Amelioration of metabolic acidosis in patients with low GFR reduced kidney endothelin production and kidney injury, and better preserved GFR
2010; Elsevier BV; Volume: 77; Issue: 7 Linguagem: Inglês
10.1038/ki.2009.519
ISSN1523-1755
AutoresSorot Phisitkul, Apurv Khanna, Jane M. Simoni, Kristine Broglio, Simon J. Sheather, M. Hasan Rajab, Donald E. Wesson,
Tópico(s)Dialysis and Renal Disease Management
ResumoMetabolic acidosis often accompanies low glomerular filtration rate and induces secretion of endothelin, which in turn might mediate kidney injury. Here we tested whether treatment of metabolic acidosis in patients with low glomerular filtration rate reduced the progression of kidney disease. Fifty-nine patients with hypertensive nephropathy and metabolic acidosis had their blood pressure reduced with regimens that included angiotensin-converting enzyme inhibition. Thirty patients were then prescribed sodium citrate, and the remaining 29, unable or unwilling to take sodium citrate, served as controls. All were followed for 24 months with maintenance of their blood pressure reduction. Urine endothelin-1 excretion, a surrogate of kidney endothelin production, and N-acetyl-β-D-glucosaminidase, a marker of kidney tubulointerstitial injury, were each significantly lower, while the rate of estimated glomerular filtration rate decline was significantly slower. The estimated glomerular filtration rate was statistically higher after 24 months of sodium citrate treatment compared to the control group. Hence it appears that sodium citrate is an effective kidney-protective adjunct to blood pressure reduction and angiotensin-converting enzyme inhibition. Metabolic acidosis often accompanies low glomerular filtration rate and induces secretion of endothelin, which in turn might mediate kidney injury. Here we tested whether treatment of metabolic acidosis in patients with low glomerular filtration rate reduced the progression of kidney disease. Fifty-nine patients with hypertensive nephropathy and metabolic acidosis had their blood pressure reduced with regimens that included angiotensin-converting enzyme inhibition. Thirty patients were then prescribed sodium citrate, and the remaining 29, unable or unwilling to take sodium citrate, served as controls. All were followed for 24 months with maintenance of their blood pressure reduction. Urine endothelin-1 excretion, a surrogate of kidney endothelin production, and N-acetyl-β-D-glucosaminidase, a marker of kidney tubulointerstitial injury, were each significantly lower, while the rate of estimated glomerular filtration rate decline was significantly slower. The estimated glomerular filtration rate was statistically higher after 24 months of sodium citrate treatment compared to the control group. Hence it appears that sodium citrate is an effective kidney-protective adjunct to blood pressure reduction and angiotensin-converting enzyme inhibition. Subjects with low glomerular filtration rate (GFR) can have metabolic acidosis1.Widmer B. Gerhardt R.E. Harrington J.T. et al.The influence of graded degrees of chronic renal failure.Arch Int Med. 1979; 139: 1099-1102Crossref PubMed Scopus (132) Google Scholar and treatment guidelines recommend alkali treatment for those with serum total CO2 (TCO2) <22 mM.2.National Kidney Foundation K/DOQI clinical practice guidelines for nutrition in chronic renal failure.Am J Kid Dis. 2000; 35: S1-S140Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar Metabolic acidosis mediates GFR decline in the five-sixths nephrectomy model of low GFR and its amelioration with oral alkali slows GFR decline in most3.Nath K.A. Hostetter M.K. Hostetter T.H. Pathophysiology of chronic tubulo-interstitial disease in rats—interactions of dietary acid load, ammonia, and complement component C3.J Clin Invest. 