SIM: Mechanism of Drug Action and Signaling Mechanisms (Cecilia A. Jimeno, MD) Flashcards
What chemicals were systemically injected into chicks in Langley’s Experiment? What did these drugs cause?
Curare and nicotine; paralysis
In Langley’s Experiment, what did additional nicotine injection produce?
Contraction of certain muscles
In Langley’s Experiment, what did co-administration of curare do?
Reduced nicotine response
What are the two distinct cell constituents revealed by Langley’s Experiment?
- Effector or target tissue/organ
2. Receptors
What substances produce their effects by combining with the receptive substance? Do axon endings matter?
- Nicotine
- Curare
- Atropine
- Pilocarpine
- Strychnine
…and other alkaloids!
A structure to which drugs bind, causing an action which results to observable effects?
Receptor
What do you call the agents that bind to receptors?
Ligands
The agent that binds to a receptor and causes an alteration in cellular function.
Agonist
The agent that is devoid of activity and prevents action of an endogenous agonist.
Antagonist
Endogenous substances that serve as signalling mechanisms to enable biologic effects
Second Messengers
Tightness with which a drug binds to its receptor.
This applies to which substances?
Affinity
Agonists and antagonists
Measure of a drug to produce physiologic effects
This applies to which substances?
Intrinsic Activity
Agonists only
What are the properties of receptors?
- Saturability
- Reversibility
- Stereoselectivity
- Agonist Specificity
- Tissue Specificity
Define: Saturability
Receptors present in FINITE numbers
Define: Reversibility
Non-covalent interactions present
Define: Stereoselectivity
Variation in +/-, l/d, and S/R forms of optical isomers
Define: Agonist Specificity
Preference for closely related agonists
Define: Tissue Specificity
Binding to tissues known to be sensitive to drug and at concentrations consistent with physiologic concentration of agent
T/F: Acetylcholine is used by all motor neurons.
True
What are the precursors of acetylcholine?
Acetyl CoA and Choline
What is the enzyme responsible for production of acetylcholine?
Choline O-Acetyltransferase
How many subunits does the Ach receptor have?
5 (2 alpha, 1 beta, 1 gamma and 1 delta)
Where do the Ach molecules bind on the receptor?
On the two alpha subunits
What is tubocurarine?
The purified form of curare that was used as a skeletal muscle relaxant.
Describe: Tubocurarine
It is a neuromuscular blocking drug and antagonist of acetylcholine.
Describe the relationship between dosage and effect of tubocurarine.
Small doses - sub threshold potentials
Larger doses - blockade
What has tubocurarine been replaced by?
Non-depolarizing anaesthetics
What are the effects of tubocurarine?
- Weakens neuromuscular transmission
- Diminishes Ach-esterase inhibitors
- Blocks pre junctional sodium channel
- Interferes with mobilisation of Ach at nerve endings
Describe the ion concentrations of Na+, K+ and Cl- inside and outside of the cell.
Na+ and Cl- concentrations are greater in the ECF.
K+ concentration is greater in the cell.
What is the resting membrane potential?
- 70mV
What is the threshold membrane potential?
- 55mV
Increased conductance of K+ leads to what?
Hyperpolarization
Increased conductance of Na+ leads to what?
Depolarization
Increase conductance of Cl- leads to what?
Hyperpolarization
What enzyme is responsible for the formation of GABA?
Glutamic acid decarboxylase
Describe: GABA
Inhibitory substance released at presynaptic terminal that causes presynaptic inhibition of postsynaptic neurons
Describe: GABA receptor
- It has five subunits (2 alpha-1, 2 beta-2 and 1 gamma-2)
- Exhibits various pentameric combination
- Attenuates excitatory effect by opening ion channels
- Two types - GABAa and GABAb
Describe: GABAa receptor
- Opens Cl- channels
2. Mediate fast IPSPs
Describe: GABAb receptor
- Inhibit Ca2+ channels or open K+ channels
2. Mediate slow IPSPs
Where does the GABA ligand bind on the receptor?
