| KEY WORDS | MEANING |
| Endocrine glands
|
Glands that secrete hormones. Hormones are secreted
by glands so they can circulate in the blood and act on target
cells.
For introduction see page 138 and Figure 6-26. |
| Hormones | Chemical messengers carried by blood from endocrine glands to cells they act upon. |
| Endocrine system | Consists of all the glands which secrete hormones. |
| Table 10-1 | Good summary of the various hormones and their mechanisms of action. Do not need to memorize. |
| Target cells | The cells hormones act upon. |
| HORMONE STRUCTURE AND SYNTHESIS | 1. AMINE HORMONES
2. PEPTIDE HORMONES 3. STEROID HORMONES |
| Table 10-2 | Outlines differences between peptide/catecholamine hormones and steroid/thyroid hormones. Know this. |
| AMINE HORMONES | Derived from the amino acid tyrosine. |
| 1. Thyroid hormones | Secreted from the thyroid gland in the lower part of the neck. |
| Fig. 10-1 | Thyroid secretes 3 hormones; 2 contain iodine, an essential element. T3 contains 3 iodines, T4 4 iodines. (Fig. 10-1). |
| Iodine | Most of the iodine ingested in food is absorbed by the intestines, transported in blood and actively transported into the thyroid cells. |
Almost all cells are effected by thyroid hormones.
Effects include:
|
|
| 2. Adrenal hormones
Fig. 10-5. |
There is an adrenal gland on top of each kidney.
This gland has an outside cortex and inside called the medulla. The cortex secretes steroid hormones. The medulla secretes amine hormones. This region is activated by the sympathetic nerves and acts much like a ganglion. |
| The medulla amine hormones are:
1. Epinephrine (secretion is 4 times the amount of NE) 2. Norepinephrine (NE) 3. Dopamine |
|
| PEPTIDE HORMONES | MOST HORMONES ARE PEPTIDES, STRINGS OF AMINO ACIDS. |
| Fig. 10-2 | The proteins are synthesized on ribosomes.
They are packaged in Golgi as prohormones and secreted in vesicles
by exocytosis when the cell is signaled by an increase in calcium
in the cell.
When the peptides are secreted into extracellular fluid, the proteins can move into capillaries through the leaky capillary walls. Many hormones are the same as neurotransmitters in the nervous system but the blood-brain barrier stops hormones in the blood from becoming neurotransmitters in the brain. |
| STEROID HORMONES | Produced in:
1. Adrenal cortex 2. Testes and ovaries 3. Placenta during pregnancy |
| Steroids are derived from cholesterol
Fig. 10-3 |
Cholesterol is the precursor of all steroid
hormones!
Steroids are very lipid-soluble so can diffuse across the membrane from the cell in which they are produced, into the blood. |
| Steroids from adrenal cortex
Fig. 10-5 |
1. Aldosterone - effects salt and mineral balance
in body.
2. Cortisol - effects glucose metabolism. 3. Androgens - leads to masculinization, includes testosterone. |
| Steroids from gonads | 1. Testes - testosterone promotes secondary
sexual characteristics in males.
2. Ovaries - estradiol, progesterone promote secondary sexual characteristics in females and cyclic changes in the uterus. |
| HORMONE TRANSPORT IN THE BLOOD | The water-soluble hormones dissolve in plasma.
Steroid and thyroid hormones are largely bound to plasma proteins. |
| A small amount of steroid and thyroid hormones
become unbound, can diffuse across target cell membranes and attach to
receptors.
For example, less than 1% of plasma thyroid hormone is free. |
|
| HORMONE METABOLISM AND EXCRETION
Fig. 10-7 |
A hormone's concentration in the blood depends
on:
1. It's rate of secretion by endocrine gland. 2. It's rate of removal from the blood. |
| Removal from blood | Either by metabolism in cells with receptors
for the hormones or excretion by liver or kidneys.
Protein-bound hormones stay in blood much longer than free soluble hormones. Protein bound may stay days compared to minutes for soluble hormones. Sometimes metabolism after secretion makes the hormones more active. ie. T4 to far more active T3. |
| MECHANISMS OF HORMONE ACTION | |
| Hormone receptors | Even though hormones can go all over the body in blood, they only act on cells with hormone receptors for that hormone. |
| Receptors inside cell vs. on cell membrane | 1. Receptors for steroid and thyroid hormones
are in the cell.
2. The receptors for peptides and catecholamines are in the cell
membrane.
|
| Up-regulation
Down-regulation |
Up-regulation is an increase in the number
of receptors on the target cell and down-regulation is a decrease.
The former often happens with prolonged exposure to a low levels of the
hormone and the latter after high levels.
Hormones may increase their own receptors or other hormone receptors. This can happen when two hormones act in tandem. |
| Effects on membrane receptors | When peptides and catecholamine hormones act on these, signal transduction mechanisms are activated: opening channels, using G proteins, cyclic GMP or AMP. This activates proteins inside the cell. |
| Effects on receptors inside the cell.
Similar to Fig. 5-9. |
Steroid and thyroid hormones increase or decrease protein synthesis by changing mRNA production. This is done by binding to the receptor, activating it, then binding to the promotor region of DNA, causing production of mRNA, which serves as a template to make protein. Sometimes a hormone will suppress mRNA production. |
| Pharmacological effects of hormones | Unnatural effects can occur when hormones are given in amounts higher than the body would produce. ie. cortisol to suppress inflammation. Steroids given in autoimmune diseases (multiple sclerosis, lupus) are used to suppress inflammation and have side effects with prolonged use. |
| CONTROL OF HORMONE SECRETION | |
| Patterns of secretion | 1. In short bursts.
2. Or 24 hour cycles |
| What signals hormone secretion?
