Endocrine System

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What is the difference between exocrine and endocrine glands? Exocrine have ducts (Ex: enzyme); endocrine lack ducts ( uses bloodstream) (hormone-chemical messengers)

In order for hormones to work (elicit their function) they must

-Source must secrete hormone

-must travel through bloodstream

-bind to receptor at target cell

Chemical Classes of Hormones

Lipid Soluble

*Would these be able to cross cell membrane? Yes.

*Would these easily circulate in bloodstream? No. Needs transport protein.

*Receptor location? inside the cell

Specific chemical composition:

*Steroid=estrogen, testosterone, cortisol, aldosterone

*Thyroid=T3 (triiodothyronine) & T4 (thyroxine)

Water soluble

*Would these be able to cross cell membrane? No.

*Would these easily circulate in bloodstream? Yes (no transport protein needed)

*Receptor location? on cell surface

*Specific chemical composition:

-Amine-epinephrine or norepinephrine

-Protein/peptide-hypothalamic or pit. hormone; insulin & glucagon; calcitonin & PTH

Mechanisms for hormone binding to the receptor:

Lipid-Soluble Hormones:

1. Hormone is transported in bloodstream to target via transport protein

2. The hormone leaves bloodstream and binds to the receptor which is located inside of the target cell

3. Hormone/Receptor Complex binds to DNA and alters gene expression (turns genes on or off)

4. DNA is transcribed then translated thus producing proteins

5. New proteins cause the physiological response

Water-Soluble Hormones:

1. Hormone circulates freely in the bloodstream to the target cell

2. Hormone binds to receptor which is located on cell membrane. (1st messenger)

3. G protein activated

4. Adenylate cyclase (enzyme because it ends in -ase) is activated and ATP is converted into cAMP (2nd messenger)

5. cAMP activates protein kinases

6. Protein kinases phosphorylate other enzyme which activates (or inactivates) the enzyme

7. These "New" enzymes (proteins) are responsible for the physiological response

Responsiveness of Target Cell to Hormone Depends On:

-hormone concentration

-Abundance of the receptors

-Up-regulation = up receptor # due to low hormone concentration

-Down-regulation = down receptor # due to high hormone concentration

-affinity for hormone to the receptor

-influence of other hormones

-Synergistic = enhance one another

-Antagonistic = opposing functions

*Possible endocrine stimuli may be:

-Humoral = blood levels (Ex. high blood glucose triggers insulin release)

-Neural = nervous system (Ex. acute stress triggers epinephrine)

-Hormonal = a hormone impacts another (Ex. TRH causes release of TSH)

*Negative Feedback versus Positive Feedback

-like thermostat (constant levels)

-levels increase - source stops production

-levels decrease - source starts production