Unable to penetrate plasma membraneBind to receptor proteins at outer surface of plasma membrane (extracellular receptors)10
A & P Fall 2013Ch-18 The Endocrine SystemOct-17-2013Mechanisms of Hormone ActionHormones and Plasma Membrane Receptors Bind to receptors in plasma membraneCannot have direct effect on activities inside target cellUse intracellular intermediary to exert effectsFirst messenger:•leads to second messenger•may act as enzyme activator, inhibitor, or cofactor •results in change in rates of metabolic reactionsImportant Second Messengers Cyclic-AMP (cAMP) : Derivative of ATPCyclic-GMP (cGMP) : Derivative of GTPCalcium ions : 11
A & P Fall 2013Ch-18 The Endocrine SystemOct-17-2013Mechanisms of Hormone ActionThe Process of Amplification Is the binding of a small number of hormone molecules to membrane receptorsLeads to thousands of second messengers in cellMagnifies effect of hormone on target cellDown-regulation Presence of a hormone triggers decrease in number of hormone receptorsWhen levels of particular hormone are high, cells become less sensitiveUp-regulationAbsence of a hormone triggers increase in number of hormone receptorsWhen levels of particular hormone are low, cells become more sensitive12
A & P Fall 2013Ch-18 The Endocrine SystemOct-17-201313Signaling Cascades:1.Transduction of first signal(can change physical properties of stimulus - elect to chem, light to chemical sound to chemical)2.Relay information into cell - (different types of molecules and signals)3.Amplification of signal:(multiple steps allow increase gain of signal - or can decrease gain of signal in some cases)4.Distribution of signal -(to various organelles, compartments or systems in cell).
A & P Fall 2013Ch-18 The Endocrine SystemOct-17-2013The activity of a GTP- binding protein generally requires the presence of a tightly bound GTP molecule (switch on).Hydrolysis of this GTP produces GDP and inorganic phosphate (Pi), and it causes the protein to convert to a different, usually inactive, conformation (switch off).Resetting the switch requires that the tightly bound GDP dissociate; once the GDP dissociates, a molecule of GTP is quickly re-bound.GTP-binding proteins form molecular switches14
A & P Fall 2013Ch-18 The Endocrine SystemOct-17-2013Mechanisms of Hormone ActionFigure 18–3 G Proteins and Hormone Activity1) G Protein Enzyme complex coupled to membrane receptorInvolved in link between first messenger and second messengerBinds GTPActivated when hormone binds to receptor at membrane surface and changes concentration of second messenger cyclic-AMP (cAMP) within cell:•increased cAMP level accelerates metabolic activity within cellHormones and Plasma Membrane Receptors 15
A & P Fall 2013Ch-18 The Endocrine SystemOct-17-2013Mechanisms of Hormone ActionFigure 18–3 G Proteins and Hormone Activity2) G Proteins and Calcium IonsActivated G proteins triggeropening of calcium ion channels
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