Chapter 18: The Endocrine System

Endocrine System

• Regulates long-term processes: – growth– development– Reproduction

• Uses chemical messengers to relay information and instructions between cells

What are the modes of intercellular communication used by the endocrine and

nervous systems?

Direct Communication

• Exchange of ions and molecules between adjacent cells across gap junctions

• Occurs between 2 cells of same type

• Highly specialized and relatively rare

Paracrine Communication

• Chemical signals transfer information from cell to cell within single tissue– paracrine factors – may enter blood stream

and function as hormones

• Most common form of intercellular communication

Endocrine Communication

• Endocrine cells – release chemicals (hormones) into

bloodstream– Alters metabolic activities of many tissues and

organs simultaneously

• Target Cells – possess receptors needed to bind and “read”

hormonal messages

• Hormones– Stimulate synthesis of enzymes or structural

proteins– Increase or decrease rate of synthesis– Turn existing enzyme or membrane channel

“on” or “off”

Synaptic Communication

• Releases neurotransmitter at a synapse that is very close to target cells

• Ideal for crisis management

What are the cellular components of the endocrine system?

• Includes all endocrine cells and tissues that produce hormones or paracrine factors

• Endocrine Cells – Glandular secretory cells that release their

secretions into extracellular fluid

• Exocrine Cells – Secrete their products onto epithelial surfaces

Figure 18–1

Endocrine System

What are the major structural classes of

hormones?

Hormones

• Can be divided into 3 groups: – amino acid derivatives– peptide hormones – lipid derivatives

Amino Acid Derivatives

• Small molecules structurally related to amino acids

• Synthesized from the amino acids tyrosine and tryptophan

• Tyrosine Derivatives – Thyroid hormones– Catacholamines: epinephrine (E), dopamine

norepinephrine (NE)

• Tryptophan Derivative – Melatonin produced by pineal gland

Peptide Hormones

• Chains of amino acids

• Synthesized as prohormones:– inactive molecules converted to active

hormones before or after secretion

• ADH, prolactin, oxytocin, growth hormone

Lipid Derivatives

• Eicosanoids: – important paracrine factors– Leukotrienes, Prostaglandins

• Steroid hormones: – derived from cholesterol– androgens (estrogens, and progestins)

corticosteroids, calcitriol

What are the general mechanisms of

hormonal action?

• Receptors are protein molecules to which a particular molecule binds strongly

• Different tissues have different combinations of receptors– Presence or absence of specific receptor

determines hormonal sensitivity

• Hormones bind to receptors in cell membrane– Cannot have direct effect on activities inside

target cell

• Hormones indirectly:– alter cellular operations by altering protein

composition and activity – stimulate protein synthesis – activate proteins – modulate current levels of protein synthesis

• First messenger:– leads to second messenger (cAMP, cGMP,

Ca+) – A single hormone promotes release of more

than 1 type of second messenger

• Down-regulation – Presence of a hormone triggers decrease in

number of hormone receptors

• Up-regulation – Absence of a hormone triggers increase in

number of hormone receptors

G Protein

• Enzyme complex coupled to membrane receptor

• Involved in link between first messenger and second messenger

• Binds GTP

• Activated when hormone binds to receptor at membrane surface

1. Activated G protein:– activates enzyme adenylate cyclase

2. Adenylate cyclase:– converts ATP to cyclic-AMP

3. Cyclic-AMP (second messenger):– activates kinase

4. Activated kinases affect target cell:– depends on nature of proteins affected

• Activated G proteins trigger:– opening of calcium ion channels in membrane– release of calcium ions from intracellular

stores

1. Activated G protein stimulates phosphodiesterase (PDE) activity

2. Inhibits adenylate cyclase activity

3. Levels of cAMP decline

4. cAMP breakdown accelerates; cAMP synthesis is prevented

Figure 18–3

G Proteins and Hormone Activity

Steroid Hormones

• Cross cell membrane

• Bind to receptors in cytoplasm or nucleus, activating or inactivating specific genes

Figure 18–4a

Steroid Hormones

Steroid Hormones

• Alter rate of DNA transcription in nucleus:– change patterns of protein synthesis

• Directly affect metabolic activity and structure of target cell

How are endocrine organs controlled?

Hypothalamus

• Integrates activities of nervous and endocrine systems in 3 ways:

1. Secretes regulatory hormones: – special hormones control endocrine cells in

pituitary gland

2. Acts as an endocrine organ• manufactures hormones released into the

bloodstream via the posterior pituitary

3. Contains autonomic centers:– exert direct neural control over endocrine

cells of adrenal medullae

Figure 18–5

Hypothalamus

Where is the pituitary gland located, and

what is its relationship to the hypothalamus?

Pituitary Gland

• Hangs inferior to hypothalamus:– connected by infundibulum

• Releases 9 important peptide hormones

• Hormones bind to membrane receptors:– use cAMP as second messenger

Figure 18–6

Pituitary Gland

What are the hormones produced by the anterior

lobe, and what are the functions of those

hormones?

Anterior Pituitary

• Thyroid-Stimulating Hormone (TSH) – Also called thyrotropin– Triggers release of thyroid hormones– hypothalamic regulatory hormone

• Thyrotropin Releasing Hormone (TRH)

• Adrenocorticotropic Hormone (ACTH) – Also called corticotropin – Stimulates release of steroid hormones by

adrenal cortex– hypothalamic regulatory hormone

• Corticotropin Releasing Hormone (CRH)

• Prolactin (PRL) – Also called mammotropin– Stimulates development of mammary glands

and milk production– hypothalamic regulatory hormone

• prolactin-inhibiting hormone (PIH)

• Growth Hormone (GH) – Also called somatotropin – Stimulates cell growth and replication – hypothalamic regulation

• growth hormone–releasing hormone (GH–RH)• growth hormone–inhibiting hormone (GH–IH)

• Melanocyte-Stimulating Hormone (MSH) – Also called melanotropin – Stimulates melanocytes to produce melanin – Inhibited by dopamine

• Gonadotropins – Regulate activities of gonads (testes, ovaries)– hypothalamic regulation

• gonadotropin-releasing hormone (GnRH)

– GnRH production inhibited by estrogens, progestins, and androgens

• Follicle-Stimulating Hormone (FSH) – Also called follitropin – Stimulates follicle development and estrogen

secretion in females – Stimulates sustentacular cells in males

• promotes physical maturation of sperm

– Production inhibited by inhibin released by testes and ovaries

• Luteinizing Hormone (LH) – Also called lutropin – Causes ovulation and progestin production in

females– Causes androgen production in males

What hormones are secreted by the posterior

lobe, and what are their functions?

• Antidiuretic Hormone – Decreases amount of water lost at kidneys– Elevates blood pressure– Release inhibited by alcohol

• Oxytocin – Stimulates contractile cells in mammary

glands– Stimulates smooth muscles in uterus

Figure 18–9

Summary: The Hormones of the Pituitary Gland