1. No category

Pathophysiology of Peritoneal
Transport
Michael F. Flessner, MD, PhD
Bethesda, Maryland, USA
No Conflicts of Interest
Major Points I
• There is no single “peritoneal membrane”. The
peritoneal barrier is made up of a microvasculature
distributed within the cell-interstitial matrix of the
tissue surrounding the peritoneal cavity.
• Trans-peritoneal transport is directly proportional to
the area of peritoneum in contact with the solution.
• Solute transport occurs via diffusion and convection
across endothelia and through interstitial matrix.
• Solute-free water transports from both blood
capillaries and cells in peritoneal tissue via specialized
water channels or aquaporins (AQP-1, AQP-3,-4?).
Water transport also depends on the structure of the
cell-interstitial matrix and lymphatics.
Major Points II
• The endothelial glycocalyx lines the inter-endothelial
clefts and limits solute transport between plasma and
interstitium and affects Starling Forces.
• The glycocalyx is sensitive to inflammatory cytokines
and hyperglycemia, which alter the trans-endothelial
permeability and may explain D/P changes with time on
dialysis.
• Inflammation alters transport by angiogenesis and
peritoneal sclerosis, often limiting fluid removal by
altering the primary structures of the barrier: the
endothelial surface area, the cell-interstitial matrix, and
the peritoneal surface area of transfer.
Peritoneal Cavity is a
potential space,
surrounded by a
multitude of different
tissues and cells.
From: The Visible
Female
Where is
the
barrier?
From: Hepinstall’s
Textbook of Anatomy
Peritoneal Barrier of Abdominal Wall (HE; 200x)
Transport of
solute and
water
Topics
• Anatomy and Physiology of the peritoneal
barrier to water and solutes
•
•
•
•
•
Importance of surface contact area
Net Ultrafiltration, lymph flow, and “wrong-way flow”
Role of Mesothelium
Interstitium: distributed osmosis
Blood Capillary: role of the aquaporin and endothelial
glycocalyx
• Response of the glycocalyx to inflammation
• Effect of angiogenesis on transport
• Sclerosis of the peritoneum limits the surface area and water
transport but not solute transport
Diffusion Equation
Rate of diffusion = -Deff x Area x dC/dx
≈ P x Area (Cblood - CPC)
Peritoneal surface area in contact
with the solution is an important
variable in solute transport.
If the solution
does not touch
the tissue,
transport does
not occur
across the
peritoneum!!
Effect of Increased Dialysate Volume on
Peritoneal Surface among Peritoneal
Dialysis Patients, Chagnac JASN 13:2554, 2002
• Measured the area of contact in 2 and 3 L
dwells in 10 adult patients by infusion of
contrast in the dialysate and multiple CT
scans and a special stereologic technique
• With ~50% increase in volume, contact area
increased by ~20% and MTACcreatinine
increased by ~25%
Conclusion: Contact area,
which is determined by the
volume instilled, is a major
determinant in rate of mass
transfer across the peritoneum.
volume drained − volumein
Net Ultrafiltration =
dialysis duration
WRONG-WAY FLOW:
Flow from the cavity to the
body: why does it occur?
Fluid in the cavity increases pressure,
which causes flow into the local tissues.
Flessner, AJP 1996
Durand Adv Perit Dial 8:22, 1992
Fluid loss during peritoneal dialysis (flow back to
the patient) can amount to ~1.5-2 L/day.
Increases in PD dwell volume will increase IP
pressure and may lead to a decrease in net UF.
Topics
• Anatomy and Physiology of the peritoneal
barrier to water and solutes
• Importance of surface contact area
• Net Ultrafiltration, lymph flow, and “wrong-way flow”
• Role of Mesothelium
• Interstitium: distributed osmosis
• Blood Capillary: role of the aquaporin and endothelial glycocalyx
• Response of the glycocalyx to inflammation
• Effect of angiogenesis on transport
• Sclerosis of the peritoneum limits the surface area and water
transport but not solute transport
Capillary (3-pore) Model of
Peritoneal Transport
Dialysate fluid
Dialysate fluid
Blood flow
Solute – water
transport
Pore Theory cannot adequately
model the peritoneal barrier!
