Endocrine System

Endocrine gland

Secretes one or more specific factors into the blood

Exocrine gland

Secretes one or more specific factors into a lumen

Chemical classes of hormone

amines

peptides+proteins

steroids

Amine hormones

• derived from amino acids

• have varying degrees of hydrophobicity/hydrophilicity

norepinephrine

epinephrine

dopamine

triiodothyroxine

tetraiodothyroxine

Peptide hormones

• come from proteins

Most hormones

ex: insulin

What form are many protein hormones secreted in?

Many are secreted as their inactive precursors that are then cleaved into an active form

Steroid hormones

• derived from cholesterol

aldosterone

cortisol

testosterone

estradiol

Steroid hormone synthesis pathway

1. extracellular molecule binds to a GPCR which signals steroid production

2. alpha subunit of G protein dissociates and binds adenylyl cyclase

3. adenylyl cyclase produces cAMP

4. cAMP activates PKA

5. PKA phosphorylates steroid-producing enzymes

Consequence of steroid hormones being derived from the same cholesterol structure

Any cell can in theory make every steroid hormone

How does the cell regulate steroid synthesis

• will activate enzymes associated with their steroid hormone of interest

• deactivate enzymes associated with other steroid hormones

Aromatase

enzyme that turns testosterone into estradiol

Expression of aromatase in males vs females

females express aromatase while males do not

Transport of peptides in plasma

Travel directly without a carrier protein

Transport of amine hormones in blood

depends on identity of hormone

What amine hormones are more steroid like

T3/T4

What amine hormones are more peptide like?

catecholamines→epinephrine/norepinephrine

Transport of steroids through plasma

Travel bound to carrier protein→albumin

General mechanism of binding of peptide and peptide-like hormones

bind to extracellular receptors

Exert short-lived, quick responses

General mechanism of binding of steroid and steroid-like hormones

bind to intracellular receptors (cytosol or nucleus)

alter gene expression

slow and long lived

Various fates of a hormone after it is secreted

• turned into an active form

• bind to a receptor

• inactivated by metabolism

• secreted in urine/feces

Categories of endocrine disorders

hyposecretion

hypersecretion

hyporesponse

hyperresponse

Types of signaling receptors that water-soluble hormones use

• ligand-gated ion channel

• GPCR

• Receptor bound directly to enzyme of interest

• Tyrosine kinase

Types of signaling receptors for lipid soluble hormones

• intracellular cytosolic or nuclear receptors

• ligand-receptor complex goes on to act as a TF that upregulates or downregulates gene expression

Permissivity

Hormones can work together to produce a more significant response→usually one hormone will upregulate the production of the other hormone’s receptor

Inputs that control hormone secretion

• ions/nutrients (ex: BGL)

• neurotransmitters

• hormones themselves (tropic)

Tropic hormone

Hormone that induces the secretion of another hormone from its target tissue

Structure of hypothalamus

• sits below thalamus

• connects to pituitary gland via hypothalamo-hypophyseal portal system

• made up of several nuclei (collection of cell bodies)

Two ways hypothalamus signal to pituitary

• Sends tropic hormones through portal system

• axons of nuclei run down from the hypothalamus into the pituitary and release hormones and neurotransmitters for pituitary to store/release

Posterior pituitary hormones

→ doesn’t synthesize its own hormones, only secretes hypothalamic-produced hormones

• oxytocin

• vasopressin

Oxytocin

controls milk ejection, emotional bonding, and birthing process

Vasopressin

regulates water balance and osmolarity

Anterior pituitary hormones

• synthesizes and secretes its own hormones

FSH

LH

ACTH

TSH

Prolactin

Endorphins

GH

Hypothalamic hormones

GnRH → LH, FSH

CRH → ACTH

TRH → TSH

Dopamine —| prolactin

GHRH → GH

SST —| GH

Short feedback loop

hormone secretes by pituitary inhibits hypothalamic hormone

Long feedback loop

Hormone secreted by a peripheral endocrine organ inhibits pituitary and hypothalamic hormone