Fig. 10-9 |
1. Plasma concentration of ions or nutrients:
- by negative feedback. ie. Glucose stimulates insulin secretion which
reduces glucose concentration.
2. Neurotransmitter: ie. the adrenal medulla is stimulated by sympathetic fibers. Also, the hypothalamus and posterior pituitary are under direct control
of neurons in the brain.
3. Another hormone: Sometimes the function of a hormone (called a tropic hormone) is to stimulate the secretion of another hormone. The tropic hormone can also cause growth of the endocrine gland it is stimulating. A combination of the above may signal hormone secretion. |
| Figs. 10-4, 10-6, 10-8 | Do not need to know. |
| Figs. 10-7, 10-9,
10-10, 10-11 |
Know these |
| HORMONES OF THE HYPOTHALAMUS AND PITUITARY | |
| Anatomy
Fig. 10-12 |
The pituitary gland lies just below the thalamus
in a pocket of bone in the skull. The pituitary is connected to the thalamus
by a stalk.
The pituitary has two lobes: 1. The anterior pituitary receives a distinct blood supply from the hypothalamus. 2. The posterior pituitary receives a nerve supply from the hypothalamus and is an outgrowth of the hypothalamus. |
| POSTERIOR PITUITARY HORMONES
|
These are produced in the hypothalamus and in
neurons that send axons into the pituitary. The hormones are packaged in
vesicles, and in response to an action potential are released from the
axon terminal into capillaries, then back to the heart and then throughout
the body.
Two posterior pituitary known hormones: 1. Oxytocin: It stimulates uterine contraction during labor and release of breast milk. 2. Vasopressin (ADH): it helps retain water (at kidneys) and raise blood pressure. Dopamine is also released from pituitary. |
| ANTERIOR PITUITARY HORMONES
Figs. 10-13, 10-14, 10-15, 10-16. |
There is typically a three hormone sequence
involving the anterior pituitary.
1. A hormone produced and released from hypothalamus into portal vessels. 2. This signals release of a hormone from the anterior pituitary. 3. This hormone goes through the circulation and acts on a target organ in the body which releases a hormone. Remember: 3 tissue sites - 3 hormones. |
| SIX WELL ESTABLISHED ANTERIOR PITUITARY HORMONES
Fig. 10-14 KNOW THIS FIGURE, |
1. FSH - follicle stimulating hormone
2. LH - luteinizing hormone 3. GROWTH HORMONE - growth hormone 4. TSH - thyrotropin 5. PROLACTIN 6. ACTH - adrenocorticotropic hormone |
| 1. FSH - follicle stimulating hormone
2. LH - luteinizing hormone *** |
Target organ: Testes and ovaries
Function: 1. Growth and development of egg and sperm cells. 2. Secretion of estrogen and progesterone in female and testosterone in male. |
| 3. GROWTH HORMONE | 1. Target organ: Liver
Function: stimulates liver to secrete IGF-I (insulin-like growth factor-I). This is a growth-promoting peptide and effects metabolism. 2. Target organ: Many organs and tissues Function: stimulates protein synthesis. Stimulates carbohydrate and lipid metabolism. |
| 4. TSH - thyrotropin
*** |
Target organ: thyroid
Function: Secretion of T3, T4 which regulate metabolic rate,
growth and brain development and function.
|
| 5. PROLACTIN | Target organ: Breasts
Function: Breast development during pregnancy, milk production during nursing. |
| 6. ACTH - adrenocorticotropic hormone
*** |
Target organ: Adrenal cortex
Function: Secretes cortisol which increases when the body is stressed. Cortisol releases nutrients into the blood, increases smooth muscle tone, is anti-inflammatory. |
| *** | Denotes organs which are part of the three chain system. These release a third hormone which acts on target organs. |
| HYPOTHALAMUS TO ANTERIOR PITUITARY
(AP) AXIS
|
1. Some CNS neurons synapse in the hypothalamus.
2. These stimulate (or inhibit) the hypothalamus to secrete hormones. 3. The hormones secreted from the hypothalamic neurons are picked up in portal blood vessels and sent to the anterior pituitary. 4. In the AP they act on cells to cause release of anterior pituitary hormones. Thus, they are generally called releasing hormones (RH). |
| Fig. 10-16 | This figure shows the releasing hormones which act on particular hormones in the AP. The releasing hormones are called hypophysiotropic hormones. Do not need to know the individual releasing hormone names. These chemicals also exist in other parts of the central nervous system. They are all peptides, except dopamine. |
| Somatostatin, dopamine | These inhibit release of AP hormones.
Somatostatin inhibits release of growth hormone. Dopamine inhibits release of prolactin. These can counterbalance other hormones: Somatostatin (-) and GHRH (+). Somatostatin (-) and TRH (+). Dopamine (-) and TRH (+).
|
| Fig. 10-17 | Summarizes the hypothalamus-anterior pituitary system. Know the hormones released from the anterior pituitary, the target organs they act upon, and the response of the target organs. |
| Negative feedback
Figs. 10-18, 10-19, 10-20
Fig. 10-18. |
An important concept:
A chemical whose level is increased by action of a hormone can suppress the further action of the hormone and thus prevent the level of the chemical from getting too high. For example: A painful stimulus can cause CRH --> ACTH --> cortisol. The cortisol feeds back to inhibit the release of CRH from hypothalamus and ACTH from pituitary. |
| Long loop negative feedback. (Fig. 10-20)
Short loop negative feedback. (Fig. 10-20)
|
Long loop negative feedback: In a three-hormone
system the third hormone can suppress the release of the first or second
hormone.
Short loop negative feedback: In a two-hormone system the second hormone can act to suppress the first hormone. Fig. 10-21 growth hormone example. Do not need to know. |