Control of proximal tubule fluid reabsorption in experimental glomerulonephritis

Development of Physiologically Based Pharmacokinetic Model for Pregabalin to Predict the Pharmacokinetics in Pediatric Patients with Renal Impairment and Adjust Dosage Regimens: PBPK Model of Pregabalin in Pediatric Patients with Renal Impairment

Pregabalin (PGB) is widely used clinically; however, its pharmacokinetics (PK) has not been studied in pediatric patients with renal impairment (RI). To design optimized PGB regimens for pediatric patients with varying degrees of RI and predict exposure to PGB, physiologically based pharmacokinetic (PBPK) models of PGB were developed and verified, and its disposition was simulated in the healthy population and adults with RI. The simulated results from the PBPK models after single-dose and multi-dose administrations of PGB were consistent with the corresponding observed data based on the fold error values of less than 2. The area under curve ratios were 1.23 ± 0.06, 2.02 ± 0.10, 3.86 ± 0.21, and 9.92 ± 0.79 in pediatric patients with mild, moderate, severe, and end-stage RI, respectively. Based on the predictions for pediatric patients with moderate, severe, and end-stage RI, the maximum dose should not exceed 7, 3.5, and 1.4 mg/kg/day, respectively, among those weighing < 30 kg, and it should not exceed 5, 2.5, and 1 mg/kg/day, respectively, among those weighing > 30 kg. In conclusion, the developed PBPK model is a valuable tool for predicting PGB dosage for pediatric patients with RI.

Collecting duct (Na+/K+)-ATPase activity is correlated with urinary sodium excretion in rat nephrotic syndromes

In puromycin aminonucleoside (PAN)-treated nephrotic rats, sodium retention is associated with increased (Na+/K+)-ATPase activity in the cortical collecting ducts (CCD). This study was undertaken to determine whether stimulation of (Na+/K+)-ATPase in the CCD is a feature of other experimental nephrotic syndromes, whether it might be responsible for renal sodium retention, and whether it is mediated by increased plasma vasopressin levels or activation of calcineurin. For this purpose, the time courses of urinary excretion of sodium and protein, sodium balance, ascites, and (Na+/K+)-ATPase activities in microdissected CCD were studied in rats with PAN or adriamycin nephrosis or HgCl2 nephropathy. The roles of vasopressin and calcineurin in PAN nephrosis were evaluated by measuring these parameters in Brattleboro rats and in rats treated with cyclosporin or tacrolimus. Despite different patterns of changes in urinary sodium and protein excretion in the three nephrotic syndrome models, there was a linear relationship between CCD (Na+/K+)-ATPase activities and sodium excretion in all three cases. The results also indicated that there was no correlation between proteinuria and sodium retention, but ascites was present only when proteinuria was associated with marked reduction of sodium excretion. Finally, the lack of vasopressin in Brattleboro rats or the inhibition of calcineurin by administration of either cyclosporin or tacrolimus did not prevent development of the nephrotic syndrome in PAN-treated rats or stimulation of CCD (Na+/K+)-ATPase. It is concluded that stimulation of Na(+/K+)-ATPase in the CCD of nephrotic rats might be responsible for sodium retention and that this phenomenon is independent of proteinuria and vasopressin and calcineurin activities.

