Regulation of Extracellular Volume

1
Regulation of Extracellular Volume or How to keep
from swelling up or shriveling away and Why you
urinate in the swimming pool Physiology in
Action Myles Akabas
2
Volume and Osmolarity are Different
volume is an absolute measure osmolarity is a
ratio, a measure of the composition of the
fluid number of osmotically active
particles/unit volume
change in volume
change in volume
change in osmolarity
3
Volume and Osmolarity are Regulated Independently
hypervolemia volume expanded
hyperosmolarity
hypoosmolarity
region of normal volume and osmolarity
hypovolemia volume contracted
4
Volume and Osmolarity are Regulated Independently
hypervolemia volume expanded
Atrial Naturetic Peptide (ANP)
hyperosmolarity
hypoosmolarity
Antidiuretic Hormone (ADH) Vasopressin
Antidiuretic Hormone (ADH) Vasopressin
Renin-Angiotensin-Aldosterone Sympathetic Nervous
System
region of normal volume and osmolarity
hypovolemia volume contracted
5
What determines body volume?
If osmolarity is fixed (290-300 mOsm/l),
then Volume is determined by the number of
osmotically active particles in a given space
osmolarity (mOsm/l) 300 300 NaCl (mM) 150
150 Amt. of NaCl (moles) 0.15 0.3 volume (liter
s) 1 2
Body regulates osmolarity Cells regulate their
individual volumes and thus the Intracellular
Volume Body as a whole regulates the
Extracellular Volume Thus, to regulate
Extracellular Volume the body regulates total
body NaCl
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Overview of Body Response to Volume Changes
volume depleted
volume expanded
euvolemic
atrial natriuretic peptide ANP increased
GFR decrease tubular NaCl reabsorption increas
ed urine volume NaCl excretion vasodilation
surpress thirst
Renin angiotensin II aldosterone maintain
GFR increase tubular NaCl reabsorption decreased
urine volume NaCl excretion vasoconstricti
on decrease NaCl in sweat salt appetite thirst
7
Why Does Water Move Through A Semi-Permeable
Membrane?

• Hydrostatic pressure
• Osmotic pressure

permeable to water but not glucose
100 mM glucose
10 mM glucose
1
2
QUESTION On which side is the concentration of
water higher?
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Why Does Water Move Through A Semi-Permeable
Membrane?
permeable to water but not glucose

• Hydrostatic pressure
• Osmotic pressure

H2O
100 mM glucose
10 mM glucose
1
2
QUESTION On which side is the concentration of
water higher? ANSWER Side 2. Osmotic movement
of water is simply the movement of water down
its concentration gradient from high
concentration of water to lower concentration of
water. It is not some mysterious force.
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Distribution of Body Water and Sodium
Total Body Water Male 65 Body Wt. Female 60
Body Wt
Intracellular
Extracellular
3Na
Plasma Volume
2K
Capillary Wall
Cell Membrane
Na 4.5 mEq/L 140 mEq/L K
140 mEq/L 4.5 mEq/L Osm 290 mOsm/L 290
mOsm/L Volume 30 L 15 L (10 and 5)
Body weight 67 kg
10
Distribution of Body Water and SodiumAddition
of NaCl
Experiment Add 12 g NaCl ( 205 mEq Na), No
water.
Capillary Wall
Cell Membrane
Na 4.6 mEq/L 145 mEq/L K
144 mEq/L 4.2 mEq/L Osm 299 mOsm/L 299
mOsm/L Volume 29.1L 15.9L (10/5)
Euvolemic Altered distribution
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Distribution of Body Water and SodiumAddition
of NaCl Water (Isotonic)
Experiment Add 12 g NaCl ( 205 mEq Na),
Allow Correction of Body Water Osmolality by
Oral Water Intake. (requires 1.4 L).
Intracellular
Extracellular
Plasma Volume
Capillary Wall
Cell Membrane
Na 4.5mEq/L 140.4 mEq/L K
140 mEq/L 4.1 mEq/L Osm 290 mOsm/L 290
mOsm/L Volume 30L 16.4L (10/5)
Extracellular volume expanded
12
In Order to Maintain Body Volume in General
and Extracellular Volume in Particular Salt
(NaCl) Intake Salt Output
Salt intake 1) mostly dietary 2) intravenous
fluids Salt Output 1) sweat 2) stool 3)
urine 4) hemorrhage - bleeding
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The Response to Increased Dietary Na Intake
sensors effectors homeostasis Why does weight
increase?
Early, LE In Clinical Disorders of Fluid and
Electrolyte Metabolism, McGraw-Hill, New York,
1972
14
What and Where are the Sensors of Body Volume
Status?
Most important short term function of blood
perfusion is O2 delivery to tissues perfusion
requires arterial-venous pressure difference Q
?P/R pressure gradient/resistance need to
maintain arterial pressure
Sensors 1) Arterial baroreceptors (wall stretch)
aortic and carotid arteries 2) Venous stretch
receptors large veins, spleen, intestines 3)
O2 sensors brain and kidney
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Relationship between Carotid Sinus Pressure
and Renal Sympathetic Nervous System Activity
Kawada et al., AJP Heart (2001) 281H1581-1590
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The Effector Systems or How the Body Responds
to Changes in Volume Status
Volume expansion Atrial Natriuretic Peptide
(ANP or ANF)
Volume depletion Renin-Angiotensin-Aldosterone
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Response to Volume Depletion An Overview
volume depletion sweat, hemorrhage respiratory
losses, inadequate intake, low Na intake
decreased perfusion reduced O2 delivery low blood
pressure
sympathetic NS activation
vasoconstriction redistribution of blood volume
from splanchnic bed to arterial system
increased renal sympathetic NS activity
decreased renal perfusion decreased tubular fluid
in JGA
increased renin secretion
increased heart rate, stroke volume,
cardiac output
18
Relationship between plasma renin activity and
sodium intake
Laragh and Sealy Handbook of Physiol. Section 8,
1992, p 1409
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Renin Secretion from the JuxtaGlomerular
Apparatus (JGA) Relationship of Macula Densa to
Afferent Arterioles
AA Afferent Arteriole GC Juxtaglomerular
cells M Mesangial Cells PO Podocytes E
Endothelial Cells GBM Glomerular Basement
Membrane US Urinary Space PE Parietal
Epithelial Cells EGM Extraglomerular
Mesangium EA Efferent Arteriole MD Macula
Densa P Proximal Tubule Cell
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The Juxta Glomerular Apparatus - JGA
afferent arteriole lumen
early distal tubule
renin containing dense-cored granules
Macula densa cells
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Coupling Between Tubular Fluid and Renin Secretion
2Cl-
tubule lumen
Na
K
phospholipase A2
PLA2
Macula Densa
Arachadonic acid
cyclooxygenase-2
COX-2
PGE2, PGI2
prostaglandin receptor
AC
G?
JG cell
renin
ATP
cAMP
arteriole lumen
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Renin Synthesis and Release

