Endocrine Flashcards
4 mechanisms of cellular communication
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Gap Junctions
- Direct cell-to-cell contact
- Pores allow movement of ions, nutrients, and signals between adjacent cells -
Neurotransmitters
- Cell-to-nearby-cell signaling
- Released by neurons into synaptic cleft to affect a specific target cell -
Paracrines
- Local signaling
- Secreted into tissue fluid to affect neighboring cells -
Hormones
- Broad, long-distance signaling
- Enter bloodstream and travel to distant cells and organs
endocrine vs nervous systems
Nervous System
- Communicates via electrical impulses and neurotransmitters
- Effects are local, specific, and fast (1–10 ms)
- Stops quickly when stimulus ends
- Adapts quickly to stimulation
- Example: reflex arc, neuron → target cell
Endocrine System
- Communicates via hormones in the bloodstream
- Effects are often widespread and slower (seconds to days)
- Can continue after stimulus ends
- Adapts slowly, may last days to weeks
- Example: blood glucose regulation via insulin/glucagon
Integration
- Some chemicals act as both (e.g., norepinephrine, vasopressin)
- Systems regulate each other
- Neuroendocrine cells (e.g., chromaffin cells / hypothalamus sells that secrete oxytocin) show shared properties
characteristics of endocrine and exocrine glands
Endocrine Glands
- No ducts
- Secrete hormones into bloodstream via fenestrated capillaries
- Effects are internal (e.g., alter metabolism of target cells)
Exocrine Glands
- Have ducts
- Secrete onto epithelial surfaces or mucosa (e.g., digestive tract)
- Effects are external (e.g., food digestion)
True endocrine secretion is always ductless, but some organs contain both endocrine and exocrine regions (like the pancreas and liver).
Mixed Function Organs
- Pancreas: endocrine (insulin) + exocrine (digestive enzymes)
- Liver: endocrine (hormones) + exocrine (bile) + non-hormonal blood proteins
“General” hormone characteristics
- Regulate:
- Extracellular fluid (e.g., water and electrolyte balance)
- Metabolism (e.g., thyroid hormones)
- Biological clock (e.g., melatonin from pineal gland)
- Contraction of cardiac and smooth muscle
- Glandular secretion (e.g., stimulation of adrenal or thyroid glands)
- Immune functions (e.g., thymosin from thymus)
- Growth and development (e.g., growth hormone, sex hormones)
- Reproduction (e.g., estrogen, testosterone, LH/FSH)
Categories of hormones
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Steroids
- Derived from cholesterol
- Hydrophobic, lipid-soluble
- Require carrier proteins in blood
- Bind intracellular receptors (affect gene expression)
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Monoamines
- Derived from amino acids (e.g., tyrosine)
- Most are hydrophilic, water-soluble
- Travel freely in blood
- Bind membrane receptors (act via second messengers)
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Peptides/Proteins
- Chains of amino acids
- Hydrophilic, water-soluble
- Travel freely in blood
- Bind to cell surface receptors (signal via second messengers)
Some peptide hormones (especially larger ones, like IGF-1) do bind carrier proteins to extend their half-life and improve stability.
Steroid Hormones
- Structure: Derived from cholesterol
- Sites of production: Only in the gonads and adrenal cortex
- Examples: Estrogens, progesterone, testosterone, cortisol, aldosterone, DHEA, calcitriol (vitamin D derivative)
- Synthesis & release: Not stored; synthesized on demand and released by simple diffusion
- Transport: Hydrophobic → require carrier proteins in blood
- Receptor location: Intracellular (cytoplasmic or nuclear)
- Mechanism: Directly affect gene transcription
- Functions: Reproduction, metabolism, stress response, fluid balance, calcium homeostasis
Monoamines
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Structure: Derived from amino acids
- Tryptophan → melatonin
- Tyrosine → catecholamines (dopamine, epinephrine, norepinephrine) and thyroid hormones (T₃ and T₄)
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Sites of production:
- Catecholamines → adrenal medulla
- Melatonin → pineal gland
- T₃ and T₄ → thyroid gland
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Solubility:
- Catecholamines & melatonin: Hydrophilic, circulate freely in blood, act on cell surface receptors
- T₃ (triiodothyronine) & T₄ (thyroxine): Hydrophobic, require carrier proteins (e.g., TBG) in blood, act on nuclear receptors
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Mechanism:
- Catecholamines act rapidly via second messenger systems
- T₃ is the biologically active form; T₄ is a prohormone converted to T₃ inside cells → binds nuclear receptor → alters gene expression
- Functions: Regulate mood, metabolism, circadian rhythm, stress response, and basal metabolic rate (via T₃/T₄)
Peptide and (Glyco)Protein Hormones
- Structure: Chains of amino acids
- Secreted by: Hypothalamus, pituitary, thyroid (calcitonin)
- Solubility: Hydrophilic, travel freely in blood
- Receptor location: Extracellular (cell surface)
- Storage: Made ahead of time, stored in vesicles, released by exocytosis when stimulated
Example: Insulin
- Synthesized as preproinsulin
- Converted to proinsulin in the ER
- Connecting peptide removed in Golgi to form active insulin
- Stored in vesicles and released when blood glucose increases
Key point: Pre-made, fast-acting hormones that act via surface receptors.
