Animal Physiology-Endocrinology

Question 1

What is a hormone?

Answer 1

A signalling molecule that travels through the circulation to affect the actions of a target tissue

Question 2

Where does the ‘endocrine’ type of hormone come from, travel via and what does it target?

Answer 2

Produced by the endocrine gland, travels via blood to distant targets/tissue

Question 3

Where does the ‘neuro-endocrine’ type of hormone come from, travel via and what does it target?

Answer 3

Produced by nervous tissue, travels via blood and targets distant tissue

Question 4

Where does the ‘neurocrine’ type of hormone come from, travel via and what does it target?

Answer 4

Produced by neurons, travels via synapses and targets neighbouring cells

Question 5

Where does the ‘paracrine’ type of hormone come from, travel via and what does it target?

Answer 5

Produced by cells, travels via interstitial fluid and affects neighbouring cells

Question 6

Where does the ‘autocrine’ type of hormone come from, travel via and what does it target?

Answer 6

Produced by cells, travels via cellular fluid and affects itself/same cell

Question 7

Where does the ‘pheromone’ type of hormone come from, travel via and what does it target?

Answer 7

Produced by animals, travels via external environment and affects other animals

Question 8

Are hormones always present?

Answer 8

Yes, they are always present in low concentrations. The change in the concentration causes the effect. This is regulated by feedback

Question 9

Describe these hormone interactions in the same tissue:

Additive
Synergistic
Non-additive
Permissive

Answer 9

Additive: A causes an effect, B causes an effect. This interaction occurs when both effects are simply added together

Synergistic: A causes an effect, B causes an effect. Both together lead to a bigger effect than if they were just added together

Non-additive: A causes an effect, B causes an effect. Both together only lead to the maximum of one of the effects

Permissive: A causes an effect but B does not cause an effect. Together, a very large effect is caused

Question 10

If a hormone is lipophilic (fat loving), what properties do they display/ what are they able to do?

Answer 10

Lipophilic hormones such as steroids can readily go through cell membranes which are essentially made of fat

Question 11

If a hormone is hydrophilic (water loving), what are they unable to do?

Answer 11

They are not able to enter cells on their own. e.g proteins/polypeptides/glycoproteins (can act as hormones)

Question 12

Amines are amino acid derivatives (a class of hormone) from which two amino acids?

Answer 12

Tryptophan and tyrosine. Side note: amines are hydrophilic

Question 13

Fatty acid derivatives are a class of hormone. They are usually derived from arachidonic acid and are lipophilic. Where are they produced and what sort of effects do they have?

Answer 13

Produced in the cell membrane and produce autocrine and paracrine effects (Self and neighbouring)

Question 14

What sort of steroid hormones result from stepwise conversion of cholesterol?

Answer 14

Estradiol, testosterone, progesterone, cortisol and aldosterone (vitamin D and A are also produced from cholesterol)

Question 15

Where are steroids produced and where does that cholesterol come from?

Answer 15

Steroids are produced by the gonad and adrenal glands; more specifically on the smooth ER and on the mitochondria in the adrenal glands. The cholesterol comes from LDLs in plasma (so the diet) but it is important to note that cholesterol can be synthesised de novo intracellularly if there is inadequate cholesterol in the diet

Question 16

Can steroids be stored? If so what is the storage form?

Answer 16

Not really, they are immediately released into the blood stream. However, in the blood stream they are bound to carrier proteins and are inactive (so this is the storage form)

Question 17

Which form of steroid hormone can stimulate a cell, be degraded and contributes to the feedback of the concentrations?

Answer 17

The active form (5-10% of all the steroid hormone in the body) which is unbound. The combination of synthesis and level of binding of the hormone is what regulates how much is circulating in the blood

Question 18

Specificity in hormones is very important. Slight changes in structure can confer massive changes in response. Name two polypeptide/protein hormones which are very similar in structure and can sometimes have a residual effect on one another.

Answer 18

Prolactin and growth hormone.

It is possible to make cows lactate by giving them growth hormone

Question 19

Are polypeptide/protein hormones synthesised in their final form?

