Relationships among protein and albumin concentrations and oncotic pressure in nephrotic plasma

Pump-Free Microfluidic Hemofiltration Device

Hemofiltration removes water and small molecules from the blood via nanoporous filtering membranes. This paper discusses a pump-free hemofiltration device driven by the pressure difference between the artery and the vein. In the design of the filtering device, oncotic pressure needs to be taken into consideration. Transmembrane pressure (TMP) determines the amount and direction of hemofiltration, which is calculated by subtracting the oncotic pressure from the blood pressure. Blood pressure decreases as the channels progress from the inlet to the outlet, while oncotic pressure increases slightly since no protein is removed from the blood to the filtrate in hemofiltration. When TMP is negative, the filtrate returns to the blood, i.e., backfiltration takes place. A small region of the device with negative TMP would thus result in a small amount of or even zero filtrates. First, we investigated this phenomenon using in vitro experiments. We then designed a hemofiltration system taking backfiltration into consideration. We divided the device into two parts. In the first part, the device has channels for the blood and filtrate with a nanoporous membrane. In the second part, the device does not have channels for filtration. This design ensures TMP is always positive in the first part and prevents backfiltration. The concept was verified using in vitro experiments and ex vivo experiments in beagle dogs. Given the simplicity of the device without pumps or electrical components, the proposed pump-free hemofiltration device may prove useful for either implantable or wearable hemofiltration.

Longitudinal study of living kidney donor glomerular dynamics after nephrectomy

BACKGROUND

Over 5,000 living kidney donor nephrectomies are performed annually in the US. While the physiological changes that occur early after nephrectomy are well documented, less is known about the long-term glomerular dynamics in living donors.

METHODS

We enrolled 21 adult living kidney donors to undergo detailed long-term clinical, physiological, and radiological evaluation pre-, early post- (median, 0.8 years), and late post- (median, 6.3 years) donation. A morphometric analysis of glomeruli obtained during nephrectomy was performed in 19 subjects.

RESULTS

Donors showed parallel increases in single-kidney renal plasma flow (RPF), renocortical volume, and glomerular filtration rate (GFR) early after the procedure, and these changes were sustained through to the late post-donation period. We used mathematical modeling to estimate the glomerular ultrafiltration coefficient (Kf), which also increased early and then remained constant through the late post-donation study. Assuming that the filtration surface area (and hence, Kf) increased in proportion to renocortical volume after donation, we calculated that the 40% elevation in the single-kidney GFR observed after donation could be attributed exclusively to an increase in the Kf. The prevalence of hypertension in donors increased from 14% in the early post-donation period to 57% in the late post-donation period. No subjects exhibited elevated levels of albuminuria.

CONCLUSIONS

Adaptive hyperfiltration after donor nephrectomy is attributable to hyperperfusion and hypertrophy of the remaining glomeruli. Our findings point away from the development of glomerular hypertension following kidney donation.

TRIAL REGISTRATION

Not applicable. FUNDING. NIH (R01DK064697 and K23DK087937); Astellas Pharma US; the John M. Sobrato Foundation; the Satellite Extramural Grant Foundation; and the American Society of Nephrology.

The association of predonation hypertension with glomerular function and number in older living kidney donors

The effect of preexisting hypertension on living donor nephron number has not been established. In this study, we determined the association between preexisting donor hypertension and glomerular number and volume and assessed the effect of predonation hypertension on postdonation BP, adaptive hyperfiltration, and compensatory glomerular hypertrophy. We enrolled 51 living donors to undergo physiologic, morphometric, and radiologic evaluations before and after kidney donation. To estimate the number of functioning glomeruli (NFG), we divided the whole-kidney ultrafiltration coefficient (Kf) by the single-nephron ultrafiltration coefficient (SNKf). Ten donors were hypertensive before donation. We found that, in donors ages >50 years old, preexisting hypertension was associated with a reduction in NFG. In a comparison of 10 age- and sex-matched hypertensive and normotensive donors, we observed more marked glomerulopenia in hypertensive donors (NFG per kidney, 359,499±128,929 versus 558,239±205,152; P=0.02). Glomerulopenia was associated with a nonsignificant reduction in GFR in the hypertensive group (89±12 versus 95±16 ml/min per 1.73 m(2)). We observed no difference in the corresponding magnitude of postdonation BP, hyperfiltration capacity, or compensatory renocortical hypertrophy between hypertensive and normotensive donors. Nevertheless, we propose that the greater magnitude of glomerulopenia in living kidney donors with preexisting hypertension justifies the need for long-term follow-up studies.

