UNIT 10 PART A: INTRO TO THE ENDOCRINE SYSTEM

Question 1

What is a hormone?

Answer 1

Hormones are chemical signals that are secreted into the blood

Question 2

What secretes hormones?

Answer 2

1. Endocrine glands
2. isolated endocrine cells in other tissues/organs
3. Neurons

Question 3

Endocrine glands

Answer 3

– ductless glands whose only function is to
synthesize and secrete hormones.

Ø E.g.1: the pituitary gland secretes Growth Hormone (amongseveral other hormones)

Question 4

. Isolated endocrine cells in other tissues/organs

Answer 4

Ø E.g.1: the Islets of Langerhans in the pancreas secrete insulin and glucagon to regulate blood sugar

Ø E.g.2: the Leydig cells in the testes secrete testosterone

Question 5

Neurons –

Answer 5

secrete neurohormones that circulate in the
blood

E.g.1: neurons in the hypothalamus secrete several different “releasing hormones”, like Growth Hormone Releasing Hormone (GHRH) that act on the anterior pituitary gland.

Question 6

List the properties of hormones:

Answer 6

1. More than one hormone can be produced in a single
   endocrine gland
   Ø E.g.1: the pituitary gland synthesizes and secretes 6 different hormones.
   Ø E.g.2: The pancreatic Islets of Langerhans contain two cell types (! and “ cells). “ (beta) cells produce insulin, while ! (alpha) cells produce glucagon.
2. Different tissues can secrete the same hormone.
   Ø E.g.1: The hypothalamus and the pancreas secrete
   somatostatin (an inhibitory hormone).
3. A single hormone can act on multiple different target cells in different organs.
   Ø E.g.1: epinephrine and norepinephrine released by the
   adrenal gland can act on cells in the heart, blood vessels,
   airways of the respiratory tract, etc
4. Multiple hormones can bind to receptors and produce responses in a single target cell.
   Ø E.g.1: norepinephrine, epinephrine, angiotensin, and vasopressin can all act on the smooth muscle cells of blood vessels to regulate blood flow and blood pressure.
5. Hormone secretion can vary over time and can be affected by changes in the environment
   Ø E.g.1: Circadian rhythms involved in release of cortisol and melatonin. Cortisol secretion from adrenal gland peaks in the early morning and decreases at night. Melatonin secretion from the pineal gland peaks at night and decreases during the day.
6. Hormones have a half life =length of time required to reduce hormone concentration in the blood by half.
   a) Single amino acid derived hormones = minutes
   b) Peptide and protein hormones = minutes to hours
   c) Steroid hormones = hours

Question 7

What do hormones control?

Answer 7

Ø Metabolism
Ø Reproduction
Ø Growth and development
Ø Water and electrolyte balance
Ø Digestion and absorption of nutrients
Ø Blood cell production and development
Ø Etc.
Ø i.e. hormones regulate homeostasis.

Question 8

How do hormones function?

Answer 8

by controlling the
1. rates of enzymatic reactions in cells
2. transport of ions or molecules across cell cell membranes
3. gene expression and protein synthesis.

Question 9

How are hormones classified?

Answer 9

based on their molecular chemical structure

Question 10

List the three classes of hormones

Answer 10

1. peptides
2. steroids
3. amino acid derivatives

Question 11

Peptide and Protein Hormones

Answer 11

Are mainly hydrophilic (water soluble), so can be transported in the blood dissolved in the blood plasma.

This prevents them from crossing the cell membrane, so they must bind to cell membrane receptors (GPCRs or tyrosine kinase receptors) on target cells and trigger signal transduction mechanisms that activate second messengers.
Ø Examples include: insulin, oxytocin, cholecyctoskinin (CCK), growth hormone (GH), etc.

Question 12

Steroid Hormones

Answer 12

Hormones made from modification of cholesterol

Ø Hydrophobic (lipid soluble) so must be transported in the blood bound to a carrier protein. They can cross cell membranes and mainly have actions on receptors in the
cytoplasm or nucleus that directly trigger the transcription and translation for synthesis
of new proteins. However some have receptors in the cell membrane.

