Narisa Futrakul

Tubular function and tubulointerstitial disease

Tubular transport determined by the fractional excretion (FE) of filtered solutes was studied in 129 nephrotic patients; 72 patients with mesangial proliferation (MesP-NS) and intact tubulointerstitium (group 1), 13 patients with MesP-NS and superimposed tubulointerstitial fibrosis (TIF; group 2), 27 patients with mild focal segmental glomerulosclerosis (FSGS; group 3), and 17 patients with severe FSGS (group 4). In the 72 nephrotic patients with MesP-NS and normal tubulointerstitium (no TIF), tubular transport was intact (FE of sodium [FENa], 0.5 +/- 0.5; FE of calcium [FECa], 0.3 +/- 0.3; FE of phosphate [FEPO4], 14 +/- 13; FE of uric acid [FEUA], 9.8 +/- 5; FE of magnesium [FEMg], 1.3 +/- 0.5). In the 13 nephrotic patients with MesP-NS and superimposed TIF (4.9% +/- 2%), there was no difference in FE solutes from those in group 1 except for FEMg (3.3 +/- 0.9; P < 0.001). In the 27 nephrotic patients with mild FSGS (TIF, 28% +/- 9%), four of five variables of FE solutes (FENa, 1.2 +/- 0.7; P < 0.001; FECa, 0.9 +/- 0.8; P < 0.001; FEPO4, 17 +/- 12; P, not significant; FEUA, 16.5 +/- 8; P < 0.001; FEMg, 4. 1 +/-1; P < 0.001) were significantly different from those of patients with MesP-NS without TIF, and two of five variables (FECa, FEMg) were statistically different from those of patients with MesP-NS with TIF. In the severe category of FSGS (TIF, 69% +/-19%), all FE solutes were statistically different from the other groups (FENa, 4.8 +/- 3; FECa, 2 +/- 1; FEPO4, 47 +/- 24; FEUA, 37 +/- 18; FEMg, 12 +/- 6). Thus, the results imply that (1) normal tubular transport reflects an underlying intact tubulointerstitial structure, whereas tubular dysfunction indicates an underlying tubulointerstitial disease, and (2) FEMg is the most sensitive index to detect an early abnormality of tubular structure and function.

PERITUBULAR CAPILLARY FLOW DETERMINES TUBULOINTERSTITIAL DISEASE IN IDIOPATHIC NEPHROTIC SYNDROME

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.

A Defective Angiogenesis in Chronic Kidney Disease

Background. A progressive reduction in peritubular capillary flow is observed in chronic kidney disease (CKD) patients as the disease severity progresses. This suggests an altered vascular homeostasis in CKD patients, but such a defective mechanism needs to be verified. Methods. To study the vascular injury as reflected by circulating endothelial cell (CEC), the balance between angiogenic factor, vascular endothelial growth factor (VEGF), and antiangiogenic factor, endostatin. Results. A deficient VEGF was observed, whereas the value of endostatin and CEC were abnormally elevated in CKD patients. Discussion. Enhanced CEC reflects an increased activity of vascular injury. A deficient VEGF in the presence of enhanced antiangiogenesis (endostatin) implies a defective angiogenesis. This may explain the progressive nature of renal microvascular disease observed in late stage of CKD patients.

Glomerular Endothelial Dysfunction Determines Disease Progression: A Hypothesis

A glomerular endothelial function with its hemodynamic impact is proposed to determine disease progression. In the clinical settings associated with an intact endothelial function, such as minimal-change steroid-sensitive nephrosis, the early phase of diabetes mellitus and the early stage of an experimental model of renal ablation in animals, it was observed that adequate renal perfusion correlates with the intact structure and function of the nephron with no evidence of disease progression. In contrast, the clinical settings associated with endothelial dysfunction, such as chronic glomerulonephropathy, the late stage of diabetes mellitus and a renal ablation model in animals, are usually associated with a reduction in renal perfusion. The magnitude of renal hypoperfusion observed in all forms of chronic glomerulonephropathies is proportional to the degree of clinical severity. A progressive pattern of renal hypoperfusion is uniquely observed when disease severity progresses. In this context, a new therapeutic maneuver aiming to improve renal perfusion is proposed for treating glomerulonephropathy with disease progression and preventing it from developing to end-stage renal disease.

