1 - intro to endocrinology

define the endocrine system

a system that integrates and controls organ function via the secretion of hormones from cells, tissues or glands which are then carried in the blood to distal targets from site of hormone synthesis to influence activity of target organ

what are the types of endocrine responses

slow - increased protein synthesis in response to GH

fast - increased heart rate in response to adrenaline

what is endocrine communication

hormones travel in blood to target

tissues detect hormones through specific receptors for that hormone

no receptor means...

no response

describe neural control

neurotransmitters are released from presynaptic neurones

travel across synaptic cleft to postsynaptic cell to influence its activity

neurotransmitter acts locally within synaptic cleft in contrast to hormones

neuroendocrine

nervous and endocrine systems combine

- nerves release hormones that enter blood stream and travel to target cells

why is hormone response highly specific

cells have specific receptors to hormones

- the same hormone can produce different effects in different cells depending on downstream pathways

insulin is released, what 2 ways can this act?

target tissue = liver - i increased glycogenesis and decreased gluconeogenesis

target tissue = skeletal muscle and adipose tissue - increased glucose uptake

what is glycogenesis

formation of glycogen from glucose

what is gluconeogenesis

formation of glucose from fats and proteins

are endocrine glands ductless or not

ductless

how to endocrine glands secrete hormones

directly into bloodstream

types of endocrine glands

pituitary

thyroid

pancreas

adrenal glands

parathyroid

what are the main features of endocrine hormones

produced by a cell or group of cells

secreted DIRECTLY into blood

transported via blood to targets distally

exert effects in LOW concentration

act by binding to receptors on target tissues

action terminated via negative feedback loop

often released in short bursts

what are the classifications of endocrine hormones

peptide or protein - composed of chains of amino acids

amine - derived from tryptophan or tyrosine

steroid - derived from cholesterol

examples of peptide hormones

TRH

FSH

insulin

what are peptide hormones synthesised as

preprohormone

- large and inactive

- produced by ribosomes

what happens to preprohormone

cleaved into inactive, smaller prohormone and stored in vesicles in golgi until required

why is the prohormone stored alongside proteolytic enzymes

proteolytic enzymes break the prohormone down into active hormone and other fragments

what is co-secretion

when hormones and other fragments are stored in vesicles of endocrine cells until release is triggered - release is into plasma

why is measuring of fragments clinically useful

inactive fragments such as C-peptide can help in diagnosis of conditions

why is measuring C-peptide important?

C-peptide is the inactive cleaved fragments from insulin prohormone

levels are often measured to indicate endogenous insulin production from pancreas

insulin is metabolised faster so C-peptide are around 5x higher than endogenous insulin

mechanisms of action of peptide/protein hormones

1. water soluble so dissolve easily in plasma - makes transport easy however bind to membrane receptors as cannot cross cell membrane

2. most hormones target either GPCR or tyrosine kinase linked receptors - phosphorylate existing proteins and modify functions - faster response

hydrophilic signal transduction of peptide hormones - rapid and slow response

hormone binds to cell surface receptor - GPCR or tyrosine kinase linked receptor

- GPCR - activation of second messenger or ion channel leading to modification of existing proteins - rapid response

- tyrosine kinase linked receptor - slow response as alters gene expression

what amino acids are amine hormones mostly derived from

tyrosine

what hormones does tyrosine make

catecholamines

thyroid hormones

what is the only amine hormone not derived from tyrosine

melatonin - derived from tryptophan

what are the main catecholamines

dopamine, norepinephrine, epinephrine

what is the main difference between steroid hormones and other hormones

steroid hormones are synthesised as needed and stored in vesicles until required because they are highly lipophilic/hydrophobic so cannot be retained in lipid membrane

since steroid hormones are poorly soluble in water, how are they transported and why

bound to carrier proteins such as albumin

- protects them from enzymatic degradation

what are steroid hormones produced by

gonads - testes and ovaries

placenta

kidney

adrenal cortex

what are steroid hormones derived from

cholesterol

mechanism of action of steroid hormones

lipophilic - readily cross plasma membrane in and out of cells - receptors located INSIDE CELLS

what does activation of steroid intracellular receptors result in

change in gene expression at level of nucleus - genomic effect

- increase or decrease protein synthesis

when are lipophilic hormones active

unbound and free in plasma

- thyroid and steroid hormones

law of mass action dictates...

as free hormone leaves the plasma more hormone is released from carriers typically in small quantities that are required for physiological functions

why are hormone carrier proteins required?

for hormones that have poor plasma solubility - steroid and thyroid hormones to increase solubility

to maintain bound : unbound ratio

- as steroid taken up more is released from carrier allowing a reservoir and prolonged activity

what is plasma hormone concentration determined by

balance between secretion and degradation/excretion

autocrine and paracrine DONT…

travel in blood

NOT endocrine

insulin is influenced by:

• Plasma glucose levels. 

• Autonomic nerve activity. 

• Presence of food in the gut. 

• Other hormones (e.g., Glucagon). 

negative feedback reflexes

most common mechanism where the output of a pathway inhibits its own further secretion (e.g., Parathyroid hormone). 

neuroendocrine pathways

Secretion is triggered by neural feedback loops, such as the release of Adrenaline from the adrenal medulla during sympathetic discharge 

List major categories of physiological function that are governed by endocrine 

pathways and mechanisms. 

metabolism and energy balance

growth and development

homeostasis

reproduction

stress response

State in logical sequence the factors that determine the availability of a hormone or neurohormone to its target cells. 

• Rate of Secretion: Determined by the balance between synthesis/secretion and degradation/excretion. 

• Transport and Solubility: * Water-soluble hormones (peptides) dissolve easily in plasma for simple transport. 

• Lipophilic hormones (steroids/thyroid) must bind to carrier proteins (like albumin) to remain soluble and protected from degradation. 

• Protein Binding (The Law of Mass Action): Only unbound (free) hormone can diffuse across capillaries to reach target cells. As free hormone is used, more is released from carriers to maintain equilibrium. 

• Half-life: The time taken for the plasma concentration to fall by half. Peptides have short half-lives (minutes), while bound steroids have much longer half-lives (hours to days).