Intercellular and Intracellular Signal Transduction
Remote Signaling
(1) Endocrine
insulin
long range, via hormones in blood
broadcast signal; only cells with receptors affected
highly diluted; pico- to nanomolar conc
difficult signal termination -> usually long-lasting effects
receptors have high affinity
lag time in concentration changes
(2) Paracrine
histamines
signal excreted into local envir only
nano- to micromolar conc
lower affinity receptors
rapid and localized signaling
(3) Synaptic
neurotransmitters
extreme form of paracrine, very localized
high concentrated; micro- to millimolar
low affinity so rapid dissociation (< 1 ms signal)
(4) Autocrine
in embryo, immune system
signal generated and received by same cell
Secreted Molecules
highly varied
endocrine: large proteins, glycroproteins, small peptides, steroids
neurotransmitters: small molecules (i.e. aa, nucleotides, subst amines, small peptides)
paracrine and autocrine very diverse (simple organics, ATP, prostaglandins)
ligand
any molecule that binds to receptor
agonist
activates signal
antagonist
prevents signal
physiological
natural
pharmacological
synthetic
receptor subtypes
one ligand can bind to many different receptors in different cells or tissues; sometimes may have counteracting effects (i.e. acetylcholine stimulates skeletal muscle but relaxes cardiac muscle)
Receptors
(1) Hydrophobic
steriod hormones, vitamin D3, retinoic acid
enter plasma membrane
bind to transcription factors in cyto and nucleus
(2) Cell surface receptors
activate effectors (or self) and generate second message or change in membrane potential
ion channel linked
G-Protein linked (largest class, regulates other proteins through G-protein)
enzyme linked (receptor itself is enzyme)
cytokine (activates another enzyme directly)
Second Messengers
ions, hydrophilic (cytoplasmic action), hydrophobic (associated with membrane)