Endocrine System Part 1

Long term regulation

Relays information and instruction between cells through chemical messengers.




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What are the components of the endocrine system?

Target Cells

Are specific cells that possess receptors needed to bind and “read” hormonal messages


Stimulate synthesis of enzymes or structural proteins

Increase or decrease rate of synthesis

Turn existing enzyme or membrane channel “on” or “off”

What are hormones?

Classes of Hormones: Hormones can be divided into three groups

1. Amino acid derivatives

2. Peptide hormones

3. Lipid derivatives

How do they get around?

Secretion and Distribution of Hormones

Hormones circulate freely or travel bound to special carrier proteins

Types of intercellular communication

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How do hormones act?

Free Hormones- Remain functional for less than 1 hour

1.Diffuse out of bloodstream and bind to receptors on target cells

2.Are broken down and absorbed by cells of liver or kidneys

3.Are broken down by enzymes in plasma or interstitial fluids

Thyroid and Steroid Hormones - Remain in circulation much longer because most are “bound”

1. Enter bloodstream

2. More than 99 percent become attached to special transport proteins

3. Bloodstream contains substantial reserve of bound hormones

Mechanismsm of a hormone

Hormone Receptor
Is a protein molecule to which a particular molecule binds strongly
Responds to several different hormones
Different tissues have different combinations of receptors
Presence or absence of specific receptor determines hormonal sensitivity

Solubility matters!

Catecholamines and Peptide Hormones

Are not lipid soluble

Unable to penetrate plasma membrane

Bind to receptor proteins at outer surface of plasma membrane (extracellular receptors)


Are lipid soluble

Diffuse across plasma membrane to reach receptor proteins on inner surface of plasma membrane (intracellular receptors)

Steroids (lipid soluable)

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Thyroid hormone (not lipid soluable or hydrophillic)

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How hormones influence - First and Second Messengers

First messenger:

Bind to receptors in plasma membrane

Cannot have direct effect on activities inside target cell

Use intracellular intermediary to exert effects

Second messenger:

May act as enzyme activator, inhibitor, or cofactor

Results in change in rates of metabolic reactions

examples of second messengers

1.Cyclic-AMP (cAMP)- Derivative of ATP

2. Cyclic-GMP (cGMP)- Derivative of GTP

3. Calcium ions

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How binding effects cAMP levels

First messenger binds to the receptor in the plasma membrane and activates a G protein,

an enzyme complex coupled to membrane receptor.

Two possible results:

1. The activated G protein uses ATP to increase the production of cAMP (second messenger) which increases metabolic activity.

2. The cell may suppress the production of cAMP, which will decrease metabolic activity

Effects on Ca2+ levels

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Overall effect of Hormones

Alter rate of DNA transcription in nucleus

Change patterns of protein synthesis

Directly affect metabolic activity and structure of target cell

Include steroids and thyroid hormones

The process of amplification

Is the binding of a small number of hormone molecules to membrane receptors

Leads to thousands of second messengers in cell

Magnifies effect of hormone on target cell

Up regulation vs Down regulation


Absence of a hormone triggers increase in number of hormone receptors

When levels of particular hormone are low, cells become more sensitive to it


Presence of a hormone triggers decrease in number of hormone receptors

When levels of particular hormone are high, cells become less sensitive to it

Hormones act with other hormones

When a cell receives instructions from two hormones at the same time, four outcomes are possible

1. Antagonistic effects – opposing

2. Synergistic effects – additive

3. Permissive effects – one hormone is necessary
for another to produce effect

4. Integrative effects – hormones produce
different and complementary results

Endocrine Reflexes

Functional counterparts of neural reflexes

In most cases, controlled by negative feedback mechanisms

Stimulus triggers production of hormone; the direct or indirect effects of the hormone reduce intensity of the stimulus

Triggered by:

1.Humoral stimuli

Changes in composition of extracellular fluid

2. Hormonal stimuli

Arrival or removal of specific hormone

3. Neural stimuli

Arrival of neurotransmitters at neuroglandular junctions

Endocrine reflexes, cont.

1. Simple Endocrine Reflex

•Involves only one hormone

•Controls hormone secretion by the heart, pancreas, parathyroid gland, and digestive tract

2. Complex Endocrine Reflex

•One or more intermediary steps

•Two or more hormones

3. The hypothalamus provides highest level of endocrine control

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Neuroendocrine Reflexes - Pathways include both neural and endocrine components

Complex Commands issued by changing:

Amount of hormone secreted

Pattern of hormone release

Hypothalamic and pituitary hormones released in sudden bursts

Frequency changes response of target cells