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Endocrine > Anatomy/Phys - Endo > Flashcards

Anatomy/Phys - Endo Flashcards

1
Q

Chemical Messengers

A

Neurotransmitters: released by axon terminals of neurons and act locally

Endocrine hormones: released by glands or specialized cells into the circulating blood and acts on target cells at another location – distant organ

Neuroendocrine hormones: secreted by neurons into the circulating blood and acts on target cells at another location

Paracrines: secreted by cells into the ECF and affect neighboring target cells other than those that secrete them– close by

Autocrines: secreted by cells into the ECF affect the same cells that secrete them

Cytokines: peptides secreted by cells into the ECF and can function as autocrines, paracrines, or endocrine hormones. (eg: interleukins, lymphokines, leptin)

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2
Q

Three main classes of hormones

A
  1. Proteins and polypeptides
    - Hormones secreted by hypothalamus, pituitary gland, pancreas, parathyroid etc.
  2. Amino acid tyrosine-based hormones
    - Hormones secreted by thyroid and adrenal medulla – epinephrine and norepinephrine
  3. Steroids
    - Synthesized from cholesterol
    - Secreted by gonads, adrenal cortex, placenta
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3
Q

TRANSPORT OF HORMONES

A
  • Water- soluble hormones (peptides and catecholamines) are dissolved in the plasma and transported from their sites of synthesis to target tissues
  • Lipid-soluble hormones (Steroid and thyroid hormones), circulate in the blood mainly bound to plasma proteins – globulins (nearly 99%)
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4
Q

Mechanisms of Hormone Action

A
  1. Water-soluble hormones (peptide hormones and biogenic amines except thyroid hormone)
    - Cannot enter the target cells/cannot cross cell memrbane
    - Act on plasma membrane receptors
    - Coupled by G proteins to intracellular second messengers that mediate the target cell’s response
    - Preformed enzymes
    - Shorter amount of time than lipid soluble
  2. Lipid-soluble hormones (steroid and thyroid hormones)
    - Cross the cell membrane easily
    - Act on intracellular receptors that directly activate genes
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5
Q

Water-Soluble Hormones

peptides and catecholamines

A

-Receptor: Outer surface of cell membrane
-Intracellular action: Production of second messengers, e.g., cAMP, Fast acting
-Storage: Stored in vesicles
-Plasma transport: Dissolved in plasma
(free, unbound), Exception: Insulin like growth factor
-Half-life: Short (minutes)

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6
Q

Lipid-Soluble Hormones

steroids, thyroid hormones

A
  • Receptor: Inside the cell, usually in the nucleus
  • Intracellular action: Stimulates the synthesis of specific new proteins, Slow acting
  • Storage: Synthesized as needed, Exception: thyroid hormones
  • Plasma transport:Attached to proteins that serve as carriers, Exceptions: adrenal androgens
  • Half-life: Long (hrs,days)
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7
Q

The Pituitary Gland

A
  • Lies in the sella turcica of the base of the skull
  • Connected to the hypothalamus by the pituitary/hypophyseal stalk
  • The pituitary gland (hypophysis) has two major lobes
    1. Posterior pituitary lobe (neurohypophysis):
  • Pituicytes (glial-like supporting cells) and nerve fibers
  • Extension of ectoderm that forms brain
    2. Anterior pituitary lobe (adenohypophysis)
  • Glandular tissue
  • Extension of roof of oral cavity
  • Ratheke’s pouch

-Between these is a small, relatively avascular zone called the pars intermedia. Produces melanocyte stimulating hormone

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8
Q

Anterior Pituitary

A
  • Originates as an out-pocketing of the oral mucosa (Rathke’s pouch*)
  • Connected to the Hypothalamus by the infundibulum
  • Regulated by the hypothalamus via releasing and inhibiting hormones that are transported in the hypophyseal portal system
  • Five cell types can be differentiated
  • Secretes six important peptide hormones
    1. Somatotropes secrete: human growth hormone (GH)
    2. Thyrotropes: Thyroid-stimulating hormone (TSH) or thyrotropin
    3. Corticotropes: Adrenocorticotropic hormone (ACTH)
    4. Gonadotropes: Follicle-stimulating hormone (FSH)
    5. Gonadotropes: Luteinizing hormone (LH)
    6. Lactotropes: Prolactin (PRL)
  • All are water soluble hormones
  • TSH, ACTH, FSH, and LH are all tropic hormones (regulate the secretory action of other endocrine glands)
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9
Q