• Renin is produced in modified vascular smooth
  muscle cells of
• afferent glomerular arterioles JG cells
• Renin is stored in electron-dense granules
• Acute renin release (hemorrhage) occurs through
  release from granules
• Chronic changes in renin release are controlled
  predominantly by changes
• in renin synthesis

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Stimulation of Renin Release

• Reduced JG cell Stretch (Baroreceptor)
• Hyperpolarization, reduced Ca2i
• Reduced renal perfusion pressure renin release
• Independent of Prostaglandins and Macula Densa
• Sympathetic NS stimulation
• ß-adrenergic receptors activate adenylate cyclase
• Independent of Prostaglandins and Macula Densa
• Macula Densa
• Reduced NaCl delivery to thick ascending limb
• Reduced Cl- in mTAL cells
• Enhanced Cox-2 expression
• Enhanced Prostaglandin 2 synthesis
• PGE2 stimulated renin release

24
Inhibition of Renin Release

• Enhanced JG cell Stretch (Baroreceptor)
• Increased Ca2i
• Increased renal perfusion pressure inhibition of
  renin release
• Atrial Natriuretic Peptide
• ANP acts directly on JG cells to inhibit renin
  release
• Macula Densa
• Enhanced NaCl delivery to thick ascending limb
• Increased Cl- in mTAL cells
• Reduced Cox-2 expression PGE2 synthesis
• Less PGE2 stimulated renin release

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What Does Renin Do? Angiotensin II Synthesis
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What does Angiotensin II do? Everything
Kidney constricts efferent arteriole
maintains GRF, increases FF increases
proximal tubular reabsorption Starling
forces increased ion transport Vascular
System - vasoconstrictor Adrenal
Glands stimulates aldosterone secretion stimula
tes increased Na reabsorption kidney (late
distal tubule and cort. collecting duct) sweat
ducts intestine (distal colon) Brain increases
salt appetite increases thirst Heart inhibits
ANP secretion
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Angiotensin II and Increased Proximal Reabsorption
QP/R
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Effect of Angiotensin II on PGC
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Angiotensin II Effects on Glomerular Filtration
and Peritubular Reabsorption
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Angiotensin II Stimulates Release of Aldosterone
from Adrenal Cortex Zona Glomerulosa Cells
Aldosterone Synthesis in Adrenal Glomerulosa
Cells
Ballermann Onuigbo, Handbook of Physiol.
Section 7 Oxford Univ. Press 2000, p124 .
31
Mineralocorticoid/Aldosterone Receptor
Typical steroid hormone receptor Binds
aldosterone and glucocorticoids with equal
affinity, both activate it Aldosterone
responsive cells achieve specificity by
expressing 11beta-hydroxysteroid-dehydrogenase
type 2 inactivates glucocorticoids thus only
allows aldosterone to reach receptor Aldosterone
responsive cells distal tubule/cortical
collecting duct distal colon sweat ducts MR
knockout mice suffer severe volume depletion due
to renal salt wasting little or no ENaC
activity need extensive salt supplementation to
survive 11beta-hydroxysteroid-dehydrogenase type
2 deficient humans and knockout mice are
hypertensive, volume expanded and hypokalemic due
to mineralocorticoid action of glucocorticoids
32
Aldosterone Actions in Distal Tubule and Cortical
Collecting Duct
Basolateral
Increases Na Reabsorption
Tubule Lumen
Aldosterone regulation increase apical Na
permeability increase apical K
permeability increase Na/K-ATPase
activity increase mitochondrial ATP synthesis
Cl- ???
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How To Separate The Role of Aldosterone in Na
Reabsorption and K Secretion

