Title: Endocrine Physiology lecture 4
1Endocrine Physiologylecture 4
- Dale Buchanan Hales, PhD
- Department of Physiology Biophysics
-
2Antidiuretic hormone and the mineralcorticoids
3Synthesis of ADH
- It is synthesized as pre-prohormone and processed
into a nonapeptide (nine amino acids). - Six of the amino acids form a ring structure,
joined by disulfide bonds. - It is very similar in structure to oxytocin,
differing only in amino acid 3 and 8. - ADH synthesized in the cell bodies of
hypothalamic neurons in the supraoptic nucleus - ADH is stored in the neurohypophysis (posterior
pituitary)forms the most readily released ADH
pool
4Hypothalamus and posterior pituitary
5Structure of ADH
6Synthesis of ADH
- Mechanical disruption or the neurohypohyseal
tract by trauma, tumor, or surgery temporarily
causes ADH deficiency. - ADH will be restored after regeneration of the
axons (about 2 weeks). - But if disruption happens at a high enough level,
the cell bodies die in the hypothalamus resulting
in permanent ADH deficiency
7Antidiuretic Hormone ADH
- ADH is also known as arginine vasopressin (AVP
ADH) because of its vasopressive activity, but
its major effect is on the kidney in preventing
water loss.
8ADH conserve body water and regulate tonicity of
body fluids
- Regulated by osmotic and volume stimuli
- Water deprivation increases osmolality of plasma
which activates hypothalmic osmoreceptors to
stimulate ADH release
9ADH increases renal tubular absorption of water
10Primary action of ADH antidiuresis
- ADH binds to V2 receptors on the peritubular
(serosal) surface of cells of the distal
convoluted tubules and medullary collecting
ducts. - Via adenylate cyclase/cAMP induces production and
insertion of AQUAPORIN into the luminal membrane
and enhances permeability of cell to water. - Increased membrane permeability to water permits
back diffusion of solute-free water, resulting in
increased urine osmolality (concentrates urine).
11ADH conserve body water and regulate tonicity of
body fluids
- Regulated by osmotic and volume stimuli
- Water deprivation increases osmolality of plasma
which activates hypothalmic osmoreceptors to
stimulate ADH release
12Secretion of ADH
- The biological action of ADH is to conserve body
water and regulate tonicity of body fluids. - It is primarily regulated by osmotic and volume
stimuli. - Water deprivation increases osmolality of plasma
which activates hypothalmic osmoreceptors to
stimulate ADH release.
13Secretion of ADH
- Conversely, water ingestion suppresses
osmoreceptor firing and consequently shuts off
ADH release. - ADH is initially suppressed by reflex neural
stimulation shortly after water is swallowed. - Plasma ADH then declines further after water is
absorbed and osmolality falls
14Pathway by which ADH secretion is lowered and
water excretion raised when excess water is
ingested
15Secretion of ADH osmolality control
- If plasma osmolality is directly increased by
administration of solutes, only those solutes
that do not freely or rapidly penetrate cell
membranes, such as sodium, cause ADH release. - Conversely, substances that enter cells rapidly,
such as urea, do not change osmotic equilibrium
and thus do not stimulate ADH release. - ADH secretion is exquisitely sensitive to changes
in osmolality. - Changes of 1-2 result in increased ADH
secretion.
16ADH and plasma osmolality
17Secretion of ADHhemodynamic control
- ADH is stimulated by a decrease in blood volume,
cardiac output, or blood pressure. - Hemorrhage is a potent stimulus of ADH release.
- Activities, which reduce blood pressure, increase
ADH secretion. - Conversely, activities or agents that increase
blood pressure, suppresses ADH secretion.
