Principles of Endocrinology (5) Flashcards
Describe the pathway of ADH (anti-diuretic hormone) (Vasopressin). NF!
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.
Describe the pathway of TSH (Thyroid stimulating hormone)
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.
Describe blood pressure homeostasis by RAAS (Renin-Angiotensin-Aldosterone System)
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.
Local vs. long-distance signaling
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.
Endocrine signalling
Secreted molecules (hormones) diffuse into the blood stream, circulate, and trigger responses in target cells anywhere in the body.
Synaptic signalling
Secreted molecules (neurotransmitters) diffuse across a synapse, triggering a response in cells of the targeted tissue.
Neuroendocrine signalling
Secreted molecules (neurohormones) from neurosecretory cellsdiffuse into the blood stream, circulate, and trigger responses in target cells anywhere in the body.
Paracrine signalling
Secreted molecules (local regulators) diffuse locally through the ECF (extracellular fluid), triggering a response in neighbouring cells.
Autocrine signalling
Secreted molecules (local regulators) diffuse locally, triggering a response in the cell which secreted them.
Regulation and homeostasis
Organisms use homeostasis to maintain a “steady state” or internal balance independent of conditions in the external environment.
Endocrine receptors/ types of hormones
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.
One hormone - multiple receptors; multiple functions
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.
The body’s long distance regulators
Nervous and endocrine system
-Signals from the nervous system initiate and regulate endocrine signals
+hypothalamus and pituitary gland in vertebrates
Feedback loops
Stimulus –> Endocrine gland –Hormone–> Target tissue –> Response
Negative feedback = reduces stimulus
Positive feedback = reinforces stimulus
Oxytocin pathway
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.
Homeostasis - Occurs by the action of antagonistic pairs of hormones. Example?
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
What are the two types of diabetes mellitus?
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.
Explain the features and functions of the posterior pituitary gland.
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
What is a hormone the kidney produces? Aside from angiotensinogen and anti-diuretic hormone.
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.
What is diabetes insipidus?
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”).
Describe the functions and features of the anterior pituitary gland.
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
What are tropic hormones?
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
What are nontropic hormones?
Hormones that target nonendocrine tissues.
Prolactin
MSH
What is a nontropic and tropic hormone?
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.
What does the adrenal gland consist of?
The adrenal cortex and the adrenal medulla
What does the adrenal cortex do?
Made up of true endocrine cells.
Release corticosteroids (steroid hormones) \+Mineralcorticoids (Aldosterone - in ADH process) \+Glucocorticoids (cortisol)
What does the adrenal medulla do?
Cells derived from neural tissue during embryonic development.
Releases catecholamines (amine hormones
+epinephrine (adrenaline)
+norepinephrine (noradrenaline)
+triggers the fight or flight response
What is the role of the hypothalamus in response to stress?
- 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
- 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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