Answer 19

No, they are synthesised as prehormones + signal peptides. Some have multiple units, some are glycosylated and some are synthesised as preprohormones

Question 20

Can protein hormones be stored?

Answer 20

Yes, in secretory vesicles. They are exocytosed during secretion. This stored hormone makes the response faster than steroid hormones

Question 21

Secretion of protein hormones is regulated by what?

Answer 21

Trophic hormones e.g GH—->IGF-1
Nervous stimuli e.g Absence of light —-> Melatonin
Levels of metabolites e.g Glucose—-> Insulin

All except from IGF-1 circulate freely

Question 22

Metabolism of hormones varies depending on how long term (e.g during pregnancy) or short term (e.g calcium levels). Half-life is an important concept in this. What is meant by a hormone’s half life?

Answer 22

The time it takes for half the level of that hormone to disappear after it is inactivated

Question 23

How are peptide hormones broken down?

Answer 23

By peptidases, deamination and reduction of disulphide bonds. This is done in lysosomes of target tissue and the liver and kidneys

Question 24

How are steroid hormones broken down?

Answer 24

By mixed function oxidases in the liver. They are excreted via urine or bile salts

Question 25

What does the rate of response depend on?

Answer 25

• The amount of active hormone arriving at the tissue (form)
• Rate of release of the hormone
• Rate of metabolism
• Affinity and number of receptors on the tissue

Question 26

What sort of receptors are needed for hydrophilic hormones such as proteins? What is the general mechanism of such a receptor?

Answer 26

Cell surface receptors;
Hormone binds to receptor, activates protein kinase (often via 2nd messenger), phosphorylates specific intracellular protein, altered protein activity, response

Rapid response- uses existing proteins

Question 27

What sort of receptors are needed for lipophilic hormones such as steroid and thyroid hormones? What is the general mechanism of such a receptor?

Answer 27

Nuclear/ intracellular receptors-acts as transcription factors;
Hormone travels through membrane, binds to receptor, translocation to nucleus, formation of a dimer hormone-receptor complex interacts with 5’ end of gene, intiates transcription, response

Slow response (hours)- has to create proteins

Question 28

Name the 3 most important endocrine glands found in the brain

Answer 28

Pineal glands, pituitary and the hypothalamus (all placed in center of brain for max. protection)

Question 29

Where is the hypothalamus located in the brain and why is it the major integration centre of the brain?

Answer 29

The hypothalamus is located just below the 3rd ventricle, above the median eminence. It receives signals from all parts of the body and regulates most of the endocrine system (and autonomous system)
It has the suprachiasmatic nucleus located inside
It also controls hormone release from pituitary and pineal glands

Question 30

What two glands does the hypothalamus control?

Answer 30

Pineal and pituitary

Question 31

What is the job of the suprachiasmatic nucleus and how is this regulated?

Answer 31

It functions as the body clock which controls circadian rhythms in mammals. This is done through neurons which have a 24 hour rhythm of activity. This is due to a regular pattern of transcription, translation and post-translation activity which allows the neurons to run the body of a 24 hour clock.

Question 32

The suprachiasmatic nucleus relies on environmental cues to tune the body clock to the correct pattern. Why is it important that the SCN lies above the optic chiasma (the cross over point of the nerve fibres from each eye)?

Answer 32

Light detecting retinal ganglion cells (Melanopsin photoreceptor cells) feed back to the SCN via the retino-hypothalamic tract to tell it whether it is dark or light outside. These cells are separate from the cells to do with vision

Question 33

To help keep the body on the same clock, signals are sent out from the SCN to cells through neural connections or sympathetic NS to the adrenal cortex. Some hormones are released in a diurnal pattern. State which ones

Answer 33

Cortisol is released in the morning to get you active (metabolism etc) and decreases when you go to sleep

Melatonin (amine- derived from tryptophan) is secreted from the pineal gland in the dark- this indicates day length-so helps body recognise the time of the year which affects seasonal breeding animals

Question 34

What is meant by a short-day breeder and a long-day breeder?

Answer 34

Short-day breeders such as sheep need to be bred in shortening days (autumn) as their gestation period is around 5 months. This means their offspring will be born in the spring where there is an abundance of food available. Long-day breeders such as horses need to be bred in days which are lengthening as their gestation period is 11 months (need to breed after winter solstice)

Question 35

Which gland is the hypothalamus linked to by a stalk (infundibulum) that releases hormones?