Between the alpha-1 and beta-2 subunits
Where does benzodiazepine bind to?
Close to alpha-1 and gamma-2 subunits
What is the clinical significance of GABA receptor drugs?
- Prevent transmission of pain signals
2. Cause coma and respiratory depression
Describe: Diazepam
- Allosteric activator for GABA
2. Lipid-soluble and long-acting benzodiazepine
When is diazepam used?
- Short-term treatment of anxiety and acute alcohol withdrawal (orally)
- Sedative, anticonvulsant, skeletal muscle relaxant, amnestic and anti-anxiety agent (parenterally)
WARNING: long-term use can result in habituation and withdrawal
Enumerate effects of GABA and Benzodiazepines
- Sedation (block cortical and limbic arousal pathways)
- Muscle relaxation (depress motor nerve function)
- Anticonvulsion
- Anesthesia (BDPs given through IV may penetrate brain)
- Heart failure (if hypovolemic)
- Medullary respiratory center depression
GPCRs are also called what?
Serpentine receptors
T/F: The same ligand can activate many different GPCR family members.
True
What is the main mechanism of action of GPCRs?
Binding and hydrolysis of GTP, which leads to amplification of transduced signal
Example of GPCRs
- Muscarinic Ach Receptor
- Adrenoreceptor
- Dopamine Receptor
- Serotonin (5-HT) Receptor
- Opiate Receptors
- Peptide Receptors
- Olfactory Chemoreceptors
Upon activation of the GPCR, the G-protein dissociates into what?
- Alpha subunit
2. Beta-gamma complex
What are the two types of G-proteins with respect to adenylyl cyclase interaction?
- Inhibitory G-protein (Gi)
2. Stimulatory G-protin (Gs)
Where does the alpha subunit of the G-protein travel to first upon activation?
Adenylyl cyclase
Describe the structure of cAMP-dependent protein kinase A (PKA)
It has two catalytic and two regulatory subunits.
Where does cAMP bind on PKA?
Regulatory subunits, which releases them
What is the function of PKA?
Catalyzes transfer of phosphates to serines and threonines of CREB, which recruits CBP for gene transcription and protein synthesis
What do CREB and CRP stand for?
CREB - cAMP response element binding protein
CBP - CRE-binding protein
Enumerate effects mediated by increase in cAMP concentration
- Thyroid hormone synthesis and secretion
- Cortisol secretion
- Progesterone secretion
- Glycogen breakdown
- Bone resorption
Describe the mechanism of action of cholera toxin on the Gs protein
It alters the alpha subunit, disabling GTP hydrolysis. This keeps it cAMP formation active.
Elevated cAMP levels leads to efflux of Cl- and water into the gut.
Describe the mechanism of action of pertussis toxin on Gi protein
ADP ribosylation of the alpha subunit of Gi. Bound GDP is retained.
T/F: Sympathetic control of the airway leads to bronchoconstriction.
False
It leads to bronchodilation.
What is the receptor of norepinephrine?
Beta-2 receptor
T/F: Beta-adrenergics are strong vasodilators.
True
If beta-2 agonists are given in high doses, what may result?
- Tachycardia
- Tremors
- Hypokalemia
What is the mechanism of action of norepinephrine?
It activates PKA, which lowers Ca2+ concentrations. Myosin Light Chain Kinase (MLCK) will not be activated due to the low Ca2+.
Function: Myosin Light Chain Kinase (MLCK)
Phosphorylation of myosin. This enables binding of actin to myosin.
Describe the action of salbutamol
It stimulates the sympathetic nervous system to induce bronchodilation. Note that it is a beta-2 adrenergic agonist.
Why does salbutamol cause tremors and tachycardia at high concentrations?
Its affinity for the beta-2 adrenergic receptor is decreased.
T/F: Salbutamol is used as a tocolytic to relax the uterine smooth muscle (delay premature labor) if administered intramuscularly.
False
This is true if it’s administered intravenously.
What molecule is responsible for mediating the muscarinic effect of the acetylcholine receptors in the lungs?
