1 Division of Nephrology, 2 Department of Radiology, 3 Department of Pathology, and 4 Department of Transplant Surgery, Stanford University School of Medicine, 5 Division of Biostatistics, Department of Health Research Policy, Stanford University, Stanford, California 94305; and 6 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
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ABSTRACT |
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Postischemic injury in 38 recipients of 7-day-old cadaveric renal allografts was classified into
sustained (n = 15) or recovering (n = 23) acute renal failure (ARF) according to the prevailing inulin
clearance. Recipients of long-standing allografts that functioned
optimally (n = 16) and living transplant donors
undergoing nephrectomy (n = 10) served as functional
and structural controls, respectively. A combination of physiological
and morphometric techniques were used to evaluate glomerular filtration
rate and its determinants 1-3 h after reperfusion and again on
day 7 to elucidate the mechanism for persistent
hypofiltration in ARF that is sustained. Glomerular filtration rate in
the sustained ARF group on day 7 was depressed by 90%
(mean ± SD); the corresponding fall in renal plasma flow was
proportionately less. Neither plasma oncotic pressure nor the
single-nephron ultrafiltration coefficient differed between the
sustained ARF and the control group, however. A model of glomerular
ultrafiltration and a sensitivity analysis were used to compute the
prevailing transcapillary hydraulic pressure gradient (P), the only
remaining determinant of
P. This revealed that
P varied between
27 and 28 mmHg in sustained ARF and 32-38 mmHg in recovering ARF
on day 7 vs. 47-54 mmHg in controls. Sustained ARF was
associated with persistent tubular dilatation. We conclude that
depression of
P, perhaps due partially to elevated tubule pressure,
is the predominant cause of hypofiltration in the maintenance stage of
ARF that is sustained for 7 days.
filtration dynamics; glomerular morphometry; tubule morphometry; ultrafiltration coefficient; filtration pressure
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INTRODUCTION |
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ALLOTRANSPLANTATION OF A
CADAVERIC kidney is invariably followed by a
postischemic-reperfusion injury (2, 8, 18). In a
substantial minority (25-30%) of cases, the injury is
sufficiently profound and prolonged to require dialytic therapy
because of inadequate allograft function. The precipitating
ischemic insult comprises renal underperfusion during the fatal
illness of the donor, followed by a period of nonperfusion from the
time the kidney is procured until completion of the vascular
anastomosis to the iliac vessels of the recipient. Despite the
cytoprotective effects of cooling the kidney to 4°C during the
protracted period of nonperfusion (often 24 h), the magnitude of the
postischemic injury is often sufficient to delay the onset of
adequate allograft function for days or weeks.
We have shown that delayed graft function is associated with all the hallmarks of postischemic acute renal failure (ARF) in native kidneys. These include a sublethal injury to proximal tubule cells, one that is associated with impairment of intercellular tight junctions and an ensuing loss of cell polarity (1, 2, 17, 18). Notwithstanding nearly normal rates of renal plasma flow (RPF) (8), the "effective" glomerular filtration rate (GFR), as determined by inulin clearance, is depressed by ~90%. This reduction is attributable, in part, to a real decline in true GFR secondary to depression of the filtration pressure. In addition, there is a transtubular backleak of inulin, which leads to an underestimation of the calculated "true" GFR (2, 18).
The aforementioned dissipation of filtration pressure has been
shown by micropuncture techniques in animal models of
postischemic ARF to be the consequence of a decline in the
glomerular transcapillary hydraulic pressure gradient (P), the
outward driving force for the formation of filtrate (4, 6,
33). In an earlier study, we used novel techniques to measure
the GFR and its remaining determinants 1-3 h after reperfusion of
cadaveric renal allografts. We then computed
P using a mathematical
model of glomerular ultrafiltration (2). Our computations
suggested that reduction of
P to a level similar to the opposing
glomerular intracapillary oncotic pressure (
GC) was the
predominant cause of filtration failure in this early or
"initiation stage" of the postischemic renal injury. The
purpose of the present study was to extend our observations to the
"maintenance" stage of the injury in transplant recipients in whom
the ARF was sustained for
7 days. By studying consecutive renal
allograft recipients 7 days after transplantation, we observed not only
sustained ARF but also transition to the "recovery" stage in those
recipients with lesser degrees of postischemic injury. Accordingly, in addition to studying the mechanism of the decrement in
GFR in sustained ARF, we have attempted to characterize the determinants of the increment in GFR that underlie the recovery from
postischemic ARF. Our findings form the basis of this report.
