Endocrine System

• The endocrine system

  • Broadcasts hormonal messages all over the body
  • Works by producing hormones in one part of the body which circulate through the whole body. Neurons are specifically connected together
• The endocrine system

  • Long acting
  • The onset of action of a hormone is seconds to minutes which is much slower than neurotransmitters that can take only milliseconds
• The endocrine system

  • Hormones are blood borne
  • Endocrine hormones are released by the “ductless glands” throughout the body into the bloodstream
• The endocrine system

  • Many hormones are also neurotransmitters
  • E.g. epinephrine and norepinephrine can cross the synapse and act as neurotransmitters by may also be released into the bloodstream by the adrenal medulla
• The endocrine system

  • Many of the top drug prescriptions are for hormones
  • SYNTHROID (levothyroxine; used for treating hypothyroidism)
  • All oral contraceptives (birth control pills) are hormones (either an estrogen and a progestin or just a progestin)
  • HUMULIN (recombinant insulin) is the treatment for the most common endocrine disease, diabetes mellitus
  • Various cortical steroids (prednisone, prednisolone, cortisone etc.) are commonly prescribed for inflammation of many types
• Endocrine Glands: these organs all produce endocrine hormones which are stimulated by drugs

  • Hypothalamus
  • Pituitary gland
  • Thyroid gland
  • Adrenal glands
  • Pancreas
  • Testes
• Which is a temporary endocrine gland in the female body

  • The placenta produces huge amounts of endocrine substances, particularly sex steroids, during pregnancy
• Gigantism: due to the hypersecretion of growth hormone (GH)
• Cushing's Syndrome: due to hypersecretion of cortisol
• Diabetes can be due to low amounts of insulin or insensitivity to insulin
• Hypothyroidism: due to a low amount of circulating thyroid hormone and depending on the cause there may be a goiter
• New hormones are constantly being discovered and new roles for old hormones are being found

  • Many are simply made for cellular communication
• Fat cells make leptin when they are “full”. Occupied leptin receptors in the hypothalamus signal satiety
• The empty stomach produced ghrelin. Occupied ghrelin receptors in the hypothalamus cause you to get hungry and eat
• GLP-1 agonists: they are glucagon-like drugs that are effective for the treatment of type 2 diabetes and cause weight loss

  • Semaglutide
• Endocrine hormones: are released by the cell, enter into circulation and interact with receptors on a distant target ce
• For certain lipophilic hormones, the receptors are intracellular but hydrophilic hormones tend to have cell-surface receptors
• Paracrine hormones; they work on nearby target cells. The PGs (prostaglandins) and NO (nitric oxide) are the most well known of these hormones
• Autocrine hormones: they work on the cell where they are produced. Growth factors like IGF1 work as autocrine/paracrines but also as endocrine hormones
• Signal Transduction for Hormones

  Hormones are chemicals, so signal transduction (converting the presence of the hormone to cellular activity) requires a receptor on the target organ
• There are 2 types or hormone receptors

  1. Water-soluble (hydrophilic) hormones

  • The receptors are on the cell surface

  1. Lipid-soluble (lipophilic) hormones

  • The receptors are intracellular and usually within the nucleus (intranuclear)
• Water-soluble (hydrophilic) hormones

  • The receptors are on the cell surface
  • The hormone binds on the exterior of the cell so the hormone message must be converted to another form within the cell (transduced) to have any effect within the cell
  • Water soluble hormones are the most common and they include oxytocin and insulin which we will see later
• Water-soluble circulating hormones

  • These hormones tend to move in the blood easily because of their water solubility
  • The binding of the hormone to the cell-surface receptor often leads to the manufacture of second messengers (cAMP or IP3) which alter cellular function
  • The second messenger concentration can be increased or decreased by hormone receptor binding
• Why is the manufacture of secondary messengers necessary?

