Hormones
Types
Amines
Derived from tyrosine
Peptides
Biggest group of endocrine hormones
Steroids
All come from cholesterol
Vitamin D also has a steroid structure
Control of Hormone Release and Mechanism of Action
Control of Hormone release
Humoral stimuli
Change of blood levels of ions and nutrients directly stimulates secretion of hormones
Neural stimuli
Sympathetic nervous system fibers stimulate adrenal medulla to secrete catecholamines
Hormonal stimuli
Hormones stimulate other endocrine organs to release their hormones
Hypothalamic hormones stimulate the release of most anterior pituitary hormones
Anterior pituitary hormones stimulate targets to secrete still more hormones
Hypothalamic-pituitary-target endocrine feedback loop:
hormones from the final target organs inhibit the release of the anterior pituitary hormones
Mechanism of Action
Target cells must have specific receptors, to which the hormone binds
ACTH receptors are only found on certain cells of the adrenal cortex
Thyroxin receptors are found on nearly all cells of the body
Target cell activation depends on three factors
Blood level of the hormone
Relative number of receptors on or in the target cell
Affinity of binding between receptor and hormone
Hormones influence the number of their receptors
Up-regulation
Target cells form more receptors in response to the hormone
Down-regulation
Target cells lose receptors in response to the hormone
Agonist
Drug that binds to a receptor and MIMICS actions of hormone or neurotransmitter
Antagonist
Drug that binds to a receptor and BLOCKS the actions of a hormone or neurotransmitter
Receptors
Ion Channel
GPCR & Enzyme-linked
Nuclear
Ligand-gated
Trigger enzyme reaction
Intracellular
Steroids and thyroid hormones bind to this receptor
Made of 5 separate polypeptides
Neurotransmitter
- Binds to receptor
- Opens (or closes) ion channel
- Increases (decreases) intracellular ion level
- Changes cell function
GPCR
Hormone
- Binds receptor
- Causes conformational change of receptor
- Activates G protein
Activated G protein
- Activates or inactivates an enzyme or an ion channel
- Changes level of second messenger
Second messenger (first messenger is hormone or drug)
Affects cell function(s)
Cyclic AMP signaling mechanism
- Activated kinases phosphorylate various proteins, activating some and inactivating others
- cAMP is rapidly degraded by the enzyme phosphodiesterase
- Intracellular enzymatic cascades have a huge amplification effect
Enzyme-Linked
Receptor Tyrosine Kinase (RTK)
Pair of two peptides linked together on surface of cell
Hormone
- Binds to receptor
- Stabilizes the receptor homodimer configuration
- Allows trans-phosphorylation of tyrosine moeties
Hormone-receptor complex
- Recruits signaling molecules
- Affects cell functions
Hormone
- Diffuses into cytoplasm
- Binds to receptor
- Forms hormone-receptor complex
Hormone-receptor complex
- Enters nucleus
- Binds to hormone response element
- Increases or decreases synthesis of specific proteins
Pituitary Gland and Hypothalamus
Pituitary Gland
Posterior (neurohypophysis)
Anterior (adenohypophysis)
Nerve fibers and pituicytes (glial cells)
Hormone secreting epithelial cells
- Downgrowth of hypothalamic neural tissue
- Neural connection to the hypothalamus (hypothalamic-hypophyseal tract)
- Nuclei of the hypothalamus synthesize the neurohormones oxytocin and antidiuretic hormone (ADH)
- Neurohormones are transported to the posterior pituitary
- Originates as an out-pocketing of the oral mucosa
- Hypophyseal portal system
Hypophyseal portal system
- Primary capillary plexus
- Hypophyseal portal veins
- Secondary capillary plexus
- Carries releasing and inhibiting hormones to the anterior pituitary to regulate hormone secretion