Download Endocrine System 1 - Napa Valley College

Biology 219 – Human Physiology
Clemens
Endocrine System 1
Text: Chs. 6 & 7 (part)
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
fast response
short duration
secretes neurotransmitters
specific, localized effects
Endocrine System
slow response
longer duration
secretes hormones
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
b. Phospholipase-C (DAG-IP3) Second Messenger System
(e.g., norepinephrine: α-adrenergic receptors)
- G-protein coupled receptor
- activation of G protein activates Phospholipase C enzyme in the cell membrane
- phospholipase C cleaves a membrane phospholipid
- forms 2 second messengers: diacylglycerol (DAG)
inositol triphosphate (IP3)
D. Stimulation of Hormone Secretion
1. neural - by neurons that synapse on endocrine cells
2. humoral - by substances in the blood or interstitial fluid
3. hormonal - by other hormones
E. Responsiveness to Hormones
1. Concentration of hormone in the blood
rate of secretion vs. rate of degradation (half-life)
2. Abundance of receptors
a. up-regulation
b. down-regulation
3. Influence of other hormones
a. synergistic effects
b. permissive effects
c. antagonistic effects