Biology 115, Lecture 14:  The Endocrine System
T. Irving 01/07/97; Revised A.Howard 02/27/00

Goals of this section:

• Name and give the location of each of the major endocrine glands
• Contrast the mechanism of action of peptide hormones with steroid hormones
• Explain negative feedback within the three-tiered control system--the hypothalamus, the anterior pituitary, and a gland controlled by the anterior pituitary
• Contrast the ways the hypothalamus controls the anterior vs. posterior pituitary
• List hormones produced by major endocrine glands and describe their effects
• Give examples of medical conditions involving over- or underproduction of hormones
• List other cells that produce hormones and describe their function

Why have an Endocrine System?

• Large organisms have trillions of cells and many different types of tissues and organs
• All must work in a coordinated way even if they are in widely dispersed parts of the body
• Nervous and endocrine systems work together so body parts can communicate

Endocrine system basics

• Glands are located various parts of the body
• Glands secrete "gland specific" hormones into blood stream
• Hormones effect only specific target organs or cells

Two types of hormones:

• Steroids
• bind to specific receptors in cytoplasm of target cells and turn on specific genes
• produced in ovaries, testes, adrenal cortex
• Peptides
• bind to specific receptors on cell surfaces
• activate certain enzymes
• produced by all other glands

Glands

Hypothalamus and Pituitary

• Located in the brain
• Pituitary:
• connected to the brain by a stalk
• has two parts, posterior and anterior
• Hypothalamus produces:
• oxytocin: causes uterine contractions, stimulates milk production
• antidiuretic hormone (ADH): regulates water content of blood via the kidney
• Both of these hormones are transferred to and released by the posterior pituitary
• Other hormones work by a three tiered system
1. Hypothalamus produces releasing or inhibitory hormones which target the anterior pituitary
2. Anterior pituitary is the "master gland":
   hormones from hypothalamus stimulates or inhibits corresponding hormones
3. Hormones effect other tissues or glands
• Physiological response: Release of other hormones
• Examples of anterior pituitary hormones:
• Growth hormone: Determine height by stimulating cell division & bone growth:
  Too little -dwarf; Too much - giant
• Prolactin: Stimulates mammary glands to produce milk
• Thyroid stimulating hormone
• Gonadotrophic hormones:
  Stimulate gonads i.e. ovaries & testes to produce sex hormones
• ACTH (adrenocorticotripic hormone): Stimulates adrenal cortex

Thyroid Gland

• Located in the front of the neck
• Produces thyroxin:
  Thyroxin doesn't have a specific target organ but stimulates most cells to increase metabolic rate
• Synthesis of thyroxin requires iodine; "iodized salt" is a good source
• Too little thyroxin: Conditions such as goiter, cretinism, myxedema
  In goiter thyroid enlarges to try to make more thyroxin
• Too much thyroxin: "Graves's disease"--Hyperactivity insomnia, irritability
• Thyroid also produces calcitonin --inhibits osteoclasts, increases Ca²⁺ deposition in bone

Parathyroid glands

• 4 glands embedded in posterior surface of thyroid
• Produces parathyroid hormone (PTH)
• As more PTH released blood [Ca²⁺] rises
• PTH targets the digestive tract to absorb more Ca²⁺; Kidneys retain more Ca²⁺
• Demineralization of bone (stimulus of osteoclasts)
• As calcitonin increases, blood [Ca²⁺] decreases
• As blood [Ca²⁺] increases, PTH production decreases
• As blood [Ca²⁺] decreases, PTH production increases

Adrenal Glands

• Located at the top of kidneys; consist of cortex and medulla
• Medulla produces epinephrine & norepinephrine -- "fight or flight"
• Increased levels of epinephrine and norepinephrine result in:
• rise in [blood glucose], metabolic heart rate
• increased respiration, increased blood supply to muscles
  --> increased muscle stamina
• Adrenal cortex produces various steroid hormones:
1. gluticorticoids, especially cortisol:
   • typical of those produced in the 3-tiered system:
     hypothalamus --> ant. pit. -> adrenal cortex ---> cortisol
   • cortisol can raise blood glucose levels (in opposition to insulin)
   • favors conversion of proteins to amino acids and thence to glucose in liver
   • counteracts inflammation
2. mineral corticoids, especially aldersterone:
   • Not under control of Anterior Pituitary
   • regulates blood [Na⁺], [K⁺]
   • Target organ the kidney
   • Renin-angiotensin- aldosterone system: Controls blood pressure
3. small amounts of sex hormones (including female in males & vice versa)
• Diseases of the Adrenal Cortex
• Underactive Adrenal Cortex - Addison disease
• Low levels cortisol ---> low [blood glucose]
• Low levels of aldosterone --> low blood pressure
• low blood pH, susceptible to stress, bronze skin color
• Overactive Adrenal Cortex - Cushing Syndrome
• High levels of cortisol can lead to diabetes mellitus
• less muscle protein, increased subcutaneous fat
• High levels aldosterone ---> high blood pressure
• trunk obese, edema of face (moon face); arms, legs normal

The pancreas as an endocrine organ

• Controls normal glucose levels
• Insulin is secreted after eating
• Stimulates uptake and metabolism of glucose by liver, muscle and fat tissue
• Stimulates liver and muscle to store glucose as glycogen
• Promotes buildup of fats and proteins and inhibits their use for catabolism.
• Blood [glucose] drops
• Glucagon is secreted between meals
  • Opposes action of insulin
  • Promotes breakdown of stored nutrients; blood [glucose] rises
• Diabetes:
  • Low blood volume and acidosis -->
    Blindness, kidney disorder, coma and death
  • Type I - Insulin dependent diabetes:
    • 2,000,000 cases/year U.S.
    • no insulin produced: Killer T-cells kill Island of Langerhans cells (viral infection?)
    • Insulin must be provided by injection
  • Type II -Insulin Independent diabetes
    • 10,000,000 cases/year U.S.
    • Pancreas produces insulin but cells unresponsive
    • Found in obese and inactive people.
    • Prevention by low fat diet, exercise best

Testes (male) & Ovaries (females)

• Ovaries produce estrogen and progesterone
  • Hypothalamus -> anterior pituitary -> ovaries
  • leads to maturation of uterus
  • Control of menstrual cycle
  • secondary sex characteristics, breasts, fat distribution
• Testes Produce Androgens, e.g. testosterone
  • Hypothalamus -> ant. pit. -> testes
  • Maturation sex organs & sperm
  • Secondary sex characteristics, beards, musculature, deep voice etc.

Other Glands & Hormones

• Thymus
  • Located in the chest
  • Thymosins are hormones that promote T-cell maturation and other immune cells
  • T-cells mature in thymus
• Pineal Gland (in brain):
  produces hormone called melatonin responsible for circadian rhythms
  Melatonin high --> sleepiness
• Growth factors:
  Produced in many different cell types; Stimulate cell growth & division
• Prostaglandins:
  
  • A large class of hormones that are produced and act locally
    in response to other hormones or to trauma;
  • various effects on many tissues
  • Some menstrual disorders - prostaglandins - treated anti-prostaglandin drugs