1985; 76: 667-675Crossref PubMed Scopus (415) Google Scholar, 4.Gadola L. Noboa O. Marquez M.N. et al.Calcium citrate ameliorates the progression of chronic renal injury.Kid Int. 2004; 65: 1224-1230Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar, 5.Phisitkul S. Hacker C. Simoni J. et al.Dietary protein causes a decline in the glomerular filtration rate of the remnant kidney mediated by metabolic acidosis and endothelin receptors.Kid Int. 2008; 73: 192-199Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar but not all6.Thorssel D. Brown J. Harris K.P. et al.Metabolic acidosis does not contribute to chronic renal injury in the rat.Clin Sci. 1995; 89: 643-650Crossref PubMed Scopus (35) Google Scholar studies. Daily oral NaHCO3 administration for 2 years slowed the decline rate of creatinine clearance in subjects with low GFR for various causes7.de Brito-Ashurst I. Varagunam M. Raferty M.J. et al.Bicarbonate supplementation slows progression of CKD and improves nutritional status.J Am Soc Nephrol. 2009; 20: 2075-2084Crossref PubMed Scopus (630) Google Scholar but no mechanisms for this alkali effect were reported. Metabolic acidosis-induced GFR decline in the five-sixths nephrectomy model is mediated by tubulointerstitial injury.3.Nath K.A. Hostetter M.K. Hostetter T.H. Pathophysiology of chronic tubulo-interstitial disease in rats—interactions of dietary acid load, ammonia, and complement component C3.J Clin Invest. 1985; 76: 667-675Crossref PubMed Scopus (415) Google Scholar,5.Phisitkul S. Hacker C. Simoni J. et al.Dietary protein causes a decline in the glomerular filtration rate of the remnant kidney mediated by metabolic acidosis and endothelin receptors.Kid Int. 2008; 73: 192-199Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar In subjects with low GFR, 6 weeks of daily oral NaHCO3 improved urine indices of kidney tubule damage but did not improve GFR.8.Rustom R. Grime J.S. Cotigan M. et al.Oral sodium bicarbonate reduces proximal renal tubular peptide catabolism, ammoniagenesis, and tubular damage in renal patients.Renal Fail. 1998; 20: 371-382Crossref PubMed Scopus (35) Google Scholar Tubulointerstitial injury is an important component of hypertensive nephropathy,9.Olsen J.L. Renal disease in hypertension.in: Jennette C.J. Olsen J.L. Schwartz M.M. Silva F.G. Pathology of the Kidney. 6th edn. Lippincott Williams and Wilkins, Baltimore, MD2007: 947Google Scholar it might predict its progression,10.D’Amico G. Tubulo-interstitial damage in glomerular diseases: its role in the progression of the renal damage.Nephrol Dial Transplant. 1998; 13: 80-85Crossref PubMed Scopus (54) Google Scholar and many subjects with hypertensive nephropathy experience progressive kidney injury despite blood pressure (BP) reduction with regimens that include angiotensin-converting enzyme (ACE) inhibition.11.Appel L.J. Wright J.T. Greene T. et al.Long-term effects of rennin-angiotensin-system-blocking therapy and a low blood pressure goal on progression of hypertensive chronic kidney disease in African Americans.Arch Int Med. 2008; 168: 832-839Crossref PubMed Scopus (137) Google Scholar Consequently, metabolic acidosis-induced tubulointerstitial injury that is not ameliorated by conventional kidney-protective strategies might contribute to kidney injury in subjects with low GFR due to hypertensive nephropathy. Furthermore, metabolic acidosis-induced tubulointerstitial injury in the five-sixths nephrectomy model is mediated through endothelin receptors5.Phisitkul S. Hacker C. Simoni J. et al.Dietary protein causes a decline in the glomerular filtration rate of the remnant kidney mediated by metabolic acidosis and endothelin receptors.Kid Int. 2008; 73: 192-199Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar and endothelins also mediate progressive tubulointerstitial injury induced by unilateral ureteral obstruction.12.Feldman D.L. Mogelesky T.C. Chou M. et al.Enhanced expression of renal endothelin-converting enzyme-1 and endothelin-A-receptor mRNA in rats with interstitial fibrosis following ureter ligation.J Cardiovasc Pharmacol. 