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Nephrology Unit, Department of Medicine, University of Rochester Medical Center, Rochester, New York 14642
Fluid loss from
the peritoneal cavity to surrounding tissue varies directly with
intraperitoneal hydrostatic pressure
(Pip). According to Darcy's law
[Q =
KA(dPif/dx)],
fluid flux (Q) across a
cross-sectional area (A) of tissue
will increase with an increase in either hydraulic conductivity
(K) or the interstitial fluid hydrostatic pressure gradient
(dPif/dx,
where x is distance). Previously, we
demonstrated that in the anterior abdominal muscle (AAM) of rats,
dPif/dx
increases by only 40%, whereas K
rises fivefold between Pip of 1.5 and 8 mmHg. Because K is a function of
interstitial volume (
if), we
hypothesized that perturbations of
Pip would change
Pif and expand the interstitium,
increasing
if. To test this
hypothesis, we used dual-label quantitative autoradiography (QAR) to
measure extracellular fluid volume
(
ec) and intravascular volume
(
iv) in the AAM of rats
within the Pip range from
2.8 to +8 mmHg.
if was
obtained by subtraction (
ec
iv).
dPif/dx
was measured with a micropipette and a servo-null system. Local
iv did not vary with
Pip and averaged 0.010 ± 0.002 ml/g, and
if averaged 0.19 ± 0.01 ml/g at Pif
1.2 mmHg.
However,
if doubled between
Pif of 1.2 and 4.2 mmHg (from 0.20 ± 0.00 to 0.39 ± 0.01 ml/g, respectively) but did not increase
with further increases in Pif.
This nonlinear pressure-volume relationship does not explain the
fivefold increase in K with
Pip. Because the interstitial
matrix contributes to the interstitial resistance to fluid flow, and
because hyaluronan (HA) is the only component of the matrix that is not
anchored to the tissue, we hypothesized that the loss of interstitial
HA was responsible for the continued decrease in interstitial
resistance to fluid flow. We determined HA concentration in the rat AAM
and adjacent subcutaneous tissue (SC) at
Pip = 0 mmHg and after 2 h of
dialysis at constant Pip = 6 mmHg.
The HA content (normalized to dry weight) in the AAM was reduced from
487 ± 16 to 360 ± 27 µg/g dry tissue
(n = 4, P < 0.05) and increased from 528 ± 72 to 1,050 ± 136 mg/g dry tissue
(n = 4, P > 0.001) in the SC. We conclude
that the mechanisms responsible for the increase in
K with
Pip include expansion of the
interstitium, dilution of interstitial macromolecules, and washout from
the AAM to SC of interstitial macromolecules responsible for resistance
to fluid flow.
convection; hydraulic conductivity; compliance; peritoneal dialysis
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