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METHODS |
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Patient Population
Thirty-eight patients undergoing renal transplantation at our institution gave informed consent to a study of allograft glomerular ultrafiltration and three of its determinants, namely, RPF, afferent oncotic pressure (On day 7, the subjects were divided arbitrarily into
two groups according to the urinary clearance of inulin, which should be regarded as the effective, rather than the true, GFR, because it
does not take into account transtubular backleak of filtrate (18). Group 1 comprised 23 subjects who were
classified as recovering from ARF by virtue of an inulin clearance 20
ml/min. Group 2 comprised the remaining 15 subjects who were
classified as exhibiting sustained ARF because of persistent depression
of inulin clearance <20 ml/min. The latter value was selected because
it represented depression of GFR by 75% below the average value in a
group of control subjects who provided an optimal range of values for
the renal allograft (24). Glomerular filtration dynamics
were evaluated 13-66 mo after transplantation on a single occasion
in these control subjects, who were 16 recipients of long-standing
renal allografts that were donated by a living sibling or parent and
had never undergone a known episode of rejection. A second control
group comprised 10 living donors of a healthy kidney for
transplantation into a relative. They provide normal values for the
quantitative assessment of tubule obstruction and of those glomerular
structures that determine Kf. Each gave
permission for two cores of kidney tissue to be taken by needle biopsy,
under anesthesia, during donor nephrectomy.
Transplantation Procedures
Management of cadaveric donors.
All cadaveric donors in this series died of a severe brain injury.
Crystalloid fluids and dopamine (0.1-0.5
µg · kg1 · min
1) were
infused in an effort to maintain systolic blood pressure at >90 mmHg
and urine flow at >50 ml/h. Cadaveric organ procurement was
coordinated by the California Transplant Donor Network. A neurologist
in each participating center diagnosed brain death using clinical
criteria. With the heart still beating, the donor was taken to an
operating room, where the renal artery and vein were exposed. To
minimize the warm ischemic time, the kidneys were first cooled
in situ by flushing the renal circulation with cold University of
Wisconsin preservation solution. The kidneys were then removed and
stored in the same solution at 4°C until transplantation.
Management of transplant recipients. All 38 recipients had end-stage renal failure that had resulted in anuria and required dialytic therapy. Thirty-four recipients in the present series were receiving maintenance hemodialysis therapy, and four were receiving chronic ambulatory peritoneal dialysis. All recipients on maintenance dialysis were dialyzed within the 24 h preceding the transplantation. Peritoneal dialysis catheters were drained and capped before surgery.
General anesthesia was induced with narcotic agents and maintained with isoflurane. An indwelling bladder catheter and a central venous line were inserted after induction of anesthesia. The extraperitoneal space was entered through a lower-quadrant abdominal incision. The external iliac artery and vein were identified, skeletonized for a distance of 8 cm, and clamped proximally and distally. Methylprednisolone (1 g) and azathioprine (10 mg/kg) were then infused intravenously. The kidney graft was removed from the iced storage solution, and the renal artery and vein were anastomosed end-to-side to the corresponding recipient iliac vessels. All clamps were then released. The "rewarming time" (from the end of cold storage until completion of the anastomoses) was recorded. Mannitol (0.5 g/kg) was infused just before release of the vascular clamps. Each recipient's bladder was filled with an irrigating solution containing neomycin, bacitracin, and heparin. The donor ureter was then spatulated, the recipient bladder mucosa incised, and a ureteroneocystostomy created. The detrusor muscle was reapproximated over the ureteroneocystostomy to create an antireflux tunnel. Crystalloid solutions were infused throughout the operative procedure to maintain central venous pressure (CVP) at >10 mmHg.Postoperative immunosuppression. All recipients received immunosuppressive therapy with prednisone and either mycophenolate mofetil or azathioprine during posttransplantation week 1. These agents are not known to impair renal blood flow. In addition, subjects received a third immunosuppressive agent during posttransplantation week 1, i.e., cyclosporine (n = 34) or tacrolimus (n = 4). The latter two agents are renal vasoconstrictors. They were used in modest dosages to achieve whole blood trough levels of 300-400 ng/ml for cyclosporine and 10-15 ng/ml for tacrolimus.