Phospholipase C
Parasympathetic control of the airway leads to (1)
(1) bronchoconstriction
How is acetylcholine exocytosed?
Ca2+ mediates this.
What receptors are responsible for bringing the heart back to normal after the action of the sympathetic nervous system?
M2 Receptors
Describe: Atropine
- Racemic mixture D- and L-hyscyamine (main effector)
2. Anti-cholinergic drug
If a person has asthma, would you give anti-cholinergics or beta-adrenergics? Why?
Beta-adrenergics
Anti-cholinergics aren’t as effective. They’re more important in resting bronchial tone like COPD, emphysema, and chronic bronchitis.
T/F: Anti-cholinergics may cause a decrease in secretion of saliva and mucus.
True
What is another name for kinase-linked receptors?
Enzyme-associated receptors
What does signal transduction involve for kinase-linke receptors?
Dimerization and autophosphorylation
Describe the tyrosine kinase insulin receptors
- Heterodimers made up of 2 alpha and 2 beta subunits linked by disulphide bonds
- The alpha subunits house the insulin-binding domains.
What does insulin receptor substrate (IRS) phosphorylation lead to?
Release of PI3P and increase in GLUT4
T/F: Insulin insensitivity or decrease in insulin receptor leads to Type 1 diabetes.
False
It leads to Type 2 diabetes.
What are the actions of tyrosine kinase receptors?
- Gene expression and growth regulation
- Glucose utilization
- Glycogen/lipid/protein synthesis
What are the most prevalent forms of oestrogen?
Estradiol and estrone
Where is estrone made?
Adrenal glands
What are the two main targets of oestrogen?
Breast and uterus
What other organs does oestrogen target?
Brain, bone, liver and heart
What stabilise oestrogen receptors?
Heat shock proteins (HSP90)
What are the domains of intracellular protein receptors?
- Cytoplasmic
- Intranuclear
- Dome-associated with cell surface membrane
T/F: The two oestrogen receptors have equal affinity for estradiol.
True
Describe: ER-alpha
- Present in endometrium and breast cancer cells
2. ER-alpha-estradiol complex activates transcription
Describe ER-beta
- Present in granulose cells and osteoblasts
- Undergoes dimerisation upon activation
- Possesses compressor proteins against antagonists
What molecule is a major autocrine mediator of estrogen-stimulated growth and estrogen-dependent growth of cancer cells?
Transforming Growth Factor alpha
Describe: TGF-alpha
It occurs in normal breast tissue, breast tumours and breast caner cells.
Describe: TGF-beta
- Regulates growth and morphogenesis
- Inhibits epithelial proliferation and induces apoptosis
- ECM deposition and modelling
What do growth factors act on?
Serine-threonine kinases (i.e. MAP kinase)
They active non-liganded ER by phosphorylation
Describe the proliferation of a tumor cell.
- Estrogen binds to estradiol.
- Tumor secretes TGF-alpha and IGF-1.
- Growth, anchorage and proliferation occur
Describe: Tamoxifen
- Competitive, partial agonist inhibitor of estorgen
- Non-steroidal prodrug
- Possesses cytostatic effects (G1 & G0 locking)
- Anti-estrogenic in breast
- Estrogen agonist in bone and uterus
How is Tamoxifen activated?
Processed in the liver to become 4-hydroxytamoxifen or endoxifen
Describe: Endoxifen
4-hydroxytamoxifen is an ER antagonist, inhibiting transcription of estorgen-responsive genes.
Describe: Raloxifen
Highly estrogenic in bone but mildly anti-estrogenic in breast and uterus.
What is Tamoxifen used for?
Treatment of early and advanced breast cancer. It also prevents its return and the emergence of new breast cancer.
What is Tamoxifen’s effect on other molecules?
- Suppresses IGF-1
- Upregulates TGF-beta (apoptosis)
- Downregulates TGF-alpha
When is Tamoxifen effective in treating breast cancer?
If the patient has ER+ breast cancer.