Transport of
solute and
water
Distributed Concept of PD Transport
Is the peritoneum a barrier to small solutes and water?
Intact peritoneum
Flessner, PDI 23:542, 2003
No peritoneum
Elimination of peritoneum does not alter water or
solute transport between cavity and tissue
mass
1.00
0.50
Flessner, PDI 23:542, 2003
Conclusion:
The anatomic peritoneum is
not a significant barrier to
small solutes.
But the peritoneum is
important for the integrity of
the barrier.
Distributed Concept of PD Transport
Does the interstitium alter solute transport?
Flessner, AJP, 1985
What is the role for the Cell-Extracellular Matrix
in osmotic filtration?
Theorizes an Osmotic
Resistance in the
Interstitial-Cell Matrix`
Distributed modeling of
glucose induced osmotic
flow Waniewski,
Stachowska-Pietka et al
AJP 296:H1960-68, 2009
c
Which Aquaporin play a
role in water transport?
• AQP1 plays an essential role in water
permeability and ultrafiltration during PD
Ni KI 69:
1518-1525, 2006.
• AQP1 is found in endothelial cells
• AQP4 is present in the entire plasma membrane
of fast muscle fibers. AQP4 expression is
associated with high water permeability and
changes in muscle fiber volume. Frigeri Faseb J 18:905; 2004
Yang and Verkman AJP 276:C76, 1999
Yang and Verkman showed
that AQP1 and AQP4
knockout mice decrease
osmotic filtration by 60%.
A
Q
P
1
A
Q
P
1
WT
WT
WT
K
O
A
Q
P
1
A
Q
P
4
AQP1 was located
primarily in endothelial
cells, while AQP4 was
located in the membrane of
the underlying muscle
cells.
}
Note Location
Hypothesized Cell-Extracellular Matrix
Mechanism of Filtration Flow
CD31 stain
- inflamed
peritoneum
Water Flow
AQP?
AQP1
Yang and Verkman AJP 276:C76, 1999
Yang and Verkman detected AQP3 in
the peritoneum AJP 276:C76, 1999.
AQP3 is present
in mesothelial
cells and some
underlying
parenchymal
cells in humans
AQP1
AQP3
HPMC
Anti-AQP3 MAb respond to
increasing
concentrations
of glucose by
increasing
mRNA for
AQP3
Lai KN KI 62:1431-39,
2002
Mechanism of Filtration Flow: upregulated
AQP3 with interstitium?
AQP3 ?
CD31 stain
- inflamed
peritoneum
Water Flow
Interstitial-cell matrix presents a
significant barrier to the transport of
solutes and water between plasma in
distributed microvessels and the
solution in the peritoneal cavity and
results in far less efficient transport
and dialysis.
The mechanism of water transport
from the capillary to the cavity is
still unknown.
Topics
• Anatomy and Physiology of the peritoneal
barrier to water and solutes
•
•
•
•
Importance of surface contact area
Net Ultrafiltration, lymph flow, and “wrong-way flow”
Role of Mesothelium
Interstitium: distributed osmosis
• Blood Capillary: roles of the aquaporin and of the
endothelial glycocalyx
• Response of the glycocalyx to inflammation
• Effect of angiogenesis on transport
• Sclerosis of the peritoneum limits the surface area and water
transport but not solute transport
Inter-Endothelial Cleft-Matrix Concept of Transport
Vink, Duling Circ
Res 79:581, 1996.
Aquaporin-1
• AQP-1 discovered Peter Agre Science 256:385, 1992.
• Trans-peritoneal UF in AQP1-KO mice demonstrated a decrease of 60% Yang AJP 276:C76, 1999.
• AQP-1 plays an essential role in water permeability
and ultrafiltration during PD Ni KI 69: 1518-1525, 2006.
• Aquaporin-1 are transendothelial pores,
but data over the last 10 years provides
extensive evidence to support an
additional barrier in the interendothelial cleft.
Re-Discovery of Luminal Endothelial
Glycocalyx
• Extracellular coating of anionic
polysaccharides discovered on luminal
surface of endothelia.
