PERITUBULAR CAPILLARY FLOW DETERMINES TUBULOINTERSTITIAL DISEASE IN IDIOPATHIC NEPHROTIC SYNDROME

Abstract

The spatial relationship between renal perfusion and nephronal structure was determined in 51 nephrotic patients consisting of 11 patients with steroid sensitive, minimal change (MC) nephrosis, 12 patients with steroid resistant, mesangial proliferative (MesP) nephrosis and without tubulointerstitial fibrosis (TIF), 11 patients with steroid resistant, MesP nephrosis and with low grade TIF and 17 patients with focal segmental glomerulosclerosis (FSGS). The intrarenal hemodynamic study revealed a unique correlation between renal perfusion and nephronal structure. A normal or slight reduction in peritubular capillary flow observed in MC or mild MesP nephrosis correlates with an intact tubulointerstitial structure. A moderate reduction in peritubular capillary flow observed in steroid resistant, MesP nephrosis induces a low incidence of TIF. A severe reduction in peritubular capillary flow denotes a higher incidence of TIF as that observed in nephrosis with FSGS. Thus, it is of notion that the reduction in renal perfusion precedes the development of tubulointerstitial fibrosis and thereby supports the concept of renal perfusion as a crucial determinant of nephronal structure.

• Renal perfusion
• Tubulointerstitial fibrosis
• Nephrosis
• Peritubular capillary flow
• Hemodynamics

INTRODUCTION

The Eastern Philosophy and its concept of natural balance emphasizes the essential of renal perfusion as a determinant of nephronal structure. This implies that in the presence of normal renal perfusion, the nephronal structure is likely to be intact. On the contrary, the reduction in renal perfusion would induce damage to the nephronal structure¹. The significance of such natural balance has recently been implicated in the aging animals in which tubulointerstitial injury is likely to be the consequence of ischemia secondary to peritubular capillary injury². In this regard, an assessment of intrarenal hemodynamics has been performed in a variety of nephrotic patients. The result of the study is then assessed as to whether such a correlation between the renal perfusion and nephronal structure as well as its cause - and - effect relationship does exist.

MATERIALS AND METHODS

Fifty-one patients associated with idiopathic nephrosis who had had [[1]] a histopathologic confirmation and morphometric analysis of the kidney tissue and [[2]] an intrarenal hemodynamic determination by the previously described method³ were included. Of these 51 nephrotics, 11 patients were categorized under minimal change steroid sensitive nephrosis (group 1), 12 patients were either steroid dependent or steroid resistant, mesangial proliferative nephrosis with IgM deposit and with intact tubulointerstitial structure (group 2), 11 patients were steroid - resistant, mesangial proliferative nephrosis and with tubulointerstitial fibrosis (group 3) and 17 patients were steroid resistant nephrosis associated with focal segmental glomerulosclerosis (group 4).

Glomerular Function

Creatinine clearance was performed by measuring the 10-hour endogenous creatinine clearance (CCr) or glomerular filtration rate (GFR) by the radioisotope technique using ^99mTc-labeled diethylene triamine pentraacetic acid (DTPA) and the value was converted to the body surface area of 1.73 m² by the method of calculation :

Vascular Function (Hemodynamic Assessment)

Simultaneous assessments of effective renal plasma flow (RPF) using ¹³¹I-labeled orthoiodohippuric acid (hippuran) and of glomerular filtration rate (GFR) using ^99mTc-labeled DTPA were determined by injecting the labeled materials intravenously into the left antecubital vein at zero time. A peritubular capillary flow (PTCF) is derived from the substraction of glomerular filtration rate from renal plasma flow.

Renal Histopathologic Study

The morphometric analysis was performed on the renal biopsied specimens by the method described elsewhere⁴. In brief, the kidney tissue was fixed in 4% buffered formalin and embedded in paraffin. Sections (2μm) were prepared and stained with haematoxylin-eosin, periodic acid Schiff reagent (PAS), silver methenamine and Massons Trichrome. At least eight serial sections were prepared from each case and examined. The number of glomeruli varied from 10 to 35. The tubulointerstitial fibrosis (TIF) was quantitated in a single blind fashion by a pathologist who had no information regarding the hemodynamic value of each individual. The widening of interstitium was assessed by a point-count technique using a counting grid. This manner would cover most area in the renal cortex which was ascertained by determining at × 100 magnification and the result obtained was expressed as percent.

STATISTICAL ANALYSIS

Comparison of the sample mean of two quantitative variables was determined by the non-parametric method using the Mann-Whitney test. The difference between groups was performed by Student's unpaired t-test. The linear regression analysis was used to correlate two quantitative variables. The scatter plot was the first step to correlate between two continuous variables. The Pearson correlation coefficient (r) was used to quantify the strength of the linear relationship. The method of least square was calculated to estimate the regression equation (y = a ± bx) if the scatter plot seemed to be linear. Some relationship of the scatter plot data which had shown to be curvilinear, was further analysed to meet the criteria of straight line. P values below 0.05 were considered to be significant.

