Introduction to the Endocrine System

What is the endocrine system?

The body system that produces and secretes hormones into the bloodstream to regulate physiological functions

What are the main endocrine organs?

• Hypothalamus

• Pituitary

• Thyroid

• Adrenals

What is endocrine?

A type of cell signalling where substances (hormones) are secreted internally into the bloodstream to act on distant target cells.

Endocrine organs release hormones that are important in 4 broad areas; what are those 4 areas?

• Reproduction

• Growth and development

• Maintenance of internal environment

• Regulation of energy

What are the key structure features of endocrine glands?

• Ductless

• Highly vascularized

• Secretes messengers (hormones) directly into circulation

• Primary glands: e.g. pituitary, thyroid, adrenals

• Other organs may have secondary endocrine function, e.g. hypothalamus, heart, kidney, GI tract

What are hormones?

Chemical messengers released by endocrine cells into the bloodstream that acts on specific target cells with appropriate receptors to regulate physiological processes.

Are hormones present in high or low concentrations?

Low concentrations (10^-7 to 10^-12 M)

What do hormones bind to?

They bind to specific, high affinity receptors on/in target cells.

Do single hormones have the same or different tissue-specific effects?

Single hormones may have different tissue-specific effects

What are the 3 major chemical classes of hormones?

1. Amino acids / amines

2. Peptides and proteins

3. Steroids

What are amine hormones?

Small hormones made from single amino acids

What are the 2 main sources of amine hormones?

1. Tyrosine

2. Tryptophan

What specific hormones are derived from tyrosine and tryptophan?

• Tyrosine is the precursor for catecholamines (e.g. adrenaline) and thyroid hormones T4 (thyroxine) and T3 (triiodothyronine)

• Tryptophan is the precursor for indoleamines (e.g. melatonin)

What is the pathway of catecholamine synthesis from tyrosine?

Tyrosine is converted to L-DOPA by tyrosine hydroxylase, then to dopamine by DOPA decarboxylase, then to noradrenaline by dopamine β-hydroxylase, and finally to adrenaline by phenyl ethanolamine N-methyltransferase.

Thyroid hormone synthesis

Name some steroid hormones

• Cortisol

• Aldosterone

• Testosterone

• Estradiol

What are peptide and protein hormones? Give examples.

• Peptide hormones: Short chains of amino acids.

  • ADH (9 amino acids)

  • Oxytocin (9 amino acids)

• Polypeptide hormones: Longer amino acid chains.

  • Insulin

  • Prolactin (198 amino acids)

• Protein hormones: Very long polypeptide chains with complex structures.

  • Thyroid-stimulating hormone (TSH)

  • Follicle-stimulating hormone (FSH)

  • Growth hormone (GH)

Which hormones are water soluble?

Peptide and amino acid hormones

They dissolve freely in plasma, and the cell receptor location is on the cell surface membrane

Which hormones are lipophilic?

Steroid and thyroid hormones

• They travel in the blood bound to carrier transport proteins and the receptor location is intracellular (cytoplasmic or nuclear).

What’s the mechanism of action for water soluble hormones?

Second-messenger system

What’s the mechanism of action for lipophilic hormones?

They directly regulate gene transcription

What are hormone receptors?

The ability of a cell to respond to a hormone depends on the presence of receptors for that hormone on the surface or in the target cells.

Can the number of receptors for a hormone change?

Yes they can increase (upregulate) or decrease (downregulate)

What do cell surface receptors do?

They activate intracellular signalling cascades

What do intracellular receptors do?

They activate gene transcription

How does direct endocrine hormone release occur?

1. A stimulus acts on endocrine cells

2. This makes the endocrine cells secrete a hormone into bloodstream

3. The hormone travels to specific target cells and produces a physiological response

How does hormone release occur through hypothalamic-pituitary axis?

1. A stimulus activates a hypothalamic neurone

2. The hypothalamic neurone releases a regulatory hormone

3. This hormone stimulates endocrine cells in the anterior pituitary gland, secreting a tropic hormone

4. The tropic hormone then acts on a target endocrine organ, leading to the release of another hormone that produces a response in target cells

What does the HPA axis do?

It controls the body’s response to stress by releasing cortisol

Which hormones are involved in the HPA axis?

CRF

ACTH

Cortisol

Which gland releases ACTH?

Pituitary gland

Which gland releases cortisol?

Adrenal gland

What is the sequence of hormones released in the HPA axis during stress?

Hypothalamus releases CRF
Pituitary gland releases ACTH
Adrenal glands release cortisol

What is homeostatic regulation of hormone release?

Hormone release is controlled by feedback mechanisms that keep internal conditions stable.

When hormone levels rise, they signal the brain to reduce further release. When levels fall, the brain increases hormone production.

How is the HPA axis regulated by homeostasis?

Cortisol levels control the system

• High cortisol reduces CRF and ACTH release

• Low cortisol increases CRF and ACTH release

What are the key features of endocrine communication?

• Hormones are released into the bloodstream from glands to target cells

• Signalling is slow compared to neural signalling

• The effects of hormones are long lasting

• All cells are exposed, but only cells with specific receptors respond

• Endocrine communication is important for the long term maintenance of homeostasis

Once a hormone has acted on its target tissue, what happens?

• The concentrations must return to normal to prevent harmful effects of prolonged exposure

• Therefore, hormones are excreted as urine or faeces (protein-bound hormones are protected from excretion and removal takes longer)

• Hormones are also inactivated by metabolism. Enzymes in the blood rapidly break down peptides and catecholamines within minutes to hours.

What are the 4 types of endocrine disorders?

• Hypo-secretion

• Hyper-secretion

• Hypo-responsive

• Hyper-responsive

Example of hypo-secretion

Type I diabetes

• Not enough insulin is being produced

Example of hyper-secretion

Pancreatic endocrine tumour

Example of hypo-responsive

Insulin resistant type II diabetes

Example of hyper-responsive

TSH binds to the TSH receptor

This tells the thyroid to make T3/T4 hormones

Constitutive activation means the TSH receptor is switched on all the time, even without TSH binding

The thyroid keeps producing hormones non stop, leading to hyperthyroidism