Body fluid dynamics: back to the future

Pioneering investigations conducted over a half century ago on tonicity, transcapillary fluid exchange, and the distribution of water and solute serve as a foundation for understanding the physiology of body fluid spaces. With passage of time, however, some of these concepts have lost their connectivity to more contemporary information. Here we examine the physical forces determining the compartmentalization of body fluid and its movement across capillary and cell membrane barriers, drawing particular attention to the interstitium operating as a dynamic interface for water and solute distribution rather than as a static reservoir. Newer work now supports an evolving model of body fluid dynamics that integrates exchangeable Na(+) stores and transcapillary dynamics with advances in interstitial matrix biology.

Effects of aging on glomerular function and number in living kidney donors

To elucidate the pathophysiologic changes in the kidney due to aging, we used physiological, morphometric, and imaging techniques to quantify GFR and its determinants in a group of 24 older (≥ 55 years) compared to 33 younger (≤ 45 years) living donors. Mathematical modeling was used to estimate the glomerular filtration coefficients for the whole kidney (K(f)) and for single nephrons (SNK(f)), as well as the number of filtering glomeruli (N(FG)). Compared to younger donors, older donors had a modest (15%) but significant depression of pre-donation GFR. Mean whole-kidney K(f), renocortical volume, and derived N(FG) were also significantly decreased in older donors. In contrast, glomerular structure and SNK(f) were not different in older and younger donors. Derived N(FG) in the bottom quartile of older donors was less than 27% of median-derived N(FG) in the two kidneys of younger donors. Nevertheless, the remaining kidney of older donors exhibited adaptive hyperfiltration and renocortical hypertrophy post-donation, comparable to that of younger donors. Thus, our study found the decline of GFR in older donors is due to a reduction in K(f) attributable to glomerulopenia. We recommend careful monitoring for and control of post-donation comorbidities that could exacerbate glomerular loss.

Determinants of decline in glomerular filtration rate in nonproteinuric subjects with or without diabetes and hypertension

BACKGROUND AND OBJECTIVES

This study investigated whether the slope of estimated GFR is different between nonproteinuric subjects with and without diabetes, and what clinical factors are associated with the GFR slope.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

An observational cohort study was performed in 923 subjects, and the predictive value of baseline variables on the GFR slope was investigated.

RESULTS

On the basis of the median 3-yr follow-up and 7 measurements of GFR, GFR slope (%/yr, median and interquartile range) was significantly larger in subjects with diabetes (-2.39 (-4.86 to 0.15), n=729) than in those without diabetes (-1.02 (-4.28 to 1.37), n=194), and this difference remained significant with or without presence of hypertension. After adjustments for confounding factors, predictors of GFR decline were found to be baseline high values of glycosylated hemoglobin A1C(HbA1C), GFR, systolic blood pressure, and low plasma total protein in subjects with diabetes, whereas only the latter two were significant in subjects without diabetes. In subjects with diabetes, the high GFR was accounted for by high HbA1C at baseline, and the predictors of GFR decline differed between those with and without hypertension, or with high and low baseline GFR. Any combination of the predictors showed increased risk for GFR decline.

CONCLUSIONS

GFR slope is substantially affected by multiple factors at various stages. The degree of chronic hyperglycemia is likely to play a crucial role in elevating GFR and accelerating the decline in patients with type 2 diabetes even from the normoalbuminuric stage.