Bennet J Histochem Cytochem 11:14, 1963
• Endothelial glycocalyx excluded blood
from a layer 1.2 µm on the luminal surface
and was suspected to influence
transcapillary transport.
Klitzman, Duling AJP 237:H481, 1979
Vink, Duling AJP 278:H285; 2000
Why change from pore theory
to the science of the glycocalyx?
• Glycocalyx limits permeation of
dextrans in a molecular size- and
charge-dependent manner.
Vink, Duling AJP 278:H285, 2000
• Damage of the glycocalyx leads to
increases in capillary permeability.
Vink AJP 290:H2174; 2006.
Revision of
Starling’s Law
JR Levick J Physiol
557.3:704, 2004.
S Weinbaum, AJP Heart
291:2950, 2006.
Decreased Glycocalyx in angiogenic
vessels in chronically exercised muscle
Brown et al Experimental Physiol 81:1043; 1996
• Examined sections of rat striated muscle stained
with ruthenium red to examine glycocalyx before
and after 2-4 days of electrical stimulation
• Before stimulation: glycocalyx continuous on
63%, absent on 13% capillaries
• After stimulation: glycocalyx continuous on 10%,
absent on 44-58% of angiogenic capillaries
• Angiogenic vessels have less glycocalyx and
therefore are more permeable. This would
dissipate the glucose more rapidly.
100%
>50%
Endothelial
Glycocalyx
<50%
absent
1 µm
EM: muscle capillaries (% coverage of Endothelium) Exp Physiol 81:1043, 1996
Glycocalyx may decrease the effective osmotic
pressure driving ultrafiltration.
Topics
• Anatomy and Physiology of the peritoneal barrier to water
and solutes
•
•
•
•
•
Importance of surface contact area
Net Ultrafiltration, lymph flow, and “wrong-way flow”
Role of Mesothelium
Interstitium: distributed osmosis
Blood Capillary: role of the aquaporin and endothelial glycocalyx
• Response of the glycocalyx to inflammation
• Effect of angiogenesis on transport
• Sclerosis of the peritoneum limits the surface area and water
transport but not solute transport
Can endothelial glycocalyx explain
observed increased transport during
inflammatory states or peritonitis?
• Damage of the glycocalyx due to:
oxidized lipoproteins, heparitinase,
fluid shear stress, adhesion of WBCs
and platelets, cytokines, and
ischemia-reperfusion leads to
increases in capillary permeability.
• Vink AJP 290:H2174; 2006.
Alteration of Glycocalyx Increases
Microvascular Permeability
Acute or chronic increase of glucose
to 25 mM in mice (6 x normal)
results in marked increase in
permeability to 70 kDa dextran and
is correlated with glycocalyx
alterations.
Zuurbier J Appl Physiol 99:1471, 2005
Damage to Glycocalyx in Clinical
Hyperglycemia
• Loss of endothelial glycocalyx during
acute hyperglycemia coincides with
endothelial dysfunction and rapid loss of a
macromolecular marker in 10 healthy
males. Nieuwdorp Diabetes 55:480; 2006
• Endothelial glycocalyx damage coincides
with microalbuminuria in Type I DM
Nieuwdorp Diabetes 55:1127, 2006
After 8 weeks of
exposure to a
glucose-based
solution
Angiogenic vessels have less glycocalyx
and therefore are more permeable. This
would dissipate the glucose more rapidly
in chronically-inflamed peritoneum,
leading ultimately to poor ultrafiltration.
High
Glucose
Low Glucose
Entire Cohort
No
Icodextrin
Icodextrin
Exposure to
Glucose increases
D/P Cr and
decreases UF over
time
Davies et al. KI 67:1609, 2005
Mechanism for the observed increase in
D/P after years on hypertonic dialysis?
• Evidence from basic research demonstrates the
importance of the glycocalyx to trans-endothelial
transport.
• Inflammation, ischemia-reperfusion,
hyperglycemia, and angiogenesis alter the
glycocalyx and increase endothelial permeability.
