Endocrine Overview

Endocrine system acts with

nervous system to coordinate and integrate activity of body cells

Endocrine system and nerouvs system work together to

• Influences metabolic activities via hormones transported in blood 

• Responses slower but longer lasting than nervous system responses

Endocrinology:

study of hormones and endocrine organs

Endocrine system controls and integrates:

• Reproduction

• Growth and development

• Maintenance of electrolyte, water, and nutrient balance of blood

• Regulation of cellular metabolism and energy balance

• Mobilization of body defenses

Exocrine glands -

• Produce nonhormonal substances (examples: sweat, saliva)

• Have ducts to carry secretion to membrane surface

Endocrine glands: pituitary, thyroid, parathyroid, adrenal, and pineal glands 

• Produce hormones

• Lack ducts

• Hypothalamus is neuroendocrine organ

• Some have exocrine and endocrine functions

  • Pancreas, gonads, placenta

• Other tissues and organs that produce hormones

  • Adipose cells, thymus, and cells in walls of small intestine, stomach, kidneys, and heart

Chemical messengers of endocrine system:

-Hormones

-Autocrines

-Paracrines

Hormones -

long-distance chemical signals; travel in blood or lymph

Autocrines -

chemicals that exert effects on same cells that secrete them

• Autocrines and paracrines are local chemical messengers; not considered part of endocrine system

Paracrines-

• locally acting chemicals that affect cells other than those that secrete them

  • Autocrines and paracrines are local chemical messengers; not considered part of endocrine system

• Two main classes of hormones:

• Amino acid–based hormones 

• Steroids

Amino acid–based hormones 

Amino acid derivatives, peptides, and proteins

Steroids -

• Synthesized from cholesterol

• Gonadal and adrenocortical hormones

• A possible third class, eicosanoids, is considered a

• hormone by some scientists, but most classify it as a paracrine

• Though hormones circulate systemically, only cells with receptors for that hormone are affected

Target cells -

tissues with receptors for a specific hormone

• Hormones alter target cell activity

Hormone action on target cells may be to:

• Alter plasma membrane permeability and/or membrane potential by opening or closing ion channels

• Stimulate synthesis of enzymes or other proteins

• Activate or deactivate enzymes

• Induce secretory activity

• Stimulate mitosis

Hormones act in one of two ways, depending on their chemical nature and receptor location

• Water-soluble hormones (all amino acid–based hormones except thyroid hormone)

  • Act on plasma membrane receptors

  • Act via G protein second messengers

  • Cannot enter cell

• Lipid-soluble hormones (steroid and thyroid hormones)

  • Act on intracellular receptors that directly activate genes

  • Can enter cell

Amino acid–based hormones, except thyroid hormone, exert effects through

second-messenger systems

Two main second-messenger systems:

Cyclic AMP

Cyclic AMP (cAMP) signaling mechanism

1. Hormone (first messenger) binds to receptor

2. Receptor activates a G protein

3. G protein activates or inhibits effector enzyme adenylate cyclase

4. Adenylate cyclase then converts ATP to cAMP  (second messenger) 

5. cAMP activates protein kinases that phosphorylate (add a phosphate) other proteins

Cyclic AMP (cAMP) signaling mechanism (cont.)

• Phosphorylated proteins are then either activated or inactivated

• cAMP is rapidly degraded by enzyme phosphodiesterase, stopping cascade

• Cascades have huge amplification effect

Other signaling mechanisms

• cGMP (cyclic guanosine monophosphate) is second messenger for selected hormones

• Other hormones work without second messenger system

  • Example: insulin receptor is a tyrosine kinase enzyme that autophosphorylates upon insulin binding

    • Activated tyrosine kinases provide docking sites for relay proteins that trigger cell responses

Lipid-soluble steroid hormones and thyroid hormone can diffuse into

target cells and bind with intracellular receptors

Receptor-hormone complex enters

• nucleus and binds to specific region of DNA

• Helps initiate DNA transcription to produce mRNA

  mRNA is then translated into specific protein

  • Proteins synthesized have various functions

  • Examples: metabolic activities, structural purposes, or exported from cell