18ADH and blood pressure
19Pathway by which ADH secretion and tubular
permeability to water is increased when plasma
volume decreases
20Secretion of ADH
- Hypovolemia is perceived by pressure receptors
-- carotid and aortic baroreceptors, and stretch
receptors in left atrium and pulmonary veins. - Normally, pressure receptors tonically inhibit
ADH release. - Decrease in blood pressure induces ADH secretion
by reducing input from pressure receptors. - The reduced neural input to baroreceptors
relieves the source of tonic inhibition on
hypothalamic cells that secrete ADH. - Sensitivity to baroreceptors is less than
osmoreceptors senses 5 to 10 change in volume
21Hypothalamus, posterior pituitary and ADH
secretion connection with baroreceptors
22Secretion of ADH
- Hypovolemia also stimulates the generation of
renin and angiotensin directly within the brain. - This local angiotensin II enhances ADH release in
addition to stimulating thirst. - Volume regulation is also reinforced by atrial
naturetic peptide (ANP). - When circulating volume is increased, ANP is
released by cardiac myocytes, this ANP along with
the ANP produced locally in the brain, acts to
inhibit ADH release.
23Secretion of ADH
- The two major stimuli of ADH secretion interact.
- Changes in volume reinforce osmolar changes.
- Hypovolemia sensitizes the ADH response to
hyperosmolarity.
24Plasma Osmolality vs. ADH
The set point of the system is defined as the
plasma osmolality value at which ADH secretion
begins to increase. Above this point slope is
steep reflecting sensitivity of system. Set
point varies from 280 to 290 mOsm/kg H2O
25Blood volume vs. ADH
When blood volume or arterial pressure decreases,
inhibitory input from baroreceptors is over
ridden and ADH secretion is stimulated.
Normally, signals from baroreceptors tonically
inhibit ADH secretion.
26Interaction between osmolar and blood
volume/pressure stimuli
With a decrease in blood volume, set point shifts
to lower osmolality and slope is steeper. During
circulatory collapse kidney continues to conserve
water despite reduction in osmolality. With
increase in blood volume, set point shifts to
higher point and sensitivity is decreased.
27Actions of ADH
- The major action of ADH is on renal cells that
are responsible for reabsorbing free (osmotically
unencumbered) water from the glomerular filtrate.
- ADH responsive cells line the distal convoluted
tubules and collecting ducts of the renal
medulla. - ADH increases the permeability of these cells to
water. - The increase in membrane permeability to water
permits back diffusion of water along an osmotic
gradient. - ADH significantly reduces free-water clearance by
the kidney
28Actions of ADH
- ADH action in the kidney is mediated by its
binding to V2 receptors, coupled to adenylate
cyclase and cAMP production. - cAMP activates protein kinase A which prompts the
insertion of water channels into the apical
membrane of the cell. - When ADH is removed, the water channels withdraw
from the membrane and the apical surface of the
cell becomes impermeable to water once again. .
29Actions of ADH
- This mechanism of shuttling water channels into
and out of the apical membrane provides a very
rapid means to control water permeability - The basolateral membrane of the ductal cells are
freely permeable to water, so any water that
enters via the apical membrane exits the cell
across the basolateral membrane, resulting in the
net absorption of water from the tubule lumen
into the peritubular blood.
30Actions of ADH
- Water deprivation stimulates ADH secretion,
decreases free-water clearance, and enhances
water conservation. - ADH and water form a negative feedback loop.
31Inputs reflexly controlling thirst.
32Actions of ADH
- ADH deficiency is caused by destruction or
dysfunction of the supraoptic and parventricular
nuclei of the hypothalamus. Inability to produce
concentrated urine is a hallmark of ADH
deficiency and is referred to as diabetes
insipidus. - ADH also acts on the anterior pituitary to
stimulate the secretion of ACTH.
33Aldosterone and the mineralocorticoids
- The mineralocorticoid, aldosterone is vital to
maintaining sodium and potassium balance and
extracellular fluid volume. - Aldosterone is an adrenal corticosteroid,
synthesized and secreted by the adrenal cortex.