Protocol
Evaluation of early allograft function.
The GFR and its determinants were evaluated during the first 3 h
after reperfusion of the allograft. Renal blood flow was determined
45-60 min after reperfusion by Doppler flow probe using an
ultrasonic transit time flowmeter (model HT 107, Transonic Systems,
Ithaca, NY). A snugly fitting 12- to 16-mm-diameter flow probe was
placed around the renal vein. The iliac fossa was then filled with
saline to optimize ultrasonic determinations. Triplicate determinations
were recorded on a precalibrated digital readout at 2-min intervals.
The coefficient of variation of the three measurements was 15%, and
renal blood flow was expressed as the median value. Mean arterial
pressure was simultaneously determined by Dynamap and CVP by
transducer. Renal vascular resistance was calculated by dividing the
arteriovenous pressure drop by renal blood flow. RPF was calculated
from the product of renal blood flow (RBF) and the hematocrit (Hct) of
venous blood (expressed as a fraction) as follows
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(1) |
Laboratory determinations.
Concentrations of inulin in urine and plasma were determined by the
autoanalyzer method of Fjeldbo and Stamey using resorcinol as the
colorimetric reagent. The concentration of creatinine in urine and
plasma was determined by an automated rate-dependent picrate method
using a creatinine analyzer (Creatinine Analyzer 2, Beckman
Instruments, Fullerton, CA). This method minimizes the influence of
slow-reacting, non-creatinine chromagens and thus provides an estimate
of the true creatinine concentration. Concentrations of sodium were
determined by ion-selective electrode (NOVA 11, NOVA Biomedical,
Waltham, MA) and osmolality by a vapor pressure osmometer (model 5500, Wescor, Logan, UT). The oncotic pressure in venous plasma was taken to
be the same as that entering the glomerular tuft (A) and
was measured directly by membrane osmometry using a colloid osmometer
(model 4400, Wescor), as described by us previously (7).
Morphological Studies
Glomerular morphometry.
Sections (1-µm thick) of the paraffin-embedded biopsy material were
cut and stained with periodic acid-Schiff reagent. A dedicated computer
system (Southern Micro Instruments, Atlanta, GA) consisting of a
videocamera, monitor, light microscope, and digitizing tablet was used
to perform measurements (2). The average number of glomeruli examined per day 0 biopsy was 18 in the recovering
group and 12 in the sustained ARF group. Corresponding numbers of
glomeruli in day 7 biopsies were 6 and 6, respectively. The
average number of glomeruli in the control group was 21. The outline of
each Bowman's capsule and glomerular tuft in the cross section was traced onto the digitizing tablet at ×900 magnification. The
cross-sectional areas within Bowman's capsule
(ABC) and of the glomerular tuft (AG) were computed using area perimeter
analysis. The difference between ABC and
AG yielded the area of Bowman's space
(ABS). Glomerular volume (VG) was
calculated from AG and corrected to account for the tissue shrinkage associated with paraffin embedding using a linear
shrinkage factor (fs) (2)
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(2) |
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(3) |
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(4) |
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(5) |
Tubule morphometry. Abnormalities of tubule structure were assessed by light microscopy using 1-µm sections stained with periodic acid-Schiff reagent. An 11 × 11 square grid was inserted into the eyepiece of the microscope. Point and intercept counting of seven grid fields at ×900 magnification was used to calculate the cross-sectional area of the lumens of all tubules in the seven fields. The percentage of proximal tubule cells that had exfoliated was also estimated. Point counting was used to count 500-1,000 tubule cells in each subject and to estimate the fraction of such cells that had sloughed off the tubular basement membrane and entered the tubular lumen (2).
Calculations
Glomerular capillary oncotic pressure.