Answer 35

Pituitary gland. (The hypothalamus never controls the body directly)

Question 36

Name the two parts/lobes of the pituitary gland and what they do

Answer 36

Posterior pituitary- aka neurohypophysis
-made out of nervous tissue
-receives hormones from neurocrine cells in hypothalamus via axons
This means the hormone actually came from the hypothalamus but it is secreted by the posterior pituitary

Anterioir pituitary- aka adenohypophysis

• made out of glandular tissue (creates its own hormone)
• produces a ‘releasing’ hormone (aka trophic) which stimulates target tissues to secrete hormones

Question 37

Name the two hormones produced by the POSTERIOR pituitary

Answer 37

Vasopressin- aka antidiuretic hormone (contains Arg+ Phe)
Oxytocin (contains Ile+ Leu)

Both are peptides and have an almost identical structure, only have two amino acid difference but these confer v. different specificity for the hormones (do very different things)

Question 38

What are the key roles of Vasopressin and Oxytocin?

Answer 38

Vasopressin regulates extracellular fluid osmolarity.
It stimulates water resoprtion from distal part of the kidney tubule if blood volume and pressure fall

Oxytocin stimulates contraction of some smooth muscle, e.g mammary gland and uterus during birth. Oxytocin has been very widely used since it was synthesised in 1955 in obstetrics and livestock (injected in animal if uterus becomes tired during birth- increases contractions to encourage it)

Question 39

How are Vasopressin and Oxytocin synthesised, secreted and released?

Answer 39

Synthesised by hypothalamic neurons as preprohormones then cleaved to active hormones during rapid axonal transport. Stored in secretory granules at nerve end. Secreted by exocytosis into capillaries and hence general circulation in response to nervous signal from the hypothalamus. This is a ‘Pulsatile release’.
Half-life in plasma = 3-5min so very rapid.

(Release in response to changes in osmotic or barometric pressure, pain, fright, stress, adrenal insufficiency, hypoxia, cardiac failure. Oxytocin also in response to teat sensory nerve stimulation – also in parturition)

Question 40

The anterior pituitary produces trophic hormones. Is it just one type of cell that secretes hormones in the ANTERIOR pituitary?

Answer 40

No, each hormone secreted by a specific cell type in the anterior pituitary e.g gonadotrope, corticope

Question 41

What sort of hormones are secreted from the anterior pituitary? How is their secretion regulated?

Answer 41

Proteins or glycoproteins which are regulated by releasing or inhibiting factors of the hypothalamus. Their half-life ranges from 10/15 mins (cortisol) -4 hours (FSH)

Question 42

Anterior pituitary hormones include;
TSH  (thyrotrophic hormone)
PRL (prolactin)
LH (luteinising hormone)
FSH (follicle stimulating hormone)
ACTH (adrenocorticotrophic hormone)
GH (Growth Hormone or somatotrophin)
MSH (melanocyte stimulating hormone)

Which organs do they target and what effect do they control?

Answer 42

TSH= Thyroid, ↑T4 and T3, ↑ metabolic rate
PRL= Mammary glands, Initiating/maintaining lactation
Many diverse functions
LH = Ovaries, Testis, Ovulation, ↑ steroid sex hormones
FSH= Ovaries, Testis, Follicle development ↑oestrogen, Regulates spermatogenesis
ACTH= Adrenal cortex, ↑Cortisol (Stress response)
GH= Liver (↑IGF-1 & binding proteins), increased growth, ↑ milk production in dairy cows
MSH=Melanocytes, CNS, Immune system, ↑skin pigmentation,↓ feed intake, Anti-inflammatory response

Question 43

How does the hypothalamus regulate anterior pituitary secretion?

Answer 43

Specific neurons in the hypothalamus produce releasing or inhibiting factors (in tiny amounts) which are secreted into the hypothalamic-hypophyseal portal system and stimulate specific cells in the anterior pituitary. Many of the hormones are then secreted as PP hormones are but into the portal system rather than systemic circulation