34Cross section through the adrenal gland cortex
and medulla
salt
sugar
sex
35Aldosterone
- The adrenal cortex is composed of three major
zones, differentiated by the histological
appearance and type of corticosteroid they
produce. - The outermost is the zona glomerulosa, is very
thin and consists of small cells with elongated
mitochondria.
36Adrenal zones
- The middle zona fasiculata is the widest zone and
consists of columnar cells that are highly
vacuolated with numerous lipid droplets. - These lipid droplets are composed of cholesterol
esters the substrate for adrenal steroid hormone
biosynthesis.
37Adrenal zones
- The innermost zona reticularis contains fewer
lipid droplets than fasiculata cells, but have
similar mitochondria. - ACTH has trophic effects on the zona fasiculata
and reticularis.
38Aldosterone synthesis
- Aldosterone is synthesized and secreted by the
zona glomerulosa . - The synthesis of aldosterone from cholesterol to
corticosterone is identical to the synthesis of
glucocorticoids in the zona fasiculata. - The C18 methyl group of corticosterone is
hydroxylated and converted to an aldehyde
yielding aldosterone.
39Aldosterone synthesis
- ACTH also stimulates aldosterone synthesis.
- However the ACTH stimulation is more transient
than the other stimuli and is diminished within
several days. - ACTH provides a tonic control of aldosterone
synthesis. - In the absence of ACTH, sodium depletion still
activates renin-angiotensin system to stimulate
aldosterone synthesis. - Aldosterone levels fluctuate diurnallyhighest
concentration being at 8 AM, lowest at 11 PM, in
parallel to cortisol rhythms.
40Aldosterone synthesis in the adrenal zona
glomerulosa
41Aldosterone function
- The principal function of aldosterone is to
sustain extracellular fluid volume by conserving
body sodium. - Aldosterone is largely secreted in response to
signals that arise from the kidney when a
reduction in circulating fluid volume is sensed.
- When body sodium is depleted, the fall in
extracellular fluid and plasma volume decreases
renal arterial blood flow and pressure.
42Aldosterone action
- Aldosterone binds to the mineralocorticoid
receptor in target cells and affects
transcriptional changes typical of steroid
hormone action. - The kidney is the major site of mineralocorticoid
activity.
43Aldosterone action
- Increased blood pressure results from excess
aldosterone. - Hypertension is an indirect consequence of sodium
retention and expansion of extracellular fluid
volume.
44Regulation of aldosterone secretion Activation
of renin-angiotensin system in response to
hypovolemia is predominant stimulus for
aldosterone synthesis.
45Components of renin-angiotensin-aldosterone system
46Aldosterone and renin-AII
- The juxtaglomerular cells of the kidney respond
to hypovolemia by secreting renin. Renin acts on
angiotensinogen (which is secreted by the liver)
to form angiotensin I which is further cleaved by
angiotensin converting enzyme (which is secreted
by the lungs) to angiotensin II.
47Aldosterone
- Angiotensin II acts on the zona glomerulosa to
stimulate aldosterone synthesis. - Angiotensin II acts via increased intracellular
cAMP to stimulate aldosterone synthesis.
48Aldosterone
- ANP reinforces the effects of the
renin-angiotensin system on aldosterone
secretion. - In response to volume expansion, artrial myocytes
secrete ANP which binds to receptors in the zona
glomerulosa to inhibit aldosterone synthesis. - ANP acts via increased intracellular cGMP which
opposes cAMP and inhibits aldosterone synthesis.
- ANP also reduces aldosterone indirectly by
inhibiting renin release.
49Action of aldosterone on the renal tubule.
Sodium reabsorption from tubular urine into the
tubular cells is stimulated. At the same time,
potassium secretion from the tubular cell into
urine is increased. Na/K-ATPase, and Na
channels work together to increase volume and
pressure, and decrease K.