Glomerular function, structure, and number in renal allografts from older deceased donors

The 5-yr survival rate of renal allografts is significantly lower for grafts from older deceased donors than from younger deceased donors. For evaluation of the potential contribution of renal senescence in this shortened graft survival, glomerular function and structure were analyzed in allografts from deceased donors older than 55 yr ("aging") or younger than 40 yr ("youthful"). Aging donors had a significantly higher prevalence of sclerotic glomeruli (P < 0.002), and their nonsclerotic glomeruli tended to be larger, had a larger filtration surface area (P = 0.02), and had a higher single-nephron ultrafiltration coefficient (K(f); P = 0.07), suggesting a compensatory response to functional loss of glomeruli. After serum creatinine reached a stable nadir in the transplant recipients, GFR and its hemodynamic determinants were evaluated and the whole allograft K(f) was computed. Compared with the allografts from youthful donors, allografts from aging donors exhibited a 32% lower GFR, which was exclusively attributable to a 45% reduction in allograft K(f) (both P < 0.001). In addition, the number of functioning glomeruli per allograft was profoundly lower in grafts from aging donors than from youthful donors (3.6 +/- 2.1 x 10(5) versus 8.5 +/- 3.4 x 10(5); P < 0.01), and this could not be explained by the relatively modest 17% prevalence of global glomerulosclerosis in the aging group. The marked reduction in overall glomerular number in many aging donors may lead to a "remnant kidney" phenomenon, potentially explaining the shorter mean survival of these allografts.

Course of preeclamptic glomerular injury after delivery

We evaluated the early postpartum recovery of glomerular function over 4 wk in 57 women with preeclampsia. We used physiological techniques to measure glomerular filtration rate (GFR), renal plasma flow, and oncotic pressure (πA) and computed a value for the two-kidney ultrafiltration coefficient ( Kf). Compared with healthy, postpartum controls, GFR was depressed by 40% on postpartum day 1, but by only 19% and 8% in the second and fourth postpartum weeks, respectively. Hypofiltration was attributable solely to depression, at corresponding postpartum times, of Kf by 55%, 30%, and 18%, respectively. Improvement in glomerular filtration capacity was accompanied by recovery of hypertension to near-normal levels and significant improvement in albuminuria. We conclude that the functional manifestations of the glomerular endothelial injury of preeclampsia largely resolve within the first postpartum month.

Adaptive hyperfiltration in the aging kidney after contralateral nephrectomy

We examined the magnitude of adaptive hyperfiltration in the remaining kidney of 16 aging (>57 yr) and 16 youthful (<55 yr) individuals who had undergone a contralateral nephrectomy. Healthy volunteers who were youthful ( n = 143) or aging ( n = 37) provided control values for the binephric condition. One-kidney glomerular filtration rate (GFR; +42%), renal plasma flow (+38%), plasma oncotic pressure (+2.8 mmHg), and mean arterial pressure (+7.0 mmHg) were all higher in youthful uninephric vs. binephric subjects. Corresponding excesses in aging uninephric vs. binephric subjects were by 38 and 36% and 1.4 and 14.0 mmHg, respectively. Modeling of these data revealed that an isolated increase in either the glomerular ultrafiltration coefficient ( Kf) by 110% or in the transcapillary hydraulic pressure gradient (ΔP) by 7 mmHg, could account for the observed level of hyperfiltration in youthful uninephric subjects. Corresponding increases for aging uninephric subjects were 61% for Kf and 5 mmHg for ΔP. We conclude that the magnitude of adaptive hyperfiltration is similar in aging to that in youthful uninephric subjects, albeit at a lower absolute GFR level. Isolated increases in either Kf or ΔP or a combination of smaller increases in both can account for the hyperfiltration. Greater adaptive arterial hypertension in aging than youthful uninephric subjects raises the possibility of a disproportionate role for glomerular hypertension and ΔP elevation in aging compared with youthful uninephric subjects. Glomerular hypertension could exacerbate the sclerosing glomerulopathy of senescence and lead to renal insufficiency. We recommend that living donors of a kidney transplantation in or beyond the seventh decade be used with caution.