Edema of the nephrotic syndrome: the role of the atrial peptide system

The nephrotic syndrome is associated with an expanded interstitial volume and edema due to sodium and water retention. The mechanisms underlying these abnormalities have been only partially clarified. Renal hypoperfusion has been considered the key event that promotes avid sodium and water reabsorption by the kidney. Hypoperfusion results from hypovolemia, a consequence of urinary protein losses and decreased oncotic pressure. However, in some patients plasma volume is normal or even increased, suggesting that in such cases the cause of sodium and water retention might be independent of systemic events and possibly originates in the kidney. Experimental evidence is now available to support this, but the intrarenal mediator(s) that promote the abnormal salt retention are still not fully clear. Atrial natriuretic peptide (ANP), which increases sodium and water excretion, has been suspected to participate in fluid retention. This is consistent with experimental and human data of a markedly blunted natriuretic and diuretic response to systemic infusion of ANP in the nephrotic syndrome. Recent studies of the mechanisms of the blunted natriuretic and diuretic response to ANP documented an increased activity of renal sympathetic nerves, but the results are controversial. The altered response to ANP also may be related to a defect in the number and affinity of receptor-binding sites for the peptide. Evidence also is available of a possible defect at the level of intracellular cyclic guanosine monophosphate, the second messenger of ANP. The gene encoding for a cyclophilin-like protein, which is increased in sodium-retaining conditions, is upregulated in the kidneys of nephrotic rats, and the infusion of ANP further increases cyclophilin-like protein mRNA. Thus, multiple factors probably act in concert to induce edema formation in the nephrotic syndrome. In this review we specifically address the tubular insensitivity to the natriuretic and diuretic action of ANP, which could be an important initiating event and could possibly contribute to sustaining the edema.

Atrial natriuretic peptide and the renal response to hypervolemia in nephrotic humans

To elucidate the abnormality of body fluid homeostasis that attends the nephrotic syndrome, we compared the atrial hormonal and renal excretory and vasomotor responses to water immersion of nephrotic patients (N = 10) with those of healthy controls (N = 9). Nephrotics exhibited depressed baseline levels of atrial natriuretic peptide (ANP, P less than 0.05) and lower rates of urine flow and sodium excretion (P less than 0.01). Although immersion-induced hypervolemia increased plasma ANP to equivalent levels (75 +/- 19 vs. 60 +/- 6 pg/ml), the disparity in corresponding urinary flow (5 +/- 1 vs. 13 +/- 2 ml/min, P less than 0.01) and sodium excretion (171 +/- 42 vs. 540 +/- 65 muEq/min, P less than 0.01) grew larger. In contrast, immersion caused an equivalent reduction of renal vascular resistance by 16 and 17%, respectively (P less than 0.01). Despite higher renal plasma flow and lower oncotic pressure of plasma, the glomerular filtration rate remained constant during immersion in both groups. Similar constancy of fractional clearances of dextrans of graded size suggests that immersion may have lowered the glomerular transcapillary hydraulic pressure difference (delta P). We conclude that renal vasomotor responsiveness to hypervolemia is preserved in nephrotics, but that the mediatory role of ANP in this response is uncertain. By contrast, diminished responsiveness of the distal nephron to the natriuretic action of endogenous ANP could contribute to edema formation in the nephrotic syndrome.

Extracellular fluid expansion and autoregulation in nephrotoxic serum nephritis in rats

Previous studies of the early autologous phase of bilateral nephrotoxic serum nephritis (NSN) in rats showed that whole kidney and single nephron glomerular filtration rate (GFR) were maintained at normal levels despite a 60% reduction in the product of surface area and hydraulic permeability (Kf). Factors responsible for this compensation were an increase of net ultrafiltration pressure, due primarily to an increased glomerular capillary pressure (PGC). This study was designed to investigate some possible causes of the compensation. Rats with bilateral NSN and normal GFR had an increased extracellular fluid volume (ECFV) 2 weeks after induction of NSN; control subjects did not change. To determine whether this ECFV expansion was responsible for triggering the compensation, we developed a unilateral NSN model with one diseased and one normal kidney. Unilaterally diseased rats did not experience an increase of ECFV. Values of Kf were 0.069 +/- 0.012 nl sec-1 mm Hg-1 in control subjects, 0.037 +/- 0.005 in bilateral NSN, and 0.043 +/- 0.006 in unilateral NSN. The elevation in PGC was the same in unilateral NSN as in bilateral NSN subjects and the same was true for the hydrostatic pressure difference across glomerular capillaries (delta P). Furthermore, in paired measurements on both kidneys of rats with unilateral NSN, PGC was significantly higher in the unilaterally diseased kidney than in the nondiseased kidney; sham control subjects had no difference. These results are interpreted to indicate that the signal that causes elevation of net ultrafiltration pressure is not a consequence of a systemic effect of NSN, but arises within the diseased kidney itself. To determine whether that signal involved some change in the mechanisms mediating autoregulation measurements were made of the response of whole kidney GFR and RBF to acute changes in arterial BP. Control rats and rats with NSN autoregulated both GFR and blood flow equally well. Tubuloglomerular feedback was studied by microperfusing loops of Henle and measuring proximal stop-flow pressure and early proximal flow rate. Stop-flow pressure was 4.0 mm Hg higher in rats with NSN at a loop perfusion rate of 10 nl/min, approximately the same difference that was found by direct measurement of PGC, but the sensitivity of response to changes in perfusion rate was the same in NSN as in control subjects. Finally, end proximal tubule flow rate was higher in NSN than in control subjects reflecting decreased proximal reabsorption. Thus, a normal feedback mechanism receives a signal that should cause afferent arteriolar constriction in NSN rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiologic basis for the maintenance of glomerulotubular balance in young growing rats