2000; 36: S255-S259Crossref PubMed Google Scholar Kidney endothelin-1 (ET-1) production is increased in the five-sixths nephrectomy model13.Benigni A. Perico N. Gaspari F. et al.Increased renal endothelin production in rats with reduced renal mass.Am J Physiol. 1991; 260: F331-F339PubMed Google Scholar,14.Wesson D.E. Endogenous endothelins mediate augmented acidification in remnant kidneys.J Am Soc Nephrol. 2001; 12: 1826-1835PubMed Google Scholar and oral alkali decreases this production.5.Phisitkul S. Hacker C. Simoni J. et al.Dietary protein causes a decline in the glomerular filtration rate of the remnant kidney mediated by metabolic acidosis and endothelin receptors.Kid Int. 2008; 73: 192-199Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar If metabolic acidosis induces tubulointerstitial injury in human hypertensive nephropathy, these animal studies support exploring endothelin as a mediator. We tested the hypothesis that oral alkali amelioration of metabolic acidosis reduces kidney endothelin production, reduces urine parameters of tubulointerstitial injury, and slows GFR decline in subjects with low GFR due to hypertensive nephropathy. Table 1 shows the characteristics of each group at study entry. There were no statistically significant differences in demographic data or systolic blood pressure (SBP), venous TCO2 (VTCO2), plasma creatinine (Pcr), plasma cystatin C (Pcys), or estimated GFR (eGFR) using either Pcr (eGFRcr) or Pcys (eGFRcys) at study entry between subjects who subsequently received no Na+ citrate (No-NaCit) or Na+ citrate (NaCit) for 24 months. The NaCit group had a higher proportion of white subjects than the No-NaCit group (23 vs 14%) but this difference was not statistically significant (P=0.59).Table 1General demographic characteristics, SBP, Pcr, and eGFR at study entry in subjects before they were not treated (No-NaCit) or treated (NaCit) with Na+ citrateNo-NaCit (n=29)NaCit (n=30)P-valueMales (%)48470.891Black/white/Hispanic (%)55/14/3153/23/230.591Mean±s.d.Mean±s.d.Age (years)53.9±5.054.1±6.40.928SBP (mm Hg)160.5±8.9161.8±10.80.611VTCO2 (mM)20.6±0.820.8±1.20.375Pcr (mg/dl)3.20±0.893.27±0.700.733eGFRcr (ml/min)33.4±8.433.0±8.50.871Pcys (mg/l)3.86±1.093.88±0.790.936eGFRcys (ml/min)32.3±8.131.7±8.30.767Abbreviations: eGFR, estimated glomerular filtration rate; N, number of subjects per group; Pcr, plasma creatinine; Pcys, plasma cystatin C; SBP, systolic blood pressure; VTCO2, venous serum total CO2. Open table in a new tab Abbreviations: eGFR, estimated glomerular filtration rate; N, number of subjects per group; Pcr, plasma creatinine; Pcys, plasma cystatin C; SBP, systolic blood pressure; VTCO2, venous serum total CO2. Table 2 shows SBP, Pcr, Pcys, and eGFR at 6 and 30 months after study entry. At 6 months, all subjects had undergone 6 months of pharmacologic SBP reduction but none had received Na+ citrate. At 30 months, NaCit but not the No-NaCit subjects had been prescribed 24 months of Na+ citrate, 1 meq/kg HCO3 equivalent daily in three divided doses and SBP reduction was maintained. Entry SBP between the NaCit and No-NaCit groups (Table 1) was not statistically different (P=0.611). All subjects received ACE inhibition (Materials and Methods) and most received it as enalapril because this was the recommended ACE inhibitor on our formulary. There was no difference in the distribution of non-ACE drugs or diuretics among subjects in the two groups. Pharmacologic antihypertensive treatment from 0 to 6 months (Materials and Methods) decreased SBP significantly (P<0.0001 for each group) to the 6-month values shown in Table 2. The SBP levels were similar between 6 and 30 months for both groups. The increase in Pcr and decrease in eGFRcr from months 6 to 30 within each group were statistically significant but there was no statistically significant difference in Pcr or eGFRcr at 6 or 30 months between groups. Similarly, there was a statistically significant increase in Pcys and a statistically significant decrease in eGFRcys from 6 to 30 months within each group. In contrast to the creatinine data, Pcys was statistically significantly lower and eGFRcys was statistically significant higher at 30 months in NaCit than No-NaCit.Table 2SBP, Pcr, and eGFR before (0 months) and after 24 months of No-NaCit vs NaCitNo-NaCit (n=29)NaCit (n=30)P-value, NaCit vs No-NaCitMonth 6Month 30P-value, 30 vs 6 monthsMonth 6Month 30P-value, 30 vs 6 monthsMonth 6Month 30SBP132.1±6.3131.9±3.80.870132.4±6.2132.7±5.70.7610.8390.490Pcr (mg/dl)3.30±0.914.24±1.55<0.00013.31±0.693.61±0.78<0.00010.9540.057eGFRcr (ml/min)32.5±8.324.9±9.7<0.000132.7±8.229.5±8.8<0.00010.9450.066Pcys (mg/l)3.94±1.105.24±1.41<0.00013.93±0.804.33±0.89<0.00010.9520.005eGFRcys (ml/min)31.7±7.923.0±6.05<0.000131.4±8.227.8±7.4<0.00010.8850.008Abbreviations: eGFR, estimated glomerular filtration rate; Pcr, plasma creatinine; Pcys, plasma cystatin; SBP, systolic blood pressure. Open table in a new tab Abbreviations: eGFR, estimated glomerular filtration rate; Pcr, plasma creatinine; Pcys, plasma cystatin; SBP, systolic blood pressure. Table 3 shows the months 6 and 30 values for additional parameters in No-NaCit and NaCit groups. VTCO2 increased significantly in NaCit and decreased significantly in No-NaCit. By contrast, urine 8 h net acid excretion decreased significantly in NaCit but remained fairly constant in No-NaCit, reflecting ingestion of the prescribed alkali in NaCit. Urine Na+ excretion (UNaV) remained similar in No-NaCit but there was a statistically significant increase in NaCit, reflecting the obligate Na+ intake with Na+ citrate. Similarly, there was almost no change urine K+ excretion (UKV) in No-NaCit but UKV increased significantly in NaCit. Plasma ionized [Ca2+] decreased significantly in NaCit but remained fairly constant in No-NaCit. The change in plasma PO4 was not significant in either group. No subject met Ca++/PO4 guidelines15.National Kidney FoundationK/DOQI clinical practice guidelines for bone metabolism and disease in chronic kidney disease.Am J Kid Dis. 2000; 42: S1-S201Google Scholar to warrant PO4 binders or vitamin D therapy.Table 3Changes in parameters after 24 months of No-NaCit vs NaCit (means±s.e.)No-NaCit (n=29)NaCit (n=30)P-value, NaCit vs No-NaCitMonth 6Month 30P-value, 30 vs 6 monthsMonth 6Month 30P-value, 30 vs 6 monthsMonth 6Month 30VTCO2 (mM)20.5±0.819.6±1.2<0.000120.5±1.123.8±1.0<0.00010.918<0.00018 h Urine NAE (meq)26.0±3.126.1±3.10.83325.4±3.211.2±3.7<0.00010.433<0.0001UNaV (meq/g Cr)75.7±6.775.3±6.60.54875.4±7.688.0±10.2<0.00010.864<0.0001UKV (meq/g Cr)36.4±5.336.4±4.50.95037.6±5.739.7±5.90.00010.4210.019Plasma [Ca2+]ionized (mM)1.12±0.061.13±0.060.3701.13±0.051.09±0.05<0.00010.5920.005Plasma PO4 (mg/dl)3.98±0.384.09±0.420.0854.11±0.314.15±0.310.2390.1660.510Abbreviations: N, number of subjects in each group; NAE, net acid excretion; UKV, urine K+ excretion; UNaV, urine Na+ excretion; VTCO2, venous serum total CO2. Open table in a new tab Abbreviations: N, number of subjects in each group; NAE, net acid excretion; UKV, urine K+ excretion; UNaV, urine Na+ excretion; VTCO2, venous serum total CO2. Figure 1 shows that despite no statistically significant differences (means±s.d.) in kidney endothelin production as measured by urine ET-1 excretion (UET-1V) between NaCit and