Endocrine System Part 1
Long term regulation
Relays information and instruction between cells through chemical messengers.
•Growth
•Development
•Reproduction
What are the components of the endocrine system?
Target Cells
Are specific cells that possess receptors needed to bind and “read” hormonal messages
Hormones
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
Structurally similar to amino acids:
Derivatives of tyrosine
•Thyroid hormones
•Catecholamines
•Epinephrine, norepinephrine
Derivatives of tryptophan
•Dopamine, serotonin, melatonin
Chains of Amino Acids
Chains of amino acids synthesized as prohormones
- Inactive molecules converted to active hormones before or after they are secreted
Glycoproteins Proteins are more than 200 amino acids long and have carbohydrate side chains
•Thyroid-stimulating hormone (TSH)
•Luteinizing hormone (LH)
•Follicle-stimulating hormone (FSH)
Short chain polypeptides
Antidiuretic hormone (ADH) and oxytocin (OXT)
Small proteins
Growth hormone (GH) and prolactin (PRL)
Lipid derived hormones
Eicosanoids
leukotrienes -secondary role as hormones
prostaglandins - coordinate local cellular activity
Steroid hormones - derived from cholsterol, released by the reproductive organs
androgens and estrogens
How do they get around?
Secretion and Distribution of Hormones
Hormones circulate freely or travel bound to special carrier proteins
Types of intercellular communication
Direct:
Paracrine:
Endocrine:
Endocrine cells release chemicals (hormones) into bloodstream that alters metabolic activities of many tissues and organs simultaneously. (Bloodstream)
Synaptic:
Ideal for crisis management
Occurs across synaptic clefts
Chemical message is “neurotransmitter”
Limited to a very specific area (neurotransmitter)
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
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 membraneBind 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)
Thyroid hormone (not lipid soluable or hydrophillic)
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
How binding effects cAMP levels
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
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
Up-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
Down-regulation
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
Neuroendocrine Reflexes - Pathways include both neural and endocrine components
Amount of hormone secreted
Pattern of hormone release
Hypothalamic and pituitary hormones released in sudden bursts
Frequency changes response of target cells