To examine the physiologic basis of preservation of glomerulotubular balance in young growing animals, we measured the various determinants for fluid transfer across the glomerular and postglomerular capillaries in young (approximately 40 days, N = 8) and adult (N = 8) male Munich-Wistar rats under mild volume expansion. The single nephron (SN) GFR increased by approximately 2.5-fold from young to adult animals. The increase in SNGFR was due to marked rises in both glomerular plasma flow rate (QA) and ultrafiltration coefficient (Kf). The increase in QA was associated with a nearly 60% reduction in afferent and efferent arteriolar resistances. As with SNGFR, the absolute proximal reabsorption rate (APR) increased by some 2.5-fold, indicating preservation of perfect glomerulotubular balance. Of the factors determining peritubular capillary uptake of APR, the mean oncotic pressure difference across the peritubular capillary was similar in young and adult animals. The mean hydraulic pressure difference was also comparable in the two groups. By contrast, the peritubular capillary reabsorption coefficient (Kr) rose markedly and accounted entirely for the increase in peritubular capillary uptake of APR during growth. These results obtained under mild volume expansion indicate that the maintenance of glomerulotubular balance in the growing rat requires harmonious growth of renal microcirculation, that is, glomerular capillary (Kf) and arteriolar (QA) maturation balances the development of peritubular capillary (Kr).

Blood flow dependence of postglomerular fluid transfer and glomerulotubular balance

The rate of blood flow entering a capillary network can, in some vascular systems, regulate capillary surface area and the rate of fluid and solute transfer. To determine whether such a mechanism exists in the renal peritubular capillary, we performed micropuncture studies in 28 rats during relatively low and high efferent arteriolar blood flow (EABF). High EABF was achieved by intravenous infusion of isoncotic plasma (group 1: from 120 +/- 11 to 301 +/- 49 nl/min [+/- SE]); whole blood with high hematocrit (approximately 75 vol %) (group 2: from 141 +/- 14 to 252 +/- 31 nl/min); or acetylcholine (group 3: from 193 +/- 20 to 266 +/- 26 nl/min). In group 1 rats, plasma infusion caused an increase in single nephron glomerular filtration rate (SNGFR), on average, from 23.2 +/- 2.4 to 45.2 +/- 3.9 nl/min, owing primarily to increased glomerular plasma flow rate (from 63 +/- 5 to 210 +/- 21 nl/min). The rate of fluid uptake by the peritubular capillary, assessed by the absolute rate of proximal fluid reabsorption (APR), also rose significantly, on average from 10.5 +/- 1.2 to 17.5 +/- 2.4 nl/min. This rise in APR was associated with near constancy in mean transcapillary hydraulic (delta Pc) and oncotic (delta IIc) pressure differences, and was therefore attributed to a significant increase in peritubular capillary reabsorption coefficient (Kr), with the mean from 0.017 +/- 0.003 to 0.030 +/- 0.005 nl/(s . mmHg). In group 2 rats, high hematocrit blood infusion led to a significant rise in APR; on average, from 10.7 +/- 0.7 to 15.0 +/- 1.2 nl/min, without changing SNGFR. This rise in APR occurred despite unfavorable changes in the physical forces, namely a significant increase in delta Pc and constancy in delta IIc. Instead, an increase in EABF was again associated with a significant rise in Kr (on average, from 0.016 +/- 0.002 to 0.030 +/- 0.06 nl/[s . mmHg]), which accounted entirely for the rise in APR, independently of SNGFR. In group 3 rats, in which an increase of EABF was induced pharmacologically with acetylcholine, a rise in EABF was also accompanied by a significant increase in Kr, on average, from 0.019 +/- 0.002 to 0.026 +/- 0.004 nl/(s . mmHg). The results indicate that: (a) Kr is modulated by EABF. (b) In view of plasma flow dependence of GFR, blood flow dependence of Kr and APR provides an important basis for glomerulotubular balance.

