Drug receptor interactions and types of receptor

Editor's Notes

• #4: extracts elicited responses in tissues that were similar to those induced by nerve stimulation A drug will not work unless it is bound’ drug molecules must be ‘bound’ to particular constituents of cells and tissues in order to produce an effect. exceptions to Ehrlich’s dictum— drugs that act without being bound to any tissue constituent (e.g. osmotic diuretics, osmotic purgatives, antacids and heavy metal chelating agents). Ehrlich envisaged molecules extending from cells that the body could use to distinguish and mount an immune response to foreign objects.
• #5: his approach was fist used successfully on the nicotinic acetylcholine receptor . The fist was that the electric organs of many fihes, such as rays (Torpedo sp.) and electric eels (Electrophorus sp.) consist of modifidmuscle tissue in which the acetylcholine-sensitive membrane is extremely abundant, and these organs contain much larger amounts of acetylcholine receptor than anyother tissue. second was that the venom of snakes of the cobra family contains polypeptides that bind with very high specifiity to nicotinic acetylcholine receptors. These substances, known as α-toxins, can be labelled and used to assay the receptor content of tissues and tissue extractsest known is α-bungarotoxin Treatment of muscle or electric tissue with nonionic detergents renders the membrane-bound receptor protein soluble, and it can then be purifid by the technique of affiity chromatography. Similar approaches have now been used to purify a great many hormone and neurotransmitter receptors, as well as ion channels, carrier proteins and other kinds of target molecules. Multiple subtypes of receptors now known, which has paved the way for clinically superior drugs
• #6: Na channel for la Dhfr, ace, cox, immunophilin for cyclosporine,, 11 PDE subtypes exist Milrinone for CHF (PDE3), Rolipram for asthma (PDE 4) and Sidenafil (PDE 5)5fu- abnormal metabolite Symport/antiport
• #8: These sites include external surfaces of skin and gastro-intestinal tract.
• #9: Endogenous agonist- hormone, NT, cytokines Exo- drugs, chemicals Act on the receptor and change in cell function producing a response
• #11: Pentameric Assembly of 4 types of subunits α, β, γ and δ 4 membrane spanning α-helices inserted into membrane 2 Ach binding sites, both must bind Ach molecules for receptor activation Lining of central transmembrane pore formed by helical segments of each subunit (negatively charged AA). 5 helices sharply kinked inwards halfway, forming a constriction Ach molecules bind, twists the α subunits, kinked helices either straighten out or swing out of the way, opening channel pore Mutation of a critical residue in helix changes channel from being cation selective (excitatory) to being anion selective (typical of receptors for inhibitory transmitters like GABA) Most excitatory neurotransmitters cause Increase in Na+ and K+ permeability net inward current carried mainly by Na+ Depolarization of the membrane (probability to generate action potential)
• #13: Generally include one transmembrane helix that contains an abundance of basic (i.e. positively charged) AA When membrane is depolarised, so that the interior of the cell becomes less negative, this region—the voltage sensor—moves slightly towards the outer surface of membrane, opening the channel Inactivation happens when an intracellular appendage of the channel protein moves to plug the channel from the inside Four six-helix domains consists of a single huge protein molecule, the domains being linked together by intracellular loops lgCs appear to assume only two states whereas VOCs undergo a third state called refractory (inactivated) state. Voltage gated channels have no major endogenous modulator (like Ach)
• #14: G protein is “guanine nucleotide binding protein” The human genome includes genes encoding about 400 GPCRs (excluding odorant receptors), which constitute the commonest single class of targets for therapeutic drugs, and it is thought that many promising therapeutic drug targets of this type remain to be identified.
