The Endocrine System

Hormones are secreted into the blood byendocrine glands

See table 1- some endocrine organs arespecialized for hormone secretion

Some (e.g., skin, stomach, liver,etc.) haveadditional functions

(specialized)

What is a hormone?

Small molecule that affects metabolism of targetorgan

Amines- from tyrosine and tryptophanadrenal medulla, thyroid, pineal glands

Polypeptides and proteinsmore than 100 amino acidsexample: growth hormone

Glycoproteinslarge chain of amino acids with carbohydratesattached (FSH and LH)

Steroidsderived from cholesterolprogesterone, cortisol, testosterone(only by adrenal gland and gonads)

Some are synthesized as a precursor and lateractivated

Some are polar, some lipid-soluble- these can passthrough cell membranes if small enough

Endocrine vs neural control

A lot of overlapsome polypeptides are hormones ANDneurotransmitters

Neurotransmitters do not travel in the blood,but across a synaptic cleft

Neural control generally considered faster-acting

Common requirementstarget cells must have specific receptorsfor the hormone/neurotransmitter

binding to receptor must trigger specificchanges in the target cell

mechanism for quickly switching offthe activity (removal or inactivationof the hormone/neurotransmitter)

Effects of hormones

One tissue can respond to many hormones

Synergistic effect- hormones work together toproduce a result

Example: epinephrine and norepinephrine havean additive effect to increase heart rate

FSH and testosterone have complementaryeffect on sperm production

Permissive effect- one hormone enhances theeffect of another

Glucocorticoids enhance effectiveness ofcatecholamines (epinephrine andnorepinephrine)

Antagonist effect- one hormone opposes theeffect of another

insulin promotes fat formationglucagon promotes fat breakdown

Modulation of hormone effect

Hormones do not usually accumulate in bloodhalf-life usually several hours; usuallydeactivated in liver

Concentration is important

Physiological range- normal activityPharmacological (high) range

may affect other cellsmay affect other hormone levels

Priminghormone bindsmore receptors synthesized

more hormone can bind cell

Downregulation- prolonged exposure to highhormone levels can reduce receptorexpression. Cells are therefore desensitized

Some hormones are therefore secreted in spurts

Mechanisms of hormone action

Lipophilic (steroids and thyroxine) pass throughmembranebind receptors inside target cells

in cytoplasm or nucleus

Water-soluble hormones can’t pass throughmembranebond to receptors on cell (membrane)surface

“Second messenger” activation requiredadenylate cyclase-cAMPphospholipase C- calcium

Some important endocrine glands

Pituitary glandanterior lobe secretes its own hormones

regulated by hypothalamusfeedback control

posterior lobe- neural tissuestores and releases products ofhypothalamus

Adrenal glands- paired organs that sit “on top”of the kidneys

Outer cortex and inner medulla have different functions

Medulla- catecholamines (epinephrine,norepinephrine)stimulated by sympathetic nerves

Cortex- controlled by ACTH from anteriorpituitary

Cortex secretes steroid hormones(corticosteroids)

Mineralocorticoids- regulate Na and K balancealdosterone

Glucocorticoids- regulate glucose metabolismcortisol

Androgens, supplement sex steroids secretedby gonads

Adrenal medulla- innervated by sympatheticnervous system

“Fight or flight”rise in blood glucoserise in blood fatty acids

Sustained stress- general adaptation syndrome1. Alarm2. Resistance3. Exhaustion

Thyroid and parathyroid glands

Thyroglobulin+ iodine = thyroxine (T4) and triiodothyronine (T3)

Released from precursor (thyroxine) throughaction of TSH

Protein synthesisMaturation of nervous systemIncrease rate of cell respiration

Calcitoninreleased by parafollicular cellsworks with parathyroid hormone

inhibits dissolution of bonestimulates excretion of calcium in urine

(lowers blood calcium levels)

Thyroid diseases

Iodine deficiency goitercan’t make enough T3 and T4no inhibition of TSHstimulates abnormal thyroid growth

Hypothyroidlow metabolic rate; inability to adapt tocoldmyxedema (swelling) in adults

Lots of possible causes for hypothyroidism

Lack of:thyrotropin-releasing hormone from hypothalamusinsufficient TSH from pituitaryiodine deficiency (goiter)

HyperthyroidGraves’ disease; tumorsmetabolic rate is too highirritability; intolerance of heathigh blood pressure

See Table 11.8 for comparison

Children with thyroxine deficiencieswill lack normal gowth and nervous systemdevelopment (cretinism)

(lack of growth hormone does not affectintelligence)

Immediate treatment with thyroxine willrestore intelligence

Children are now routinely tested at birthfor thyroid function

Review other hormones for:where produced and what are targetorgans/tissues

how regulated (feedback, pituitary, etc.)

disorders associated with overproductionor underproduction