Microalbuminuria--a biomarker of renal microvascular disease.

Microalbuminuria (amount greater than 30–300 mg/day) reflects an abnormal glomerular capillary permeability to protein. It is usually dependent upon three mechanisms. First, loss of negatively charged surface of the glomerular capillary wall secondary to circulating toxic substances injury—namely, oxidative stress and proinflammatory cytokines—allows the albumin with negatively charged surface to freely escape into the urine. Second, intraglomerular hypertension and hemodynamic maladjustment secondary to glomerular endothelial dysfunction increases filtration pressure and enhances sized selective proteinuria leakage. Third, podocyte injury leads to a vicious cycle of hemodynamic maladjustment and endothelial and podocyte injuries. All three of these mechanisms induce glomerular endothelial injury and microalbuminuria, which reflects renal microvascular disease.

Improvement of renal function in type 2 diabetic nephropathy.

Background. Therapeutic failure in preventing renal disease progression in type 2 diabetic nephropathy (DN) is due to a failure in the early detection of DN by microalbuminuria and the inappropriate correction of renal hemodynamic maladjustment secondary to glomerular endothelial dysfunction. Methods. Thirty patients associated with normoalbuminuric type 2 DN were subject to the following studies: tubular function by means of fractional excretion of magnesium (FE Mg), vascular function by means of determining the circulating endothelial cell, VEGF, VEGF/TGF B ratio, and intrarenal hemodynamic studies. Results. FE Mg, circulating endothelial cells, and TGF B were abnormally elevated, and VEGF/TGF B ratio was decreased in these normoalbuminuric patients. The intrarenal hemodynamic study revealed a hemodynamic maladjustment characterized by a preferential constriction at the efferent arteriole and a reduction in peritubular capillary flow. Following treatment with vasodilators, a decrease in efferent arteriolar resistance and increase in peritubular capillary flow as well as glomerular clearance were observed. Conclusion. FE Mg appears to be a more sensitive marker than microalbuminuria for the early detection of DN. Increased endothelial cell injury is reflected by enhanced circulating endothelial cell loss in conjunction with the increased TGF B and the decreased ratio between VEGF and TGF B. This is further supported by the dysfunctioning glomerular endothelium, which is characterized by hemodynamic maladjustment and a reduction in the peritubular capillary flow. A correction of such hemodynamic maladjustment by multidrug vasodilators effectively improves renal perfusion and restores renal function in type 2 DN.

Vascular Homeostasis and Angiogenesis Determine Therapeutic Effectiveness in Type 2 Diabetes

Under common practice, recognition and treatment of type 2 diabetic nephropathy (DN) are usually revealed at a rather late stage (CKD stages 3–5) due to the insensitiveness of available diagnostic markers. Accumulating data obtained from vascular homeostasis in late stage DN demonstrated (1) a defective angiogenesis and impaired NO production which explains the therapeutic resistance to vasodilators and the inability to correct chronic renal ischemia and (2) an abnormally elevated antiangiogenesis and a progressive vascular disease which correlates with the altered renal hemodynamics characterized by a progressive reduction in renal perfusion as the disease severity progressed. In contract, the vascular homeostasis is adequately functional in early stage DN. Thus, vasodilator treatment at early stage DN (CKD stages 1-2) can enhance renal perfusion, correct the renal ischemia, and restore renal function.

OXIDATIVE STRESS AND HEMODYNAMIC MALADJUSTMENT IN CHRONIC RENAL DISEASE: A THERAPEUTIC IMPLICATION

Hemodynamic maladjustment with predominant constriction at the efferent arteriole has been encountered in a variety of clinical settings of glomerulonephropathy. In essence, it induces not only intraglomerular hypertension but also exaggeratedly reduces the peritubular capillary flow, which supplies the tubulointerstitial compartment. The hemodynamic maladjustment is believed to reflect a glomerular endothelial cell dysfunction. In this regard, oxidative stress and antioxidant defect are likely responsible for the glomerular endothelial dysfunction. Improvement in renal function was accomplished following the correction of oxidant and antioxidant imbalance with antioxidant therapy and vasodilators. Following such therapy, there was a correction in hemodynamic maladjustment with a decline in intraglomerular hydrostatic pressure and an increase in renal perfusion with a subsequent increase in renal functions namely creatinine clearance, glomerular filtration rate and a decline in FEMg.