Role for intrarenal mechanisms in the impaired salt excretion of experimental nephrotic syndrome

A unilateral model of puromycin aminonucleoside (PAN)-induced albuminuria was produced in Munich-Wistar rats to examine the mechanisms responsible for renal salt retention. 2 wk after selective perfusion of left kidneys with PAN (n = 8 rats) or isotonic saline (control, n = 7 rats), increases in albumin excretion and decreases in sodium excretion were demonstrated in PAN-perfused but not in nonperfused kidneys of PAN-treated rats although systemic plasma protein concentration remained at control level. Total kidney glomerular filtration rate (GFR) and superficial single nephron (SN) GFR were also reduced selectively in PAN-perfused kidneys, on average by approximately 30%, due primarily to a marked decline in the glomerular capillary ultrafiltration coefficient (Kf), which was also confined to PAN-perfused kidneys. Values for absolute proximal reabsorption (APR) were also selectively depressed in PAN-perfused kidneys, in keeping with a similarly selective decline in peritubular capillary oncotic pressure measured in these kidneys, the latter also a consequence of the fall in Kf. In a separate group of seven PAN-treated rats, however, no differences were detected between PAN-perfused and nonperfused kidneys in the absolute amount of sodium reaching the early (0.77 +/- 0.09 neq/min vs. 0.74 +/- 0.08, P greater than 0.40) and late portions of superficial distal tubules (0.31 +/- 0.02) neq/min vs. 0.32 +/- 0.05, P greater than 0.50), despite the lesser filtered load of sodium in PAN-perfused kidneys. Suppressed sodium reabsorption in both proximal convoluted tubules and short loops of Henle of PAN-perfused kidneys contributed to this equalization of sodium delivery rates to the late distal tubule, as did comparable reabsorption along distal convolutions. In two additional groups of PAN-treated rats, infusion of saralasin (0.3 mg/kg per h, i.v.) led to substantial increases in total kidney GFR and SNGFR in PAN-perfused but not in nonperfused kidneys. Despite these increases in total and SNGFR, urinary sodium excretion by PAN-perfused kidneys remained at a level far below that for nonperfused kidneys, again indicating that the antinatriuresis characterizing the PAN-perfused kidney is due to alterations in sodium handling by the tubules rather than changes in GFR. These results therefore indicate (a) that reductions in Kf and depressed sodium reabsorption by proximal tubules and Henle's loop segments in this model are brought about by intrarenal rather than circulating or systemic factors, and (b) assuming that superficial nephrons are representative of the entire nephron population, renal salt retention in this model is due primarily to intrarenal factor(s) acting beyond the distal convolution.

