BIOL 22000 Lecture Notes - Lecture 5: Hyperthyroidism, Heat Intolerance, Endometrium
8 Jun 2018
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Chapter 5 Endocrine System
5.1 Mechanisms of Hormone Action
• The endocrine system consists glands, that secrete hormones
• Hormones are signaling molecules that are secreted directly into the bloodstream to
travel to a distant target tissue
CLASSIFICATION OF HORMONES BY CHEMICAL STRUCTURE
• Hormones can be classified by their chemical identities, peptides, steroids, or amino
acid derivatives
Peptide Hormones
• Peptide hormones are made up of amino acids, with various sizes
• Peptide hormones are from larger precursor polypeptides that was cleaved during
posttranslational modification
• The small units are transported to the Golgi apparatus for modification that activate the
hormone and direct it to the correct location in the cell
• Hormones are released by exocytosis after being packaged into
vesicles (out of the GA?)
• Peptide hormones are charged and cannot pass though plasma
membrane, these hormones must bind to an extracellular
receptor
• The peptide hormone is considered the first messenger; it
binds to the receptor and triggers the transmission of a second
signal, known as the second messenger
• There are many types of receptor, and each receptor
determines what happens once the hormone has stimulated the
receptor
• Signaling cascade is a stimulus (first messenger) acting on a
receptor that is transduced to the cell interior through second
messengers (which amplify the initial signal) and ultimately to
effector molecules, resulting in a cell response to the initial
stimulus
• At each step, there is the possibility of amplification
o One hormone molecule may bind to multiple receptors
before it is degraded
o Thus, each step can result in an increase in the intensity of the signal
• Some common second messengers are cyclic adenosine monophosphate (cAMP),
inositol triphosphate (IP3), and calcium
• The activation of a G protein-coupled receptor is shown on the right
o Peptide hormone binding triggers the receptor to either activate or inhibit an
enzyme called adenylate cyclase, raising or lowering the levels of cAMP
accordingly
o cAMP can bind to intracellular targets, such as protein kinase A, which
phosphorylates transcription factors like cAMP response element-binding protein
(CREB) to exert the hormone’s ultimate effect
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• The effects of peptide hormones are usually rapid but short-lived because these hormones
act through transient second messenger systems
• It is quicker to turn them on and off, but their effects do not last without relatively
constant stimulation
• Because peptides are generally water-soluble, peptide hormones can travel freely in the
bloodstream and usually do not require carriers
Steroid Hormones
• Steroid hormones are derived from cholesterol and are produced primarily by the
gonads and adrenal cortex
• Steroid hormones are derived from nonpolar molecules, they can easily cross the cell
membrane
• Their receptors are usually intracellular (in the cytosol) or intranuclear (in the nucleus)
• Upon binding to the receptor, steroid hormone–receptor complexes undergo
conformational changes
• The receptor can then bind directly to DNA, increase or decrease transcription of
particular genes, depending on the hormone
• One common form of conformational change is dimerization, or pairing of two
receptors–hormone complexes
• Steroid hormones’ effect is slower but last longer than peptide hormones
o Because steroid hormones cause alterations in the amount of mRNA and protein
present in a cell
• KEY CONCEPT
o Peptide hormones have surface receptors and act via second messenger systems
o Steroid hormones bind to intracellular receptors and function by binding to DNA
to alter gene transcription
• Steroid hormones are not water-soluble, so it must be carried by proteins in the
bloodstream to be able to travel around the body
• Some of these proteins are very specific, and carry only one hormone, while other
proteins are nonspecific
• Hormones are generally inactive while attached to a carrier protein and must dissociate
from the carrier to function
• Levels of carrier proteins can change the levels of active hormone
o Example: some protein that carries a hormone (hormone doesn’t function while
on the protein), this cause the body to perceive lower level of hormone available,
Amino Acid-Derivative Hormones
• Amino acid-derivative hormones derived from one or two amino acids, usually with a
few additional modifications
o Less common than peptide and steroid hormones
• The catecholamines (epinephrine and norepinephrine) bind to G protein-coupled
receptors while the thyroid hormones bind intracellularly
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• MCAT EXPERTISE
o Mechanism of action of the amino acid-derivative hormones should be
memorized because it is so unpredictable
▪ Epinephrine and norepinephrine have extremely fast onset but are short-
lived, like peptide hormones
▪ Thyroxine and triiodothyronine have slower onset but a longer duration,
like steroid hormones—they regulate metabolic rate over a long period of
time
CLASSIFICATION OF HORMONES BY TARGE TISSUE
• Direct hormones, are secreted and then act directly on a target tissue
o For example, insulin released by the pancreas causes increased uptake of glucose
by muscles
• Tropic hormones require an intermediary to act
o Stimulate the production of another hormone by another endocrine gland that acts
on these target tissues
5.2 Endocrine Organs and Hormones
HYPOTHALAMUS
• Hypothalamus is a bridge between the nervous and endocrine systems
• Regulating the pituitary gland through tropic hormones, hypothalamus can have
organism-wide effects
• The hypothalamus is in the forebrain, directly above the pituitary gland and below the
thalamus
• Because the hypothalamus and the pituitary are close to each other, the hypothalamus
controls the pituitary through paracrine release of hormones into a portal system that
directly connects the two organs
• The hypothalamus receives input from a wide variety of sources
o Receives light input from the retinas and helps to control sleep–wake cycles
o Respond to increases in blood osmolarity (concentration)
o Regulate appetite and satiety
• The release of hormones by the hypothalamus is regulated by negative feedback
o Negative feedback occurs when a hormone (or product) later in the pathway
inhibits hormones (or enzymes) earlier in the pathway
• This type of feedback maintains homeostasis and prevents wasted energy by restricting
production of substances that are sufficient
Interactions with the Anterior Pituitary
• The hypothalamus secretes compounds into the hypophyseal portal system, a blood
vessel system that directly connects the hypothalamus with the anterior pituitary
• Hormones released from the hypothalamus travel directly to the anterior pituitary and
cannot be found in other parts of the body
• Hypophysis is an alternative term for the pituitary
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Document Summary
5. 1 mechanisms of hormone action: the endocrine system consists glands, that secrete hormones, hormones are signaling molecules that are secreted directly into the bloodstream to travel to a distant target tissue. Classification of hormones by chemical structure: hormones can be classified by their chemical identities, peptides, steroids, or amino acid derivatives. It is quicker to turn them on and off, but their effects do not last without relatively constant stimulation: because peptides are generally water-soluble, peptide hormones can travel freely in the bloodstream and usually do not require carriers. Tropic hormones: these hormones work by causing the release of another hormone at the organ level. Posterior pituitary: the posterior pituitary contains the nerve terminals of neurons with cell bodies in the hypothalamus, posterior pituitary receives, and stores two hormones produced by the hypothalamus: Increase t3 and t4 increased cellular respiration: greater amount of protein and fatty acid production by speeding up both synthesis and degradation.