Growth Hormone (GH)

A
  • Produced by somatotrophs
  • Stimulates most cells, but targets bone and skeletal muscle
  • Promotes protein synthesis and encourages use of fats for fuel
  • Most effects are mediated indirectly by insulin-like growth factors (IGFs)
  • Stimulates liver, skeletal muscle, bone, and cartilage to produce insulin-like growth factors
  • Mobilizes fats, elevates blood glucose by decreasing glucose uptake and encouraging glycogen breakdown (anti-insulin effect of GH)
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10
Q

insulin-like growth factor

A
  • Are tightly bound to proteins hence half- life is long (20hrs)
  • Serves as a better reflection of 24hr GH secretion, than GH itself as GH is secreted in pulses and mainly at night
  • Exerts its anabolic effect by increasing the synthesis of cartilage (chondrogenesis) in the epiphyseal plates of long bones; thereby increasing bone length
  • Increases lean body mass, hence with aging the decrease in IGFs lead to decrease in lean body mass
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11
Q

Regulation of GH secretion

A

Stimulate Growth Hormone Secretion

  • hypoglycemia
  • decreased blood free fatty acdis
  • exercise
  • starvation or fasting, protein def
  • trauma, stress, excitement
  • androgens during puberty
  • growth hormone-releasing hormone

Inhibit Growth Hormone Secretion

  • hyperglycemia
  • increased blood free fatty acids
  • aging
  • obesity
  • growth hormone inhibitory hormone (somatostatin)
  • IGFs
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12
Q

Thyroid-Stimulating Hormone (Thyrotropin)

A
  • Produced by thyrotrophs of the anterior pituitary
  • Stimulates the normal development and secretory activity of the thyroid

Regulation of TSH release

  • Stimulated by thyrotropin-releasing hormone (TRH)
  • Inhibited by rising blood levels of thyroid hormones that act on the pituitary and hypothalamus
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13
Q

Adrenocorticotropic Hormone (Corticotropin)

A
  • Secreted by corticotrophs of the anterior pituitary
  • Stimulates the adrenal cortex to release glucocorticoids and androgens

Regulation of ACTH release

  • Triggered by hypothalamic corticotropin-releasing hormone (CRH) in a daily rhythm
  • Internal and external factors such as fever, hypoglycemia, and stressors can alter the release of CRH
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14
Q

Gonadotropins

A
  • Follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
  • Secreted by gonadotrophs of the anterior pituitary
  • FSH stimulates gamete (egg or sperm) production
  • LH promotes production of gonadal hormones
  • Absent from the blood in prepubertal boys and girls

Regulation of gonadotropin release

  • Triggered by the gonadotropin-releasing hormone (GnRH) during and after puberty
  • Suppressed by gonadal hormones (feedback)
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15
Q

Prolactin (PRL)

A
  • Secreted by lactotrophs of the anterior pituitary
  • Stimulates milk production
  • Regulation of PRL release: Primarily controlled by prolactin-inhibiting hormone (PIH) (dopamine)
  • Blood levels rise toward the end of pregnancy
  • Returns to nonpregnant level few weeks post partum
  • Suckling stimulates PRL release and promotes continued milk production
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16
Q

The Posterior Pituitary

A
  • A 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
  • Contains axons of hypothalamic neurons
  • Stores antidiuretic hormone (ADH) and oxytocin
  • ADH and oxytocin are released in response to nerve impulses
  • Not synthesized in Pituitary
17
Q

Oxytocin

A
  • Stimulates uterine contractions during childbirth
  • Also triggers milk ejection (“letdown” reflex) in women producing milk
  • Plays a role in sexual arousal and orgasm in males and females
18
Q

