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Topic 4: Endocrine SystemMajor glands and Hormones
- Endocrine: refers to hormone that is produced and secreted within the body
- Exocrine: refers to hormone that is produced and secreted outside the body
- Paracrine: acts on nearby cells
- Auotcrine: acts on its own secreting cell
- Neuroendocrine: synthesized by nervous tissue and carried in the blood
- Gland: cluster of cells organized for synthesis / release of compound, can be endocrine or
exocrine
- Hormones: regulatory chemicals produced in an endocrine gland or scattered cells, secreted into
the blood and carried to its target cell that responds by altering its metabolism
- Involved in maintaining homeostasis
- Subjected to tight regulation by feedback (mainly negative)
Structural Classification
- Steroid hormone:
- Step wise Conversion of cholesterol by multiple enzymes
- Lipid-soluble = leave production cells and enter target organ by diffusion through the
membrane
- Peptides, proteins, glycoproteins:
- Chains of amino acids = sequence determines primary structure and nature
- Genetic code
- After translation (ribosome/cytosol), protein can be modified:
- Glycosylation = carbohydrates chains attached
- Phosphorylation = phosphate group attached
- Modified, packaged in the ER and golgi
- Secreted by exocytosis
- AA derivatives (thyroid hormones, catecholamines):
- Derived from tyrosine and tryptophan
- FA derivatives or eicosanoids (prostaglandins):
- Derived from cell membrane phospholipids (arachidonic acid)
- Mainly autocrine and paracrine effects
Major Glands
- Pineal gland
- Encapsulated by meninges
- Secretes melatonin
- Under indirect influence of the hypothalamus circadian center
- Sleep patterns and recognition of seasons
- Kidneys
- Renin = increased aldosterone secretion by adrenal cortex
- Adipose Tissue
- Adipocytes release leptins and adipokines
, - Give information about energy storage status
- Regulates appetite
Hormone Receptors
- Hormones binding to receptors initiated the effects
- Lipophilic hormones
- Diffuse out of producing cells
- Must be bound to carriers since they are not water soluble
- Carrier proteins are big: keep hormone in vessel, prevent hormone degradation
- Diffuse in target cells = intracellular receptors
- regulation of bioactivity can be thru altering the concentration of carrier proteins
- Enters target cells, finds Nuclear hormone receptors (NHR = transcription factors)
- Hormone receptor complex translocates to the nucleus and binds to specific DNA
sequence = stimulates gene expression
- Slow acting hormones
- In the case of thyroid hormone
- Major circulating is T4 (thyroxine)
- When it enters target cell, its converted to T3 (active form, tri-iodo-thyronine)
- Binds to its specific receptor after entering the nucleus
- Water soluble hormones
- Secreted via exocytosis (packaged first)
- Circulate free in the blood
- Stay out of target cell, cannot enter phospholipid membrane barrier = bind to surface
receptors (extracellular)
- Receptor is the key mediator bw hormone and downstream effects
- 4 main classes of receptors:
- G-proteins coupled
- Messenger: adenylate cyclase; cAMP, Phospholipase C; Ca2+
- Tyrosine kinase
- Autophosphorylation
- Interleukin/cytokine family
- JAK-STAT
- Lacks ability to self phosphorylate
- Serine kinase (TGFbeta)
- Phosphorylation with SMADs
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