H1.1 State that hormones are chemical messengers secreted by endocrine glands into the blood and transported by the blood to specific target cells

A hormone is a regulatory chemical that is secreted in to the blood by an endocrine gland or an organ of the body exhibiting an endocrine function The blood carries the hormone to every cell in the body, but only target cells for a given hormone can respond to it

H1.2 State that hormones can be steroids, peptides, and tyrosine derivatives, and provide one example of each Hormones secreted by endocrine glands belong to four different chemical categories: 1. polypeptides  composed of chains of amino acids that are shorter than about 100 amino acids  examples include insulin and antidiuretic hormone (ADH)  lipophobic or polar (water-soluble)

2.glycoproteins (modified proteins)  composed of a polypeptide much longer than 100 amino acids to which is attached a carbohydrate  include follicle stimulating hormone (FSH) and luteinizing hormone (LH)  lipophobic or polar (water-soluble)

3.amines  derived from the amino acids tyrosine and tryptophan  include the hormones from the adrenal medulla (epinephrine and norepinephrine), thyroid (thyroxine and calcitonin), and pineal (melatonin) glands  epinephrine and norepinephrine are known as catacholamines and are derived from tyrosine  thyroxine which is secreted by the thyroid gland is also derived from tyrosine and is lipophilic (fat-soluble)  all the others are polar, or water-soluble  melatonin is derived from tryptophan

4.steroids  lipids derived from cholesterol that are lipophilic  include testosterone, estradiol (estrogen), progesterone, aldosterone, and cortisol

 may be subdivided into sex steroids secreted by the testes, ovaries, placenta, and adrenal cortex (estrogens, progestins, and androgens)  also subdivided into corticosteroids secreted by the adrenal cortex (cortisol) which regulates glucose balance, and mineralocorticoids (aldosterone) which regulates salt balance

H1.3 Distinguish between the mode of action between steroid hormones and protein (peptide) hormones Lipophilic (fat-soluble) and lipophobic or polar (water- soluble) hormones regulate their target cells by different means Lipophilic hormones which include all of the steroid hormones and thyroxine, can easily enter the cell because the lipid portion of the plasma membrane does not present a barrier to entry since they are chemically similar Because these hormones are not water-soluble, they will not dissolve in plasma and travel through the blood attached to protein carriers

At the target cells, they detach from their carriers and pass through the plasma membrane Some steroid hormones bind to specific receptor proteins in the cytoplasm forming a hormone-receptor complex which then moves into the nucleus Other steroids travel directly into the nucleus before finding their receptor (T4) Once the hormone receptor is activated through attachment of the hormone, it binds to specific regions of the DNA (hormone response elements)

This directly affects the level of transcription at that site by activating or deactivating transcription Activation produces mRNA which then codes for specific proteins which will change the metabolism of the target cell

The thyroid hormone’s mechanism of action is similar Thyroxine contains 4 iodines (T4) and upon entering the cell is changed into triiodothryonine with 3 iodines (T3) which then enters the nucleus and binds to nuclear receptors This hormone-receptor complex binds to regions of the DNA and stimulates gene transcription by regulating genes in the target cell

Hormones that are too large or too polar to cross plasma membranes include the peptide and glycoprotein hormones as well as the catecholamine hormones epinephrine and norepinephrine (amine hormones derived from tyrosine) They bind to receptor proteins on the outside surface of the plasma membrane and do not enter the cell; they are the “first messenger” A number of different molecules inside the cell serve as “second messengers” to produce the effects of the hormone An example of a second messenger is cyclic AMP (cAMP) See Fig.47.7 pg. 998

The interaction between the hormone and its receptor activates mechanisms that increase the concentration of the second messengers within the target cell cytoplasm The binding of the hormone to its receptor is reversible and brief

After it activates the second messenger, it detaches from the receptor and may travel in the blood to another target cell somewhere else Eventually liver enzymes break down the hormone into inactive derivatives Some of the larger polar hormones act by causing a change in the shape of membrane proteins called ion channels A change in shape opens normally closed channels allowing a particular ion to enter or leave the cell or the change closes normally opened channels Examples of this type of second messenger include inositol triphosphate (IP3) and calcium ions (Ca++)

In many cases, the second messengers activate previously inactive enzymes

H1.4 Outline the relationship between the hypothalamus and the pituitary gland The hypothalamus controls the production and secretion of the anterior pituitary hormones The secretion of the anterior pituitary product is hormonally controlled, instead of through nerve axon stimulus Neurons in the hypothalamus secrete hormones that are carried by short blood vessels (primary capillaries which connect to portal venules) directly to the anterior pituitary

These hormones will either stimulate or inhibit the secretion of anterior pituitary hormones The hormones secreted by some endocrine glands feed back to inhibit the secretion of hypothalamic releasing hormones and anterior pituitary tropic hormones The posterior pituitary contains axons that originate in cell bodies within the hypothalamus and extend along the stalk of the pituitary as a tract of fibers This relationship results from the way the posterior pituitary is formed during embryonic development Neural tissue from the hypothalamus grows downward during development to produce the posterior pituitary resulting in the hypothalamus and posterior pituitary remaining interconnected by a tract of axons See Text Fig pg. 1003

H1.5 Explain the control of ADH (vasopressin) secretion by negative feedback The posterior pituitary gland contains axons originating from neurons in the hypothalamus These neurons produce ADH and oxytocin which are stored and released from the posterior pituitary in response to neural stimulation from the hypothalamus In response to increased blood plasma osmolality by osmoreceptors in the hypothalamus, ADH is released by the posterior pituitary into the blood ADH promotes water retention by the kidneys and increases vasoconstriction which will raise blood pressure This works as a negative feedback loop to correct the initial disturbance of homeostasis See Fig pg. 1000