Introduction to Endocrine Physiology Flashcards
The classical definition of a hormone defines one as a chemical messenger from an endocrine gland into the bloodstream and acts on a distant tissue. How does this differ from the modern definition of hormone?
The modern definition defines a hormone as any substance released from a cell that acts on another cell, near or far, regardless of the singularity or ubiquity of the source and regardless of the means of conveyance
Define autocrine action. Example?
Secreting cell and target cell are the same
Ex: prostaglandins
Define paracrine action. Example?
A hormone is secreted into extracellular space and targets cells in the surrounding area; diffuse short distances
Ex: Somatostatin
Define endocrine action. Example?
A hormone is secreted by specialized endocrine glands/cells directly into the blood; travel long distances
Ex: thyroxine
Define neural action. Example?
A product of a neuron that is secreted into synaptic spaces; travel short distances
Ex: Acetylcholine & Norepinephrine
Define neuro-endocrine action. Example?
A product of a neuron that is released into the blood
Ex: Oxytocin
What six hormones does the anterior pituitary secrete?
Follicle-stimulating hormone (FSH)
Leutinizing hormone (LH)
Adrenocorticotropic hormone (ACTH)
Thyroid-stimulating hormone (TSH)
Prolactin (PRL)
Growth hormone (GH)
Somatostatin is secreted from what two locations?
Hypothalamus
Pancreas
What two receptors are responsible for modulating the secretion of antidiuretic hormone (ADH)? What do they respond to?
Stretch receptors signal ADH secretion at low atrial pressures (hypovolemia)
Osmoreceptors signal ADH secretion when ECF osmolarity increases (dehydration)
What five hormones function to increase plasma glucose?
Growth hormone
Norepinephrine
Epinephrine
Cortisol
Glucagon
When is norepinephrine considered a hormone and when is it considered a neurotransmitter?
Hormone: when secreted by the adrenal medulla
Neurotransmitter: when released by sympathetic postganglionic neurons
What are the four types of hormones? Which ones are hydrophilic and which ones are hydrophobic?
Peptide and protein hormones (Hydrophilic)
Amino acid derivatives (Hydrophilic)
Steroid hormones (Hydrophobic)
Fatty Acid derivatives (Hydrophobic)
Where are peptide hormones produced? Where are their receptors typically located?
Produced by the pituitary gland as releasing factors (except insulin)
Act on receptors on cell surfaces of peripheral endocrine glands
What are the “pre” and “pro” segments of a preprohormone? What are their functions?
Pre: signaling peptide sequence; bind to signal recognition complex to pause translation until docked on the ER
Pro: Intervening peptide; holds the final product in the proper orientation; keeps hormone inactive until it is cleaved
What is the “C-Peptide”? What is its clinical significance?
“Pro” segment of pro-insulin
Unknown physiological function
Marker of insulin levels (used to diagnose diabetes)
How are peptide hormones stored and transported?
Secretory vescicles
Which hormone type requires use of a secondary messenger? Why?
Peptide/protein hormones
Receptors are located the cell surface; cannot freely diffuse through the cell membrane
True or false: protein/peptide hormones can be administered orally.
False. They will degrade in the stomach.
What hormones are derivatives of tyrosine?
Thyroid hormones
Catecholamines
Where are catecholamines synthesized within the cell (2)? What tissues synthesize them?
Synthesized in the cytosol in:
Adrenal medulla
Post-ganglionic fibers of the sympathetic nervous system
In the synthesis of catecholamines, what reaction does tyrosine hydroxylase catalyze?
Tyrosine –> DOPA
In the synthesis of catecholamines, what reaction does DOPA decarboxylase catalyze?
DOPA –> Dopamine
In the synthesis of catecholamines, what reaction does Dopamine β-hydroxylase catalyze?
Dopamine –> Norepinephrine
In the synthesis of catecholamines, what reaction does PNMT catalyze?
Norepinephrine –> Epinephrine
What is the rate-limiting step of steroid hormone synthesis? Where does it occur in the cell? What enzyme catalyzes this step?
Cholesterol –> Pregnenolone
occurs in the mitochondria
side-chain cleavage enzyme (desmolase)
What are the three major types of steroid hormones?
Corticosteroids
Sex hormones
Vitamin D
What are the two subtypes of corticosteroids? Examples?
Glucocorticoids (ex: cortisol)
Mineralcorticoids (ex: aldosterone)
Where are corticosteroids synthesized?
