Ans: D (Substance...) SECOND MESSENGERS Ganong23nd/446-48; Guyton 11th/912-15Second messenger, a molecule inside cells that acts to transmit signals from a receptor to a target. The term second messenger was coined upon the discovery of these substances in order to distinguish them from hormones and other molecules that function outside the cell as "first messengers" in the transmission of biological information. Many second messenger molecules are small and therefore diffuse rapidly through the cytoplasm, enabling info rotation to move quickly throughout the cell. As elements of signaling pathways, second messengers can serve to integrate information when multiple independent upstream inputs influence the rates of synthesis and degradation of the second messenger. In addition, second messengers can have multiple downstream targets, thereby expanding the scope of signal transmissionHormones exert intracellular actions is to stimulate formation of the second messenger inside the cell membrane. The second messenger then causes subsequent intracellular effects of the hormone.The specific action that occurs in response to increases or decreases of cAMP in each type of target cell depends on the nature of the intracellular machinery-some cells have one set of enzymes, and other cells have other enzymes. Therefore, different functions are elicited in different target cells, such as initiating synthesis of specific intracellular chemicals, causing muscle contraction or relaxation, initiating secretion by the cells, and altering cell permeability.Second messengers are molecules that relay signals from receptors on the cell surface to target molecules inside the cell, in the cytoplasm or nucleus. Secondary messengers are a component of signal transduction cascades.Secondary messenger systems can be synthesized and activated by enzymes, like the atlases that synthesize cyclic nucleotides, or by opening of ion channels to allow influx of metal ions, like Ca2+ signaling. These small molecules bind and activate protein kinases, ion channels, and other proteins, thus continuing the signaling cascade.Types of Secondary Messenger MoleculesThere are three basic types of secondary messenger molecules:Hydrophobic molecules: water-insoluble molecules, like diacylglycerol, and pbosphatidylinositols, which are membrane-associated and diffuse from the plasma membrane into the intermembrane space where they can reach and regulate membrane-associated effector proteinsHydrophilic molecules: water-soluble molecules, like cAMP, cCMP, IP3, and Ca2+, that are located within the cytosolGases: nitric oxide (NO), carbon monoxide (CO) and hydrogen sulphide (H2S) which can diffuse both through cytosol and across cellular membranes.The binding of the neurotransmittertothe receptor changes the receptor and causes it to expose a binding site for a G-protein. The G-protein is known as the "transducer."The alpha subunit, of G-protein is free to move along the inner membrane, eventually contacts another membrane-bound protein - the "primary effector."The primary effector then has an action, which creates a signal that can diffuse within the cell. This signal is called the "secondary messenger" (The neurotransmitter is the first messenger.) The secondary messenger may then activate a "secondary effector" whose effects depend on the particular secondary messenger system. Calcium ions are responsible formany important physiological functions, such as in muscle contraction. It is normally bound to intracellular components, even though a secondary messenger is a plasma membrane receptor.I.Hormones that bind to intracellular receptors AndrogensCalcitriol (1, 25 2-D3)EstrogensGlucocorticoidsMineralocorticoidsProgestinsRetinoic addThyroid hormones (T3 and T4)IIHormones that bind to cell surface receptorsAThe second messenger is cAMPa-Adrenergic catecholaminesb-Adrenergic catecholaminesAdrenocorticotropic hormone (ACTH,Corticotrpin) Antidiuretic hormone (Vasopressin)CalcitoninChorionic gonadotropin, humanCorticotropin-releasing hormoneGlucagonLipotropinLuteinizing hormoneMelanocyte-stimulating hormoneParathyroid hormoneSomatostatinThyroid-stimulating hormoneBThe second messenger is cGMPAtria/ natriuretic factorNitric oxideCThe second messenger is calcium or pbosphatidylinositols (or both)Acetylcholine (muscarinic)al-Adrenergic catecholaminesAngiotensin IIAntidiuretic hormone (vasopressin)CholecystokininGastrinGonadotropin-releasing hormoneOxytocinSubstance PThyrotropin-releasing hormoneDThe second messenger is a kinase or phosphatase cascadeAdiponectinChorionic somatomammotropinEpidermal growth factorErythropoietinFibroblast growth factorGrowth hormoneInsulinInsulin-like growth factors I and IILeptinNerve growth factorPlatelet-derived growth factorProlactinThese intracellular messengers have some properties in common:They can be synthesized/released and broken down again in specific reactions by enzymes or ion channels.Some (like Ca2+) can be stored in special organelles and quickly released when needed.