Reduced endothelial factor VIII staining in renal microcirculation correlates with hemodynamic alteration in nephrosis

Endothelial factor VIII staining in renal microcirculation was performed in eight nephrotic patients associated with mesangial proliferation (MesP) and six nephrotic patients associated with focal segmental glomerulosclerosis (FSGS). The result in MesP revealed a greater staining for glomerular endothelial factor VIII (35 ± 15%) and for postglomerular capillary endothelial factor VIII (65 ± 21%) than that observed in FSGS, which revealed a 11 ± 8% staining for glomerular endothelial factor VIII and 19 ± 15% staining for postglomerular capillary endothelial factor VIII. This finding implies that there is a greater loss of endothelial cell in renal microcirculation in FSGS. Such a finding correlates with the intrarenal hemodynamics which illustrated (Futrakul, P.; Sitprija, V.; Yenrusi, S. Glomerular endothelial dysfunction determines disease progression: a hypothesis. Am. J. Nephrol. 1997, 17, 533–540.) a mild reduction in renal plasma flow (535 ± 106 mL/min/1.73 m2, normal 600 mL/min/1.73 m2) and in peritubular capillary flow (422 ± 80 mL/min/1.73 m2, normal 480 mL/min/1.73 m2) in MesP and (Futrakul, P. Coagulation in glomerulonephritis and nephrotic symdrome: Its therapeutic intervention. In Asian Manual of Nephrology, Takeuchi, T.; Sugino, N.; Ota, K., Eds.; SEAMIC Publication, Tokyo, 1981; pp. 89–95.) a greater reduction in renal plasma flow (108 ± 50 mL/min/1.73 m2) and in peritubular capillary flow (87 ± 42 mL/min/1.78 m2) in FSGS. Therefore the study has emphasized both the structural and functional defects of endothelium in renal microcirculation in particular in FSGS.

Treatments of Hemodynamic Maladjustment and Oxidative Stress Prevent Renal Disease Progression in Chronically Severe Glomerulonephritides

Hemodynamic maladjustment is a unique observation in chronically severe glomerulonephritides. It is characterized by a markedly elevated efferent arteriolar resistance (RE), an elevated intraglomerular hydrostatic pressure (PG) and a markedly decreased renal plasma flow (RPF), and peritubular capillary flow (PTCF). A correction of such hemodynamic maladjustment can be accomplished by administering a combination of vasodilators (angiotensin receptor antagonist, angiotensin converting enzyme inhibitor, and calcium channel blocker) in 14 chronic glomerulonephritides with severe renal function impairment (mean serum creatinine 3.6 + 1.3 mg/dL). Doses titration aim for maximal renal perfusion effect (increased RPF, PTCF) or maximal renal function improvement (increased CCr, reduced FE Mg) usually higher than needed for maximal blood pressure reduction. Evidence of oxidative stress is also corrected with high doses of vitamins C and E. After a mean period of treatment for 13.5 months, improvements in CCr (pre Rx 22 ± 10 vs. post Rx 32 ± 13 mL/min/1.73 m2), and FE Mg (pre Rx 11.9 ± 4% vs. post Rx 10 ± 3%) were observed in conjunction with the improvement in intrarenal hemodynamics namely RPF (pre Rx 201 ± 71 vs. post Rx 288 ± 99 mL/min/1.73 m2), PTCF (pre Rx 161 ± 57 vs. post Rx 242 ± 90 mL/min/1.73 m2), PG (pre Rx 56.7 ± 0.5 vs. post Rx 51 ± 0.1 mm Hg), and RE (pre Rx 12085 ± 6503 vs. post Rx 6550 ± 1872 dyne.s.cm−5).