated immune response
(This is the equivalent of the police shutting down cell phone service to prevent teenagers from getting together and terrorizing people on Michigan ave)
Does ion movement through ion channels affect their intracellular or extracellular concentrations?
NO! Bulk concentrations are not impacted by tiny amounts of ion moving across the membrane.
This movement DOES however, affect membrane potential
What is the mechanism of action of _____-setron drugs?
What can they treat?
_____-setron drugs are competitive antagonists of serotonin 5HT-3 receptors
They decrease excitiation to treat nausea and vomiting, especially related to chemotherapy drugs
What is the mechanism of action of sleep aids such as eszopiclone (lunesta) or zolpidam (ambien)?
These drugs are positive allosteric modulators of GABAA. They increase the frequency of Cl- channel opening, thus enhancing the inhibitory response
Which drug has the off-target effect of uncompetitive inhibition of GABAA receptors? What are the consequences?
Penicillin; Seizures due to impairment of inhibitory GABAA Cl- channels
What is the mechanism of action of ____-sone drugs? What conditions can they treat?
These drugs are usually corticosteroids that act on glucocorticoid receptors to alter gene transcription. They can be used to treat inflammation related to immune responses
Describe desensitization of GPCRs
GPCRs are desensitized within minutes of continuous exposure to the lignd.
GRK phosphorylates the GPCR at the c-terminus, preventing the G-protein from binding to the receptor. This recruits Beta-arrestin to bind to the phosphorylated C-terminus. The receptor is internalized.
If the agonist is removed, Beta-arrestin releases the GPCR and re-sensitization occurs
- If the agonist persists, the GPCR is transported to the lysosome and degrated. Re-sensitization cannot occur
What is the role of GRK in GPCR desensitization?
GRK phosphorylates the GPCR at the c-terminus. This prevents the G-protein from binding and recruits beta-arrestin
What is the role of beta-arrestin in the action of biased ligands?
Based on the different phosphorylation “barcodes” established by GRKs on G-proteins, different Beta-arrestin conformations can produce different responses
This either increase or decrease beta-arrestin mediated degradation of GPCRs
What is a biased ligand?
A ligand that preferrentially activates teh GPCR pathway or the Beta-arrestin pathway
What is the role of GRK in the action of biased ligands?
Different GRKs create different phosphorylation patterns (barcodes) at the GPCR c-terminus. This results in differential beta-arrestin activity
In what drug is Beta-arrestin evil?
Morphine; the action of beta-arrestin results in rapid desensitization
Give an example of a drug that promotes beta-arrestin
Beta-blockers and Angiotensin II; beta-arrestin is cytoprotective
Drug: carvedilol
A receptor with a “2” subtype is usually involved in…
Inhibitory activity mediated by Gbeta/Ggamma
Ex: smooth muscle relaxation
A receptor with a “1” or “3” subtype is usually involved in…
Activation of 2nd messengers mediated by Galpha
ex: IP3/Ca2+ and smooth muscle contraction
Describe GPCR-mediated smooth muscle contraction
The main (water soluble pathway)
GPCR-> PLC-> IP3 -> Ca2+ release, resulting in smooth muscle contraction
This is supplemented by the GPCR -> PLC -> DAG -> PKC pathway that enhances contration
What process is initiated when Galpha activates PLC?
Smooth muscle contraction
Mediated by 2nd messenger synthesis
List the 4 receptor types from fastest to slowest desensitization
Ligand-gated ion channels
GPCRs
RTKs and related
Cytoplasmic/nuclear receptors
What are the three key determinants of drug movement across cell membranes?
- The type of membrane
- The active membrane transporters
- Physio-chemical properties (size, lipid solubility, acidic/basic)
How are ABC and SLC transporters different?
ABC transporters are directly energy dependent, SLC are not
ABC transporters are only active in efflux from cells, SLC are active in uptake and efflux
ABC transporters are pumps, SLC are symporters or antiporters
What kind of transporters are implicated in drug resistance?
ABC transporters
MDR1 (P-glycoprotein) and MRP1
True or false: “SLC transporters do not depend on ATPase pumps”
False; SLC transporters do not hydrolyze ATP directly, but they do take advantage of concentration gradients created by energy-dependent processes such as ATPases
What is the ion-trapping theory?
