Principles of Endocrinology (5)

Question 1

Describe the pathway of ADH (anti-diuretic hormone) (Vasopressin). NF!

Answer 1

Increased osmolarity (stimulus) is detected by osmoreceptors on the hypothalamus. (The hypothalamus generates thirst, drinking water reduces blood osmolarity). The hypothalamus sends Neurosecretory hormones down axons to release ADH into the blood stream from the posterior pituitary gland. The ADH is detected by GPCRs on collecting duct cells (!!). This increases the number of aquaporins through the exocytosis of the synaptic vessels in the collecting duct cell to the inner wall to increase the number of aquaporins and allows more water to be absorbed. The negative feed back loop stops the hypothalamus from causing the secretion of ADH.

Alcohol inhibits ADH production, causing dehydration.

The other side of the story: Ecess water consumption = reduced blood osmolarity = reduced vasopressin secretion and diuresis.

Question 2

Describe the pathway of TSH (Thyroid stimulating hormone)

Answer 2

A hormone cascade pathway

A decrease in temperature causes afferent signals to be sent to the hypothalamus (through sensory neurons) releases TRH (thyrotropin releasing hormone) in the portal system (blood). These hormones trigger the release of TSH (thyroid stimulating hormone) from the anterior pituitary gland into the systemic blood. TSH binds to the thyroid gland (important word - gland) which causes it to release T3 and T4 hormones. These bind to all cells which increase cellular respiration to increase system temperature and maintain homeostasis. The T3 and T4 are part of a negative feedback loop with the hypothalamus to prevent the release of thyrotropin releasing hormone, and a part of a short feedback loop with the anterior pituitary (!!) gland to prevent production of TSH.

Question 3

Describe blood pressure homeostasis by RAAS (Renin-Angiotensin-Aldosterone System)

Answer 3

Blood pressure, blood volume or sodium concentration (?) drops which is detected by sensors in the juxtaglomerular apparatus (JGA). The juxtaglomerular apparatus (in the kidneys) releases renin (enzyme). Renin acts on (cleaves it) angiotensinogen (-ogen usually means pre-cursor) released by the liver to produce angiotensin I. Angiotensin I is acted upon by angiotensin cleaving enzyme (ACE) to produce angiotensin II. This causes arterioles to constrict (vasoconstriction) to increase blood pressure and stops the JGA from producing renin in a negative feedback loop. Angiotensin II also stimulates the adrenal gland (adrenal cortex) to produce aldosterone which allows more Na+ and H2O to be absorbed into distal tubules from filtrate (flows from the tubules in the kidney nephron) into the blood, increasing blood volume. Prevent production of renin by the JGA.

Question 4

Local vs. long-distance signaling

Answer 4

Local signalling:

• Paracrine signalling = A secreting cell acts on nearby target cells by secreting molecules of a local regulator
• Synaptic signalling = A nerve cell releases a neurotransmitter molecule into a synapse, stimulating the target cell, such as a muscle or nerve cell.

Long-distance signalling:
-Endocrine (hormonal) signalling = Specialised endocrine cells secrete hormones into body fluids, often blood. Hormones reach virtually all body cells but are bound only by some cells.

Question 5

Endocrine signalling

Answer 5

Secreted molecules (hormones) diffuse into the blood stream, circulate, and trigger responses in target cells anywhere in the body.

Question 6

Synaptic signalling

Answer 6

Secreted molecules (neurotransmitters) diffuse across a synapse, triggering a response in cells of the targeted tissue.

Question 7

Neuroendocrine signalling

Answer 7

Secreted molecules (neurohormones) from neurosecretory cellsdiffuse into the blood stream, circulate, and trigger responses in target cells anywhere in the body.

Question 8

Paracrine signalling

Answer 8

Secreted molecules (local regulators) diffuse locally through the ECF (extracellular fluid), triggering a response in neighbouring cells.

Question 9

Autocrine signalling

Answer 9

Secreted molecules (local regulators) diffuse locally, triggering a response in the cell which secreted them.

Question 10

Regulation and homeostasis

Answer 10

Organisms use homeostasis to maintain a “steady state” or internal balance independent of conditions in the external environment.

Question 11

Endocrine receptors/ types of hormones

Answer 11

Endocrine receptors have specificity.

Types of hormones:
-Water-soluble (hydrophilic)
+Polypeptides (Insulin)
+Amines (Adrenaline)
+Receptor location in plasma membrane
+Goes through gene regulation and then causes cytoplasmic response or immediately causes a cytoplasmic response.
-Lipid soluble
+Steroids (Cortisol)
+Amines (Thyroxine)
+Receptor location in nucleus or cytoplasm
+Goes through gene regulation and then causes cytoplasmic response.

Question 12

One hormone - multiple receptors; multiple functions

Answer 12

Liver cell: Adrenaline binds to beta receptor which causes glycogen to break down into glucose which is released from the cell. Blood glucose level increases.

Smooth muscle cell in wall of blood vessel that supplies skeletal muscle:
Adrenaline binds to a beta receptor which causes the smooth muscle to relax. Blood vessel dilates, increasing flow to skeletal muscle.

Smooth muscle in wall of blood vessel that supplies intestines:
Adrenaline binds to alpha receptors on the smooth intestine muscles which causes them to contract. Blood vessel constrict, decreasing flow to the intestines.