We computed GC from the arithmetic mean of
A and
E, which are the respective oncotic
pressures of plasma entering the afferent and efferent arterioles,
respectively. The
A was assumed to be the same as that
measured directly in systemic venous blood. The
E was
calculated as follows
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(6) |
Glomerular ultrafiltration coefficient. The overall Kf for the transplanted kidney is the product of the glomerular capillary hydraulic permeability (k) and the total surface area available for filtration in all glomeruli. The total surface area was computed from the single nephron value (S, determined as described above) and estimates of the total number of nephrons. The baseline value of the total number of nephrons was taken to be 0.7 or 1.0 × 106 (14, 25). The effective hydraulic permeability was estimated from the individually measured values of basement membrane thickness, FSF, and W by using the structural-hydrodynamic model of Drumond and Deen (12). Briefly, that model approximates the glomerular capillary wall as consisting of a large number of repeating structural units, each unit being based on a single filtration slit. Within a structural unit are representations of the individual layers of the capillary wall, namely, the fenestrated endothelium, the basement membrane, and the epithelial filtration slits with slit diaphragms. By solving the differential equations describing viscous flow through each of these layers and using the concept of resistances in series, a value for k is obtained. In addition to the values of basement membrane thickness, FSF, and W measured in the present study, a number of other quantities are needed as inputs for the calculations of k. The other quantities, which include the intrinsic (Darcy) permeability of the glomerular basement membrane and the dimensions of various other structures, were estimated from data reported for normal rats, as described in detail previously (13). The values of the other inputs used here are identical to those given by Drumond et al. (see Table 1 in Ref. 13).
Glomerular transcapillary hydraulic pressure difference.
The P was computed from GFR, RPF,
A, and
Kf (10), as described in detail by
Alejandro et al. (2). In the model employed, the
glomerular capillary network is idealized as a number of identical capillaries in parallel, and steady-state mass balance equations are
used to compute variations in plasma flow rate and protein concentration with distance along a representative capillary. It was
assumed that
P is constant along a capillary.
Statistical Analysis
Because the distribution of many of the measured variables was not Gaussian, comparisons between and among groups of measurements were made with one- or two-sample Wilcoxon statistics or, where there were three groups to compare (control, sustained, and recovering ARF), by their extension, the Kruskal-Wallis statistic. The latter is a nonparametric substitute for the one-way, fixed-effects analysis of variance (19). Each of the three-group comparisons done separately for days 0 and 7 was supplemented by three pairwise comparisons of groups using the two-sample Wilcoxon statistic. For these comparisons, significance was judged according to the Bonferroni technique (20). Thus P < 0.017 for a particular comparison meant P < 0.05 for the set of three comparisons. For five variables (urine flow, fractional sodium excretion, urine-to-plasma osmolality ratio, and cold and warm ischemic times), there were no control data, so two-sample Wilcoxon statistics with standard notions of significance were used. Matching on a patient-by-patient basis within each ARF group allowed us to compute changes between day 0 and day 7 for each group. These changes were evaluated with paired, one-sample Wilcoxon signed-ranks tests. ![]() |
RESULTS |
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Initial Allograft Function
Initial allograft function was measured 1-3 h after reperfusion. The duration of cold ischemia averaged 921 ± 369 min in the group destined to exhibit recovering ARF and 1,221 ± 349 min in the group destined to exhibit sustained ARF (P = 0.014). The corresponding durations of rewarming times, during performance of the vascular anastomosis, were 29 ± 9 and 32 ± 10 min, respectively [P = not significant (NS)]. In keeping with their classification according to the inulin clearance on day 7, those in the recovering ARF group tended to have a significantly higher effective GFR on day 0 than those classified as sustained ARF (Table 1; P = 0.0017). Moreover, initial GFR was profoundly and significantly depressed below the control value (80 ± 20 ml/min) in both groups, averaging 17 ± 11 and 7 ± 6 ml/min, respectively.