  1. The cell is surrounded by a lipid membrane and neurotransmitters, and most hormones, are NOT lipophilic. In order to have an effect internally the 2nd messenger serves the purpose of bringing/amplifying in the signal
  2. The interaction of a few hormones or neurotransmitters (first messengers) can cause the formation of a lot of 2nd messenger so amplification of the signal occurs
  3. Some hormones, such as insulin, cause direct activation of the cell without second messengers
• Some water-soluble hormones have no second messengers

  • Hormones (like insulin) have transmembrane receptors
  • When insulin binds it moves the transporter
  • Sodiums have enough potential to pull glucose in with them, so thats how they get in the cell cytoplasm
• Lipid-soluble (lipophilic) hormones

  • The receptors are intracellular and usually within the nucleus (intranuclear)
  • The hormones bing the intracellular receptors and the hormone-receptor complex bind to DNA to cause effects
  • E.g. steroids like estrogen and testosterone but thyroid hormone as well
• Lipid soluble circulating hormones

  • The hormones tend to move in the blood stream bound to a transport protein
  • The hormone-receptor complex binding to the DNA may lead to cessation of DNA transcription
  • You can often guess which gene is being read with a certain hormone
  • E.g. testosterone leads to the transcription of genes associated with muscles growth
  • Hit the hormone receptor inside the nucleus and binds to DNA
  • Mainly steroids but sometimes thyroid hormones
• The pituitary is made of 2 distinct areas

  1. Anterior pituitary
  2. Posterior pituitary
• Anterior pituitary

  • This gland arises from an outgrowth of the ectoderm of the roof of the mouth
  • It is sometimes adenohypophysis because it is more of a gland than anything else
• Posterior pituitary

  • This gland arises from and outgrowth of ectoderm at the base of the hypothalamus
  • It is sometimes called the neurohypophysis because it arises from neurological tissue
• The anterior pituitary is stimulated by hormones in the blood made by neurosecretory cells in the hypothalamus

  • These hormones 1st travel in the axons to the pituitary portal system and then enter the blood. The hormones travel in the blood to the anterior pituitary where they stimulate hormone release to the blood
• The posterior pituitary releases hormones into the blood which are made by neurosecretory cells in the hypothalamus

  • These hormones travel in axons to the posterior pituitary where they enter the blood stream
• Turk’s saddle or sella turcica

  • Tuberculum sellae
  • Hypophyseal fossa
• Pituitary Tumors

  • The pituitary adenoma is removed and the depression of the hypothalamus is revealed. This depression is what is called a mass effect
• The pituitary tumor pushes the pituitary and it pushes on the optic chiasm resulting in vision loss (bilateral hemaniopsia) because they can’t go down because of the bone
• Pituitary Blood Supply

  • The portal veins have blood under pressure and are very susceptible to damage after hemorrhage
  • The pressure of blood goes from really low, to really high, then to really low
• Controlling Hormone Secretion

  • All of the hormones normally work with a negative feedback mechanism
  • A stimulus leads to increased levels of hormone that then leads to decreases stimulus and lower levels of hormone secretion
• • The types of signal that lead to hormone release are very numerous but can be:
  • Chemical alteration in the blood
  • Ca2+ decrease in the blood leads to parathyroid hormone release
  • Nervous
  • Parasympathetic stimulus leads to insulin release while sympathetic stimulation leads to glucagon release
  • Other hormones
  • The hormone progesterone is released because of luteinizing hormone (LH) release. LH is released because of gonadotropin releasing hormone (GnRH) release
• Control Systems for Anterior Pituitary Hormones

  • There may be releasing and inhibiting hormones
  • Not all anterior pituitary hormones are tropic, some work directly

  1. Releasing hormone: in the hypothalamus
  2. Stimulating hormone: (a tropic hormone) produced in the anterior pituitary
  3. Hormone production: by the target organ
  4. Hormone effects in target issues
• Regulation of Thyroid Hormone

  1. Thyrotropin Releasing Hormone (TRH) produced in, and released by, the hypothalamus
  2. Thyroid Stimulating Hormone (TSH), produced in, and released by, the anterior pituitary
  3. Thyroid hormone produced in and released by the thyroid gland
  4. Increased metabolic rate
  5. Increased protein synthesis
  6. Increased fat breakdown