Diabetic nephropathy is associated with increased albumin and fibrinogen production in patients with type 2 diabetes

AIMS/HYPOTHESIS

Hyperfibrinogenaemia and albuminuria are cardiovascular risk factors, often coexisting in diabetic and non-diabetic people. Albuminuria in turn is associated with a compensatory albumin overproduction in non-diabetic patients. It is not known whether the presence of albuminuria in patients with type 2 diabetes mellitus is associated with greater albumin and fibrinogen production rates than in normoalbuminuric patients. SUBJECTS, MATERIALS, AND METHODS: Using leucine isotope methods, we measured fractional and absolute synthesis rates (FSR, ASR) of albumin and fibrinogen in post-absorptive type 2 diabetic patients with either normal (n=11) or increased (n=10) urinary albumin excretion.

RESULTS

In albuminuric patients, albumin FSR (16.2+/-1.5%/day) and ASR (20.5+/-1.9 g/day) were greater (p<0.02 and p<0.05, respectively) than in normoalbuminuric patients (FSR=11.5+/-1.1%/day; ASR=15.7+/-1.2 g/day). Fibrinogen FSR was similar between patients with normal and increased albumin excretion, but concentration, the circulating pool and ASR of fibrinogen were 40 to 50% greater (p<0.035) in patients with albuminuria. Albuminuria was positively correlated with albumin ASR, with fibrinogen concentration, the fibrinogen pool and ASR, whereas albumin synthesis was inversely correlated with calculated oncotic pressure.

CONCLUSIONS/INTERPRETATION

Synthesis of albumin and fibrinogen is upregulated in type 2 diabetic patients with increased urinary albumin excretion. Albuminuria is associated with enhanced fibrinogen and albumin synthesis.

The dynamics of glomerular filtration in the puerperium

We evaluated the glomerular filtration rate (GFR) during the second postpartum week in 22 healthy women who had completed an uncomplicated pregnancy. We used physiological techniques to measure GFR, renal plasma flow, and oncotic pressure and computed a value for the two-kidney ultrafiltration coefficient (K(f)). We compared these findings with those in pregnant women previously studied on the first postpartum day as well as nongravid women of reproductive age. Healthy female transplant donors of reproductive age permitted the morphometric analysis of glomeruli and computation of the single-nephron K(f). The aforementioned physiological and morphometric measurements were utilized to estimate transcapillary hydraulic pressure (Delta P) from a mathematical model of glomerular ultrafiltration. We conclude that postpartum day 1 is associated with marked glomerular hyperfiltration (+41%). A theoretical analysis of GFR determinants suggests that depression of glomerular capillary oncotic pressure, the force opposing the formation of filtrate, is the predominant determinant of early elevation of postpartum GFR. A reversal of the gestational hypervolemia and hemodilution, still evident on postpartum day 1, eventuates by postpartum week 2. An elevation of oncotic pressure in the plasma that flows axially along the glomerular capillaries to supernormal levels ensues; however, GFR remains modestly elevated (+20%) above nongravid levels. An analysis of filtration dynamics at this time suggests that a significant increase in Delta P by up to 16%, an approximately 50% increase in K(f), or a combination of smaller increments in both must be invoked to account for the persistent hyperfiltration.

Determinants of glomerular hypofiltration in aging humans

BACKGROUND

The purpose of the present study was to confirm the extent to which glomerular filtration rate (GFR) is depressed in healthy, aging subjects and to elucidate the mechanism of such hypofiltration.

METHODS

Healthy volunteers aged 18 to 88 years (N = 159) underwent a determination of GFR, renal plasma flow (RPF), afferent oncotic pressure, and arterial pressure. Glomeruli in renal biopsies of healthy kidney transplant donors aged 23 to 69 years (N = 33) were subjected to a morphometric analysis, so as to determine glomerular hydraulic permeability and filtration surface area. The aforementioned GFR determinants were then subjected to mathematical modeling to compute the glomerular ultrafiltration coefficient (Kf) for two kidneys and individual glomeruli.

