Endocrine System 1
A. Overview
endocrine gland - secretes hormones; no ducts
hormone - chemical messenger, carried in blood,
affects target cells (i.e., cells that have receptors for the hormone)
receptor - binds to hormone, required for response to the hormone
1. Comparison of Communication Systems
Nervous System Endocrine System
fast response slow response
short duration longer duration
secretes neurotransmitters secretes hormones
specific, localized effects widespread effects
2. Classes of Chemical Signal Molecules
neurotransmitters - affect a specific cell (the postsynaptic cell) at a synapse
hormones - affect target cells throughout the body
neurohormones - hormones secreted by neurons
paracrine substances (e.g., prostaglandins) - exert local effects on neighboring cells
B. Chemical Classification of Hormones
1. Lipophilic (lipid-soluble)
e.g., steroid hormones, thyroid hormones, eicosanoids
- cross the plasma membrane
- bind to intracellular (cytoplasmic) receptors
2. Lipophobic (water-soluble)
e.g., epinephrine, insulin, antidiuretic hormone
- do not cross plasma membrane
- bind to receptors on the PM
C. Cellular Mechanisms of Hormone Action
1. Lipophilic Hormones
- hormone binds to cytoplasmic receptor
intracellular hormone-receptor complex → binds to DNA, acts as transcription factor
→ regulates (↑ or ↓) transcription of specific genes → mRNA → protein synthesis
→ metabolic and developmental effects
2. Lipophobic (water-soluble) Hormones
- hormone binds to membrane receptor
hormone-receptor binding activates a signal transduction pathway
→ activates enzymes within the cell
→ mostly metabolic effects
a. Enzyme-linked Receptors (e.g., insulin receptor - tyrosine kinase)
b. G Protein-Coupled Receptors
3. Second Messenger Systems
a. Cyclic AMP Second Messenger System
(e.g., epinephrine: β-adrenergic receptors)
1. Hormone binds to receptor
2. G protein associated with receptor is activated
3. Activated G protein activates adenylyl cyclase in the plasma membrane
4. Adenylyl cyclase converts ATP to cyclic AMP (cAMP)
5. Cyclic AMP acts as a second messenger, activates protein kinase enzymes in the cell
6. Protein kinase activates other enzymes via phosphorylation
→ amplification of cellular response
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