Aggravation of experimental glomerulonephritis by superimposed clip hypertension

To evaluate the possible enhancing effect of hypertension on the clinical and morphologic features of glomerulonephritis, two-kidney clip hypertension (CH) was superimposed on a mild form of nephrotoxic serum nephritis (NSN) in female Sprague-Dawley rats. The following parameters were assessed regularly over a 6-month period: blood pressure (BP), heart weight, proteinuria (UpV), and renal morphology. Blood pressure and heart weights were increased equally in clip hypertension and in nephrotoxic serum nephritis combined with clip hypertension. While only moderate proteinuria occurred in nephrotoxic serum nephritis (49 +/- 28 mg/24 hr) or clip hypertension (40 +/- 22 mg/24 hr) alone, the superimposition of clip hypertension on nephrotoxic serum nephritis resulted in heavy proteinuria (161 +/- 36 mg/24 hr) (P less than 0.001) after 5 months of hypertension. Glomerular histology in nephrotoxic serum nephritis showed infrequent focal and segmental proliferation and minimal sclerosis; vessels were normal. Clip hypertension was characterized by infrequent and mild vascular sclerosis and glomerular proliferation and sclerosis. Severe glomerular endo- and extracapillary proliferation and widespread glomerular and vascular sclerosis occurred in the majority of rats when nephrotoxic serum nephritis was combined with clip hypertension. The data demonstrate that clip hypertension enhances glomerular proliferation and sclerosis and results in the development of vascular sclerosis in experimental nephritis.

Formation and structure of human Hageman factor fragments

Autodigestion of activated Hageman factor (HFa) yields a 40,000-mol wt activated enzyme as well as Hageman factor fragment (HFf); HFf consists of two molecular weight species of 28,500 and 30,000. We have investigated the structure of these active fragments and demonstrate that upon reduction, each possesses a heavy chain of 28,000. The associated light chains were identified by subjecting iodinated proteins to two-dimensional slab gel electrophoresis in which the second dimension is run reduced. The 40,000-dalton enzyme has a light chain of 15,000, the 30,000-dalton form of HFf has a light chain of 2,000 and we have suggestive evidence of a light chain associated with the 28,500-dalton form of HFf (putative mol wt approximately 500). We also demonstrate that the 30,000-dalton form of HFf precedes the 28,500 form. These data indicate that digestion of native HF to form HFa precedes cleavages that fragment the molecule and diminish its molecular weight. The 28,500-dalton light chain of HFa becomes the heavy chain of each of the fragmentation products while cleavage at different points along the heavy chain of HFa determines which fragments will be produced. In contrast to autoactivation, kallikrein digestion of HFa yields primarily HFf; however, the 40,000-dalton enzyme may be seen when prekallikrein-deficient (Fletcher trait) plasma is activated.

A micropuncture study of renal sodium retention in nephrotic syndrome in rats: evidence for increased resistance to tubular fluid flow

Micropuncture studies were carried out in surface nephrons of rats with nephrotoxic-serum (NTS)-induced nephrotic syndrome during a period of active sodium and water retention. It was found that hydrostatic pressure and tubular diameter were increased in the proximal tubules (13.4 +/- 0.2 vs. 10.4 +/- 0.2 mm Hg; 31.3 +/- 0.9 vs. 18.4 +/- 0.7 mu), whereas pressure and tubular diameter were normal in the distal tubules. Single nephron glomerular filtration rate (SNGFR) was decreased and fractional reabsorption of fluid was markedly increased in the proximal tubules (74.1 vs. 61.7%). The increased pressure gradient between the proximal and distal tubules suggests a condition of increased resistance to flow between the proximal and distal tubules. Microinfusion of proximal tubules with an isotonic "equilibrium" solution led to little or no rise in intratubular pressure in normal rats but it led to a significant rise in nephrotic rats. When proximal tubules of normal rats were infused with a solution containing 100 mg/100 ml albumin, pressure rose to levels observed in nephrotic rats. The mechanism of the increased resistance to flow appeared to be related, therfore, to the presence of protein in the tubular fluid. Sodium retention in the nephrotic animals might be attributed to the reduction in GFR. In other types of renal disease in animals and man with comparable or greater reductions in GFR, sodium retention does not occur, however, and fractional excretion of sodium in the urine is increased in proportion to the reduction in GFR. Thus, the rise in proximal fractional reabsorption secondary to impaired fluid flow could be an important factor in the sodium retention of this disease.