Drugs that are weakly acidic or basic are absorbed in their uncharged form and “trapped” on one side of the membrane if they are charged.
Differences in pH of stomach, intestine, and interstitial fluid can impact whether a drug is absorbed or trapped
Suppose Drug X is a weak base with a pKa of 3.5
Is Drug X more likely to be absorbed in the stomach (pH = 1.4) or the intestine (pH 6.5)? Why?
Drug X is more likely to be absorbed in the intestine
In the stomach, the charged BH+ form will dominante, and the drug will be trapped in the stomach
(~100:1 ratio of charged:uncharged)
In the intestine, the uncharged B form will dominate, allowing the drug to be absorbed
(~1:1000 ratio of charged:uncharged)
Suppose Drug Y is weakly acidic, with a pKa of 4.5
Is Drug Y more likely to be absorbed in the stomach (pH = 1.4) or the intestine (pH 6.5)? Why?
Drug Y is more likely to be absorbed in the stomach
In the stomach, the uncharged HA form will dominante, and the drug is able to be absorbed
(~1:1000 ratio of charged:uncharged)
In the intestine, the charged A- form will dominate, trapping the drug
(~100:1) ratio of charged:uncharged
What kind of transporters are most active in primary active transport?
ABC transporters
What kind of transporters are most active in secondary active transport?
SLC transporters
They may also participate in facilitated diffusion
Which (specific) transporters are most active in creating the blood-brain barrier?
ABC; P-glycoprotein
These transporters pump drugs out of the endothelial cells back into the interstitum from whence they came
Which route of drug administration results in the slowest and most variable distribution?
Oral
Which route of drug administration results in 100% bioavailability?
Intravenous
What is the term for the “percentage of unchanged drug that reaches systemic circulation?”
Bioavailability
What do two drugs that are pharmaceutical equivalents have in common?
Same active ingredients, dosage, concentration, and route of administration
Two drugs that have the same active ingredients, dosage, concentration, and route of administration are…
Pharmaceutical equivalents
What do two drugs that are bioequivalents have in common?
Same active ingredients and bioavailability
Drugs that have the same active ingredients and bioavailability are…
Bioequivalents
What is the relationship of a generic drug to its pharmaceutical parent?
A generic drug is a pharmaceutical equivalent (Same active ingredients, dosage, concentration, and route of administration) of its parent, and it must have 80-125% bioequivalence to its parent
Suppose your friend buys the store-brand ibuprofen because it’s $2 cheaper than the name brand. You take the prescribed dose, but you swear it’s not working as well. Your friend tells you to stop being a baby, the drugs are exactly the same. Who is right?
You are! (obviously)
Although generic drugs are pharmaceutical equivalents to their parents and must have the same amount of the active ingredient, they may not be bioequivalents. This means that they may not have the same bioavailability; perhaps less of the generic drug actually reaches the bloodstream
What factors may impact the oral absorption of drugs?
FIT-BAD
First-pass metabolism
Interfering substances ingested simultaneously
Transporter activity; is something blocking them? Polymorphism?
Blood flow to GI tract
Age
Destruction by gastric pH (may be exacerbated if gastric emptying is slow due to full stomach)
What properties make the blood-brain barrier difficult to cross?
Tight junctions
Non-fenestrated, continuous endothelium
P-Glycoprotein pumps drugs out of the endothelium back into the blood stream (transcellular tranport usually isn’t feasible)
What is the apparent volume of distribution (VD)?
VD = Total amount of drug administerd/plasma concentration
Why are drugs administered intrathecally more likely to cross the blood-brain barrier?
The barrier between the brain and the cerebrospinal fluid is easier to cross than the blood brain barrier.
Epithelial cells lining the brain are leaky, and allow paracellular transport from CSF to brain.
Describe the blood-CSF barrier
The epithelial cells of the chorid plexus have tight junctions, but fenestrated capilaries.
Lipid-soluble drugs can penetratete the CSF
From the CSF, drugs and other substances can more easily get to the brain
What might cause a large difference in effective concentration and total plasma concentration of a drug?
Binding to plasma proteins;
Binding has no effect on the plasma concentration of the drug, but it lowers the effective (free drug) concentration
What is albumin?