• #15: First GPCR to be fully characterized was β-Adrenoceptor. Family A is largest. C smallest. differ in length of extracellular N-terminus and location of agonist binding domain. For small molecules, NA, the ligand-binding domain of class A receptors is buried in the cleft between the α-helical segments within the membrane. Peptide ligands bind more superficially to the extracellular domain
• #16: Coupling of the α subunit to an agonist-occupied receptor causes the bound GDP to exchange with intracellular GTP; the α–GTP complex then dissociates from the receptor and from the βγ complex, and interacts with a target protein (target 1, which may be an enzyme, such as adenylyl cyclase, or an ion channel) The GTPase activity of the α subunit is increased when the target protein is bound, leading to hydrolysis of the bound GTP to GDP, whereupon the α subunit reunites with βγ
• #18: catalyses formation of the intracellular messenger cAMP cAMP activates various protein kinases that control cell function in many different ways by causing phosphorylation of various enzymes, carriers & other proteins increased activity of voltage gated calcium channels in heart muscle cells. Phosphorylation of these channels increases the amount of Ca2+ entering the cell during the action potential, and thus increases the force of contraction of the heart In smooth muscle, cAMP-dependent protein kinase phosphorylates (thereby inactivating) another enzyme, myosin-light-chain kinase, which is required for contraction. This accounts for the smooth muscle relaxation Adenylyl cyclase can be activated directly by certain agents, including forskolin and fluoride ions 11 PDE subtypes exist; inhibited by theophylline & caffeine, Milrinone for CHF (PDE3), Rolipram for asthma (PDE 4) and Sidenafil (PDE 5)
• #19: Receptor-mediated activation of phospholipase C results in the cleavage of phosphatidylinositol bisphosphate (PIP2), forming diacylglycerol (DAG) (which activates protein kinase C) and inositol trisphosphate (IP3) (which releases intracellular Ca2+). The role of inositol tetraphosphate (IP4), which is formed from IP3 and other inositol phosphates, is unclear, but it may facilitate Ca2+ entry through the plasma membrane. IP3 is inactivated by dephosphorylation to inositol. DAG is converted to phosphatidic acid, and these two products are used to regenerate PI and PIP2. many events, including contraction, secretion, enzyme activation and membrane hyperpolarization.
• #20: Direct G-protein–channel interaction first shown for cardiac muscle, In cardiac muscle, mAChRs enhance K+ permeability (thus hyperpolarising the cells and inhibiting electrical activity). Similarly in neurons, many inhibitory drugs such as opioid analgesics reduce excitability by opening K+ channels or inhibiting Ca2+ channels. By enhancing hypoxia-induced pulm artery vasoconstriction, Rho kinase active. thought to be imp pathogenesis of pulmonary HT. Specific Rho kinase inhibitors e.g. fasudil. Rho–GDP, the resting form, is inactive, but when GDP–GTP exchange occurs, Rho is activated and controls smooth muscle contraction, proliferation, angiogenesis and synaptic remodeling.
• #21: Toll like receptor Janus kinase/signal transducers and activators of transcription
• #22: Generally involves dimerization autophosphorylation of tyrosine residues, act as acceptors for the SH2 domains of intracellular proteins Involved mainly in events controlling cell growth and differentiation, and act indirectly by regulating gene transcription Two important pathways are:– the Ras/Raf/ MAP kinase pathway - cell division, growth and differentiation.– the Jak/Stat pathway activated by many cytokines - controls the synthesis and release of many inflammatory mediators 
• #24: Class I : In the absence of their ligand, these NRs are predominantly located in the cytoplasm, complexed with heat shock and other proteins and possibly reversibly attached to the cytoskeleton. Liganded receptors generally form homodimers and translocate to the nucleus, where they can transactivate or transrepress genes by binding to ‘positive’ or ‘negative’ hormone response elements. Large numbers of genes can be regulated in this way by a single ligand Class II : LXR = Liver Oxysterol Receptor, FXR = Farsenoid (Bile acid) receptor, RXR = Retinoid receptor. Xenobiotic receptor = SXR. Bind to thiazolidinediones and Fibrates Hybrid Class = Retinoic Acid receptor (RAR)
• #25: they bind only molecules of a certain precise type. angiotensin (Ch. 22). This peptide acts strongly on vascular smooth muscle, and on the kidney tubule, but has very little effect on other kinds of smooth muscle or on the intestinal epitheliumA small chemical change, such as conversion of one of the amino acids in angiotensin from L to D form, or removal of one amino acid from the chain, can inactivate the molecule altogether, because the receptor fails to bind the altered form. tricyclic antidepressant drugs act by blocking monoamine transporters but are notorious for producing side effects (e.g. dry mouth) lower the potency of a drug and the higher the dose needed, the more likely it is that sites of action other than the primary one will assume signifiance. In clinical terms, this is often associated with the appearance of unwanted side effects, of which no drug is free.
