Mechanism of Hormone Regulation Flashcards
Five Functions of the Endocrine System
- Differentiate reproductive and CNS during fetus development
- Coordinate male and female reproductive systems
- Stimulate growth and development during children and teens
- Maintained optimal internal environment
- Initiate corrective responses when emergency demand occurs
Hypothalamic-pituitary- target glands axis
forms the structural and functional basis for central integration of the neurologic and endocrine systems, creating what is called the neuroendocrine system
Hormone release is regulated by one or more of the following mechanism
(1) chemical factors (i.e., blood glucose or calcium levels)
(2) endocrine factors (a hormone from one endocrine gland controlling another endocrine gland)
(3) neural control.
An example of chemical regulation
insulin is secreted following chemical stimulation by increased plasma glucose levels
An example of endocrine regulation
Cortisol from the adrenal cortex regulates and stimulates insulin secretion from beta cells within the pancreas.
An example of neural control
The autonomic nervous system directly stimulates the insulin-secreting cells of the pancreas
Negative feedback
most common
a decrease in the production of hormones as s result of a signal from another hormone
Positive Feedback
occurs when a neural, chemical, or endocrine response increases the synthesis and secretion of a hormone
The protein (peptide) hormones
are water soluble
generally circulate in free (unbound) forms
have a short half-life of seconds to minutes because they are catabolized by circulating enzymes.
e.g. insulin
Lipid-soluble hormones
are transported bound to a carrier protein and can remain in the blood for hours to days.
e.g. cortisol
Hormone receptors of the target cell have two main functions:
(1) to recognize and bind with high affinity to their particular hormones
(2) to initiate a signal to appropriate intracellular effectors
upregulation
Low concentrations of hormone increase the number of receptors per cell
downregulation
high concentrations of hormone decrease the number or affinity of receptors
Water-soluble hormones
are proteins that are polarized with a high molecular weight
cannot diffuse across the lipid layer of the cell (plasma) membrane
interact or bind with receptors in or on the cell membrane and activate a second messenger to mediate short-acting responses
lipid-soluble steroids
diffuse freely across the plasma and nuclear membranes and bind the cytosolic or nuclear receptors
first messenger
Water-soluble hormone binding with the plasma membrane receptor initiates a complex cascade of intracellular effects
second messenger
conveys the signal from the receptor to the cytoplasm and nucleus of the cell and mediates the effect of the hormone on the target cell
Second messengers include
1) cyclic adenosine monophosphate (cAMP)
(2) cyclic guanosine monophosphate (cGMP), calcium
(3) inositol triphosphate (IP3) and membrane-associated diacylglycerol (DAG)
(4) the tyrosine kinase system
lipid-soluble hormones
synthesized from cholesterol
include glucocorticoids, androgens, estrogens, progestins, mineralocorticoids, vitamin D, and retinoid
steroid hormones can cross the plasma and nuclear membranes by simple diffusion because
they are relatively small, nonpolar, lipophilic, hydrophobic molecules
The binding of hormones with their receptors stimulates three general types of effects by:
- Acting on preexisting channel-forming proteins to alter membrane channel permeability
- Activating preexisting proteins through a second-messenger system
- Activating genes to cause protein synthesis
Direct effects
obvious changes in cell function that specifically result from stimulation by a particular hormone
e.g. insulin has a direct effect on skeletal muscle cells, causing increased glucose transport into these cells
Permissive effects
less obvious hormone-induced changes that facilitate the maximal response or functioning of a cell
e.g. insulin has a permissive effect on mammary cells, facilitating their response to the direct effects of prolactin
hypothalamic-pituitary axis (HPA)
forms the structural and functional basis for the central integration of the neurologic and endocrine systems
produces a number of releasing/inhibitory hormones and tropic hormones that affect a number of diverse body functions
Hypothalamus
contains special neurosecretory cells that can synthesize and secrete the hypothalamic-releasing hormones that regulate the release of hormones from the anterior pituitary
synthesize the hormones ADH and oxytocin that are then stored and released from the posterior pituitary gland.
HYPOTHALAMIC HORMONES (HYPOPHYSIOTROPIC HORMONES)
The Anterior Pituitary
accounts for 75% of the total weight of the pituitary gland
Name the three parts of the anterior pituitary gland
(1) the pars distalis: major component of the anterior pituitary and the source of the anterior pituitary hormones
(2) the pars tuberalis: a thin layer of cells on the anterior and lateral portions of the pituitary stalk.
(3) the pars intermedia: lies between the two
The anterior pituitary is composed of two main cell types:
(1) the chromophobes, which appear to be nonsecretory
(2) the chromophils, which are considered the secretory cells of the adenohypophysis.