Hormonal Stimulation
- Definition: One hormone stimulates the release of another hormone.
- Mechanism: Tropic hormones from one gland trigger hormone release from a second gland.
- Example: Hypothalamus → anterior pituitary → thyroid/adrenal/gonads.
Humoral Stimulation
- Definition: Hormone release in response to changes in blood levels of ions or nutrients.
- Mechanism: Endocrine cells detect blood composition (e.g., calcium, glucose) and respond.
- Example: Low Ca²⁺ → parathyroid gland releases PTH.
Neural Stimulation
- Definition: Nerve fibers stimulate endocrine cells to release hormones.
- Mechanism: Usually part of sympathetic nervous system.
- Example: Preganglionic neuron → adrenal medulla → releases epinephrine/norepinephrine.
Hormone Transport in Blood
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Peptides & Most Monoamines:
- Hydrophilic
- Freely circulate in plasma
- Some (e.g., GH) bind carrier proteins for stability
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Steroid & Thyroid Hormones:
- Hydrophobic
- Bind to transport proteins (e.g., albumins, globulins)
- Bound hormones = longer half-life
- Protected from liver enzymes & kidney filtration
- Only free hormone is biologically active (can exit capillaries)
Hydrophilic Hormone Action
- Receptor Location: Cell surface
- Mechanism: Cannot enter target cell; binds to membrane receptor
- Signal Cascade: Triggers a 2nd messenger system (e.g., cAMP or Ca²⁺)
- Result: Activates enzymes or other intracellular processes (e.g., epinephrine increases cAMP in liver to promote glycogen breakdown)
- Speed: Fast onset, short duration
- Hormone Classes: Peptides and most monoamines
Hydrophobic Hormone Action
- Receptor Location: Intracellular (cytoplasm or nucleus)
- Mechanism: Diffuses through membrane, binds intracellular receptor
- Signal Cascade: Hormone-receptor complex binds DNA → alters gene transcription
- Result: Protein synthesis → long-term metabolic changes (e.g., cortisol alters glucose metabolism)
- Speed: Slower onset, longer duration
- Hormone Classes: Steroids and thyroid hormones
Hormone Signal Amplification
- Definition: A single hormone molecule triggers a cascade that massively amplifies its effect inside the cell.