Antidiuretic Hormone (ADH)-Action

A
  • The main target tissue is the renal collecting duct
  • ADH increases the permeability of the duct to water
  • Urea can pass with the water, but electrolytes cannot
19
Q

ADH-Regulation of ECF volume and osmolarity

A

Volume Regulation:

  • Stimuli arising from stretch receptors in arteries and veins chronically inhibit ADH secretion
  • Decrease in blood volume causes loss of this stimuli resulting in loss of the inhibitory affect

Osmoregulation:
-Hypothalamic osmoreceptors respond to changes in the solute concentration of the blood

If solute concentration is high

  • Osmoreceptors depolarize and transmit impulses to hypothalamic neurons
  • ADH is synthesized and released, inhibiting urine formation

If solute concentration is low

  • ADH is not released, allowing water loss
  • Alcohol inhibits ADH release and causes copious urine output
20
Q

Thyroid Gland

A
  • Consists of two lateral lobes connected by a median mass called the isthmus
  • Composed of follicles that produce the glycoprotein thyroglobulin
  • Colloid (thyroglobulin + tyrosine + iodine) fills the lumen of the follicles and is the precursor of thyroid hormone
  • Parafollicular cells produce the hormone calcitonin
21
Q

Thyroid Hormone (TH)

A

Two Hormones:

1) T4 (thyroxine)
2) T3 (triiodothyronine)
- Major metabolic hormone
- Increases metabolic rate and heat production (calorigenic effect)
- Thyroid hormones do not act directly on many tissues but acts by being a permissive hormone, e.g.: Growth hormone functions normally only in presence of thyroid hormones.

Plays a role in

  • Maintenance of blood pressure
  • Regulation of tissue growth
  • Development of skeletal and nervous systems
  • Reproductive capabilities
22
Q

Synthesis and secretion of Thyroid Hormone

A

1) Thyroglobulin Synthesis:
- Thyroglobulin is synthesized in the follicular cells and discharged into the follicle lumen
2) Oxidation of I– to I2 :
- Iodides (I–) are actively taken into the cell, oxidized to iodine (I2) by peroxidase enzymes, and released into the lumen
3) Iodination:
- Iodine attaches to tyrosine, mediated by peroxidase enzymes
4) Coupling:
- Iodinated tyrosines link together to form T3 and T4, mediated by peroxidase enzymes

Secretion:

  1. Endocytosis: Pieces of follicular colloid is endocytosed and taken back into the follicle
  2. Fusion: Endocytosed material fuses with lysosomes
  3. Proteolysis of thyroglobulin: Within the lysosomes, the thyroglobulin is cleaved into T3,T4, DI and MIT
  4. Secretion: T4 and T3 are secreted into the blood, the ratio usually being 20 T4 to 1 T3 (this ratio shifts toward T3 in iodine deficiency).
  5. Deiodination: Microsomal deiodinase removes the iodine from iodinated tyrosines (DIT and MIT). The iodine is reused for hormone synthesis.
23
Q

Transport and Regulation of TH

A
  • T4 and T3 are transported by thyroxine-binding globulins (TBGs)
  • Both bind to target receptors, but T3 is ten times more active than T4
  • Peripheral tissues convert T4 to T3

Negative feedback regulation of TH release

  • Rising TH levels provide negative feedback inhibition on release of TSH
  • Hypothalamic thyrotropin-releasing hormone (TRH) can overcomethe negative feedback during pregnancy or exposure to cold
24
Q

Calcitonin

A
  • Produced by parafollicular (C) cells
  • Antagonist to parathyroid hormone (PTH)

Lowers plasma Ca2+ by-:

  • Inhibiting osteoclast activity and release of Ca2+ from bone matrix
  • Stimulating Ca2+ uptake and incorporation into bone matrix
  • Regulated by a humoral (Ca2+ concentration in the blood) negative feedback mechanism
  • No important role in humans; removal of thyroid (and its C cells) does not affect Ca2+ homeostasis
25
Q

Parathyroid Glands

A
  • Four to eight tiny glands embedded in the posterior aspect of the thyroid
  • Contain oxyphil cells (function unknown) and chief cells that secrete parathyroid hormone (PTH) or parathormone
  • PTH—most important hormone in Ca2+ homeostasis