RESULTS

GFR was significantly depressed (P < 0.0001) by 22% in aging (>or=55 years old) compared to youthful subjects (<or=40 years old). Corresponding reductions of the following GFR determinants in the aging vs. youthful subsets were RPF by 28% (P < 0.0001), two-kidney Kf by 21% to 53% (P < 0.005), glomerular hydraulic permeability by 14% (P = 0.03), and the single-nephron Kf (SNKf) by 30% (P = 0.09). There were no significant differences between aging and youthful subsets for afferent oncotic pressure and filtration surface area per glomerulus.

CONCLUSION

We conclude that a reduction in overall, two-kidney Kf contributes to GFR depression in aging subjects. We infer that this is due in part to structural changes that lower SNKf, and in part to the reduction in the actual number of functioning glomeruli that has been demonstrated by others at autopsy.

Determinants of GFR depression in early membranous nephropathy

We evaluated the glomerular filtration rate (GFR) in 34 subjects with membranous nephropathy (MN) of new onset. We used physiological techniques to measure GFR, renal plasma flow, and oncotic pressure and computed a value for the two-kidney ultrafiltration coefficient (K(f)). A morphometric analysis of glomeruli in the diagnostic biopsy permitted computation of the single-nephron ultrafiltration coefficient (SNK(f)). MN subjects were divided into two groups: moderate or severe, according to whether GFR was depressed by less or more than 50%. SNK(f) was subnormal but similar in moderate and severe MN. In contrast, two-kidney K(f) was significantly more depressed in severe than in moderate MN. We estimated the total number of functioning glomeruli (N(g)) by dividing two-kidney K(f) by SNK(f). Whereas mean N(g) was similar in controls and moderate MN (1.5 and 1.4-1.7 x 10(6), respectively), it was significantly lower in severe MN (0.5 x 10(6)). This degree of glomerulopenia was not reflected in the rate of global sclerosis. We conclude that a combination of depressed SNK(f) (due to foot process broadening) and profound glomerulopenia accounts for GFR depression of >50% early in the course of MN. The cause of the glomerulopenia remains to be elucidated.

Maintenance and recovery stages of postischemic acute renal failure in humans

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 (DeltaP), the only remaining determinant of DeltaP. This revealed that DeltaP 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 DeltaP, 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.

Plasma protein synthesis in patients with low-grade nephrotic proteinuria

Overt nephrotic syndrome is characterized by albumin and fibrinogen hyperproduction and reduced very low density lipoprotein apolipoprotein B-100 (VLDL apoB-100) clearance. Whether similar changes also occur in low-grade proteinuria is not known. Thus we measured albumin, fibrinogen, and VLDL apoB-100 kinetics in six patients with modest proteinuria and normal creatinine clearance (P) and in ten control subjects (C) by leucine tracer infusion and precursor-product relationships. In P, plasma albumin concentration was decreased (P < 0.003), whereas concentrations of fibrinogen and VLDL apoB-100 were increased (P < 0.001). In P, albumin fractional secretion rate (FSR) was increased (P < 0.01), fibrinogen FSR was normal, and VLDL apoB-100 FSR was decreased (P < 0.03). As a result, in P, absolute secretion rates (ASR) of albumin and fibrinogen were increased (P < 0.03), whereas VLDL apoB-100 ASR was normal. Albumin FSR was inversely correlated to oncotic pressure in P but not in C. These findings suggest that low-grade nephrotic proteinuria is characterized by simultaneous multiple alterations in turnover rates of albumin, fibrinogen, and VLDL apoB-100. Their pathogenesis, however, appears to be multifactorial.

Glomerular hemodynamics in severe obesity

Differential solute clearances were used to characterize glomerular function in 12 nondiabetic subjects with severe obesity (body mass index >38). Nine healthy subjects served as the control group. In the obese group, glomerular filtration rate (GFR) and renal plasma flow (RPF) exceeded the control value by 51 and 31%, respectively. Consequently, filtration fraction increased. The augmented RPF suggested a state of renal vasodilatation involving, mainly or solely, the afferent arteriole. Albumin excretion rate and fractional albumin clearance increased by 89 and 78%, respectively. Oral glucose tolerance tests were suggestive of insulin resistance. Insulin resistance was positively correlated with GFR (r = 0.88, P<0.001) and RPF (r = 0.72, P <0.001). Mean arterial pressure was higher than in the control group. Fractional clearances of dextrans of broad size distribution tended to be lowered. The determinants of the GFR were estimated qualitatively by using a theoretical model of dextran transport through a heteroporous membrane. This analysis suggests that the high GFR in very obese subjects may be the result of an increase in transcapillary hydraulic pressure difference (DeltaP). An abnormal transmission of increased arterial pressure to the glomerular capillaries through a dilated afferent arteriole could account for the augmentation in DeltaP.