• #26: Classification of Ligands a.  agonist b.  partial agonist c.  antagonist Effect of drug attributed to two factors
• #27: Efficacy Potential maximum therapeutic response that a drug can produce. ----Stephenson (1956) that describes effiacy describes the tendency of the drug–receptor complex to adopt the active (AR*), rather than the resting (AR) state Lower the potency of a drug and the higher chances of unwanted side effects, of which no drug is free
• #28: Emil fischer
• #29: FIRST model to be put forward by CLARK’S//// 2--Quantifies the relationship b/w drug conc and observed effct// based on law of mass action kinetics Limitations--Not all compounds that bind to a receptor elicit a response (Eg: antagonist). does not explain about partial agonism, spare receptors concept of partial agonist, constitutional receptor activity, inverse agonism allosteric modulation. lead to the development of agonist models of drug action --binding and activation phenomenon were explained by Ariëns and Stephenson in 1956 to account for the intrinsic activity (efficacy) of a drug (that is, its ability to induce an effect after binding).
• #30: The active site and substrate are exact matches for each other, similar to puzzle pieces fitting together. LOCK & KEY This theory maintains that the active site and the substrate are, initially, not perfect matches for each other. Rather, the substrate induces a change in shape of the receptor Similar to getting a hand in glove, placing ist finger might be difficult but once we pass the initial step the rest gets easier. The glove slides on easily
• #31: the duration of Receptor occupation determines whether a molecule is agonist, partial agonist of antagonist Agonist- high K2 fast association and fast dissociation Partial agonist- intermediate Antagonist- low K2 fast association and slow dissociation Correlation was poor This explians phenomenon “ fade” which is observed when an agonist produces a very transient peak (E peak ) effect followed by steady state response (equilibrium effect i.e equal to E max). Hypothesis of Paton
• #32: as well as through interpretation of the results of direct binding experiments This model has gained wide acceptance because it provides an explanation for the phenomenon of positive cooperativity often seen with neurotransmitters, and is supported by studies of conformational mutants ofthereceptor with altered equilibrium.
• #33: Del Castillo & Katz in 1957 was based on their work on the action of acetylcholine at the motor endplate effiacy as a property determined by the relative affiity of a ligand for R and R*, In contrast to the classical occupation theory the agonist in the two-state model does not activate the receptor but shifts the equilibrium toward the Rform.
• #36: (1+1=2)
• #38: (1+1=3)
• #39: Even though drugs may occupy the same number of receptors, the magnitude of their effects may differ. Naloxene on opioid receptors saralasin on angiotensin receptors 
• #40: naloxone and naltrexone are also partial inverse agonists at mu opioid receptors. This constitutive activity is usually minimal in natural receptors but is markedly observed in wild type and mutated (naturally or induced) receptors. According to conventional two-state drug receptor interaction model, binding of a ligand may initiate activity (agonist with varying degrees of positive intrinsic activity) or prevent the effect of an agonist (antagonist with zero intrinsic activity). metoprolol, carvedilol, propranolol, nebivolol, and bisoprolol, have shown inverse agonist activity Nearly all H1 and H2 antihistaminics (antagonists) have been shown to be inverse agonists. Among the β-blockers, carvedilol and bucindolol demonstrate low level of inverse agonism as compared to propranolol and nadolol. Several antipsychotic drugs (D2 receptors antagonist), antihypertensive (AT1 receptor antagonists), antiserotoninergic drugs and opioid antagonists have significant inverse agonistic activity t
• #42: Chemical antagonism Ex: -heparin(-ve) protamine +ve, Chelating agents, infliximab(tnf), mab interaction of two drugs whose opposing action in the body tend to cancel each other example – Histamine and Omeprazole on parietal cell of gastric mucosa.
