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Transcript
Chapter 5 Chemical Messengers Chapter Outline 5.1 Mechanisms of Intercellular Communication • General mechanisms o Direct: Gap junctions (smooth muscle and cardiac muscle) o Indirect: Chemical messengers 5.2 Chemical Messengers (Table 5.2) The messenger is produced by the source cell. It binds to the receptor on a target cell to trigger a cell response o Chemical Messenger Classification by Function (Table 5.1) o Paracrine chemical messenger signals a nearby cell. Example: Histamine, responsible for an inflammation response o Autocrine chemical messenger is a subclass of paracrines. Chemical that signals the same cell that secreted it o Neurotransmitter is the messenger produced by neurons. It is released into the ECF of the synaptic cleft. Examples: Acetylcholine, GABA, serotonin o Hormone is the messenger produced by endocrine cells. It is secreted into the blood via the interstitial fluid. Examples: Insulin, estrogen, thyroxin o Neurohormone is a special class of hormone that is produced by neurons and secreted into the blood. Examples: Antidiuretic hormone (ADH), oxytocin • Chemical Classification of Messengers by Chemical Properties (Table 5.2) o Solubility properties o Chemical class o A lipophobic ligand is water soluble; not lipid soluble. It does not cross the cell membrane. It binds with receptors on the cell membrane. It activates enzyme and change membrane permeability. o A lipophilic ligand can easily crosses the cell membrane bind with intracellular location of receptors. General action of target response is via gene activation • Synthesis and Release of Chemical Messengers o Amino acids neurotransmitters are synthesized within a neuron stored in vesicles and released by exocytosis o Amines are derived from amino acids with exception of thyroid hormones. o Peptide and proteins are formed as other proteins. o Steroids are derived from the cholesterol molecule o Eicosanoids is derived from a membrane phospholipid called arachidonic acid • Transport of Messengers o Paracrines, autocrines, neurotransmitters, most cytokines diffuse through interstitial fluid. Ligand is quickly degraded o Hormones, neurohormones, some cytokines is transported in the blood either in dissolved form or by carrier protein o Messenger half-life is the time for a chemical to decrease its concentration by half Messengers dissolved in plasma have a relatively short half-life. Example: half-life of insulin is less than 10 minutes Messengers bound to plasma protein have a relatively long half-life Example: Half-life of cortisol is 90 minutes 1 5.3 Signal Transduction • Signal transduction is the process of producing a response afer a messenger binds to a receptor in the target • Receptor Properties o Specificity: One messenger may bind to many receptor types o One target may have many types of receptors o The number of receptors per cell varies and is dynamic • Magnitude of target response depends on three factors: o Concentration of the messenger (ligand) o Number of receptors per target cell o Receptor affinity for the messenger Agonists and Antagonists o Agonist is when a ligand binds to a receptor triggers a normal response o Antagonist is when a ligand binds to a receptor not result in a response. Its response is the opposite of that to an agonist Mechanisms: Signal Transduction (Table 5.4) o Intracellular-mediated responses Characteristic of lipophobic ligands (except thyroid hormones) Receptors are found in the cytosol or nucleus Cell response is via gene activation o Membrane-bound receptor-mediated responses Channel-linked receptors (Chapter 8) Enzyme-linked receptors Receptor and enzyme—same protein Ligand binding activates the enzyme Action is direct Activated enzyme causes the target response Examples: Tyrosine kinases and guanylate cyclases G protein–linked receptors (Chapter 8) • Signal Amplification=Small amounts of ligand can cause a huge response in the target 5.4 Long-Distance Communication (Table 5.6) Endocrine Communication (Chapter 6) Nervous Communication (Chapter 7 and 8) 2