Endocrine Glands
Thyroid Gland
- 2 lobes of endocrine tissue joined in middle located over trachea below larynx
- follicular cells and C cells secrete thyroid hormones and calcitonin
Function:
t3 + t4: metabolism, mood, body temp
calcitonin: reduces calcium in body (inhibits osteoclasts)
Follicular Cells
- functional unti formed into hollow spheres
- lumen filled with colloid that stores thyroid horms
- produces iodine containing horms from tyrosine (t3, t4)
Thyroid synthesis
Occurs on thyroglobulin molecules in colloid
1. tyrosine with thyroglobulin exported from follicular cells into colloid by exocytosis
2. thyroid captures iodine from blood and transfers it into colloid by iodine pump
3. within colloid, iodine attaches to tyrosine
4. coupling occurs between iodinaated tyrosine molecules to form thyroid horms
Thyroid Hormone Storage
- remain in colloid until splitting and secreted
- enough thyroid horm stored to supply body's need for several months
Thyroid Hormone Secretion
- follicular cells phagocytize thyroglobulin-laden colloid
- process frees T3. and T4 diffusing across plasma membrane into blood
- most prod is t4 converted to T3
Thyroglobulin-Laden Colloid
- secretion of thyroid needs follicular cells to bite off a piece of colloid
- breaking thyroglobulin into free t3 and t4 into blood
- thyroglobulin with colloid taken by follicular cells via phagocytosis
Follicular Cell Phagocytosis of Thyroglobulin
1. lysosomes fuse with vesicles and release digestive enzymes releasing t3, with iodotyrosines
2. thyroid horms pass freely through outer membranes of follicular cells and into blood
3. once release into blood, high lipphilic thyroid horm molecule quickly bind with sevreal plasma porteins
4. t3,4 is moved by throxine-binding globulin
Thryoid Hormone Actions
Sympathomimetic Effect
- increases target-cell responsiveness to catecholamines
Cardiovascular Effect
- increases heart rate and force of contraction
Neural Effect
- normal growth and development of NS
SKM Effect
- increased muscle size, Ca uptake and shortening velocity
Hypothalamus-Pituitary-Thyroid Axis
- regulated by negative-feedback system between hypothalamic TRH, anterior pituitary TSH, and thyroid gland T3,4
- feedback loop maintains constant output of thyoid homrs
Adrenal Glands
- embedded above each kidney in capsule of fat
adrenal cortex: outer portion
- zona glomerulosa (aldosterone)
- zona fasciculata (cortisol)
- zona reticularis (DHEA)
Adrenal Medulla: inner portion
- medulla (catecholamines)
Zona Glomerulosa
Mineralocorticoids
- aldosterone
- influence mineral balance, Na and. K.
- action sites on distal and collecting tubules of kidneys
Secretion Increased by:
- Activation of RAAS by reduction in Na and BP
- direct stim of adrenal cortex by rise in plasma K concentration
Zona Fasciculata
Glucocorticoids
- cortisol
Primary Metabolic Effects
- stim hepatic gluneogenesis
- inhibits glucose uptake and use by many tissues but not brain
- stims protein degradation in many tissues like muscle
- facilitates liplysis
- adaptation to stress
- diurnal rhythm
Cortisol Secretion
- activated by stress and dirunal rhythm
- regulated by neg-feedback with hypthalamus CRH and anterior pituitary ACTH
- ACTH stims adrenal cortex to secrete cortisol
Zona Reticularis
Sex Hormones
- adrenal cortex secretes same sex horms as gonads
- androgens, estrogens and progesterone
Dehydroepiadrosterone
- only adrenal sex hormone
Male:
- converted to a number of androgens but overpowered by testicular testosterone
Female;
- growth of pubic and axillary hair
- enhanement of pubertal growth spurt
- development and maintenance of semale sex drive
- 20% total test prod
Adrenal Medulla
- modified part of SNS
- primary stim for increased adrenomedullary secretion act of SNS by stress
Storage and Release of Catecholamines
1. syn within cytosol of adrenomedullar secretory cells
2. catecholamines stored in chromaffin granules