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RPF and A were also similar in the two ARF groups. The
A tended to be lower than in the control group, a
finding that should enhance, and not depress, GFR (Table 1). RPF was
also below control: by 37% in the recovering group (P = 0.0009) and by 41% in the sustained group (P = 0.00003; Table 1). As indicated by profound and significant depression
of corresponding values of the filtration fraction to only 9 ± 6% (P = 0.00001) and 3 ± 3% (P = 5 × 10
6) in the recovering and sustained ARF
groups, respectively, compared with 22 ± 5% in controls, the
modest depression of RPF does not explain the extent of initial
glomerular hypofiltration in the two ARF groups. The value for
k was significantly depressed below control levels by a
similar amount in the recovering and sustained ARF groups on day
0: 2.4 ± 0.4 vs. 1.9 ± 0.3 (P = 0.005)
and 1.8 ± 0.4 × 10
9
m · s
1 · Pa
1
(P = 0.002), respectively (Table
2). The low k, in turn, was a
consequence of foot process broadening with an ensuing reduction in FSF
(Table 2). A trend to enhanced filtration surface area, owing to
enlargement of glomerular volume after transplantation, offset the low
k, however (Table 2). As a result, Kf
did not differ from the control value of 4.6 ± 1.8 nl · min
1 · mmHg
1,
averaging 5.4 ± 2.8 and 3.9 ± 2.1 nl · min
1 · mmHg
1 in the
recovering and sustained groups, respectively (P = NS; Table 2). Thus the present findings confirm precisely our earlier observations that changes in
A, RPF, and
Kf do not explain GFR depression in the
immediate wake of an ischemic-reperfusion injury to the renal
allograft. That tubules also suffer a severe initial injury is
indicated by excretion of a massive fraction of the filtered sodium
load (
20% on average) and isosthenuria in each ARF group (Table
3).
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Late Allograft Function
Late allograft function was measured on day 7. There was negligible change in effective GFR between day 0 and day 7 in the sustained ARF group (7 ± 6 vs. 8 ± 8 ml/min). The following measured determinants of GFR also remained constant: RPF = 225 ± 53 vs. 239 ± 98 ml/min,The recovering ARF group was characterized by a large increase in
effective GFR between day 0 and day 7: 17 ± 11 vs. 50 ± 20 ml/min (P = 0.00003). Whereas
A remained relatively constant, RPF increased between
the two examinations, albeit not significantly and less than in
proportion to the corresponding change in GFR (Table 1). There was,
however, a large and significant increase between day 0 and
day 7 in Kf: 5.4 ± 2.8 vs.
7.0 ± 5.3 nl · min
1 · mmHg
1
(P < 0.02; Table 2). A significant (P = 0.02) increase in k by 14% as foot process conformation
was restored to normal contributed to the higher
Kf (Table 2). Also contributing was a trend
toward glomerular hypertrophy (Table 2) with an ensuing numerical
enhancement of filtration surface area to a mean value that exceeded
that in controls by 72% (Table 1). Noteworthy is that, despite their improving GFR, the recovering ARF group continued to exhibit
postischemic tubule injury on day 7 similar to that
in the sustained ARF group, as judged by a persistently high fractional
sodium excretion and isosthenuria (Table 3).
Evaluation of Ultrafiltration Pressure
We applied the values for effective GFR, RPF,
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Using morphometric techniques superior to those used by Dunnill and
Halley (14), Nyengaard and Bendtsen (25)
suggest that 0.7 × 106 is likely a more accurate
estimate of mean glomerular number per kidney than 1.0 × 106. Using the former value and assuming 50% backleak in
sustained ARF but no backleak in controls and recovering ARF, we have
selected a "best-case" value for P in each group (Fig.
1). It can be compared with the opposing
mean glomerular intracapillary oncotic pressure, which we have computed
using Eq. 5 (Fig. 1). Whereas net ultrafiltration pressure
(best-case
P
GC) approximates 27 mmHg in
controls, the corresponding pressure falls to only ~4 mmHg in
sustained ARF on days 0 and 7 (Fig. 1). A
similarly low initial ultrafiltration pressure approximates 5 mmHg on
day 0 in the recovering ARF group. The best-case
P suggests that ultrafiltration pressure increases to 12 mmHg in the
recovering ARF group on day 7, however. Although the latter
is not yet as high as in controls, this increment, combined with
the significant 30% increase in computed Kf,
appears to account for the corresponding increment in GFR on day
7.
