3.1 Receptors PDF - MAHSA University

3.1 Pharmacodynamics: Receptors Ligand-gated ion channel Enzyme-linked GPCR receptor Enzyme Voltage-gated ion...

3.1 Pharmacodynamics: Receptors Ligand-gated ion channel Enzyme-linked GPCR receptor Enzyme Voltage-gated ion channel Nuclear receptor DNA Dr. Yamuna Sucedaram, PhD (UM) Faculty of Pharmacy MAHSA University  Outline ❖ Basic concept – receptors ❖ Types of receptor ▪ Ligand-gated ▪ G-protein coupled ▪ Enzyme-linked ▪ Intracellular ❖ Comparison of receptor types ❖ Down-regulation of receptors 2  Basic Concept Receptors – Cellular macromolecules that a ligand binds to initiate its effects. – Drug binds and transduce extracellular signal to intracellular response. – Has specific site for ligands to bind (key & lock model) Effect of drug determined by 2 factors: i. Affinity of drug = tendency to bind to receptor to form D-R complex ii. Intrinsic activity of drug = ability to trigger a response after forming D-R complex 3 4  Major Receptors Types Ligands generally : Hydrophilic or hydrophobic (lipophilic). Hydrophilic Hydrophilic Hydrophilic Hydrophobic Transmembrane signalling mechanisms. A. Ligand binds to the extracellular domain of a ligand-gated ion channel. B. Ligand binds to a domain of a transmembrane receptor, which is coupled to a G protein. C. Ligand binds to the extracellular domain of a receptor that activates a kinase enzyme. D. Lipid-soluble ligand diffuses across the membrane to interact with its intracellular receptor. R = inactive protein. 5  A) Ion channels Localized on cell membrane Channel usually close until activated by agonist. Ligand-binding site = extracellular domain Duration of response = rapid (only few milliseconds). Generate action potential to mediate diverse functions, including neurotransmission (how neurons communicate) and muscle contraction (cardiac muscle, skeletal muscle). The concentration of Na+ outside the cell is 10 times > inside. The concentration of K+ inside the cell is 30 times > outside. The cytosol contains a high concentration of anions (-ve), in the form of phosphate ions and negatively charged proteins. i) Ligand-gated ion channel ii) Voltage-gated ion channel 6  A) Ion channels Ligand-gated ion channel E.g. Stimulation of nicotinic acetylcholine receptor by acetylcholine cause Na+ & Ca2+ influx, potassium efflux, generating an action potential in a neuron or muscle cell. 7 8  Ion channels Mechanically-gated ion channel: - Opens due to a physical distortion of the cell membrane. E.g.: as pressure and temperature changes are applied to the skin, these channels open and allow ions to enter the cell. 9  Ion channels Leakage ion channel: - it opens and closes randomly. There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states. Leakage channels contribute to the resting transmembrane voltage. 10  B) G Protein-Coupled Receptors (GPCR) ▪ Composed of 7 transmembrane helices. ▪ Ligand-binding site = extracellular domain ▪ Duration of response = secs to mins ▪ Intracellular domain = linked to G protein ▪ G protein has 3 subunits (α subunit that binds to guanosine diphosphate, β & γ subunits) GPCR Main Variants Function Gs Stimulate Adenylyl Cyclase Gi Inhibit Adenylyl Cyclase Gq Stimulate Phopholipase C activity 11  B) G Protein-Coupled Receptors (GPCR) File:Second Messenger Mechanism.jpg Second Messenger Mechanism Second messengers = essential in conducting and amplifying signals coming from GPCR. There are 2 well-characterized second messenger cascade mechanisms: intracellular process Cyclic AMP (cAMP) Pathway: G protein Effector 2nd Function E.g. Messenger Gs Adenylyl Cyclase cAMP Activate protein kinase A β adrenoceptor Gq Phospholipase C IP3 & DAG Activate Ca2+ release & Angiotensin receptor protein kinase C Phosphoinositide Pathway (IP3/DAG) pathway: 12 B) G Protein-Coupled Receptors (GPCR) 1. Binding of the ligand of the receptor activates the G protein so that GTP Relay Protein replaces GDP on the α subunit. Effector 2. Dissociation of the G protein occurs, and both the α-GTP subunit and the β, γ subunit subsequently interact with other cellular effectors, usually an enzyme, a protein, or an ion channel. 