RESULTS

Of the 11 nephrotic patients associated with minimal change, steroid - sensitive nephrosis; group 1, the mean values of glomerular filtration rate (GFR), renal plasma flow (RPF) and peritubular capillary flow (PTCF) were 127 ± 26 mL/min/1.73 m², 711 ± 92 mL/min/1.73 m² and 584 ± 85 mL/min/1.73 m² respectively. These values appeared to be within normal limits of the control (Table 1). Of the 12 nephrotic patients associated with mesangial proliferation and with intact tubulointerstitial structure; group 2, the mean values of GFR, RPF and PTCF were 99 ± 14 mL/min/1.73 m², 491 ± 52 mL/min/1.73 m² and 392 ± 50 mL/min/1.73 m² respectively. The difference between groups 1 and 2 was statistically significant, p < .05. Of the 11 nephrotic patients associated with mesangial proliferative, steroid resistant nephrosis and with mild tubulointerstitial fibrosis (5 ± 2%); group 3, the mean values of GFR, RPF and PTCF were 78 ± 36 mL/min/1.73 m² 313 ± 67 mL/min/1.73 m² and 235 ± 49 mL/min/1.73 m² respectively. The difference between groups 2 and 3 was statistically significant; p < .05. The lowest values of hemodynamic study were documented in the nephrotic patients associated with focal segmental glomerulosclerosis and with severe tubulointerstitial fibrosis (58 ± 14%); group 4, and the mean values of GFR, RPF and PTCF were 36 ± 18 mL/min/1.73 m², 179 ± 80 mL/min/1.73 m² and 142 ± 69 mL/min/1.73 m² respectively. The difference between groups 3 and 4 was statistically significant, p < .001.

Table 1. Depicts the Correlation between Renal Perfusion and Tubulointerstitial Structure. GFR = Glomerular Filtration Rate, RPF = Renal Plasma Flow, PTCF = Peritubular Capillary Flow, TIF = Tubulointerstitial Fibrosis, MC-NS = Minimal Change Nephrosis, MesPNS = Mesangial Proliferative Nephrosis, NS-FSGS = Nephrosis Associated with Focal Segmental Glomerulosclerosis

Patients	RPF mL/min/1.73m^2	PTCF mL/min/1.73m^2	TIF%	GFR mL/min/1.73m^2
Normal	600	480	0	120
Group 1:MC-NS (steroid-sensitiveness)	711 ± 92	584 ± 85	0	127 ± 26
Group 2:MesP-NS with no TIF (steroid-resistance)	491 ± 52	392 ± 50	0	99 ± 14
Group 3:MesP-NS with TIF (steroid-resistance)	311 ± 67	235 ± 49	5 ± 2	78 ± 36
Group 4:NS-FSGS (severe)	179 ± 80	142 ± 69	58 ± 14	36 ± 18

DISCUSSION

The result of our study in the idiopathic nephrotic syndrome indicates that there is a spatial relationship between the renal perfusion and nephronal structure. As illustrated in Table 1, the presence of a normal perfusion (normal peritubular capillary flow) observed in minimal change nephrosis (group 1) or of a slight reduction in peritubular capillary flow observed in steroid resistant, mesangial proliferative nephrosis (group 2) was associated with an intact structure (absence of tubulointerstitial disease). However, a further but substantial reduction in peritubular capillary flow as that observed in steroid resistant, mesangial proliferative nephrosis (group 3) correlated with a mild degree of tubulointerstitial fibrosis. A greater or severe reduction in peritubular capillary flow as that observed in nephrosis associated with focal segmental glomerulosclerosis (group 4) was also denoted in association with a higher incidence of tubulointerstitial fibrosis.

The preceding information would not only support the relationship between the renal perfusion and structure but also address the cause-and-effect relationship. A mild reduction in peritubular capillary flow (mean 392 ± 50 mL/min/1.73 m²; normal 480 mL/min/1.73 m²) was clearly been documented in the steroid-resistant, mesangial proliferature nephrosis (group 2) and such a mild reduction in renal perfusion is generally unable to induce tubulointerstitial fibrosis. With a greater reduction in peritubular capillary flow (mean 235 ± 49 mL/min/1.73 m²) in this severe category, such degree of perfusion deficit is then capable of inducing tubulointerstitial fibrosis. The above evidence simply implies that a substantial reduction in renal perfusion is prerequisited and precedes the development of nephronal death or tubulointerstitial fibrosis. Such a conceptual view of renal perfusion deficit inducing tubulointerstitial fibrosis is well illustrated by the multiple regression analysis approach in which the peritubular capillary flow is inversely proportional to the incidence of tubulointerstitial fibrosis (Figure 1). The development of tubulointerstitial fibrosis would occur as the reduction in peritubular capillary flow approaching the fifty per cent area of normal value. As the reduction in peritubular capillary flow becomes progressive, there is also a steady increase in the incidence of tubulointerstitial fibrosis. Such reduction in renal perfusion has been uniquely observed in a variety of chronic glomerulonephritides⁵.

Figure 1. Illustrates the correlation between peritubular capillary flow and tubulointerstitial fibrosis.

The result of this intrarenal hemodynamic study appears to be somewhat contradictory to the general belief that the nephronal death such as tubulointerstitial fibrosis is the cause rather than the effect of the renal perfusion deficit. A similar observation to us was also remarked by Bohle and associates⁶ who had denoted that the degree of tubulointerstitial fibrosis correlated inversely with the intensity of postglomerular capillary patency. In addition, Truong and Farhood⁷ demonstrated in their experimental model of renal ischemia inducing nephronal death by which it also supported our observation. The preceding observation of renal perfusion deficit as a determinant of the nephronal damage appear to have a clinical impact on the therapy as well as preventive implication. The significance of such hemodynamic impact would likely explain the refractoriness to the conventional therapy among many steroid resistant nephrotic patients in whom the crucial issue of severe reduction in renal perfusion had unintentionally been disregarded. The failure in correcting such renal perfusion deficit among these nephrotic patients associated with severe reduction in peritubular capillary flow would indeed, allow the continuation of progressive development of tubulointerstitial fibrosis and eventually the entrapment into the end stage renal disease.

On the contrary, a therapeutic approach aiming to improve the renal perfusion with a variety of drugs would likely be able to retard, improve or even prevent the renal disease progression. The result of such therapeutic approach with so called “enhanced renal perfusion formula which has been launched to treat a variety of nephrotic patients, appears to be superior to the conventional regimen^8–15