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One possible mechanism by which postischemic renal injury could
lower P is obstruction of tubule lumens by exfoliated cells, cell
debris, and/or casts with a subsequent increase in the upstream pressure in Bowman's space (11, 21). Tubule morphometry
provides equivocal information about this possibility. Fractional
Bowman's space area and tubule luminal cross-sectional area were
significantly higher in the recovering group on day 0 than
in controls: 34 ± 10 vs. 22 ± 6% (P = 0.003) and 704 ± 206 vs. 362 ± 127 µm2
(P = 0.003), respectively (Fig.
2). Corresponding values in the sustained
group on day 0 were similar at 34 ± 11% for
fractional Bowman's space area (P = 0.006 vs.
controls) and intermediate at 548 ± 205 µm2 for
tubule luminal area (P = NS vs. controls). Each of the
latter values remained unchanged in the sustained group on day
7. In contrast, increasing GFR and
P in the recovering group on
day 7 were associated with a significant reduction in tubule
luminal area (P = 0.004) toward normal values (Table 2;
Fig. 2). There was a parallel, albeit nonsignificant, reduction in
fractional Bowman's space area (Table 2; Fig. 2). Despite the apparent
distension of Bowman's space and tubule lumens, however, intratubular
casts were rarely observed, and there was only scant tubule cell
exfoliation. On day 0 the percentage of exfoliated cells in
the recovering (4.0 ± 2.5%) and sustained groups (3.7 ± 2.6%) was similar to that associated with nephrectomy in the control
group (3.7 ± 3.5%). The percentage of exfoliated cells by
day 7 was not significantly different in the sustained group
(2.6 ± 2.1%) but declined significantly by 75% to 1.0 ± 1.0% in the recovering group (P = 0.02; Table 3).
Thus, although tubule and Bowman's space distension correlate with GFR
and computed
P depression during posttransplantation week
1, corresponding evidence of substantial cell exfoliation as a
potential cause of downstream tubule obstruction is scanty.
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DISCUSSION |
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We recently showed that transtubular backleak contributes to lowering of the urinary clearance of inulin during the maintenance stage of postischemic ARF in the renal allograft (18). We used a differential solute clearance technique to evaluate the renal handling of nonreabsorbable polysaccharide molecules of graded size. Our analysis suggested that normal tubule impermeability to the filtration markers inulin and dextran was lost and that ~50% of filtered inulin leaked back across damaged tubule walls (18). A morphometric and histochemical analysis pointed to a paracellular pathway for backleak between proximal tubule cells owing to impairment of tight junctions and cell-cell adhesion (18). Correcting the observed urinary inulin clearance in the present study by our previous estimate of the inulin backleak rate results in computation of a mean true GFR that is only 20% of the corresponding value observed in our control subjects with excellent allograft function.
The present study was designed to determine what accounts for the
remaining disparity (~60-70 ml/min) between GFR "corrected" for backleak in sustained (maintenance stage) ARF and the control value
for GFR. To do this, we determined GFR and three of its four
determinants: RPF, A, and Kf. We
then subjected the foregoing quantities to mathematical modeling and a
sensitivity analysis to estimate
P, the single remaining determinant
of GFR (2). Our findings indicate that
A
and Kf after 7 days of sustained ARF do not
differ from control values. Furthermore, although it is modestly
lowered below control by 38% on average, depressed RPF cannot be
invoked to explain the observed hypofiltration. The extreme depression
of filtration fraction, even after correction for backleak (4 vs. 22%
in controls), indicates that GFR depression in sustained ARF is
disproportionate to the corresponding depression of RPF. Precisely the
same findings for GFR, RPF,
A, and
Kf during the initiation stage of ARF on
day 0 indicate that a remarkable constancy of GFR and its
aforementioned three determinants accompanies the phenomenon of
sustained ARF on day 7 (Tables 1 and 2). It is evident by
exclusion, and our model of glomerular ultrafiltration confirms, that a
profound lowering of
P is the predominant cause of GFR depression in
the initiation and maintenance stages of sustained postischemic
ARF (Fig. 1).
