Endocrine System Flashcards
What is the endocrine system?
Cell to cell communication
What is exocrine system?
Cell to environment communication
Classical Definition of Hormones
A chemical released by specialized cells into the blood stream to elicit a specific response at tissues throughout the body
Broad definition of hormones
A chemical released by a cell which signals another cell
Direct Cell Signaling
Done by gap junctions between cells
Autocrine/Paracrine Cell Signaling
A cell sends a chemical messenger out and that messenger goes across a space in between cells and signals a receptor on the target cell (or itself). Happens with cells near each other
Endocrine Cell Signaling
Message is a hormone that travels through the blood stream throughout the body, targeting different cells. Different cells may give different responses to the same hormone as well
Neural Cell Signaling
Dendrites receive message, travels down the axon, fires through the axon terminal to another neuron’s receptors (dendrites)
Exocrine glands
Have cuts, poorly vascularized; release things to the external environment, ex: sweat, milt, saliva, etc. (gut is part of external environment)
Endocrine Glands
Highly vascularized, no ducts; release things into the internal environment (usually the blood stream). Ex: Hypothalamus, testes, ovaries, etc.
Peptide/Protein Hormones
- Often end in “-in”
- Large
- Tend to act quickly
- Hydrophilic
- Between 20-120 amino acids long
- Travels to target cell dissolved in extracellular fluid
- Binds to transmembrane receptors (ligand binding domain) and changes their shape
- Rapid effects on target cells
- Leaves cell via exocytosis
- Ex: Oxytocin, human growth hormone, etc.
- Production and release:
○ Created in the rough ER via ribosomes
○ Is made as a preprohormone
§ Pre portion: Signal sequence, gets cleaved off in the rough ER
§ Pro portion: Transports through vesicles to the Golgi apparatus which modifies it
□ Is stored in secretory vesicle until it needs to be released
® While in this vesicle the hormone is cleaved from the pro portion, producing peptide fragments - Synthesis and Secretion of Vasopressin
○ Follows the above mechanism
○ Works to control blood pressure by increasing H20 reabsorption and constricts arterioles which increase BP
Amine Hormones
- Often end in “-ine”
- Small
- Have and amine (NH2) portion
- Some are endocrine hormones, some neurotransmitters, some both
- Mostly hydrophilic (except thyroid hormones)
- Have diverse effects
- Ex. Melatonin, histamine, thyroid hormones, norepinephrine, etc.
Steroid Hormones
- Often end in “-one_
- Lipids derived from cholesterol
- Synthesized by the smooth ER or mitochondria
- Three classes:
○ Mineralocorticoids for electrolyte balance (water ion balance)
○ Glucocorticoids are stress hormones
○ Reproductive hormones with regulated sex-specific characteristics - Ex. Cortisol, testosterone, progesterone, etc.
- Hydrophobic
- Made on demand; cannot be stored
- Need a carrier protein (usually albumin)
- Bind to intracellular (sometimes transmembrane) receptors
- Effects the target cell slowly (except cortisol)
Receptors
○ Hydrophilic messengers bind to transmembrane receptors
○ Hydrophobic messengers bind to intracellular receptors
○ Receptors change shape/confirmation when bound by the ligand
○ Receptors become saturated at a certain amount (saturation) of ligand, you would need more receptors to go beyond this point
- A high-affinity receptor will reach saturation faster than a low-affinity one
Ligand
○ Chemical that binds to receptor
○ Agonist: Binds to and activate receptors, not necessarily the natural ligand
- Antagonist: Binds to and does not activate the receptor, prevents ligand binding
Inactivation of Ligand-Receptor Complex
○ Can happen by removal/destruction of receptor
§ Ex. For insulin receptors to not have too low of blood sugar
Intracellular Receptors
- Hydrophobic ligand diffuses across cell membrane
- Binds to receptor in cytoplasm or nucleus
○ Shape changes - L-R complex binds to specific DNA sequences
- Regulates the transcription of target genes
○ Increases or decreases production of specific mRNA - Transcriptional Cascades:
○ Receptor is activated by hormone, causes transcription of mRNA, and that mRNA can split, with the different parts effecting multiple pathways
- Binds to receptor in cytoplasm or nucleus
- Basic/Hypothetical Receptor Enzyme
○ Has a ligand binding domain, trans-membrane domain, and catalytic domain in that order from top to bottom
Amplification
○ One hormone can bind to the receptor causing A to happen, which causes more things to happen, which causes more things to happen, etc.
Guanylate Cyclase
○ Receptor Enzyme
○ Uses GTP, converts it to cGMP
○ Steps:
1. Ligand binds to receptor guanylate cyclase, changing its conformation
2. The activated receptor catalyzes the conversion of GTP to GMP
3. The cGMP acts as a second messenger, and binds to PKG (phosphokinase G)
4. The activated G-kinase phosphorylates proteins on serine or threonine residue
Tyrosine Kinase
○ Receptor enzyme
○ Phosphorylates tyrosine
○ Steps:
1. Ligand binds to receptor
2. Receptors dimerize and auto-phosphorylate
3. Phosphorylated receptors interact with protein kinases
4. Protein kinases signal to Ras protein
Ras switches between the active and inactive forms