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Mechanism (Hydrophilic hormones):
- Binds membrane receptor → activates G-proteins
- G-proteins activate adenylate cyclase → produces 1000s of cAMP
- cAMP activates protein kinases → activate 1000s of enzymes
- Final result: enormous metabolic change (e.g., epinephrine → glycogen breakdown in liver)
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Hydrophobic hormones:
- Do not rely on amplification via 2nd messengers
- Instead, act directly on gene transcription, producing fewer but longer-lasting effects
- Key difference: Hydrophilic hormones amplify via cascade; hydrophobic hormones act at the DNA level with no amplification chain
Modulation of Target-Cell Sensitivity
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Up-regulation:
- Increases number of receptors on target cell
- Increases cell’s sensitivity to a hormone
- Occurs when hormone levels are low or when more responsiveness is needed
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Down-regulation:
- Decreases number of receptors on target cell
- Decreases sensitivity to hormone
- Common with prolonged high hormone levels (e.g., insulin resistance: receptors down-regulate after chronic high insulin levels)
Hormone Interactions
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Synergistic effects:
- Two or more hormones act together to produce a stronger response
- Example: Testosterone + FSH → high sperm production
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Permissive effects:
- One hormone enhances the target cell’s response to a second hormone
- Example: Estrogen stimulates production of progesterone receptors in uterus
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Antagonistic effects:
- One hormone opposes the action of another
- Example: Insulin promotes glucose uptake, glucagon promotes glucose release
Hormone Clearance
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Hormones must be inactivated once they’ve served their purpose
- Broken down by enzymes in plasma or interstitial fluid
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Most are degraded by the liver and kidneys
- Then excreted in bile or urine
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Metabolic Clearance Rate (MCR)
- Measures how quickly hormones are removed from the blood
- Half-life = time to remove 50% of hormone
- Higher MCR → shorter half-life
Regulation of Hormone Secretion
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Feedback Loops
- Negative feedback (most common): hormone secretion is inhibited once levels are sufficient
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Positive feedback: original hormone release triggers more hormone
- Example: oxytocin during childbirth
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Control by higher brain centers
- Hypothalamus can influence hormone release via the autonomic nervous system (ANS)
- Example: baby’s cry triggers milk ejection reflex
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Pathology
- Can occur due to hypersecretion or hyposecretion
Hypothalamus and Pituitary Gland Overview
- Hypothalamus: Master endocrine gland; regulates the anterior pituitary via tropic hormones.
- Infundibulum: Stalk that connects the hypothalamus to the pituitary gland.
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Anterior pituitary regulation:
- TRH → TSH → thyroid
- CRH → ACTH → adrenal cortex
- GnRH → FSH/LH → gonads
- GHRH → GH → liver (→ IGF)
- PIH → inhibits PRL
- Somatostatin → inhibits GH and TSH
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Posterior pituitary hormones:
- Oxytocin (OT) → made by paraventricular nucleus of hypothalamus
- Antidiuretic hormone (ADH/vasopressin) → made by supraoptic nucleus of hypothalamus
- Both are stored and released by the posterior pituitary (not synthesized there)
Tropic Hormones of the Anterior Pituitary
- Definition: Tropic hormones stimulate other endocrine glands to release hormones.
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Examples:
- TSH → stimulates thyroid (→ T3 and T4)
- ACTH → stimulates adrenal cortex (→ cortisol, corticosterone)
- FSH/LH → stimulate testes (→ testosterone) and ovaries (→ estrogen, progesterone)
- GH → stimulates liver to release IGF (growth effects on bones, muscles)
- PRL → stimulates milk production in mammary glands
- MSH → stimulates melanocytes for pigment regulation
- Note: Posterior pituitary hormones (OT, ADH) are not tropic; they act directly on targets. They are made in hypothalamus and stored in posterior pituitary
Embryonic Origin of Pituitary Gland
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Anterior pituitary (adenohypophysis)
- Derived from an outgrowth of pharyngeal epithelium (Rathke’s pouch)
- Glandular tissue in origin
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Posterior pituitary (neurohypophysis)
- Derived from neural tissue
- Outgrowth from hypothalamus (infundibulum)
- Retains neural connection and function (stores/releases hormones from hypothalamus)
hypophyseal portal system
Definition:
- A specialized blood vessel network connecting the hypothalamus to the anterior pituitary gland.
- Consists of two capillary beds connected by portal venules.
Function:
- Allows hypothalamic releasing and inhibiting hormones to travel directly to the anterior pituitary without entering systemic circulation. The hypothalamic hormones enter the bloodstream locally through the primary capillary bed in the hypothalamus, but instead of circulating through the whole body, they are shuttled directly to the anterior pituitary via portal venules.
- Enables fast, targeted regulation of anterior pituitary hormone secretion.
Key Hormones Involved:
- Releasing hormones: TRH, CRH, GnRH, GHRH
- Inhibiting hormones: PIH (dopamine), Somatostatin
Importance:
- Ensures that anterior pituitary can respond quickly and specifically to signals from the hypothalamus.
- Critical for maintaining hormonal control over thyroid, adrenal cortex, gonads, and growth processes.
