1. INTRODUCTION TO ENDOCRINOLOGY I
ENDOCRINE SYSTEM M321
INTRODUCTION TO ENDOCRINOLOGY I

2. Homeostasis Homeostasis is the property of a system.
Homeostatic system is one in which variables are regulated so that internal conditions remain stable and constant
Human homeostasis is what maintains body's internal environment in response to changes in external conditions
In humans, homeostasis is maintained by the concerted action of nervous and endocrine system

3. Endocrine glands: 1. Hypothalamus 2. Pituitary 3. Epiphysis 4. Thymus
5. Thyroid gland
6. Parathyroid glands
7. Langergans’ islands of pancreas
8. Adrenal glands
9. Gonads 3

4. Hormone Classification
Hormones

5. Hormone Classification
Hormones classified by source

6. Hormones
What are they?
Chemical messengers secreted by specialized cells
From endocrine cells that make up the diffuse endocrine system
From neurons
From immune system cells producing cytokines
Where do they go?
Into blood (most)
Into environment (ectohormones or pheromones)

7. Hormones Involved in Act by Growth Development Metabolism Reproduction
Altering rates of enzyme mediated reactions
Control the movement of molecules across the plasma membrane (Ion channels)
Regulating the rate of gene expression (& therefore protein production)

8. Hormones How do hormones get from point A to B?
To be classified as hormone & not a paracrine or autocrine hormone
Travel in blood
May require water soluble (protein) transport mechanism if hormone is lipid soluble
Hormones act by binding to receptor on target cell surface or inside target cells
Hormones need a mechanism to terminate their effect
Stop/reduce production of hormone
Degrade hormones
Enzymatic removal from receptor
Endocytosis of receptor-hormone complex

9. Hormone
Hormones are mainly classified by their chemical structure

11. Hormone Hormones classified by structure Peptide/protein hormones
Steroid hormones
Amino acid based hormones
Derived from tyrosine and may be
Catecholamines
Thyroid hormones

12. Hormone Classification
Peptide/protein hormones
Classification that includes most hormones
If a hormone is not a steroid hormone or an amino acid derived hormone, then it is a protein/peptide hormone!
Concerns with these hormones
How are they made, stored and released
How are they transported in blood
Their mechanism of action
How long does the signal last?

13. Hormone Classification
Peptide/protein hormones & how they are made, stored and released

14. Communication Hormone Classification
Examples of hormone processing
PreproTSH (thyrotropin releasing hormone) has six copies of the 3-amino acid hormones TRH
Prohormones may contain several peptide sequences that have biological activity
The peptide chain of insulin’s prohormone folds back and is cleaved into insulin & C-peptide

15. Hormone Classification
Peptide/protein hormones
Cellular action mechanism
Water soluble – must bind to receptors on target cell membrane
Work via second messenger system

16. Hormones Steroid Hormones Lipophilic – creates problems No storage
Production is on an “as needed” basis
Can have the precursors in cytoplasm ready to go
Require protein transporters in blood
prolongs duration of hormone
Blocks entrance into cell… it must disengage from carrier
this follows law of mass action…

17. Hormones Steroid hormones Based on cholesterol
Produced using SER in the
Adrenal cortex
Gonads
Placenta
Secretion is by simple diffusion

18. Hormones Steroid Hormones Cellular mechanism of action
Diffuses into cytosol
Bind with nuclear receptors ( a superfamily of transcription factors)
Nuclear receptors are bound with heat shock proteins
Binding of hormone releases heat shock proteins
This allows nuclear receptor to translocate into the nucleus
Nuclear receptors have DNA binding domains (DBDs) that allows them to bind on the DNA on hormone response element (HRE)

19. New(er) research indicates that steroid hormones have membrane receptors and signal transduction pathways similar in process to peptide/protein hormones
This concept is known as Nongenomic action of hormones

20. Steroid hormones
Steroid Hormone Action

21. Hormone Classification
Amino acid derived hormones may be
Derived from tyrosine
Produces
T3 & T4
Epinephrine & norepinephrine

22. Hormone Classification
Amino acid derived hormones may be
Derived from tyrptophan
Produces
Melatonin
Serotonin*
*serotonin is classified as a neurohormone as it is synthesized and secreted by neurons of the GI tract for regulation of motility and CNS

23. Regulation of hormone levels
Hormones are controlled by negative feedback loops!

24. Feedback Loops Hypothalamus Anterior Pituitary Corticotropin
releasing factor
Adrenal
Cortex +
ACTH +
Cortisol