50Aldosterone mechanism
- The aldosterone-induced proteins serum and
glucocorticoid-inducible kinase (Sgk),
corticosteroid hormone-induced factor (CHIF), and
Kirsten Ras (Ki-Ras) increase the activity and/or
no. of these transport proteins during the early
phase of action
51Aldosterone action
- Aldosterone stimulates the active reabsorption of
sodium from the tubular urine back into the
nearby capillaries in the distal tubule. - Water is passively reabsorbed with sodium which
maintains sodium concentrations at a constant
level. - Hence extracellular fluid volume expands in a
virtually isotonic fashion
52Pathway by which aldosterone secretion and
tubular sodium reabsorption is increased when
plasma volume is decreased
53Aldosterone clears potassium
- Aldosterone facilitates the clearance of
potassium from the extracellular fluid, and
potassium stimulates aldosterone synthesisthus
providing a feedback control mechanism to control
potassium levels. - Conversely, potassium depletion lowers
aldosterone secretion. - Potassium stimulates aldosterone synthesis by
depolarizing zona glomerulosa cell membranes to
stimulate aldosterone synthesis.
54Aldosterone action
- Aldosterone stimulates the active secretion of
potassium from the tubular cell into the urine. - Most potassium that is excreted daily results
from distal tubular secretion. - Hence aldosterone is critical for disposal of
daily dietary potassium load at normal plasma
potassium concentrations.
55Pathway by which an increased potassium intake
induces greater potassium excretion mediated by
aldosterone
56Summary of aldosterone system
57Aldosterone time course of action
58Aldosterone genomic vs. non-genomic
59Aldosterone Action
60Cortisol is at 1000 fold higher concentrations
than aldosterone
61Aldosterone action
- Cortisol binds well to the mineralocorticoid
receptor and plasma cortisol levels are orders of
magnitude higher than aldosterone. - Target tissues for aldosterone are protected from
glucocorticoid excess via the action of
11b-hydroxysteroid dehydrogenase, the enzyme that
converts cortisol to cortisone, a biological
inactive metabolite. - Aldosterone is not a substrate for 11b-HSD and
thus only it can bind to its receptor.
62Integrated control of water and sodium homeostasis
- All of the renal, adrenal, cardiac, vascular,
brain and endocrine influences on body fluid
homeostasis converge on kidney as final site of
regulation - AII and aldosterone are anti-natiuretic
- Increase Na, result in water retention, increase
plasma volume and perfusion pressure - Counter regulator effects of ANP
- Stimulate urinary Na and water excretion
- Inhibit antidiuretic effects of ADH
-
63Integrated control of water and sodium homeostasis
- Imbalances in any one of these hormones affects
volume status and plasma osmolar state
64Hyperaldosteronism
- Hyperaldosteronism is known to be caused by
primary overproduction of aldosterone in
conditions such as Conns syndrome. - Conditions of low cardiac output are also known
to stimulate synthesis of aldosterone. - Both conditions result in sustained hypertension.
65Hyperaldosteronism
- Previously, the hypertension associated with
hyperaldoseteronism was thought to be mediated
exclusively through the mineralocorticoid
receptor located in classical epithelial target
tissues and result from renal sodium retention. - We now know that cardiac hypertension associated
with hyperaldosteronism is far more complex.
66Hyperaldosteronism
- The treatment of patients with severe congestive
heart failure with spironolactone
(mineralocorticoid antagonist) produced
significant reduction in mortality and morbidity,
despite the very modest diuretic effect of the
drug. - The demonstration of local synthesis of
aldosterone by cardiac and vascular cells - The demonstration of high affinity, low capacity
binding proteins for mineralocorticoids in
cardiac myocytes and vascular endothelial cells.
67Hyperaldosteronism
- The demonstration that 11-beta-hydroxysteroid
dehydrogenase is expressed in cardiac myocytes. - The identification of classic mineralocorticoid
receptor in paraventricular nuclei and amygdala
in the brain, regions known to be associated with
salt intake.