Anterior Pituitary Hormones
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ACTH (Adrenocorticotropic hormone)
- Target: Adrenal cortex
- Function: Stimulates secretion of corticosteroids (regulate glucose, fat, protein metabolism)
- Note: Tropic hormone, stress response
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TSH (Thyroid-stimulating hormone)
- Target: Thyroid gland
- Function: Stimulates thyroid growth and TH secretion
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FSH (Follicle-stimulating hormone)
- Target: Ovaries/testes
- Function: Stimulates production of eggs or sperm
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LH (Luteinizing hormone)
- Target: Ovaries/testes
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Function:
- Females: triggers ovulation and secretion of estrogen/progesterone
- Males: stimulates testosterone secretion
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PRL (Prolactin)
- Target: Mammary glands (females), testes (males)
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Function:
- Females: milk synthesis after delivery
- Males: ↑ LH sensitivity → ↑ testosterone
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GH (Growth hormone)
- Target: Most tissues
- Function: Promotes tissue growth via mitosis and cellular differentiation
Growth Hormone (GH): Overview and Secretion Patterns
- Function: Stimulates bone and tissue growth, remodeling, and cellular metabolism
- Secretion: Peaks during first 2 hours of sleep and with vigorous exercise
- Age: Declines with age (avg. 6 ng/mL in adolescence, 1.5 ng/mL in old age)
- Decline Effects: ↓ protein synthesis → tissue aging, ↓ bone and muscle mass, ↑ fat
Growth Hormone: Mechanism and IGF Role
- Direct effects: Stimulates mitosis, differentiation, and metabolic activity
- Liver activation: GH stimulates liver to release IGF-I and IGF-II
- IGF actions: Promote tissue growth and prolong GH effects
- Half-life: GH = 6–20 min; IGF-I = ~20 hours
Metabolic Functions of Growth Hormone
- Protein metabolism: ↑ DNA transcription, mRNA, amino acid uptake; ↓ protein breakdown
- Lipid metabolism: ↑ lipolysis by adipocytes (protein-sparing effect)
- Carbohydrate metabolism: Glucose-sparing effect → ↑ fatty acid use, ↓ glucose use by most cells (reserving it for brain)
- Electrolyte balance: Promotes Na⁺, K⁺, Cl⁻ retention in kidneys and ↑ Ca²⁺ absorption in intestines
Posterior Pituitary (Neurohypophysis)
- Type of tissue: Composed of nervous tissue, not glandular. It is an extension of the hypothalamus.
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Hormone synthesis: Hormones are not synthesized in the posterior pituitary, but in the hypothalamus:
- Oxytocin is produced by the paraventricular nucleus
- ADH (antidiuretic hormone/vasopressin) is produced by the supraoptic nucleus
- Transport: These hormones travel from the hypothalamus to the posterior pituitary via axonal transport along neurons.
- Storage and release: Hormones are stored in axon terminals within the posterior pituitary and released into the bloodstream when triggered by neural signals.
- Function: Acts as a neuroendocrine release site, connecting the nervous system to the endocrine system by secreting hormones into the blood in response to neuronal signals.
ADH (Antidiuretic Hormone / Vasopressin)
- Source: Produced in the supraoptic nucleus of the hypothalamus
- Transport: Delivered via axonal transport through the infundibulum to the posterior pituitary
- Target: Kidneys, sweat glands, arterioles
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Actions:
- Increases water retention by kidneys → ↓ urine output
- Stimulates vasoconstriction at high levels → ↑ blood pressure
- Reduces water loss through sweat
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Trigger (neuroendocrine reflex):
- Dehydration → osmoreceptors in hypothalamus activate → ADH released
- Overhydration → osmoreceptors inhibited → ADH secretion stops
- Clinical note: Also called vasopressin for its vasoconstrictor effect
- Additional: Osmoreceptors are located in circumventricular organs, which lack a BBB and monitor blood osmolarity
Oxytocin (OT)
- Source: Produced in the paraventricular nucleus of the hypothalamus
- Transport: Delivered via axonal transport to the posterior pituitary
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Actions:
- Stimulates uterine contractions during labor (positive feedback loop)
- Triggers milk ejection (let-down reflex) during lactation
- Promotes emotional bonding and trust between partners
- Released during sexual arousal and orgasm
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Feedback: Uses a positive feedback loop during childbirth:
- Baby’s head stretches cervix
- Nerve signals sent to brain
- Brain releases oxytocin
- Uterus contracts harder → cycle continues until birth
- Nickname: Known as the “love hormone” due to its role in bonding and affection