Adrenal cortex
How are steroid hormones transported?
Synthesized and immediately diffuses through the cell membrane
Bound to albumin or a specific binding globulin for transport through the blood
Where are receptors for steroid hormones located?
Intracellularly
True or false: steroid hormones can be administered orally.
True
Aside from peptide and protein hormones, what specific hormones also have cell surface receptors? (2)
Triiodothyronine (T3)
Thyroxine (T4)
CYP11A1 is what enzyme? What reaction does it catalyze?
Side-chain cleavage enzyme (desmolase)
catalyzes the first reaction in steroidogenesis:
cholesterol –> pregnenolone
CYP17 is what enzyme? What reactions does it catalyze? (3)
17 alpha-hydroxylase
Pregnenolone –> 17α-OH pregnenolone
Progesterone –> 17α-OH progesterone
cleavage of C17,20 bonds to yield DHEA or androstenedione
CYP21A2 is what enzyme?
21-hydroxylase
CYP19 is what enzyme? What reaction does it catalyze?
Aromatase
Testosterone –> estradiol
Steroid hormones are synthesized from the enzymatic modification of what molecule?
cyclopentanoperhydrophenanthrene (CHL)
How are steroid hormones transported?
Immediately released from the cell after synthesis
Circulates protein-bound in blood
Interacts with cytoplasmic or nuclear receptors (activate DNA for protein synthesis)
The physiologic effects of hormones depend on?
Concentration in blood
The concentration of hormone as seen by target cells is determined by what three factors? Which is the most highly regulated (*)?
*Rate of production
Rate of delivery
Rate of degradation and elimination
How is the rate of production and secretion of hormones mediated? (3)
Neuroendocrine reflexes
Circadian rhythms
Positive and negative feedback
What is the main determinant of the rate of delivery of hormones?
Blood flow to a target organ
High blood flow delivers more hormone
True or false: rate of degradation (liver) and elimination (kidney) is normally meant to fluctuate.
False: any changes in rate of degradation and/or elimination are bad
What are the three types of stimuli that control rate of production of hormones?
Humoral: pertaining to ECF, blood, or lymph
Neural: direct stimulation of glands by the nervous system
Hormonal: hormone is released in response to another
T4 levels are _______ in the winter and _______ in the summer.
Increased in the winter
Decreased in the summer
What time of the day does cortisol concentration peak?
At the beginning of the light cycle
What is the pacemaker for most circadian rhythms? Where is it located?
Suprachiasmatic nucleus
located in the hypothalamus, superior to the optic chiasma
What are some examples of positive feedback?
Opening of Na+ channels during depolarization
Ovulation (progesterone)
Birth (oxytocin)
What is the HPA axis?
Hypothalamic-pituitary-adrenal axis
Negative feedback loop between the hypothalamus, anterior pituitary, and the adrenal cortex that controls the secretion of CRH, ACTH, and cortisol
How do binding proteins affect half-life of hormones?
Prevent hormones from being degraded, metabolized, or bound to a receptor
Which thyroid hormone (T3 and T4) has a greater affinity for its binding protein and which has a greater affinity for its receptor? What are their half-lives?
T4: binding protein (6-7 day half-life)
T3: receptor (18 hour half-life)
What is half-life?
The amount of time required to reduce the amount of a hormone to 1/2 of the original amount
What are the three characteristics of receptors?
Specificity: receptor can bind a given hormone (+ agonists & antagonists) but not others
Affinity: how well the receptor binds to the hormone (affected by receptor conformation)
Density: changing the number of receptors on the cell surface
Define permissiveness. Example?
One hormone must be present in adequate amounts for the full effect of another hormone
Ex: thyroid hormone increases epinephrine receptors
Define synergism. Example?
The combined effect of several hormones is greater than the sum of their separate effects
Ex: counterregulatory hormones on blood glucose
Define antagonism. Example?
One hormone causes the loss of another hormone’s receptors
Ex: Progesterone causes loss of estrogen receptors on uterine smooth muscle (decreases uterine contractions)
Hormone-producing cells in tumors usually do not respond to negative feedback except for _____
tumors of the pituitary gland
What is the difference between functional and nonfunctional endocrine tumors?
Nonfunctional: do not secrete hormone (~25% of endocrine tumors)
Functional: secrete hormones and are named after hormones they secrete (ex: insulinomas secrete insulin)
