Chapter 5: The Endocrine System Flashcards
Insulin
Pancreas produced peptide hormone that induces the transport of glucose into organs and the storage of excess glucose when blood glucose concentrations are high.
Glucagon
Pancreas produced peptide hormone that triggers the release of sugar stores which raises blood glucose concentration. Glucagon triggers gluconeogenesis (process of harvesting glucose from non carbohydrate sources such as lactate, AA, and glycerol)
Diabetes mellitus (type one and two)
Type I: Auto immune disease in which insulin producing cells in the islets of Langerhans are destroyed.
Type II: caused by end organ insensitivity to insulin.
Glands
Organs in the endocrine system that secrete hormones.
Hormones
Signaling molecules that are secreted directly into the bloodstream to distant target tissues. Hormones bind to receptors, inducing a change in gene expression or cellular functioning.
Classification of hormones by chemical structure/identity (three kinds)
Peptide hormones
Steroid hormones
Amino acid derivative hormones
Peptide hormones
Hormones that are made up of amino acids.
Peptide hormones contain a charge and are therefore polar and water soluble.
Derived from precursor polypeptides and transported to the Golgi apparatus for further modification that activate the hormones and direct them to the correct location in the cell.
First and second messenger
The peptide hormone is considered the first messenger.
After the peptide hormone binds to the receptor and triggers the transmission of a second signal, known as the second messenger.
Because peptide hormones are charged and cannot pass through the plasma membrane, they must bind to an extra cellular receptor.
Signaling cascade
The connection between the hormone at the surface and the effect brought about by second messengers.
Signal amplification
An increase in signal intensity. One hormone molecule may bind to multiple receptors before it is degraded, and each receptor may activate multiple enzymes, each of which will trigger the production of large quantities of second messengers.
Three types of common second messengers
Cyclic adenosine monophosphate (cAMP)
Inositol triphosphate (IP3)
Calcium
G-protein coupled receptor
Integral membrane proteins that form the fourth largest superfamily in the human genome. G protein coupled receptors mediate most cellular responses to hormones and neurotransmitters.
Activation of a G protein-coupled receptor example
The binding of a peptide hormone triggers the receptor to either activate or inhibit an enzyme, called adenylate cyclase, raising or lowering the levels of cAMP (cyclic adenosine monophosphate) accordingly. cAMP combined to intacellular targets, such as protein kinase A, which phosphorylates transcription factors like cAMP Response element – binding protein (CREB) to exert the hormones ultimate effect of expressing a gene.
Transient (medical term)
Temporary
Steroid hormones
Derived from cholesterol and are produced primarily by the gonads and adrenal cortex.
Nonpolar molecules that can easily cross the cell membrane.
Receptors are usually intracellular or intranuclear. The steroid hormone – receptor complexes bind directly to DNA resulting in either increased or decreased transcription of particular genes.
Dimerization
Pairing of two receptor – hormone complexes.
Dimer means two parts
Steroid carriers
Steroid hormones are nonpolar, and therefore must be carried by proteins in the bloodstream to travel around the body. Some carriers are very specific and carry only one hormone (such as sex hormone binding globulin), while other protein carriers are non-specific (such as albumin).
Key concept regarding action of peptide and steroid hormones.
Peptide hormones have surface receptors and act via a second messenger systems.
Steroid hormones bind to intracellular receptors and function by binding to DNA to alter gene transcription.
Key concept and mnemonic for onset and duration of peptide and steroid hormones.
Insulin is a peptide hormone, and it has to be released at every meal in order to be active. Thus, it has fast onset but is short acting (like most peptide hormones).
Estrogen and testosterone are steroid hormones that promote sexual maturation. This is a slower, but longer lasting change (as is true for most steroid hormones).
Thyroxine-binding globulin (TBG) as an example of levels of carrier proteins changing the levels of active hormones.
Hormones are generally inactive while attached to a carrier protein and must dissociate from the carrier to function. Some conditions increase the quantity of a protein that carries thyroid hormones, such as TBG. This causes the body to perceive a lower level of thyroid hormone because the increased quantity of TBG binds a larger proportion of the hormone, meaning there is less free hormone available.
Real world example of thyroxine-binding globulin (TBG) causing increased levels of thyroid hormones in pregnancy.