• #43: D==selectively inhibits metabolism of the pharmacologically active (S) isomer the reduction of the anticoagulant effect of warfarin when an agent that accelerates its hepatic metabolism, such as phenobarbital, is the rate of absorption of the active drug from the gastrointestinal tract may be reduced, or the rate of renal excretion may be increased.
• #44: because the two are in competition, raising the agonist concentration can restore the agonist occupancy (and hence the tissue response). The antagonism is therefore said to be surmountable hallmark Atropine is a competitive antagonist of Ach. Captopril competes with angiotensin 1 for angiotensin convertin enzyme (ACE). • Finasteride competes with testosterone for Sa-reductase
• #45: agonist is able to displace the antagonist molecules from the receptors, although it cannot, of course, evict a bound antagonist molecule. Displacement occurs because, by occupying a proportion of the vacant receptors, the Surmountability of the block by the antagonist may be important in practice, because it allows the functional effect of the agonist to be restored by an increase in concentration
• #46: These are mainly used as experimental tools for investigating receptor function, and few are used clinically AcetazolamideDisulfiramindomethacin - A- CyclooxygenasearbonicldehydeanhydraseOmeprazoleDigoxin; Theophylline PropylthiouracilLovastatinSildenafilII. ION CHANNELSdehydrogenase- H• K• ATPase- Na• K• ATPase- Phosphodiesterase- Peroxidase in thyroid- HMG-CoA reductase- Phosphodiesterase-S
• #47: Log concentration–effect curves for a series of α-adrenoceptor agonists causing contraction of an isolated strip of rabbit aorta. Phenylephrine is a full agonist.The others are partial agonists with different effiacies. [B] The relationship between response and receptor occupancy for the series. Note that the full agonist, phenylephrine, produces a near-maximal response when only about half the receptors are occupied, fraction of receptors occupied (known as occupancy) as a function of drug concentration. whereas partial agonists produce submaximal responses even when occupying all of the receptors. The effiacy of tolazoline is so low that it is classifid as an α-adrenoceptor antagonist
• #48: Stephenson (1956), studying the actions of acetylcholine analogues in isolated tissues, found that many full agonists were capable of eliciting maximal responses at very low occupancies, often less than 1%. Allows maximal response without total receptor occupancy – Common with drugs that elicit smooth muscle contraction but less so for other types of receptor-mediated response, such as secretion, smooth muscle relaxation or cardiac stimulation, where the effect is more nearly proportional to receptor occupancy. agonist–receptor complexes can be minimized, with a lower concentration of hormone or neurotransmitter .
• #51: Well-established paradigm for GPCR sigNAL relative efficacy of PACAP1-27 for cyclic AMP elevation is higher than that of PACAP1-38 but lower for elevation of IP3 clearly indicate that agonist activation of multiple signaling mechanisms is not uniform but rather is often ‘biased’ toward some but not all signaling pathways
• #52: Activation of signalling pathways depends on the conformational change induced by the agonist receptor complex and it is highly selective Not all agonist ll induce the same conformational change and hence lead to differential activation of downstream signalling pathways 3..Taking natural agonist for the receptor as useful point of reference ; this willhave a natural signaling bias can be used as astandard with which other agonists can be compared
• #53: Extensive research done on Receptor pharmacology -lead to discovery of new drug targets for treatment of several diseases. Still requires discovery of new receptor types and the mechanisms of many orphan receptors that can result in effective treatment of many diseases. Much to be discovered about the nuclear receptors.