Chapter 18 Flashcards
hormone
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
receptors
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
paracrine
local hormones that act on neighbouring cells
autocrine
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
- make lipid soluble hormones temporarily water soluble (increasing solubility in blood)
- they retard passage of small hormone molecules through the filtering mechanism in the kidneys (slowing the rate of hormone loss in the urine)
- 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
- 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.
- 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.
- 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.
- The new proteins alter the cell’s activity and cause the responses typical of that hormone.
Action of Water Soluble Hormones
- 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.
- 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.
- 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.
- 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.
- 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.
- 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)
synergistic effect
when the effect f two hormones acting together is greater or more extensive than the effect of each hormone acting alone, the two hormones are said to have a synergistic effect
antagonistic effect
when one hormone opposes the actions of another hormone, the two hormones are said to have antagonistic effects
3 factors that affect responsiveness of a target cell to a hormone
- the hormone’s concentration in the blood
- the abundance of target cell’s hormone receptors
- influences exerted by other hormones
control of hormone secretion (3 factors)
- signals from the nervous system
- chemical changes in the blood
- other hormones
pituitary gland (hypophysis)
“master” endocrine gland since it secretes several hormones that control other endocrine glands
hypothalamus
master of the pituitary gland
infundibulum
a stalk that attaches the hypothalamus to the pituitary gland
anterior pituitary or anterior lobe or adenohypophysis
composed of epitelial tissue; consists of 2 parts: the pars distalis (larger portion) and the pars tuberalis (forms a sheath around the infundibulum)
releasing hormones
stimulates release of anterior pituitary hormones
inhibiting hormones
suppresses release of anterior pituitary hormones; from the hypothalamus
hypophyseal portal system
blood flows from capillaries int he hypothalamus into portal veins that carry blood to capillaries of the anterior pituitary
neurosecretory cells
clusters of specialized neurons above the optic chiasm
human growth hormone (hGH) or somatotropin
stimulates several tissues to secrete insulinlike growth factors (hormones that stimulate general body growth and regulate aspects of the metabolism
thyroid-stimulating hormone (TSH) or thyrotropin
controls the secretions and other activities of the thyroid gland
follicle-stimulating hormone (FSH)
secreted by gonadotrophs; stimulate secretion of estrogens and progesterone and the maturation of oocytes in the ovaries and they stimulate sperm production and secretion of testosterone in the testes
Luteinizing hormone (LH)
secreted by gonadotrophs; stimulate secretion of estrogens and progesterone and the maturation of oocytes in the ovaries and they stimulate sperm production and secretion of testosterone in the testes
Growth Hormone Releasing Hormone (GHRH)
promotes secretion of human growth hormone
Growth Hormone Inhibiting Hormone (GHIH)
suppresses secretion of human growth hormone
Thyrotropin Releasing Hormone (TRH)
from the hypothalamus; controls TSH secretion
gonadotropins
secreted by gonadotrophs; FSH and LH