• The effect of hyperglycemia on endothelial
permeability could be the mechanism for increase in
D/P creatinine and decrease of D/D0 for glucose
with time on dialysis.
Does inflammation
result in a loss of
Aquaporin?
Are all of the new
vessels in the subcompact zone
perfused?
Do they contain
aquaporin?
Dark brown = CD31
Angiogenesis resulting
from chronic inflammation
100 µm
Water channels play a fundamental role
in cell migration. Saadoun Nature 434:786792, 2005
• Aortic endothelia, harvested from wild-type
and from AQP-1 deficient mice, were
grown in primary cultures.
• Cell adhesion and proliferation were similar
• Cell migration was severely impaired in
AQP-1 deficient cells.
• Transfection of AQP-1 into nonendothelial cells accelerates cell
migration and wound healing, in vitro.
Aquaporins in Endothelia Verkman KI 69:1120-3, 2006
Conclusion from Studies of
Endothelial Proliferation
• Angiogenesis resulting from
inflammation absolutely
depends on the presence of
AQP1. Therefore AQP1
deficiency is unlikely in chronic
inflammation in the peritoneum.
Normal Expression of Aquaporin-1 in a Long-Term
Peritoneal Dialysis Patient with Impaired
Transcellular Water Transport Goffin AJKD 33:383, 1999
Note fibrotic layer
~500 µm
AQP1-Staining
Avascular “Tanned” Peritoneum
Expression of Aquaporin-1 in a Long-Term
Peritoneal Dialysis Patient with Impaired
Transcellular Water Transport Goffin AJKD 33:383, 1999
controls
case
peritonitis
Topics
• Anatomy and Physiology of the peritoneal barrier to water
and solutes
•
•
•
•
•
Importance of surface contact area
Net Ultrafiltration, lymph flow, and “wrong-way flow”
Role of Mesothelium
Interstitium: distributed osmosis
Blood Capillary: role of the aquaporin and endothelial glycocalyx
• Response of the glycocalyx to inflammation
• Effect of angiogenesis on transport
• Sclerosis of the peritoneum limits the
surface area and water transport but not
solute transport
Long-Term Effects of PD
Normal Control
Compact mesothelial
zone
After 9 years of PD
How does the
avascular,
acellular scar
alter transport of
solute and water?
500 µm
500 µm
Williams JD et al, JASN 13:470, 2002.
Abnormal
interstitial-cell
matrix does not
transport water
to the cavity
• Avascular submesothelial compact zone
markedly decreases the effective osmotic
pressure near the exchange microvessels
• Increased perfused vascular area, which
may be hyper-permeable, exacerbates the
UF rate by dissipating the osmotic
gradient rapidly in the vicinity of the
exchange microvessels
Major Points I
• There is no single “peritoneal membrane”. The
peritoneal barrier is made up of a microvasculature
distributed within the cell-interstitial matrix of the
tissue surrounding the peritoneal cavity.
• Trans-peritoneal transport is directly proportional to
the area of peritoneum in contact with the solution.
• Solute transport occurs via diffusion and convection
across endothelia and through interstitial matrix.
• Solute-free water transports from both blood
capillaries and cells in peritoneal tissue via specialized
water channels or aquaporins (AQP-1, AQP-3,-4?).
Water transport also depends on the structure of the
cell-interstitial matrix and lymphatics.
Major Points II
• The endothelial glycocalyx lines the inter-endothelial
clefts and limits solute transport between plasma and
interstitium and affects Starling Forces.
• The glycocalyx is sensitive to inflammatory cytokines
and hyperglycemia, which alter the trans-endothelial
permeability and may explain D/P changes with time on
dialysis.
• Inflammation alters transport by angiogenesis and
peritoneal sclerosis, often limiting fluid removal by
altering the primary structures of the barrier: the
endothelial surface area, the cell-interstitial matrix, and
the peritoneal surface area of transfer.
Thank you for your attention!
Questions?
Glycocalyx-Endothelial Cleft Theory of TransCapillary Transport
(dense glycocalyx)
Vink, Duling Circ
Res 79:581, 1996.
(less dense
glycocalyx)