Tropic hormones
affect the the physiologic function of specific target organs
What are the 3 categories of tropic hormones
- corticotropin-related hormones (adrenocorticotropic hormone [ACTH] and melanocyte-stimulating hormone [MSH])
- glycoproteins (luteinizing hormone [LH], follicle-stimulating hormone [FSH], and thyroid-stimulating hormone [TSH])
- somatotropins (growth hormone [GH] and prolactin)
Melanocyte-stimulating hormone (MSH)
promotes the pituitary secretion of melanin, which darkens skin color
Corticotropin related hormone
follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
influence reproductive function
are glycoproteins
Adrenocorticotropic hormone (ACTH)
regulates the release of cortisol from the adrenal cortex
Corticotropin related hormone
Growth hormone (GH)
GH is essential to normal tissue growth and maturation
peaks during adolescence
GH also impacts aging, sleep, nutritional status, stress, and reproductive hormones
In the bone, GH stimulates epiphyseal growth and increases osteoclast and osteoblast activity, resulting in increased bone mass
GH also increases amino acid transport in muscles.
Other functions of GH include lipolysis and enhancement of hepatic protein synthesis.
Prolactin
functions to induce milk production during pregnancy and lactation
It has immune stimulatory effects and modulates immune and inflammatory responses with both physiologic and pathologic reactions.
posterior pituitary (neurohypophysis)
secretes two polypeptide hormones: (1) ADH, also called arginine-vasopressin; and (2) oxytocin
Name the three parts of the posterior pituitary
(1) median eminence -contains at least 10 biologically active hypothalamic-releasing hormones, as well as the neurotransmitters dopamine, norepinephrine, serotonin, acetylcholine, and histamine
(2) pituitary stalk - secretes the hormones of the posterior pituitary
(3) the infundibular process, also known as the pars nervosa or neural lobe.
ADH
Antidiuretic Hormone- acts on the vasopressin 2 (V2) receptors of the renal tubular cells to increase their permeability.
Increased permeability leads to increased water reabsorption into the blood, thus concentrating the urine and reducing serum osmolality.
These effects may be inhibited by hypercalcemia, prostaglandin E, and hypokalemia.
Oxytocin
contraction of the uterus and milk ejection in lactating women
may affect sperm motility in men
functions near the end of labor to enhance the effectiveness of contractions, promote the delivery of the placenta, and stimulate postpartum uterine contractions, thereby preventing excessive bleeding
Pineal Gland
secretes melatonin
Melatonin
is stimulated by exposure to dark and inhibited by light exposure
regulates circadian rhythms and reproductive systems, including the secretion of gonadotropin-releasing hormones and the onset of puberty
increases nitric oxide release from blood vessels, removing toxic oxygen free radicals, and decreasing insulin secretion
Melatonin has been used therapeutically in humans to help with sleep disturbances, jet lag, and inflammatory and psychological disorders.
Thyroid Gland
produces hormones that control the rates of metabolic processes throughout the body
consists of follicles that contain follicular cells surrounding a viscous substance called colloid
normally produces 90% T4 and 10% T3
a 2 month’s supply of thyroid hormone is stored in this gland
follicular cells
synthesize and secrete thyroid hormone
Thyroid hormone (TH)
is regulated through a negative-feedback loop involving the hypothalamus, the anterior pituitary, and the thyroid gland
Thyroid-stimulating hormone (TSH)
glycoprotein hormone synthesized and stored within the anterior pituitary.
Effects of TSH
(1) an immediate increase in the release of stored thyroid hormones
(2) an increase in iodide uptake and oxidation,
(3) an increase in thyroid hormone synthesis
(4) an increase in the synthesis and secretion of prostaglandins by the thyroid.
Effects of TH
essential for normal growth and neurologic development in the fetus and infant and affects metabolic, neurologic, cardiovascular, and respiratory functioning across the lifespan
is required for the metabolism and function of blood cells, as well as normal muscle functioning and the integrity of skin, nails, and hair
affect cell metabolism by altering protein, fat, and glucose metabolism, and, as a result, heat production and oxygen consumption are increased.
Parathyroid Glands
produce parathyroid hormone (PTH), which is the single most important factor in the regulation of serum calcium concentration
Parathyroid Hormone (PTH)
increases serum calcium concentration and decreases the concentration of serum phosphate by stimulating the mechanism of breaking down bone to increase serum calcium
at a low dose, it stimulates bone formation
acts on the kidney to increase calcium reabsorption while phosphate reabsorption is decreased
Pancreas
the endocrine gland that produces hormones and an exocrine gland that produces digestive enzymes
houses the islets of Langerhans
What 4 hormone-secreting cells are found in the islets of Langerhans?
alpha cells, which secrete glucagon
beta cells, which secrete insulin and amylin
delta cells, which secrete gastrin and somatostatin
F (or PP) cells, which secrete pancreatic polypeptide that stimulates gastric secretion and antagonizes cholecystokinin.
Insulin
an anabolic hormone that promotes glucose uptake primarily in the liver, muscle, and adipose tissue
Secretion of insulin is regulated by
chemical, hormonal, and neural control.
Factors influencing insulin secretion
when the beta cells are stimulated by the parasympathetic nervous system, usually before eating a meal
increased blood levels of glucose, amino acids (leucine, arginine, and lysine), and gastrointestinal hormones (glucagon, gastrin, cholecystokinin, secretin)
Insulin secretion decreases
in response to low blood levels of glucose (hypoglycemia)
in response to high levels of insulin (through negative feedback to the beta cells)
in response to sympathetic stimulation of the beta cells in the islets
in the presence of prostaglandins
Insulin sensitivity
is affected by age, weight, abdominal fat, and physical activity
Amylin
peptide hormone cosecreted with insulin by beta cells in response to nutrient stimuli
It regulates blood glucose concentration by delaying gastric emptying and suppressing glucagon secretion after meals.