Our finding that RPF is well preserved in the initiation (day 0) and maintenance (day 7) stages of sustained ARF is not widely recognized. This is because injury to proximal tubules inactivates the organic anion transporter, thereby precluding the use of PAH, the standard clearance marker for RPF in humans (8). Profound impairment of tubular PAH secretion results in substantial underestimation of RPF by the urinary clearance of PAH (5, 8, 15, 23). Although the number of observations is small, more invasive techniques have been used to reliably estimate RPF in the presence of ARF in humans. These include application of an electromagnetic flowmeter to the renal artery (3, 23) or cannulation of the latter vessel to measure flow by dye dilution (29) or by washout of inert radioactive gases (16). Each of these studies has reported reductions in RPF that were modest and of proportions remarkably similar to those observed by us in the present study. We have carefully validated the accuracy of our noninvasive cine-PC-MRI technique by comparison with PAH clearance in the healthy kidney, in which the organic anion transporter is unimpaired (24, 32). To the extent that the former method is also accurate in the presence of ARF, it is interesting to note the similarity of RPF in the sustained ARF group on day 7 when cine-PC-MRI was used (239 ± 98 ml/min) to that on day 0 during surgery, as determined directly by Doppler flow probe (225 ± 153 ml/min). That RPF is unlikely to have changed substantially between the two examinations is consistent with unchanging levels of corresponding serial determinations of GFR and its remaining determinants (Tables 1 and 2).
The process of recovery from allograft ARF is also characterized by
relative constancy of A and RPF (Table 1). In contrast to ARF that is sustained, however, the recovery stage is accompanied by
a substantial increment in Kf on day
7. This is attributable partly to a significant increase in
glomerular hydraulic permeability as foot process conformation is
restored toward normal (Table 2). Also contributing is adaptive
glomerular enlargement (see glomerular volume, Table 2) at this time,
with an ensuing enhancement of filtration surface area (Table 2). The
resulting 30% increase in Kf above day
0 levels is insufficient to account by itself for the observed
increase in GFR. Applying the observed values of
A, RPF,
and Kf to the model of ultrafiltration reveals
that it is also necessary to invoke a substantial increase in
P to explain the observed level of GFR on day 7 as the allograft
recovers from the postischemic injury. Finn and Chevalier
(15) used the micropuncture technique to demonstrate that
the recovery from postischemic ARF in the rat is associated
with progressive and parallel increases in GFR and
P over a period
of >8 wk. It seems likely that a similar process of recovery lasting
several weeks also applies to postischemic injury of the human
renal allograft. Presumably, subsequent increases in
P to a normal,
or even supernormal, range should eventuate beyond day 7. In
combination with parallel increases in Kf and
RPF due to adaptive hypertrophy and hyperperfusion of glomeruli in the
uninephric condition, respectively, such increases in
P should lead
eventually to the marked elevation of single-kidney GFR that is
observed in our control group of uninephric transplant recipients with
optimally functioning allografts of long standing (24).
There are two potential mechanisms for the depression of P that we
compute in our subjects with allograft ARF. One is a rise in pressure
in Bowman's space consequent on downstream obstruction of tubules
(4, 6, 15, 33). The other is a fall in perfusion pressure
of glomerular capillaries consequent on afferent arteriolar vasoconstriction. Micropuncture studies of the aforementioned pressures
suggest that each contributes to
P depression in the initiation and
maintenance stages of postischemic ARF in rats and dogs
(4, 6, 15, 33). Finn and Chevalier (15)
extended their observations into the recovery stage. They found that
early recovery, 2 wk after injury, was associated with a parallel but partial improvement in GFR and
P owing to a decline in Bowman's space pressure. Only beyond 2 wk was continued recovery of GFR and
P
attributable to a late rise in glomerular capillary pressure. The
elevation of Bowman's space pressure in the maintenance stage of ARF
was associated with sluggish tubule fluid flow, distension of the
tubule lumen, and extensive necrosis and exfoliation of proximal tubule
cells. The decline in Bowman's space pressure in the recovery stage,
on the other hand, was associated with less tubule luminal distension
and a reduction of intraluminal cells and casts (15).
Because hydraulic pressures in Bowman's space and glomerular
capillaries cannot be determined in humans, only the above-described morphological features can be used to provide indirect insights into
the mechanism of P depression in our subjects with allograft ARF.