Question 13

The body’s long distance regulators

Answer 13

Nervous and endocrine system

-Signals from the nervous system initiate and regulate endocrine signals
+hypothalamus and pituitary gland in vertebrates

Question 14

Feedback loops

Answer 14

Stimulus –> Endocrine gland –Hormone–> Target tissue –> Response

Negative feedback = reduces stimulus

Positive feedback = reinforces stimulus

Question 15

Oxytocin pathway

Answer 15

Simple neurohormone pathway

Suckling stimulates hypothalamus/posterior pituitary gland through a sensory neuron. Neurosecretory cell in the posterior pituitary gland secrets oxytocin neurohormone into the blood. Oxytocin acts on receptors of smooth muscle cells (target cells) in mammary glands to cause milk release (response). Through a positive feedback loop oxytocin results in the increase of stimulation of the hypothalamus/posterior pituitary gland.

Question 16

Homeostasis - Occurs by the action of antagonistic pairs of hormones. Example?

Answer 16

Stimulus: Blood glucose level rises (after eating a carb rich meal)

Beta cells of the pancreas release insulin into the blood

This causes the body cells to take up more glucose and the liver to take up glucose and store it as glycogen.

Blood glucose level declines.

-ve feed back loop stops pancreas from producing insulin (pancreas detects homeostatic blood glucose levels).

Stimulus: Blood glucose level decreases.

Alpha cells of the pancreas release glucagon into the blood stream.

Glucagon causes liver to break down glycogen and release glucose into the blood.

Blood glucose level rises

-ve feedback loop stops production of glucagon by the pancreas

Question 17

What are the two types of diabetes mellitus?

Answer 17

Type 1 (Insulin-dependent):
An autoimmune disorder in which the immune system destroys pancreatic beta cells

Type 2 (non-insulin dependent): 
Involves insulin deficiency or reduced response of target cells due to change in insulin receptors.

Question 18

Explain the features and functions of the posterior pituitary gland.

Answer 18

Neurosecretory cells of the hypothalamus secrete neurohormones down axons to the posterior pituitary gland. This causes the posterior pituitary to produce hormones.

Hormones produced (two polypeptide hormones):

Anti-diuretic hormone (ADH) - Kidney tubules

Oxytocin
- Mammary glands, uterine muscles

Question 19

What is a hormone the kidney produces? Aside from angiotensinogen and anti-diuretic hormone.

Answer 19

Erythropoietin is produced in response to low oxygen carrying capacity (low oxygen levels?) in the blood. This hormone changes stem cells in bone marrow into erythrocytes (RBCs) to increase oxygen carrying capacity. Once homeostatic level of Oxygen is reached, the kidney stops producing erythropoietin.

Question 20

What is diabetes insipidus?

Answer 20

Mutations that prevent ADH production or inactivate ADH receptor gene, cause diabetes insipidus (“to pass through”).

Mutations in aquaporin genes can also cause diabetes insipidus.

Consequences: Severe dehydration, solute imbalances due to increased (dilute) urine volume.

Not always associated with genetic changes - alcohol can also inhibit ADH release, excess urine and dehydration (“hangover”).

Question 21

Describe the functions and features of the anterior pituitary gland.

Answer 21

Neurosecretory cells in the hypothalamus secrete hypothalamic releasing and inhibiting neurohormones in response to a stimulus. These neurohormones pass through portal vessels to cause the production of hormones by the anterior pituitary gland.

Hormones produced:
Thyroid stimulating hormone (TSH)

FSH and LH
+Testes or ovaries

ACTH
+Adrenal cortex

Prolactin
+Mammary glands

MSH
+Melanocytes

GH
+liver, bones, other tissues

Question 22

What are tropic hormones?

Answer 22

Hormones that regulate the function of other endocrine cells or glands.

LH and FSH (gonadotropins) stimulate the activities of the male and female gonads

TSH acts on the thyroid to stimulate the release of thyroid hormones

ACTH stimulates the production and secretion of steroid hormones by the adrenal cortex

Question 23

What are nontropic hormones?

Answer 23

Hormones that target nonendocrine tissues.

Prolactin
MSH

Question 24

What is a nontropic and tropic hormone?

Answer 24

GH (growth hormone)
Targets liver, bones, other tissues

Growth hormones exerts diverse metabolic effects to raise blood glucose.

Also acts on liver to release Insulin-like Growth Factors (IGFs), which circulate in blood and directly stimulate bone and cartilage growth.

Question 25

What does the adrenal gland consist of?

Answer 25

The adrenal cortex and the adrenal medulla

Question 26

What does the adrenal cortex do?

Answer 26

Made up of true endocrine cells.

Release corticosteroids (steroid hormones)
\+Mineralcorticoids (Aldosterone - in ADH process)
\+Glucocorticoids (cortisol)

Question 27

What does the adrenal medulla do?

Answer 27

Cells derived from neural tissue during embryonic development.

Releases catecholamines (amine hormones
+epinephrine (adrenaline)
+norepinephrine (noradrenaline)
+triggers the fight or flight response

Question 28

What is the role of the hypothalamus in response to stress?

Answer 28

1. Autonomic neural signal to the adrenal medulla.
   
   • Nerve signals sent from hypothalamus to spinal chord to adrenal medulla through a nerve cell.
   • Epinephrine and norepinephrine is secreted which act on alpha and beta adrenoreceptors in target tissues
2. Neuroendocrine signal (CRH - corticotropin releasing hormone) to the anterior pituitary.
   
   • Hypothalamus releases tropic hormone (CRH)through portal vessels to the anterior pituitary. The anterior pituitary releases ACTH into the blood stream to the adrenal cortex
   • Adrenal cortex secrets mineralocorticoids and glucocorticoids.

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