Albumin is a plasma protein that binds to weak-base drugs.
How might hypoalbuminemia affect one’s response to a weakly-basic drug?
Hypoalbuminemia would increase the free drug concentration relative to what doctors might expect; this may lead to drug overdose or toxicity
What is first-pass metabolism?
Metabolism by enzymes in intestinal epithelium or liver following absorption after oral administration
This lowers biovailability of the drug
How does albumin affect bioavailability of a drug?
Albumin does not affect bioavailability; it can reduce the effective concentration of the drug once in the bloodstream, but it doesn’t prevent unchanged drug from reaching the bloodstream
What is bioavailability?
The fraction of the administered drug that reaches the systemic circulation unchanged
In what scenarios does Kd = EC50?
Ligand binding to receptor
(Kd is not equal to EC50 in biological response due to spare receptors)
What kind of ligand-gated ion channels allow Cl- into the cell when activated?
GABA/GABAA Receptor
Glycine/Glycine Receptor
This is associated with an inhibitory response
What kind of ligand-gated ion channels allow Ca2+ into the cell when activated?
NMDA receptors for Glutamate
This is associated with an excitatory response
What kind of ligand-gated ion channels allow (mostly) Na+ into the cell when activated?
Ach/Nicotinic
Serotonin/5HT-3
ATP/P2X
What is the effect of an antagonist to the GABAA Receptor?
Seizures/convulsion
Epinephrine is a non-selective adrenergic agonist, however many of the receptors do very different things when activated.
How is it possible for an agent that acts so differently on different receptors result in a coordinated “fight or flight” response?
Receptor localization!
Not all tissues have the same receptors
For example: Beta 1 receptors in the heart, Beta 2 receptors in the airways arteries supplying skeletal muscle, and Alpha 1 receptors in arteries supplying the gut.
When you’re running away from the bear, your cardiac output increases (Beta 1), you get more oxygen into your lungs and skeletal muscles (Beta 2), and you’re not wasting any blood trying to digest food (Alpha 1)
Why are SNARE proteins important in the somatic and autonomic nervous systems?
SNARE protiens prime secretory vesicles in the nerve ending that contain ACh.
They have Ca2+ sensors (synaptotagmins) that allow them to respond to the increased intracellular Ca2+ that results from an action potential.
When Ca2+ binds to the synaptotagmin, the ACh-containing vesicle fuses with the plasma membrane and releases ACh into the synpatic cleft
What kind of receptors are found at the motor end plate of the somatic nervous system?
Nicotinic cholinergic receptors; these are fast ligand-gated, nonspecific cation channels
(although Na+ si the ion that flows in due to its nernst potential)
How is an end-plate potential generated?
- Action potential propagates down neuron
- The AP opens Ca2+ channels in the nerve ending
- Ca2+ enters the cell and binds to synaptotagmins
- The SNARE-primed vesicle fuses with the plasma membrane
- The vesicle releases ACh into the synaptic cleft
- ACh binds to its nicotinic receptors on the motor end plate
- The non-selective cation channel associated with the receptor opens
- Na+ flows into the muscle cell and triggers an end-plate potential (EPP)
- The EPP triggers a muscle action potential that propagates along the muscle fibers
- The signal is terminated by AChE
Name an agent that interferes with the conduction of action potentials into nerve terminals
Tetrodotoxin (TTX) blocks voltage-gated Na+ channels, preventing action potential initiation and propagation
- This acts on nerve and muscle
Name 2 agents that prevent Ca2+ influx into the nerve ending
Mg2+ and other polyvalent cations compete with Ca2+
Aminoglycoside antibiotics block voltage-gated Ca2+ channels
Both prevent ACh release
Name an agent that interferes with SNARE proteins
Botulinum Toxins; Botox A cleaves SNARE SNAP-25 and prevents the ACh-containing vesicle in the nerve terminal from becoming fusion-competent, thus preventing ACh release
Name 3 agents that prevent ACh release from the nerve ending
Polyvalent cations, aminoglycoside antibiotics, botulinum toxins
Name 3 pharmachologic agents that act as nicotinic receptor blockers
Tubucurarine (non-depolarizing)
-curonium drugs (non-depolarizing)
Succinylcholine (depolarizing)
Name a disease that interferes with nicotinic receptor blockers
Myasthenia Gravis - antibodies destroy the nicotinic receptors
Name an agent that reversibly inhibits Acetylcholinesterase
Neostigmine; used to enhance nicotinic receptor signaling
Treats Myasthenia Gravis and speeds recovery from non-depolarizing blockers during surgery
All receptors at motor end plates of sympathetic neurons are…
Alpha- or Beta- andrenergic GPCRs that respond to norepinephrine or epinephrine
All receptors at motor end plates of the parasympathetic nervous system are…
Muscarinic cholinergic GPCRs
The action of which receptors slows heart rate?