Significant or nearly significant (P = 0.03-0.06)
distension of Bowman's space and tubule lumens in the initiation and
maintenance stages and reduced distension in the recovery stage could
be interpreted as consistent with intratubular obstruction and an
upstream rise in pressure in Bowman's space. In contrast to
postischemic ARF in the rat, however, our morphometric analysis
indicates that tubule cells exhibiting overt necrosis or exfoliation
are sparse in this form of human ARF (Table 3), a finding that has been demonstrated by others and by us previously (2, 18, 27, 31). Unlike other forms of postischemic ARF in humans,
intratubular casts, another potential source of luminal obstruction
(26-28, 30), are also rare in allograft ARF in our
experience (18). We wish to emphasize, however, that we
cannot exclude the presence of casts or other structural changes that
favor obstruction in inner medullary or papillary segments of tubules,
because such segments are not sampled during a renal biopsy.
Nevertheless, it is conceivable that a high rate of tubule fluid flow,
rather than mechanical obstruction, leads to elevation of proximal
tubule pressure and is solely responsible for the distension of
Bowman's space and tubule lumens observed by us in the initiation and
maintenance stages of allograft ARF. That proximal tubule fluid flow
rate could have been very high in our subjects is suggested by the enormous fraction of filtered sodium that was excreted in the initiation and maintenance stages of allograft ARF (Table 3). We
previously used fractional lithium excretion as a surrogate for the
fraction of filtered sodium delivered out of the proximal tubule to the
macular densa (17). We showed that proximal reabsorption of sodium was profoundly depressed. This phenomenon is consistent with
a high rate of tubule fluid flow and could have a mechanical effect to
elevate pressure in and distend the proximal tubule.
Increased delivery of sodium to the macula densa could, of course, also
activate tubuloglomerular feedback and lower P by mediating afferent
vasoconstriction with a downstream fall in glomerular capillary
hydraulic pressure (34). Table 1 shows that although RPF
in the maintenance stage of ARF was depressed by only 38% on average,
the corresponding increase in mean renovascular resistance was by a
factor of 2: 346 ± 158 vs. 168 ± 48 units in controls
(P = 0.0009). This change seems large enough,
particularly if accompanied by selective afferent vasoconstriction
alone or in combination with efferent vasodilatation, to account for
the apparent reduction of computed
P.
In the absence of methods to determine Bowman's space and glomerular
capillary pressures in the human kidney, the potential contribution of
tubule obstruction or afferent vasoconstriction to the filtration
failure that typifies postischemic allograft ARF in humans
cannot be determined. Nevertheless, a consideration of proximal tubule
compliance suggests that our measured changes in luminal area are
consistent with the changes we inferred for P. Converting the
luminal areas in Table 3 to diameters gives 21.5 µm for control, 26.4 µm for sustained day 0, 29.9 µm for recovery day
0, 25.9 µm for sustained day 7, and 22.8 µm for
recovery day 7. As reported by Cortell et al.
(9), proximal diameter varies linearly with pressure, with
a slope of 0.45 µm/mmHg. For example, for recovery day 7 vs. day 0, the diameter reduction is 29.9-22.8 = 7.1 µm. The corresponding reduction in proximal tubule pressure,
based on the rat compliance data, is 7.1/0.45 = 16 mmHg. Because
that matches or exceeds the increases in
P in our Table 4, it tends
to give credence to the idea that increases in proximal tubule
pressure, secondary to tubule obstruction and/or increased flow, are a
major contributor to acute reduction and eventual recovery of
P. Any
additional contribution by afferent vasoconstriction must remain a
matter for speculation.
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ACKNOWLEDGEMENTS |
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This study was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grants DK-50712 and DK-48051 and by General Clinical Research Center Grant M01-PR-00070-DK. D. Ramaswamy's postdoctoral fellowship was supported by the Satellite Dialysis Centers Postdoctoral Fellowship and Educational Fund and by the Northern California Chapter of the National Kidney Foundation. G. Corrigan's postdoctoral fellowship was supported by a grant from Chiron.
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FOOTNOTES |
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Address for reprint requests and other correspondence: B. D. Myers, Div. of Nephrology/S201, Stanford University Medical Center, Stanford, CA 94305-5114 (E-mail: h.takagishi{at}leland.stanford.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
First published August 15, 2001; 10.1152/ajprenal.00068.2001
Received 27 February 2001; accepted in final form 22 August 2001.
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