Mechanism of inhibition of proximal tubule fluid reabsorption after exposure of the rat kidney to the physical effects of expansion of extracellular fluid volume

The natriuresis and concomitant decline in absolute proximal reabsorption (APR) that occur in rats in response to saline loading are blunted markedly when renal perfusion pressure is reduced immediately before, but not after, the volume load. To ascertain the mechanism responsible for these differences between early clamp (EC) vs. late clamp (LC), intracapillary and interstitial determinants of peritubular capillary uptake of APR were measured in seven LC and seven EC Munich-Wistar rats before and after isotonic saline loading (80% body wt). With volume expansion in LC animals, we observed a marked decline in APR (averaging 11+/-1 nl/min), associated with large increases in urinary sodium excretion rate, which averaged 8+/-2 mueq/min. In EC, the changes in urinary sodium excretion rate (+1+/-0 mueq/min) and APR (-3+/-1 nl/min) with volume expansion were smaller in magnitude. Since peritubular capillary reabsorption coefficient and mean peritubular transcapillary hydraulic pressure difference did not change with saline loading in LC, the marked fall in APR was attributed primarily to a measured large decline in mean peritubular transcapillary oncotic pressure difference (deltapi). Despite an equivalent mean fall in deltapi with volume expansion in EC, near-constancy of APR was found to be associated with a simultaneous and equivalent decline in mean peritubular transcapillary hydraulic pressure difference (a consequence of decreased mean peritubular capillary hydraulic pressure), which effectively offset the fall in deltapi. These results demonstrate the importance of hydraulic pressure patterns of the peritubular capillaries in modulating APR and are consistent with the view that Starling forces across the postglomerular microcirculation play a fundamental role in determining APR.

Structural and functional adaptation after reduction of nephron population

This review of adaptive changes in renal structure and function in subjects with reduced renal mass has two primary goals. One is to provide a description of the remarkable compensatory increases in glomerular filtration rate (GFR), and renal blood flow, at the level of individual nephrons, and the alterations in water and electrolyte transport by tubular epithelium. These processes preserve fluid and electrolyte balance in subjects with progressive renal failure, until whole kidney GFR is reduced to about 20 percent of normal, and provide the basis for conservative clinical medical management. The other aim is an attempt to provide an understanding of the mechanisms involved in compensatory adaptation, since this information, in addition to amplifying our understanding of renal transport processes, helps to elucidate the functional limitations placed on subjects with renal insufficiency. An attempt has been made to analyze both clinical observations and relevant experimental models and an effort has been made to correlate renal function with different patterns of renal injury.

Renal sodium retention during volume expansion in experimental nephrotic syndrome

We have studied sodium retention during volume expansion in rats with autologous immune complex nephropathy (AICN), a model of nephrotic syndrome (NS) in which GFR after volume expansion was not different from that in adjuvant-injected controls (C). AICN rats developed heavy proteinuria (298 +/- 27 vs. less than 10 mg/day), hypoalbuminemia (2.14 +/- 0.15 vs. 3.08 +/- 0.12 g/100 ml) and hypercholesterolemia (181 +/- 22 vs. 58 +/- 4 mg/100 ml). After saline, there were no significant differences in blood pressure (119 +/- 2 vs. 114 +/- 2 mm Hg), renal plasma flow (4.9 +/- 0.41 vs. 4.1 +/- 0.28 ml/min), inulin clearance (1.37 +/- 0.06 vs. 1.55 +/- 0.10 ml/min), or SNGFR (47 +/- 2 vs. 53 +/- 4 nl/min). Sodium excretion, however, was significantly lower in NS rats (4.7 +/- 1.1 vs. 9.2 +/- 1.2 muEq/min). Proximal sodium reabsorption was decreased in NS rats (35 +/- 2 vs. 41 +/- 2%, 2.5 +/- 0.2 vs. 3.3 +/- 0.2 nEq/min). Sodium delivery into the loop, however, was equal in NS and C, since the slightly lower filtered load in NS rats offset the depression in proximal reabsorption. Sodium reabsorption by the loop and by the distal convoluted tubules were equal in NS and C. Thus, sodium delivered into the cortical collecting ducts was the same in both groups (0.33 +/- 0.17 vs. 0.34 +/- 0.07 nEq/min; 4.5 +/- 0.6% of filtered sodium vs. 4.4 +/- 0.3%). The percent of filtered sodium excreted in the urine, however, was significantly lower in the NS rats, 2.18 +/- 0.48% vs. 4.0 +/- 0.58%. We conclude that antinatriuresis in this model of NS is determined beyond the superficial late distal convoluted tubule. The inability to excrete the sodium load during volume expansion is due to either enhanced reabsorption by the collecting duct or to abnormal function in deep nephrons.