3. carecholamines secreted into blood by exocytosis of chromaffin granules
4. E and NE releaed by adrenla medulla at same time
Adrenal Medulla Sympathetic Stimulation
- controlled by sympathetic input to adrenal gland
- SNS is activated under fear or stress, triggering a surge of adrenomedullary catecholamine release
Epinephrine and SNS
- fortifies SNS mounting general fight or flight responses
Organ Effects:
- increases cardiac output, maintains ABP, dilates respiratory airways
Metabolic Effects:
- increases blood glucose and FA
Neural Effects:
- promote state of arousal and increases CNS alertness
Anabolism
- buildup or syn of larger organic macromolecules from small organic subunits
- requires ATP
Result:
- manufacture of materials needed by cell
- storage of excess ingested nutrients not immediately needed for energy prod
Catabolism
- brekdown of large, energy-rich organic molecules in cells
2 levels:
1. hydrolysis of large cellular molecules into smaller units
2. oxidation of smaller units to yield energy for ATP prod
Interconversion
- transformation of substrate by enzyme to a product in a metabolic pathway (glucose to glycogen)
- mostly occurs in liver
- essential nutrients
- nutrients must be stored for use between meals
Excess circulating glucose:
- stored in liver and muscle as glycogen
- additional glucose is made into FA and glycerol stored in adipose tissue
Excess circulating FA:
- incorporated into triglycerides
Excess circulating AA:
- converted to glucose and FA
Brain and Glucose
- brain relies on blood glucose for energy
- glucose levels must be maintained above some level
- liver glycogen maintains glucose levels
Absorptive State
Fed state
1. carbs absorbed as simple sugars sent to liver; glucose
2. glucose travels in blood to be used as energy for cells or converted to glycogen or fat
Postabsoriptive State
Fasting State
1. protein and fat syn curtaileld as body enters catabolic state
2. endogenous energy stores mobilized to provide energy
Insulin and Glucagon
- important for regulating fuel metabolism
- produced in pancreas
Islets of langerhans
- type of endocrine cell
Beta cells:
- site of insulin syn and secre
Alpha cells:
- prod glucagon
Delta cells:
- pancreatic site of somatostatin syn
PP cells:
- least common
- secrete pancreatic polypeptide
- modulate digestion of food by inhibition of gastric emptying
Insulin: Actions on blood glucose concentration
- promo cellular uptake of glucose, FA and AA
- enhances their conversion into glycogen, TG and proteins
- lowers blood concen of molecules
- secretion increased during absorptive state
- prime stim for scretion = increase in blood glucose concen
Insulin: Actions on Fat
Signals body to lower bloody FA + store TG:
1. enhances entry of FA from blood to adipose
2. increases transport of glucose into adipose
3. uses glucose as precursor for FA and glycerol
4. promos chemical reactions that use FA + glucose for TG syn
5. inihibits lipolysis, reducing release of FA from adipose into blood
Insulin: Actions on Protein
Signals body to lower blood AA lvls and enhances protein Syn:
1. promos active transport of AA from blood to muscles
2. increases rate of AA incorporation into protein stiming syn
3. inihibits protein degradation
Regulation of Blood Glucose Lvls
- primary control of insulin secretion is through direct negative feedback
- between pancreatic beta cells
Glucagon and Insulin
- both insulin and glucagon control metabolic adjustments between the absorptive and postabsorptive states
- Glucagon affects many of the same metabolic processes that insulin influences
Glucagon: Actions on Tissues
CARBS:
- increased hepatic glucose prod + release
- increase blood glucose
Fat:
- antagonizes actions of insuling
- promo fat breakdown + inhibiting TG syn
Protein:
- inhibits heptic protein syn
- promo heptic protein degradation
- promos protein catabolism in liver