Ø Examples include: estrogen, testosterone, vitamin D3

Question 13

Amine Hormones

Answer 13

Ø Hormones made from modification of the individual amino acids tyrosine or
tryptophan
Ø Hormones derived from tyrosine:
a) Thyroid Hormones
b) Catecholamines

Question 14

Thyroid Hormones

Answer 14

(e.g. Thyroxine/ T4)
Ø control cellular metabolism
Ø Hydrophobic (lipid soluble) – must be transported in the blood bound to a carrier protein; can cross the cell membrane and bind to receptors in the nucleus to directly trigger transcription and translation (synthesis of new proteins).

Question 15

Catecholamines

Answer 15

(epinephrine and norepinephrine)

Ø Hormones secreted by the adrenal medulla involved in short term stress response (as part of the fight-or-flight response).

Ø Hydrophilic (water soluble) – transported in the blood dissolved in plasma; bind to cell membrane receptors and trigger second messenger systems via GPCRs

Ø Hormone derived from tryptophan = melatonin

Question 16

Describe or diagram the process of peptide hormone synthesis

Answer 16

1. Translation of mRNA by a ribosome on the
   endoplasmic reticulum to form an inactive
   preprohormone.
2. Enzymes cut off the signal sequence from the
   preprohormone to form an inactive prohormone.
3. The prohormone passes through the ER and Golgi,
   and undergoes postranslational modification.
4. The prohormone is packaged into a secretory vesicle
   that contains enzymes that cleave the prohormone
   into the active peptide hormones. The active
   hormone is stored in the vesicles in the cell until the
   cell is stimulated to release it.
5. In response to a release signal exocytosis of the
   active hormone from the vesicle occurs into the ISF
   (interstitial fluid).
6. The hormone crosses the capillary wall (endothelium)
   into the blood and is transported to the target cells

Question 17

differentiate between preprohormones, prohormones, and hormones.

Answer 17

Peptide hormones are created from large, inactive
precursor proteins called preprohormones.

Preprohormones contain one or several prohormone
amino acid sequences and a signal sequence that
helps move the protein into the lumen of the
endoplasmic reticulum where posttranslational
modifications will take place

Question 18

Describe or digram the process of steroid hormone synthesis

Answer 18

Steroid hormones are synthesized from cholesterol in the adrenal cortex or gonads (ovaries
and testes)

Unlike peptide and catecholamine hormones that can be stored in secretory vesicles inside the cell, steroid hormones are lipophilic and immediately cross cell
membranes after being made (so they are not stored, but made on demand and then
diffuse out of the cell).

Question 19

Describe the process of amine hormone synthesis

Answer 19

1. Tryptophan
   Ø E.g. Melatonin from the pineal gland – helps
   to regulate body’s circadian rhythms.
   Produced according to the amount of light a
   person is exposed to (less light = more
   melatonin). Plays a role in sleep-wake cycles.
2. Tyrosine
   Ø E.g.1: Catecholamines are derived from a
   single tyrosine, and so most behave like
   peptide hormones
   Ø E.g.2: Thyroid Hormones (T3 and T4) are
   derived from 2 tyrosine molecules, and so
   behave like steroid hormones.

Answer 20

1. Stimulus – change in the internal or external environment
2. Sensor – cells of the endocrine gland
3. Input signal – signal transduction pathway in endocrine cell
4. Integration – biochemical processes within the endocrine gland
5. Output (efferent) signal = hormone released into blood
6. Target Cells – Hormone interacts with Target cell receptors – leads to activation of second messenger systems or activation of genes for transcription and translation.
7. Response – can be within the tissue and systemic = changes caused by altering cellular activity
8. Negative feedback – Systemic change opposes the original stimulus and shuts down signaling by the endocrine gland

Question 21

Describe or diagram the reflex pathways of regulation of blood calcium levels

Answer 21

1. Stimulus – low blood calcium concentration
2. Sensor – cells of the parathyroid gland
3. Input signal – signal transduction pathways in parathyroid cells
4. Integration – biochemical processes within the parathyroid gland
5. Output (efferent) signal = parathyroid hormone released into blood
6. Target Cells – Bone and kidney cells.
7. Response –
   a) Tissue Response: Bone = increased resorption (bone break down);
   kidney increases reabsorption of calcium (less lost in urine) and
   increased calcitriol (vitamin D3) production which increases
   absorption of calcium from the GI tract (ie. from the foods we eat)
   b) Systemic response: increased plasma calcium concentration.
8. Negative feedback – higher plasma calcium levels feed back to the parathyroid gland and become the new stimulus. Once calcium levels are high enough, the parathyroid gland will no longer be stimulated to secrete