• Requirements for K Secretion
• Aldosterone
• Na delivery to distal tubule
• Volume delivery to distal tubule

• Volume Depletion
• Aldosterone
• Na reabsorption proximally
• low volume delivery

Clinical Problem To excrete K in a
significantly volume depleted state. Solution
Volume replacement
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Angiotensin II is a Potent Vasoconstrictor
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Amount of Angiotensin Infused (ng)
Ferguson Washburn ( 1998) Prog. Neurobiol. 54
169
35
Volume Expansion Atrial Natriuretic Peptide
36
Response to Volume Expansion An Overview
Volume expansion Intake of salty food and
fluids excessive IV fluids
Right atrial distension increase venous
capacitance
secretion of Atrial Natriuretic Peptide (ANP)
inhibit renin secretion inhibit aldosterone
secretion
vasodilation
increased renal NaCl and H2O excretion
37
Infusion of atrial, but not ventricular extracts
produces a brisk increase in renal NaCl excretion
atrial ventricular
38
Cardiac atria contain abundant electron-dense
granules ANP storage sites
Jamieson Palade J. Cell Biol. 23 151, 1964
39
ANP Primary Structure
pro-ANP synthesized as 126 aa protein
C-terminal 28 aa are ANP
Levin et al., NEJM (1998) 339321-328
40
Atrial Natriuretic Peptide Release

• Acute ANP release from cardiac atria
• Atrial distension
• Acute ECF volume expansion
• Saline infusion
• Delivery at the end of pregnancy
• Water immersion
• Congestive Heart Failure
• Chronic Increase in ANP Synthesis
• Atrial and Ventricular hypertrophy/stretch

41
ANP Receptors (NPR-A/B) are Guanylate Cyclases
Brenner et al Physiol. Rev. 70 665, 1990
42
NPR-A/B Mediates ANP Functional Effects NPR-C is
Clearance Mechanism
Levin et al., NEJM (1998) 339321-328
43
Renal atrial natriuretic peptide receptors are
located in glomeruli and in the renal medulla
Mendelsohn et al Can. J. Physiol. Pharmacol.
65 1517, 1987
44
Systemic Effects of ANP
Kidney dilates afferent arteriole decreases
renal resistance, increase PGC, increase
GFR decreases proximal tubular
reabsorption inhibits amiloride-sensitive Na
channel in distal tubule inhibits renin
release Adrenal Gland inhibits aldosterone
synthesis Vascular Smooth Muscle relaxes smooth
muscle gt vasodilation gt increased vascular
capacity increases vascular permeability
45
ANP Increases GFR
ANP
afferent arteriole dilation
RPF
Rafferent
Rtotal
total renal resistance RE RA
PGC
QP/R
renal plasma flow (RPF)
GFR
GFR
GFR
RPF
GFR increases
filtration fraction
efferent arteriolar oncotic pressure
FF GFR/RPF
proximal tubular reabsorption
46
Effect of ANP on PGC
ANP
MAP
PGC
PGC
Pressure (mm Hg)
MVP
renal artery
afferent arteriole
glomerular capillary
efferent arteriole
peritubular capillary
47
Summary of ANP Actions in Response to Volume
Expansion
Levin et al., NEJM (1998) 339321-328
48
The effect of water immersion on venous return
and atrial volume OR Why you urinate in the
swimming pool? OR ANP in Action
Hydrostatic pressure on large veins increases
venous return to Right Atria Atrial distension
induces ANP secretion and you know the rest
49
Chest X-Rays Upright and Immersed Note
the increase in heart volume on immersion
Echt et al., (1974) Pflugers Arch. 352211-217
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51
Distribution of Body Water and SodiumAddition
of Water
Experiment Add 3 L Water (no solute) to
Vascular Compartment
Intracellular
Extracellular
3Na
Plasma Volume
2K
Capillary Wall
Cell Membrane
Na 4.2 mEq/L 131 mEq/L K
131 mEq/L 4.2 mEq/L Osm 267 mOsm/L 267
mOsm/L Volume 32L 16L (10/5)
52
Volume and Osmolarity are Independently Regulated
volume can change without changing osmolarity and
conversely osmolarity can change without changing
volume
osmolarity is a ratio of particles per unit
volume volume is an amount of particles