The Endocrine System

Types of Chemical Messengers

• Hormone

– Regulatory chemical that is secreted into

extracellular fluid and carried by the blood

– Can act at a distance from source

• Endocrine system

– Organs and tissues that produce hormones

• Only targets with receptor can respond

2

Endocrine System

• The endocrine system includes all the

organs that secrete hormones

– Endocrine – product secreted into

extracellular fluid and carried in blood

– Exocrine – secrete product into a duct

• 2 basic hormone characteristics

1. Must be sufficiently complex to convey

regulatory information to their target cells

2. Must be adequately stable to resist

destruction before reaching their target cells3

4

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Adenohypophysis

HypothalamusHypothalamus Pineal gland

Neurohypophysis Pituitary

gland

Thymus

Thyroid gland

Pancreas

Testes

(in males)

Ovaries (in females)

Pineal gland

Parathyroid glands

(behind thyroid)

Adrenal

glands

5

6

7

8

9

Hydrophilic

ransport proteins

5

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Activated4

Target cell

Nontarget

cells

3

Activated

3

Target cellNuclear

receptor

5. Unused,

deactivated

hormones

are removed

by the liver

and kidney.

4. Target cells

possess

receptors,

and are

activated by

hormones.

Lipophilic

3. Nontarget

cells lack

receptors,

and cell

stimulation

does not

occur.

Nontarget

cells

T

1 1

Blood

vessels

Hormones

Secretory

vesicles

Extracellular

fluid

Extracellular

fluid

Endocrine gland B

1. Hormones

secreted into

extracellular

fluid and

diffuse into

bloodstream.Hormones

4

Endocrine gland A

Membrane

receptors

2. Hormones

distributed by

blood to all

cells. Diffuse

from blood to

extracellular

fluid.

2

Hydrophilic Hormones

• Peptide, protein, glycoprotein, and

catecholamine hormones

• Too large or polar to cross cell membrane

• Bind to receptors on plasma membrane

• Initiate signal transduction pathways

• Activation of protein kinases

– Activate or deactivate intracellular proteins by

phosphorylation

• Production of second messengers 10

11

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

GTPGTP

1. Receptors Function as Kinase Enzymes

γ

αβ

2. Receptors Activate G Proteins

Hormones

GPCR GPCR

Second messenger-

generating enzyme

Active G

protein

Second

messenger

Active

protein kinase

Target

proteins

Inactive

protein kinase

Inactive G

protein

Cellular

response

ATP

Hormone

Receptor

Inactive

Active kinase

domain

ADP

Phosphorylated

proteinTarget

protein

Cellular

response

GDP

Hydrophilic Hormones

• Receptor kinases

– For some peptide hormones (like insulin) the

receptor itself is a kinase

• Can directly phosphorylate intracellular proteins

that alter cellular activity

– For other peptide hormones (like growth

hormone) the receptor itself is not a kinase

• Rather, it activates intracellular kinases

12

Hydrophilic Hormones

• Second-messenger systems

– Many hydrophilic hormones work through

second messenger systems

– Two have been described

• One involving cyclic adenosine monophosphate

(cAMP)

• One that generates 2 lipid messengers: inositol

triphosphate (IP3) and diacyl glycerol (DAG)

13

Hydrophilic Hormones

• Second-messenger systems

– Receptors are linked to a second-messenger-

generating enzyme via membrane proteins called G

proteins

• G protein–coupled receptors (GPCRs)

– When the G protein activates the enzyme, the

second-messenger molecules increase

• Cellular response depends on the type of G

protein activated

– Some activate while others inhibit their second-

messenger-generating system

– Single hormone can have distinct actions in 2

different cells 14

The Posterior Pituitary

• Antidiuretic hormone (ADH)

– Peptide hormone that stimulates water

reabsorption by the kidney, and thus inhibits

diuresis (urine production)

• Oxytocin

– Like ADH, composed of 9 amino acids

– In mammals, it stimulates the milk ejection

reflex and uterine contractions during labor,

and it regulates reproductive behavior

15

Anterior Pituitary Disorders

• Growth Hormone

– Stimulates protein synthesis and growth of

muscles and connective tissues

– Stimulates production of insulin-like growth

factors that stimulate cell division in

epiphyseal growth plates – elongation of bone

• Gigantism vs pituitary dwarfism

– Also functions in adults to regulate protein,

lipid, and carbohydrate metabolism

• Acromegaly16

Other anterior pituitary hormones

• Prolactin

– Acts on glands that are not endocrine glands

– Actions appear diverse

• Milk production in mammals, “crop milk” and brood patch in

birds, electrolyte balance in kidneys

• TSH stimulates thyroid

• ACTH stimulates only adrenal cortex

• FSH and LH act only on the gonads

• MSH regulates melanophores or melanocytes

that contain melanin

17

The Thyroid Gland

• In humans, the thyroid gland is shaped like

a bow tie, and lies just below the Adam’s

apple in the front of the neck

• Secretes

– Thyroid hormones

• Thyroxine – metabolic rate and body temperature

• Triiodothyronine

– Calcitonin

18

The Pancreas

• Exocrine and endocrine glands

• Connected to the duodenum of the small

intestine by the pancreatic duct

• Islets of Langerhans are scattered clusters

of cells throughout the pancreas

• These govern blood glucose levels

through two hormones with antagonistic

functions

19

The Pancreas

• Insulin

– Secreted by beta (b) cells of the islets

– Stimulates cellular uptake of blood glucose

and its storage as glycogen in the liver and

muscle cells, or as fat in fat cells

• Glucagon

– Secreted by alpha (a) cells of the islets

– Promotes the hydrolysis of glycogen in the

liver and fat in adipose tissue

20

21

( – )( – )Blood glucose increased

Stimulus

Blood glucose decreased

Stimulus

After a Meal Between Meals

EffectorEffector

Ne

ga

tive

fe

ed

ba

ck

Ne

ga

tive

fe

ed

ba

ck

Increased glucagon

production

By a cells

Increased insulin

production

by β cells

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

ResponseResponse

Glucose moves from

blood into cells,

reducing blood glucose

Glycogen hydrolized to

glucose, then secreted into

blood, increasing blood glucose

( + )( + )

Sensor

Pancreatic

Islets

Diabetes Mellitus

• Diabetics cannot take up glucose from

blood

• Type I (insulin-dependent diabetes)

– Individuals lack insulin-secreting b cells

– Treated by daily injections of insulin

• Type II (noninsulin-dependent diabetes)

– Most patients have this form

– Very low number of insulin receptors

– Treated by diet and exercise22

The Gonads

• Ovaries and testes in vertebrates

• Produce sex steroids that regulate

reproductive development

• Estrogen and progesterone

– “Female” hormones

• Androgens

– “Male” hormones

– Testosterone and its derivatives

23

The Pineal Gland

• Located in the roof of the third ventricle of

the brain

• Secretes hormone melatonin

• Functions of melatonin

– Reduces dispersal of melanin granules

– Synchronizes various body processes to a

circadian rhythm

• Secretion of melatonin activated in the dark

24