Mechanisms of filtration failure during postischemic injury of the human kidney. A study of the reperfused renal allograft

Postischemic filtration failure in experimental animals results primarily from depression of the transcapillary hydraulic pressure difference (delta P), a quantity that cannot be determined in humans. To circumvent this limitation we determined the GFR and each of its remaining determinants in transplanted kidneys. Findings in 12 allografts that exhibited subsequent normofiltration (group 1) were compared with those in 11 allografts that exhibited persistent hypofiltration (group 2). Determinations were made intraoperatively in the exposed graft after 1-3 h of reperfusion. GFR (6 +/- 2 vs 29 +/- 5 ml/min) and renal plasma flow by Doppler flow meter (140 +/- 30 vs 315 +/- 49 ml/min) were significantly lower in group 2 than group 1. Morphometric analysis of glomeruli obtained by biopsy and a structural hydrodynamic model of viscous flow revealed the glomerular ultrafiltration coefficient to be similar, averaging 3.5 +/- 0.6 and 3.1 +/- 0.2 ml/(min.mmHg) in group 2 vs 1, respectively. Corresponding values for plasma oncotic pressure were also similar, averaging 19 +/- 1 vs 21 +/- 1 mmHg. We next used a mathematical model of glomerular ultrafiltration and a sensitivity analysis to calculate the prevailing range for delta P from the foregoing measured quantities. This revealed delta P to vary from only 20-21 mmHg in group 2 vs 34-45 mmHg in group 1 (P < 0.001). Further morphometric analysis revealed the diameters of Bowman's space and tubular lumens, as well as the percentage of tubular cells that were necrotic or devoid of brush border, to be similar in the two groups. We thus conclude (a) that delta P depression is the predominant cause of hypofiltration in this form of postischemic injury; and (b) that afferent vasoconstriction rather than tubular obstruction is the proximate cause of the delta P depression.

Structural basis for reduced glomerular filtration capacity in nephrotic humans

Previous studies have established that in a variety of human glomerulopathies the reduced glomerular filtration rate (GFR) is due to a marked lowering of the ultrafiltration coefficient (Kf). To identify the factors which lower Kf, we measured the filtering surface area per glomerulus, filtration slit frequency, basement membrane thickness, and GFR and its determinants in patients with minimal change and membraneous nephropathies and in age-matched healthy controls. Overall values of Kf for the two kidneys were calculated from GFR, renal plasma flow rate, systemic colloid osmotic pressure, and three assumed values for the transcapillary pressure difference. "Experimental" values of the glomerular hydraulic permeability (kexp) were then calculated from Kf, glomerular filtering surface area, and estimates of the total number of nephrons of the two kidneys. Independent estimates of the glomerular hydraulic permeability (kmodel) were obtained using a recent mathematical model that is based on analyses of viscous flow through the various structural components of the glomerular capillary wall. Individual values of basement membrane thickness and filtration slit frequency were used as inputs in this model. The results indicate that the reductions of Kf in both nephropathies can be attributed entirely to reduced glomerular hydraulic permeability. The mean values of kexp and kmodel were very similar in both disorders and much smaller in the nephrotic groups than in healthy controls. There was good agreement between kexp and kmodel for any given group of subjects. It was shown that, in both groups of nephrotics, filtration slit frequency was a more important determinant of the water flow resistance than was basement membrane thickness. The decrease in filtration slit frequency observed in both disorders caused the average path length for the filtrate to increase, thereby explaining the decreased hydraulic permeability.