Amylin also has a satiety effect which reduces food intake
Glucagon
is an antagonist to insulin and acts to increase blood glucose during fasting, exercise, and hypoglycemia
acts primarily in the liver to increase blood glucose concentration by stimulating glycogenolysis
How does glucagon work
High glucose levels cause glucagon release to be inhibited
low glucose levels and sympathetic stimulation promote glucagon release
Pancreatic Somatostatin
a hormone essential in carbohydrate, fat, and protein metabolism
is involved in regulating alpha-cell and beta-cell function within the islets by inhibiting the secretion of insulin, glucagon, and pancreatic polypeptide.
Incretins
control post meal glucose levels by promoting glucose-dependent insulin secretion, inhibiting glucagon synthesis, promoting hepatic glucose secretion, and delaying gastric emptying
Gastrin
stimulates the secretion of gastric acid.
Ghrelin
intestinal hormone
stimulates GH secretion, controls appetite, and plays a role in obesity and the regulation of insulin sensitivity and glucose tolerance
Pancreatic polypeptide
promotes gastric secretion, antagonizes cholecystokinin, and is frequently increased in pancreatic tumors and in diabetes.
The adrenal cortex is made up of
- The zona glomerulosa, the outer layer, constitutes about 15% of the cortex and primarily produces the mineralocorticoid aldosterone
- The zona fasciculata, the middle layer, constitutes 78% of the cortex and secretes the glucocorticoids cortisol, cortisone, and corticosterone.
- The zona reticularis, the inner layer, constitutes 7% of the cortex and secretes mineralocorticoids (aldosterone), adrenal androgens and estrogens, and glucocorticoids.
glucocorticoids
have metabolic, neurologic, anti-inflammatory, immunosuppressive, and growth-suppressing effects
are released under stress
how do glucocorticoids work
increase blood glucose concentration by promoting gluconeogenesis in the liver
conserving glucose by decreasing the uptake of glucose into muscle cells, adipose cells, and lymphatic cells by antagonizing insulin
results in increased glucose for the brain during stress.
effects of glucocorticoids
depression of cellular immunity than humoral immunity
decrease immune and inflammatory responses by decreasing natural killer cell activity
suppress the synthesis, secretion, and actions of chemical mediators involved in inflammatory and immune responses, including histamine
glucocorticoids cause poor wound healing and increase chances for infection by
suppression of innate and adaptive immunity
Name 3 additional effects of glucocorticoids
inhibition of bone formation
inhibition of ADH secretion
stimulation of gastric acid secretion
cortisol
most potent of the naturally occurring glucocorticoids
main secretory product of the adrenal cortex
is needed to maintain life and regulate the body during both positive and negative stress
cortisol secretion is regulated primarily by
the hypothalamus and the anterior pituitary gland
3 factors that regulate ACTH
(1) negative feedback effects of high circulating levels of cortisol and synthetic glucocorticoids suppress both CRH and ACTH, whereas low cortisol levels stimulate their secretion
(2) diurnal rhythms affect ACTH and cortisol levels (ACTH peaks 3 to 5 hours after sleep begins and declines throughout the day. Cortisol levels peak just before awakening)
(3) psychologic and physiologic stress increases ACTH secretion, leading to increased cortisol levels
Mineralocorticoid
directly affect ion transport by epithelial cells, causing sodium retention and potassium and hydrogen loss
Aldosterone
most potent of the naturally occurring mineralocorticoids
acts to conserve sodium by increasing the activity of the sodium pump of the epithelial cells in the nephron
is degraded in the liver and is excreted by the kidney.
renin-angiotensin-aldosterone system
regulates synthesis and secretion of aldosterone
activated by sodium and water depletion, increased potassium levels, and a diminished effective blood volume
Adrenal Medulla
store and secrete the catecholamines epinephrine (adrenaline) and norepinephrine
Catecholamines
epinephrine (adrenaline) and norepinephrine
Their release and the body’s response is the “fight or flight” response
Radioimmunoassay (RIA),
uses antibodies and radiolabeled hormones to determine the quantity of hormone in the plasma.
Enzyme-linked immunosorbent assay (ELISA)
used to determine circulating hormone levels
bioassay
involves the use of graded doses of hormone in a reference preparation and then comparison of the results with an unknown sample
Endocrine changes that may be associated with aging include
altered biologic activity of hormones
altered circulating levels of hormones
altered secretory responses of endocrine glands
altered metabolism of hormones
loss of circadian control of hormone release
changes in the secretion of hypothalamic regulatory hormones.
atrophy of the thyroid gland
is associated with infiltrative glandular changes
secretion of thyroid hormones may diminish with age
Klotho proteins
The kidney, choroid plexus, and parathyroid gland secrete this, which has antiaging effects
These decrease with aging, leading to decreased bone and muscle mass.
Growth hormone levels