Increases Blood Calcium level:

  • Stimulates osteoclasts to digest bone matrix
  • Enhances reabsorption of Ca2+ and secretion of phosphate by the kidneys
  • Promotes activation of vitamin D (by the kidneys); increases absorption of Ca2+ by intestinal mucosa

Negative feedback control: rising Ca2+ in the blood inhibits PTH
release

26
Q

Adrenal (Suprarenal) Glands

A
  • Paired, pyramid-shaped organs atop the kidneys
  • Structurally and functionally, they are two glands in one
  • Adrenal cortex—three layers of glandular tissue that synthesize and secrete corticosteroids
  • Adrenal medulla—nervous tissue; part of the sympathetic nervous system
27
Q

Adrenal Cortex

A

Three layers and the corticosteroids produced
G: Zona glomerulosa—mineralocorticoids
F: Zona fasciculata—glucocorticoids
R: Zona reticularis—sex hormones, or gonadocorticoids

28
Q

Mineralocorticoids

A

-Aldosterone is the most potent mineralocorticoid

Regulate electrolytes (primarily Na+ and K+) in ECF

  • Importance of Na+: affects ECF volume, blood volume, blood pressure, levels of other ions
  • Importance of K+: sets RMP* of cells
29
Q

Aldosterone - Action

A
  1. Increases Na+ reabsorption by the principal cells of the late distal tubule and proximal collecting ducts of kidney
  2. Promotes secretion of H+ by the intercalated cells, and K+ secretion by the principal cells
  3. The Na+ conserving action is also seen in salivary ducts, sweat glands and the distal colon
30
Q

Regulation of Aldosterone Secretion

A
  1. Renin-angiotensin mechanism: decreased blood pressure stimulates kidneys to release renin, triggers formation of angiotensin II, a potent stimulator of aldosterone release
  2. Plasma concentration of K+: Increased K+ directly influences the zona glomerulosa cells to release aldosterone
  3. ACTH: is necessary for aldosterone secretion but has little effect in controlling the rate of secretion in most physiological conditions
  4. Increased Na+ concentration in the ECF very slightly decreases aldosterone secretion
  5. Atrial natriuretic peptide (ANP): blocks renin and aldosterone secretion, to decrease blood pressure
31
Q

Glucocorticoids

A
  • Cortisol is the most significant glucocorticoid

- Released in response to ACTH, patterns of eating and activity, and stress

32
Q

Cortisol-Action

A
  1. Metabolic Actions, Promotes mobilization of energy stores specifically:
    - Protein: promotes degradation and increase delivery of amino acids
    - Lipids: promotes lipolysis and increased delivery of free fatty acids and glycerol
    - Carbohydrate: raises blood glucose, making more glucose available for nervous tissue by - Inhibiting glucose uptake in most tissues - Increasing gluconeogenesis—formation of glucose from fats and proteins in the liver
    2) Permissive Actions-
    - On Glucagon : promotes glycogenolysis in the liver( without cortisol fasting hypoglycemia rapidly develops)
    - On Catecholamines : promotes vasoconstriction and bronchodilation( without cortisol, blood pressure decreases)
  • Keeps blood sugar level relatively constant
  • Maintain blood pressure by increasing the action of vasoconstrictors (catecholamines)
33
Q

Gonadocorticoids (Sex Hormones)

A

-Most are androgens (male sex hormones) that are converted to testosterone in tissue cells or estrogens in females

May contribute to

  • The onset of puberty
  • The appearance of secondary sex characteristics
  • Sex drive
34
Q

Adrenal Medulla

A

-Chromaffin cells secrete epinephrine (80%) and norepinephrine (20%)

These hormones cause

  • Blood glucose levels to rise
  • Blood vessels to constrict
  • The heart to beat faster
  • Blood to be diverted to the brain, heart, and skeletal muscle
  • Epinephrine stimulates metabolic activities, bronchial dilation, and blood flow to skeletal muscles and the heart
  • Norepinephrine influences peripheral vasoconstriction and blood pressure

Endocrine (12 decks)

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