3. Effectors then activate second messengers that are responsible for further actions within the cell. 4. Stimulation of GPCR results in responses that last several secs to mins. 13 14 ▪ Involves the G-protein activation of phospholipase C (PLC), which breaks down phosphatidylinositol biphosphate (PIP2) to Inositol trisphosphate (IP3) and diacylglycerol (DAG). ▪ DAG acts as a second messenger to stimulate protein kinase C ; IP3 stimulates the release of Ca2+ from ER.  C) Enzyme-Linked Receptors consists of a protein that may form dimers or multisubunit complexes. Upon binding of a ligand (at extracellular domain), receptor undergoes conformational changes (activated), resulting in increased cytosolic enzyme activity (The activated receptor phosphorylates tyrosine residues on itself, then other specific proteins). Duration of responses = minutes to hours. E.g. Epidermal growth factor (EGF), Insulin, various growth factors. 16  C) Enzyme-Linked Receptors Insulin receptor Diabetic patients inject insulin to increase the glucose uptake by the cells 18  D) Intracellular Receptors Receptor entirely intracellular. Ligand must have sufficiently high lipid solubility to diffuse across membrane to interact with the receptor. Primary drug targets are transcription factors. The activated ligand–receptor complex then translocates to the nucleus, where it binds to specific DNA sequences, causing transcription (mRNA), then translation (protein). Other intracellular targets = enzyme, ribosomes, protein (tubulin). Duration of responses = hours to days. E.g. Tamoxifen (antagonist) binds to the estrogen receptors, thereby preventing the growth of breast cancer cells. 19  D) Intracellular receptors Tamoxifen Estrogenic response and Tamoxifen in breast cancer cells. Upon the binding of estrogen (E2) to ERα, the receptor dissociates from the heat shock proteins in the cytoplasm and forms a homodimer, which can translocate to the nucleus. ERα homodimers transcribe target genes to promote cancerous growth. Tamoxifen is an antagonist of the estrogen receptor, inhibiting the growth of breast cancer cells. 20 Comparison of Receptor Types 21  Desensitization & Down-Regulation of Receptors ▪ Repeated administration of drugs (agonist or antagonist) may lead to change in responsiveness of receptor. ▪ Tachyphylaxis = desensitization that occurs very rapidly, sometimes with the initial dose. ▪ Mechanism of drug desensitization: ❖ Receptor mediated ❖ Non-receptor mediated 23  Desensitization & Down-Regulation of Receptors Receptor Mediated i. Lose of receptor function ✓ Rapid desensitization = change in receptor conformation ✓ Usually due to feedback of cellular effect of drug ✓ E.g. phosphorylation of specific aas in GPCR blocks coupling of G protein. ii. Reduction in receptor number ✓ Slower desensitization = change of receptor number ✓ Receptor down regulation due to internalized within the cell** ✓ E.g. Phosphorylation of specific aas in GPCR causes removal from cell surface. ** Receptor may be recycled to cell surfaces, restoring sensitivity or degraded (decrease total # of receptor available. Aas = amino acids. 24  Desensitization & Down-Regulation of Receptors Non-Receptor Mediated i. Reduction in receptor signaling components ✓ Depletion of signaling molecules. ✓ E.g. depletion of second messenger. ii. Increase metabolic degradation ✓ Increase metabolism rate / elimination rate. ✓ Lower blood plasma conc. iii. Physiological adaptation ✓ Due to opposing homeostatic response. Receptor / non-receptor dependent factors contribute to drug response variability between individuals. 25 26