However, there may be Tamoxifen resistance in women.
What causes Tamoxifen to fail?
- Tamoxifen resistance
2. Failure of endoxifen metabolism (CYP2D6 and CYA3P4 variants)
What are slow mechanisms of receptor regulation?
- Transcriptional/translation changes in level
2. Changes in migration to surface
What are fast mechanisms of receptor regulation?
- Covalent modification
- Association with other proteins
- Changes in subcellular location
Define: Homologous Desensitisation Response
Loss of responsiveness of receptors exposed to sustained activation by agonist
Give an example of a homologous desensitisation response.
Phosphorylation by G protein-coupled receptor kinase (GRK)
In the homologous desensitisation response, prolonged exposure to the agonist leads to what?
Delivery of internalised receptors to lysosomes
Define: Heterologous Desensitisation Response
Desensitisation of one receptor results in desensitisation of another receptor not directly activated by agonist
Give an example of a heterologous desensitisation response.
Second messenger feedback (PKA phosphorylation of beta receptors)
What happens when beta-2 receptors couple to Gq proteins in the heterologous desensitisation response?
PKC phosphorylates serine residues in the 3rd cytoplasmic loop and COOH terminal tail of beta-2 receptor.
What is used to reverse the toxic effects of heavy metals?
Chelating agents
When is CaNaEDTA used intravenously?
Cases of lead poisoning and lead encephalopathy
Why is there limited clinical use for CaNaEDTA?
Its affinity for Ca2+
What happens to the calcium component of CaNaEDTA when it encounters a metal in the body?
It is displaced by divalent/trivalent metal to form water-soluble hexadentate chelate.
What other chelating agent is used in treating lead poisoning?
Succimer
What chelator is used against copper poisoning?
Penicillamine
What is dimercaprol used for?
Arsenic, mercury, gold, lead, antimony and other toxic metal poisoning.
Define: Osmotic Diuretics
- Filtered but not reabsorbed by the kidneys
2. Causes osmotic pressure that pulls water into lumen
Describe the ADME, onset, and dosage of mannitol.
A - 20% through IV over 30 mins
D - ECS and extracts water from IC compartment
M - Liver
E - 80% by the kidney
Onset: 30 - 60 mins post-administration; 1.5 - 3 hrs duration of action
Dosage: 1 - 2 gm/kg body weight
Describe osmotic effect of mannitol on the kidneys
Major effect in proximal tubule and descending loop of Henle
Can mannitol relieve edema? If yes, how?
Yes, by increasing plasma oncotic pressure, which will pull fluid back into the blood vessels.
Describe mannitol’s effect on urine flow rate and Na+ reabsorption.
It increases urine flow rate, leading to decreased contact of filtrate and thus decreased Na+ reabsorption.
What are the therapeutic uses of mannitol?
- Treat oliguric phase of acute renal failure
- Promotes excretion of toxins
- Reduce intracranial pressure
What are the adverse effects of mannitol?
- Convulsions
- Thrombophlebitis
- Pulmonary congestion
- Headaches, chest pain, tachycardia, blurred vision, chills and fever
Function: Alkalinizing Drugs
- Increases absorption of other drugs (non-ionized form)
2. Used in acid-peptic diseases, where mucosal erosions can be pathologic
Give the chemical reaction involving use of antacids
HCl + NaHCO3 -> NaCl + H2CO3 -> NaCl + CO2 + H2O
What are the symptoms of excess CO2?
Abdominal distension, belching and flatulence
What are the symptoms of excess NaCl absorption?
Fluid retention leading to heart failure, HPN and renal insufficiency
Give examples of antacid formulations
- Calcium carbonate
- Alka seltzer
- Magnesium hydroxide
- Aluminum hydroxide
Function: Acidifying Drugs
Correct metabolic alkalosis
How does ammonium chloride lower blood pH?
Metabolism to urea and HCl
How does ascorbic acid act as an acidifying drug?
Directly acidifies urine
How does acetazolamide act as an acidifying drug?
Increases excretion of bicarbonate
