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Chemical messengers
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Outline Mechanisms of intercellular communication Chemical messengers Signal transduction mechanisms
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Mechanisms of intercellular communication Direct through gap junctions Cells exchange chemicals through connections between cytoplasm Indirect through chemical messengers The messenger must bind to a matching receptor and stimulate a response
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Figure 5.1a Indirect messengers Autocrine: the cell secretes a chemical which can bind on its own membrane and stimulates a response Paracrine: a cell secretes a chemical which can bind to a receptor on a neighboring cell trigger a response Hormone: a chemical travels long distance through the blood before binding to its own target organ and stimulating a response Neurotransmitter: influences a distant organ by secreting chemicals which travel through axons
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Chemical classification of messengers Lipophilic molecules = lipid soluble can cross the phospholipid bilayer cannot be stored by the cell synthesized and released immediately. ex: steroids (from cholesterol), eicosanoids (from fatty-acids) Lipophobic molecules = not soluble in lipid cannot cross the phospholipid bilayer can be stored in vesicles by the cell. ex: amino acids, peptides, proteins
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Transport and elimination For autocrines, paracrines and neurotransmitters: diffusion For non-water soluble hormones: transport into in the blood bound to carrier proteins for transport (steroids and thyroid hormones) Hormones will eventually be degraded by the liver and eliminated by the kidneys each has a characteristic ½ life.
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Hormone binding The hormones must bind to a matching receptor The binding is specific for this hormone receptor affinity (a receptor might have differing affinity for various hormones (ex: epinephrine and norepinephrine)
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Signal transduction: the binding of the hormone to its receptor triggers a cell response - steroid hormone: cross the phospholipid bilayer receptors are inside the cytoplasm or nucleus activates transcription and synthesis of new proteins response slow but sustained - Lipophobic hormones bind to receptors located on the surface of the cell membrane activates a cascade of enzymes response amplified rapid response but not sustained
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Mechanisms of transduction (do not memorize these mechanisms The binding of the hormone to its receptor can trigger the opening of a channel, induce enzymatic activation
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Application Cholera
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Readings Chp. 5, p. 126-145. Discovery: p.128, Clinical connections: p. 140. Not expected to be known: Clinical connections: p. 134
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