Chapter 5 chapter notes
Chapter 5: Chemical Messengers (Selected based on lecture)
Functional Classification of Chemical Messengers
Classes of Chemical Messengers
1. Paracrines – chemicals that communicate with neighboring cells. E.g. histamine –
responsible for allergic rxns and inflammations
Target cells must be closed enough
Secreted to ECF and reach target cell by diffusion
Includes Growth Factors – proteins that stimulate proliferation and differentiation of
Clotting Factors – formation of blood clot
Cytokines – peptides release from immune system functioning as body defense and
function like paracrines and hormones – travel thru bloodstream
Presynaptic Neuron – releases NT
Post synaptic – target cell
3. Hormones – chemicals release from endocrine glands into IF whre they can diffuse to
blood to target cells.
Neurohormones – release by special class of neurons called neurosecretory cells though similar
mechanism to NT release. E.g. ADH or vasopressin which is synthesized by neurosecretory cells
originating in hypothalamus. ADH located in posterior pituitary glands travels in blood to its
Chemical Classification of Messengers
Lipophilic – hydrophobic lipid soluble and ready to cross plasma membrane e.g. steroids,
Eicosanoids – paracrines that is lipids and insoluble to water.
Lipophobic – hydrophilic water soluble and not cross plasma membrane. E.g. amino acids,
amines, peptides or proteins
Receptor agonists – ligands bind to receptors and produce biological response and antagonists –
ligands bind to receptors without response
Signal Transduction Mechanisms for responses mediated by Intracellular Receptors - see p.138
Channel – Linked Receptors –type of ligand – gated channel in which the ligand is a messenger
that binds to receptor. Fast channel – receptor and channel are the same proteins and Slow
channel – receptor and channel are the same proteins but coupled with third type of protein.
Second messenger – intracellular messenger produced by binding of an extracellular messenger
(1st messenger) to receptor. E.g. cAMP
G – Protein – Linked receptors – activates special membrane protein called G proteins.
G proteins – have 3 subunits: Alpha (20 alpha subunits), Beta (5), and Gamma (13). = 38
Located on intracellular side of plasma memb. Where they function as links bet. G
protein – linked receptor and other proteins in plasma memb. called Effectors
Releases GDP, binds a molecule of GTP active state
Also function as an enzyme GTPase that hydrolyze GTP inactive state
Are assoc. with activation and inhibition of enzymes Amplifier Enzymes.
Amplifier Enzymes – catalyze the production of second messenger in ICF
Slow Ligand – gated ion channels - regulated by G-proteins which causes channels to open or
close bec.of messenger binding to receptor. (i.e. alpha moves to ion channel causing
conformational change that causes it to open or close)
G protein – regulated enzymes – are assoc. w/ production of 2nd messengers in cytosol. E.g.
cAMP, cGMP, inositol triphosphate, diacylglycerol, Ca+.
Signal Amplification in Chemical Messenger Systems
Signal Amplification – ability of small changes in conc. Of chemical messenger to elicit mark
response in target cells
Cascade –sequence of rxns – series of sequential steps that progressively inc. in magnitude. E.g.
from 1 product to 2,500,000 proteins
Nervous system – direct cell to cell communication of chemical signals – wired system
Endocrine System – communicates through chemical messengers – hormones – IF- bloodand
hormones communicate via G-Proteins – slower than electrical and chemical used by nervous