During pregnancy, high levels of estrogen and progesterone cause increased production of TBG. In order to compensate, people who are pregnant secrete much higher levels of thyroid hormones. Thus, in order to diagnose thyroid disease during pregnancy, different reference values must be used.
Amino acid – derivative hormones
Derived from one or two amino acids, usually with a few additional modification.
Less common than peptide and steroid hormones, but include some of the most important hormones.
Four amino acid derivative hormones, their source, and actions.
Epinephrine and Norepinephrine (the catecholamines), source from the adrenal medulla, causes increase blood glucose concentrations and increase heart rate, dilate bronchi, alter blood flow patterns. Extremely fast onset but are short-lived like peptide hormones. Think adrenaline rush.
Triiodothyronine (T3) and Thyroxine (T4), source from the thyroid (follicular cells), stimulate metabolic activity, Slow onset but long duration like a steroid hormone. They regulate metabolic rate over long period of time.
Mnemonic for deciphering peptide, amino acid-derivative, and steroid derivative hormones.
In general, most peptide and amino acid derivative hormones have names that end in -IN or -INE (insulin, vasopressIN, thyroxINE, triiodothyronINE, etc.)
In general, most steroid hormones have names that end in -ONE, -OL, or -OID (testosterONE, aldosterONE, cortisOL, aldosterONE, estradiOL, and other mineralcorticOIDS, etc.)
Nonexhaustive, but may help for test day.
Classification of hormones by target tissue (two types)
Direct hormones and tropic hormones
Direct hormones
Secreted and then act directly on a target tissue.
For example, insulin released by the pancreas causes increased uptake of glucose by muscles.
Tropic hormones
Require an intermediary to act. Usually originate in the brain and anterior pituitary gland, allowing for the coordination of multiple processes within the body.
For example, gonadotropin releasing hormone (GnRH from chapter 2) from the hypothalamus stimulates the release of luteinizing hormone (LH) and follicle – stimulating hormone (FSH). LH then act on the gonads to stimulate testosterone production in males and estrogen production in females. GnRH and LH do not cause direct changes in the physiology of muscle, bone, and hair follicles; rather they stimulate the production of another hormone by another endocrine gland that acts on these target tissues.
Endocrine organs
Hypothalamus, pituitary, thyroid, parathyroid glands, adrenal glands, pancreas, gonads, pineal gland. Called endocrine organs because hormone production is their main function.
Hypothalamus
The bridge between the nervous and endocrine system.
The release of hormones by the hypothalamus is regulated by NEGATIVE FEEDBACK. When hormone levels reach a certain level, the hypothalamus and pituitary gland reduce the production of hormones to maintain a balance.
Example: When thyroid hormone levels are high, the hypothalamus and pituitary gland reduce the production of thyroid-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH)
Nuclei in the three sections of the hypothalamus
Lateral
Ventromedial
Anterior
These nuclei play roles in emotional experience, aggressive behavior, sexual behavior, metabolism, temperature, regulation, and water balance.
Hypothalamus (deeper)
Regulates the anterior pituitary through tropic hormones, and through direct neural connection to the posterior pituitary, capable of having organism wide effects. Located in the forebrain, directly above the pituitary gland and below the thalamus. Because of the close proximity between the hypothalamus and the pituitary gland, the hypothalamus controls the pituitary through peregrine release of hormones.
Among other things, it receives some of the light input from the retina helps to control sleep-wake cycles and also responds to increases in blood osmolarity (concentration of solutes in blood).
Negative feedback regarding the hypothalamus
The release of hormones by the hypothalamus is regulated by 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 conserve energy by restricting production of substances that are already present and sufficient quantities.
Hypophyseal portal system
Blood vessel system that is a direct inextricable link between the hypothalamus and the anterior pituitary. Note: the posterior pituitary does not receive tropic hormones through the hypophyseal portal system.
Thus, hormones released from the hypothalamus travel directly to the interior pituitary and cannot be found in appreciable concentrations in the systemic circulation.
Hypophysis
Alternative term for the pituitary
Some (four) tropic hormones released by the hypothalamus and the hormone released by the anterior pituitary in response:
GnRH - FSH and LH
Growth hormone-releasing hormone (GHRH) - Growth hormone (GH)
Thyroid releasing hormone (TRH) - thyroid stimulating hormone (TSH)
Corticotropin-releasing factor - adrenocorticotropic hormone (ACTH)
All of the hormones from the hypothalamus cause an increase in hormonal response from the anterior pituitary, except for one: dopamine (PIF, prolactin inhibiting factor). Dopamine inhibits release of prolactin from the anterior pituitary.