Parasympathetic: muscarinic receptors that activate the membrane-delimited G-beta/G-gamma motif
How does vagal stimulation affect heart rate?
Describe the pathway
Vagal (parasympathetic) stiumlation results in decreased heart rate
- ACh binds to its muscarinic receptor
- G-Beta/G-Gamma subunits activated
- G-Beta/G-Gamma activates K+ channesl
- K+ flows in and hyperpolarizes the membrane, thus inhibiting contraction
This happens in milliseconds
How does vagal stimulation affect airway?
Describe the pathway
Vagal (parasympathetic) stimulation constricts the airways
- ACh binds to its muscarinic receptor
- G-Alpha is activated
- G-Alpha stimulates Phospholipase C (PLC)
- PLC induces PIP2 to turn into IP3 (the water-soluble smooth muscle contraction motif)
- IP3 induces Ca2+ release from the ER
- Increased smooth muscle contraction = airway constriction
No phosphorylation is required; this happens in seconds
What is the effect of sympathetic stimulation on the heart?
Describe the pathway
Sympathetic stimulation increases heart contractions
- Norepinephrine/epinephrine binds to its Beta-1 receptor in the heart (Beta 1 = #1 in our hearts)
- G-Alpha is activated
- G-Alpha activates Adenylyl Cyclase
- Adenylyl Cyclase produces cAMP
- cAMP activates Protein Kinase A
- Lots of downstream phosphorylation
- Cardiac stiumulation and increased contractile force
This happens in minutes (cAMP/PKA pathway is slowest)
What is the effect of sympathetic stimulation on blood vessel radius in the GI system?
Describe the pathway
Sympathetic stimulation of smooth muscle in the arterioles of the GI system is mediated by Alpha-1 receptors, and results in vasoconstriction
- Norepinephrine/epinephrine binds to its Alpha-1 receptor
- G-Alpha is activated
- G-Alpha activates PLC
- PLC induces PIP2 to turn into IP3
- IP3 induces Ca2+ release from the ER
6. Smooth muscle contraction = vasoconstriction in the arterioles of the GI system (we don’t need to digest when we’re fighting for our lives)
This happens in seconds
What is the effect of sympathetic stimulation on our airways?
Describe the pathway
Sympathetic stimulation in the pulmonary system results in activation of Beta-2 receptors -> dilation of the airway and the arterioles supplying the airway.
- Epinephrine binds to its receptors on bronchioles and arterioles
- G-Alpha is activated
- G-Alpha activates Adenylyl cyclase
- Adenylyl cyclase increases cAMP
- cAMP activates PKA
- PKA phosphorylates many things
- Smooth muscle relaxation: bronchio- and vaso-dilation in the pulmonary system
(Beta2 = we have 2 lungs)
Why does epinephrine affect the heart and lungs differently?
cAMP has different effects on cardiac and smooth muscle
In cardiac muscle, cAMP augments Ca2+ entry and storage in the cell, resulting in more forceful contractions
In smooth muscle, cAMP decreases intracellular calcium
What is the effect of a Beta 2 agonist?
Dilation of the airways and arteriole in the pulmonary system
What is the effect propanolol on the cardiopulmonary system?
Propanolol = a nonselective beta blocker
Beta 1: Decreased contractility of the heart
Beta 2: Decreased dilation of the bronchiole and arterioles of the pulmonary system (it constricts the airways)
What would you prescribe to increase heart rate?
Epinephrine (promotes contractility via Beta-1 agonism)
Atropine (inhibits decreased contractility via muscarinic antagonism)
Which of the following is under sympathetic control only?