Acute effects of antiglomerular basement membrane antibody on the process of glomerular filtration in the rat

Nehron filtration rate (sngfr) and the factors controlling filtration were examined before and with 60 min of the intravenous infusion of 225-450 mug of antiglomerular basement membrane antibody (AGBM Ab) (greater than 50% antigenic saturation) in plasma-expanded (2.5% body wt) Munich-Wistar rats. Pressures in glomerular capillaries (PG) and bowman's space (Pt) were measured with a servo-nulling device, systemic (piA) and efferent arteriolar oncotic pressures (piE) were measured by microprotein methods, and nephron plasma flow (rpf) and sngfr were measured by micropuncture techniques in both control and post-AGBM Ab conditions in each rat. The sngfr fell from 52.7+/-2.9 to 24.1+/-1.9 nl/min per g kidney wt (n = 7, P less than 0.001). Both afferent and efferent arteriolar resistances increased and rpf fell from 221+/-25 to 90+/-9 nl/min per g kidney wt (P less than 0.001) but the hydrostatic pressure gradient across the glomerular membrane deltaP = PG - Pt) increased from 37+/- 1 to 50+/-2 mm Hg (P less than 0.001). The increase in deltaP and a numerical decrease in piA both acted to maintain sngfr after AGBM Ab and effectively nullified the influence of decreased rpf upon sngfr. The mean effective filtration pressure (EFP = deltaP - pi) increased from 14+/-2 to 30+/-3 mm Hg (P less than 0.001) while sngfr decreased. The major and critical reason for this reduction in sngfr was a decrease in the glomerular permeability coefficient from 0.077+/-0.017 to 0.014+/-0.001 nl/s per g kidney wt per mm Hg P less than 0.001) where sngfr=EFP-LpA.

Functional adaptation of nephrons in dogs with acute progressing to chronic experimental glomerulonephritis

Although a diminished fractional excretion of sodium (FENa) is the hallmark of acute proliferative glomerulonephritis (APGN), an enhanced natriuresis per glomerular filtration rate (GFR) in the chronic phases of this disease has been reported. We studied this adaptive response utilizing two different split-bladder dog models with unilateral, and a third group of dogs with bilateral Masugi's nephritis. Group I. Six dogs with unilateral nonaccelerated APGN studied a mean of 6 days after induction had a mean base-line APGN/intact kidney GFR of 31/50 ml/min (P less than 0.005) and FENa of 0.2/0.75% (P less than 0.005). Acute volume expansion caused a smaller absolute increase in FENa from the APGN kidney, 1.6%, than from the intact kidney, 4.0%, (P less than 0.01). Maximum tubular secretion of rho-aminohippuric acid/GFR (TmPAH/GFR) measured in three dogs was higher in the APGN kidney than intact kidney, 13.1 vs. 9.3 mg/dl. Subsequent studies on three of the six dogs when the disease had become chronic demonstrated a reversal in the pattern of sodium excretion in response to volume expansion. Group II. Six dogs with accelerated unilateral APGN (dogs presensitized to antibody source) studied a mean of 5 days after induction had a mean base-line APGN/intact kidney GFR of 16/57 ml/min and FENa of 0.22/0.12% (P less than 0.1). Contrary to group I, volume expansion caused a greater absolute increase in FENa from the APGN kidney, 5.8%, than from the intact kidney, 2.9% (P less than 0.05). TmPAH/GFR studied in four dogs was similar for both kidneys, 17.9 and 18.5 mg/dl for the APGN kidney and intact kidney, respectively. Group III. Sequential studies were performed on seven dogs with bilateral nonaccelerated APGN. Initially each demonstrated sodium retention and a smaller absolute increase in FENa in response to volume expansion compared to a predisease control study. With disease progression, volume expansion induced a greater absolute increase in FENa than in the control study. We concluded that (a) the fractional excretion of sodium from the APGN kidney will be less or greater than the contralateral intact kidney or control study depending on the severity and/or chronicity of the disease, possibly as the result of morphologic alterations; (b) the degree of extracellular fluid volume expansion is an important variable influencing similarity of glomerulotubular balance between the APGN and contralateral intact kidney; and (c) the "intact nephron hypothesis" applies in a limited fashion to kidneys with APGN in the absence of volume expansion just as it does for kidneys with chronic glomerulonephritis or pyelonephritis.

