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a mediator molecule that is released in one part of the body but regulates the activity of cells in other parts of the body; most enter the bloodstream; bind to receptors on or in the target cells


exocrine gland

secrete products into ducts that carry the secretions into body cavities, into the lumen of an organ, or to the outer surface of the body; i.e. sudoriferous (sweat), sebaceous (oil), mucous and digestive glands


endocrine glands

secrete their products (hormones) into interstitial fluid surrounding the secretory cells rather than into ducts, from the interstitial fluid, hormones diffuse into the blood which carries the hormones to the target cells



protein that only specifically recognize and bind to one hormone


down regulation

if a hormone is present in excess, the number of target cell receptors may decrease; makes target cell less sensitive to the hormone


up regulation

when a hormone is deficient, the number of target cell receptors may increase; makes target cell more sensitive to a hormone


circulating hormones

most endocrine hormones; pass from secretory cells that make them into interstitial fluid and into the blood


local hormone

act locally on neighbouring cells or on the same cell that secreted them without first entering the blood stream



local hormones that act on neighbouring cells



local hormones that act on the same cell that secreted them


lipid soluble hormones

soluble in lipids; include steroid hormones, thyroid hormones and nitric oxide


steroid hormone

derived from cholesterol; each is unique due to the presence of different chemical groups attached at various sites on the four rings at the core of its structure (allows for different function)


thyroid hormones

synthesized by attaching iodine to the amino acid tyrosine


nitric oxide

gas; both a hormone and a neurotransmitter


water soluble hormones

soluble in water; include amine hormones, peptide and protein hormones and eicosanoid hormones


amine hormones

synthesized by removing a molecule of CO2


peptide and protein hormones

amino acid polymers


glycoprotein hormones

protein hormones with attached carbohydrate groups ; i.e. thyroid stimulating hormone


eicosanoid hormones

derived from arachidonic acid (20 C fatty acid)


prostaglandins and leukotrienes

two types of eicosanoid hormones


3 functions of transport proteins

1. make lipid soluble hormones temporarily water soluble (increasing solubility in blood)
2. they retard passage of small hormone molecules through the filtering mechanism in the kidneys (slowing the rate of hormone loss in the urine)
3. provide a ready reserve of hormone (already ready in the bloodstream)


free fraction

0.1-10% of the molecules of a lipid soluble hormone are not bound to a transport protein


Action of Lipid-Soluble Hormones

1. A free lipid-soluble hormone molecule diffuses from the blood, through interstitial fluid, and through the lipid bilayer of the plasma membrane into a cell.
2. If the cell is a target cell, the hormone binds to and activates receptors located within the cytosol or nucleus. The activated receptor–hormone complex then alters gene expression: It turns specific genes of the nuclear DNA on or off.
3. As the DNA is transcribed, new messenger RNA (mRNA) forms, leaves the nucleus, and enters the cytosol. There, it directs synthesis of a new protein, often an enzyme, on the ribosomes.
4. The new proteins alter the cell’s activity and cause the responses typical of that hormone.


Action of Water Soluble Hormones

1. A water-soluble hormone (the first messenger) diffuses from the blood through interstitial fluid and then binds to its receptor at the exterior surface of a target cell’s plasma membrane. The hormone–receptor complex activates a membrane protein called a G protein. The activated G protein in turn activates adenylate cyclase.
2. Adenylate cyclase converts ATP into cyclic AMP (cAMP). Because the enzyme’s active site is on the inner surface of the plasma membrane, this reaction occurs in the cytosol of the cell.
3. Cyclic AMP (the second messenger) activates one or more protein kinases, which may be free in the cytosol or bound to the plasma membrane. A protein kinase is an enzyme that phosphorylates (adds a phosphate group to) other cellular proteins (such as enzymes). The donor of the phosphate group is ATP, which is converted to ADP.
4. Activated protein kinases phosphorylate one or more cellular proteins. Phosphorylation activates some of these proteins and inactivates others, rather like turning a switch on or off.
5. Phosphorylated proteins in turn cause reactions that produce physiological responses. Different protein kinases exist within different target cells and within different organelles of the same target cell. Thus, one protein kinase might trigger glycogen synthesis, a second might cause the breakdown of triglyceride, a third may promote protein synthesis, and so forth. As noted in step 4 , phosphorylation by a protein kinase can also inhibit certain proteins. For example, some of the kinases unleashed when epinephrine binds to liver cells inactivate an enzyme needed for glycogen synthesis.
6. After a brief period, an enzyme called phosphodiesterase inactivates cAMP. Thus, the cell’s re- sponse is turned off unless new hormone molecules continue to bind to their receptors in the plasma membrane


first messenger

when a water soluble hormone binds to its receptor at the outer surface of the plasma membrane, it acts as the first messenger


second messenger

the first messenger (the hormone) then causes the production of a second messenger inside the cell, where hormone specific stimulated responses take place


cyclic AMP (cAMP)

common second messenger


G protein

a membrane protein that activates edentate cyclase


adenylate cyclase

converts ATP into cyclic AMP


permissive effect

the actions of some hormones on target cells require a simultaneous or recent exposure to a second hormone (second hormone is said to have a permissive effect)