A. The heart
B. The blood vessels
C. The lungs
B: The blood vessles
This is why sympathetic tone is super important for blood pressure!
What is the relationship between total peripheral resistance (TPR) and blood vessel diameter?
TPR is proportional to (1/r4)
This means that:
- Small increases in the radius result in a large drop in TPR (decreased BP)
- Small decreases in the radius result in a large increase in TPR (increased BP)
What is the relationship between TPR and blood pressure?
BP = CO * TPR
TPR = total peripheral resistance
BP = blood pressure
CO = cardiac output
What is the role of the adrenal medula in sympathetic signaling?
The adrenal medula is basically a post-synaptic ganglion.
The presynaptic sympathetic neuron releases ACh to adrenal chromaffin cells.
This triggers systemic release of epinephrine (80%) and norepinephrine (20%)
The result is global amplification of the fight or flight response
Order the following from fastest to slowest
A) cAMP/PKA pathway
B) PLC/IP3 pathway
C) Ligand-gated ion channel
D) Muscarinic K+ opening
- Fastest: C) ligand-gated ion channel
- D) Muscarinic K+ opening
- B) PLC/IP3
- A) cAMP/PKA
Which (general) pathway is activated in the “smooth muscle contraction motif?”
PLC/IP3
Which (general) pathway is activated in both cardiac stimulation and smooth muscle relaxation via Beta-adrenergic receptors?
cAMP/PKA
A drug that is an alpha-1 agonist acts via which mechanism?
Alpha-1 agonists stimulate the PLC/IP3 pathway
What is the mechanism of action of Botox A?
Botox A cleaves SNARE SNAP-25, preventing the priming of ACh release machinery. The ACh-containing vesicle cannot fuse with the plasma membrane, and therefore cannot release its contents into the synaptic cleft.
The result is ihibition of muscle contraction
What is the result of a competitive inhibitor of ACh at a muscarinic GPCR?
Increased heart rate if interfereing with G-Beta/G-Gamma
Opening of the airways if interfering with G-Alpha/PLC/IP3
Which drugs promote airway opening? By which mechaism?
- erols: sympathetic Beta-2 agonists
- tropiums: parasympathetic G-Alpha/PLC/IP3 vasoconstriction antagonists
What are 2 receptors that act via a phosphorylation pathway?
Beta-1 and Beta-2: Both activate the G-Alpha/cAMP/PKA pathway
What are 2 receptors that act via a G-alpha pathway that does not involve phosphorylation?
Alpha-1 (constricts smooth muscle)
Muscarinic G-alpha (constricts airways)
Which autonomic signaling receptors are found in the heart?
Sympathetic: Beta-1 -> increased contraction
Parasympathetic: Muscarinic G-Beta/G-Gamma (membrane delimited) -> Decreased contraction
Which autonomic signaling receptors are found in blood vessels?
Sympathetic: Alpha-1 -> constriction (in GI tract, kindeys)
Sympathetic: Beta-2 -> Dilation (Arterioles of the pulmonary system)
No parasympathetic control
Which autonomic signaling receptors are found in the pulmonary system?
Sympathetic: Beta-2 -> dilation of bronchioles
Parasympathetic: Muscarinic G-Alpha (PLC/IP3) -> constriction of broncioles
Which ligand activates inhibitory ligand-gated receptor channels and GPCRs?
GABA
What is the effect of inhibiting the Na/K ATPase?
Depolarization of the cell membrane
The leak force of K+ will be attenuated, resulting in more positive charge on the inside of the membrane
What is the difference between L-type and T-type Ca2+ channels?
L-type = Large Long Lasting current; Important for excitation-contration coupling in muscle and heart, cardiac potential plateau, smooth muscle contraction
T-type = Tiny Transient current; low-voltage activated. Involved in pacemaker activity of cardiac and neuronal cells
What are some processes that T-type Ca2+ channels are involved in?
Involved in pacemaker activity of cardiac and neuronal cells.
(Tiny Transient Current; Low-voltage activated)
What are some processes that L-type Ca2+ channels are involved in?
Excitation-contration coupling in muscle and heart, cardiac potential plateau, smooth muscle contraction
(L-type = Large Long Lasting current; high-voltage activated)