Permselectivity of the glomerular capillary wall. Studies of experimental glomerulonephritis in the rat using dextran sulfate

To determine whether the increased filtration of serum proteins after glomerular injury is the consequence of altered electrostatic properties of the glomerular capillary wall, we measured fractional clearances of the anionic polymer, dextran sulfate, in nine Munich-Wistar rats in the early autologous phase of nephrotoxic serum nephritis (NSN). In agreement with previous studied from this laboratory, whole kidney and single nephron glomerular filtration rates were normal in NSN rats despite histological evidence of glomerular injury, and despite a marked reduction in the glomerular capillary ultrafiltration coefficient to approximately one-third of normal. In the companion study (9), it was shown that in NSN rats the mean fractional clearances of neutral dextrans over the range of effective molecular radii from 18 to 42 A were reduced, compared to normla. In contrast, in the present study the mean fractional clearances for dextran sulfate over the same range of molecular radii were significantly greater than those found previously for normal Munich-Wistar rats. The fractional clearance of dextran sulfate molecules of the same molecular radius as serum albumin (approximately 36 A) was increased markedly, from 0.015 +/- 0.005 (SEM) in nonnephritic controls to 0.24 +/- 0.03 in NSN (P less than 0.001). The sialoprotein content of glomeruli, estimated by the colloidal iron reaction, was reduced in NSN rats as compared to normal controls. It is concluded that the abnormal filtration of anionic serum proteins, such as albumin, seen in glomerulopathies is, at least in part, the consequence of loss of fixed negative charges from the glomerular capillary wall.

Permselectivity of of the glomerular capillary wall. Studies of experimental glomerulonephritis in the rat using neutral dextran

Polydisperse [3h] dextran was infused into eight Munich-Wistar rats in the early autologous phase of nephrotoxic serum nephritis (NSN), thereby permitting direct measurements of pressures and flows in surface glomeruli and fractional clearances for dextrans [(U/P) dextran/(U/P) inulin] ranging in radius from 18 to 42 A. Despite glomerular injury, evidenced morphologically and by a marked reduction in the glomerular capillary ultrafiltration coefficient, the glomerular filtration rate remained normal because of a compensating increase in the mean net ultrafiltration pressure. In NSN rats, as in normal controls, inulin was found to permeate the glomerular capillary wall without measurable restriction, and dextrans were shown to be neither secreted nor reabsorbed. For dextran radii of 18, 22, 26, 30, 34, 38, and 42 A, (U/P) dextran/(U/P) inulin in NSN and control rats, respectively, averaged 0.90 vs. 0.99, 0.81 vs. 0.97, 0.63 vs. 0.83, 0.38 vs 0.55, 0.20 vs. 0.30, 0.08 vs. 0.11, and 0.02 vs. 0.03. Using a theory based on macromolecular transport through pores, the results indicate that in NSN rats, effective pore radius is the same as in controls, approximately 50 A. In NSN, however, the ratio of total pore surface area to pore length, a measure of the number of pores, is reduced to approximately 1/3 that of control, probably due to a reduction in capillary surface area. These results suggest that proteinuria in glomerular disease is not due simply to increases in effective pore radius or number of pores, as previously believed. Using a second theoretical approach, based on the Kedem-Katchalsky flux equations, dextran permeability across glomerular capillaries was found to be slightly lower, and reflection coefficient slightly higher in NSN than in control rats.