-tropic suffix
Turned to, or attracted to
One exception to the pattern of increased hormonal reaction of the anterior pituitary from the hypothalamus
Prolactin – inhibiting factor (PIF), which is actually dopamine, is released by the hypothalamus and causes a DECREASE in prolactin secretion. It is the absence of PIF that allows prolactin to be released.
Negative feedback effects of the hypothalamus and anterior pituitary: corticotropin-releasing factor (CRF), adrenocorticotropic hormone (ACTH), and cortisol EXAMPLE
Release of CRF from the hypothalamus will stimulate the anterior pituitary to secrete ACTH. ACTH will then cause the adrenal cortex to increase the level of cortisol being secreted into the blood. Cortisol is detrimental when levels become too high. Cortisol inhibits the hypothalamus and interior pituitary from releasing CRF and ACTH.
CRF and ACTH goal is getting more cortisol in the blood. Since cortisol receptors are present in the hypothalamus and pituitary, they recognize the presence of cortisol and therefore discontinue production of CRF and ACTH.
Relationship between the hypothalamus and the posterior pituitary
Neurons in the hypothalamus send their axons down the pituitary stock directly into the posterior pituitary, which can then release oxytocin and anti-diuretic hormone.
Note: The posterior pituitary does not receive trophic hormones through the hypophysial portal system. The hypophyseal portal system is strictly between the hypothalamus and the anterior pituitary.
Oxytocin
Stimulates uterine contractions during labor, as well as milk letdown during lactation. May also be involved in bonding behavior. Released by posterior pituitary.
Oxytocin is unusual in that it has a positive feedback loop: the release of oxytocin promotes uterine contraction, which promotes more oxytocin release, which promotes stronger uterine contractions, and so on.
Antidiuretic hormone (ADH or vasopressin)
Increases reabsorption of water in the collecting ducts of the kidneys. ADH (or vasopressin) is secreted in response to increased plasma osmolarity (increased solute concentrations within the blood).
Note: hypothalamus detects increased plasma osmolarity, synthesizes ADH, sends a direct signal to posterior pituitary, ADH is secreted by posterior pituitary and increases reabsorption of water in the kidneys by increasing the permeability of the collecting duct.
Remember: the posterior pituitary does not make hormones.
Pituitary tumor, release of prolactin
A tumor in the pituitary gland may result in compression of the portal system that connects to the hypothalamus. This may block the ability of prolactin-inhibiting factor (PIF or dopamine) from reaching the anterior pituitary and exerting its effects. Thus more prolactin will be released, resulting in lactation. Milk production in a male or non-pregnant female should lead a physician to suspect the presence of a pituitary tumor. Interesting example.
Triiodothyronine (T3) and thyroxine (T4)
Produced by iodination of tyrosine (an amino acid) in the follicular cells of the thyroid. These hormones reset the basal metabolic rate by making energy production more or less efficient, as well as altering the utilization of glucose and fatty acids.
Increase in triiodothyronine (T3) and thyroxine (T4) leads to increase in cellular respiration. Negative feedback loop: high levels of T3 and T4 leads to decreased synthesis of triiodothyronine and thyroxine, prevents excessive secretion of triiodothyronine and thyroxine.
Calcium and phosphorus homeostasis
Calcitonin
Parathyroid hormone (PTH)
Renin-angiotensin-aldosterone system
Controls aldosterone in the body.
Decreased blood pressure causes the juxtaglomeral cells (baroreceptors) of the kidneys to release renin.
Renin cleaves an inactive plasma protein called angiotensinogen to its active form angiotensin I.
Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme in the lungs.
Angiotensin II stimulates the adrenal cortex to produce aldosterone.
Aldosterone increases sodium and reabsorption of water, increasing plasma volume, increasing cardiac output, increasing blood pressure, negative feedback of the juxtoglomerular (baroreceotors).
Pancreas
Located in upper abdomen, behind the stomach and in front of the spine. About the size of your hand.
Has both endocrine and exocrine functions.
Insulin
Antagonist to glucagon. Secreted when blood glucose levels are high. Makes muscle and liver cells to take up glucose and store it as glycogen for later use. Stimulates anabolic (building) processes such as fat and protein synthesis.