No-NaCit at study entry (5.97±2.31 vs 6.17±1.96 ng/g Cr, respectively, P=0.73) and at 6 months after SBP reduction (6.06±1.97 vs 5.95±1.55 ng/g Cr, respectively, P=0.81), UET-1V was statistically significantly lower in NaCit than No-NaCit (4.83±1.47 vs 6.92±1.67 ng/g Cr, respectively, P<0.0001) at month 30, that is, after 24 months of Na+ citrate. Also, 30- vs 6-month values for UET-1V decreased significantly in NaCit (P<0.0001) but increased significantly in No-NaCit (P<0.0001). Figure 2 shows that despite no statistically significant differences in tubulointerstitial injury as measured by urine N-acetyl-β-D-glucosaminidase (NAG) excretion (UNAGV) between NaCit and No-NaCit at study entry (9.32±3.53 vs 9.26±3.19 U/g Cr, respectively, P=0.95) and at 6 months (8.93±2.92 vs 9.01±2.70 U/g Cr, respectively, P=0.92), UNAGV was statistically significantly lower in NaCit than No-NaCit (7.72±2.14 vs 10.37±3.15 ng/g Cr, respectively, P=0.0004) at month 30. Also, 30- vs 6-month values for UNAGV decreased significantly in NaCit (P<0.004) yet increased significantly in No-NaCit (P<0.0001). These data support that kidney tubulointerstitial injury decreased with alkali therapy but increased without it, following the same directional pattern described for kidney ET-1 production.Figure 2Box plots in the previously described format showing the course of urine N-acetyl-β-D-glucosaminidase (NAG) excretion (UNAGV) as U/g creatinine in spot morning specimens at study entry, after 6 months of blood pressure (BP) reduction, and after an additional 24 months of daily Na+ citrate (NaCit) or no Na+ citrate (No-NaCit) with continuation of BP reduction, 30 months after study entry. *P<0.05 vs No-NaCit at the indicated time point; +P<0.05 vs respective 6-month value within groups.View Large Image Figure ViewerDownload (PPT) Figures 3 and 4 show the course of change for the remaining two urine indices of kidney injury. Figure 3 shows that despite no statistically significant differences in urine albumin excretion (UalbV) between NaCit and No-NaCit at study entry (160.6±74.8 vs 145.6±54.7 mg/g Cr, respectively, P=0.38) and at 6 months (132.0±66.3 vs 119.1±46.2 mg/g Cr, respectively, P=0.39), UalbV was statistically significantly lower in NaCit than No-NaCit (107.1±48.7 vs 146.9±44.6 mg/g Cr, respectively, P=0.002) at month 30. Also, 30- vs 6-month values for UalbV decreased significantly in NaCit (P<0.0001) but increased significantly in No-NaCit (P<0.0001). In addition, entry compared to 6-month UalbV was lower in both No-NaCit (P<0.0001) and NaCit (P<0.0001) groups before either received alkali therapy, likely reflecting the effects of SBP reduction. Figure 4 shows that despite no statistically significant differences in urine transforming growth factor-β excretion (UTGFV) between NaCit and No-NaCit at study entry (97.0±29.1 vs 99.1±32.1 ng/g Cr, respectively, P=0.79) and at 6 months (91.5±28.3 vs 96.6±29.1 ng/g Cr, respectively, P=0.78), UTGFV was statistically significantly lower in NaCit than No-NaCit (80.8±30.1 vs 102.2±26.5 ng/g Cr, respectively, P=0.005) at month 30. Also, 30- vs 6-month UTGFV decreased significantly in NaCit (P<0.003) but increased significantly in No-NaCit (P=0.004). Together, these data support that kidney injury worsened with time in these hypertensive nephropathy subjects with metabolic acidosis but that daily Na+ citrate for 24 months decreased kidney injury, possibly mediated through decreased kidney ET-1 production.Figure 4Box plots in the previously described format showing the course of urine transforming growth factor-β1 excretion (UTGFV) as ng/g creatinine (ng/g Cr) in spot morning specimens at study entry, after 6 months of blood pressure (BP) reduction, and after an additional 24 months of daily Na+ citrate (NaCit) or no Na+ citrate (No-NaCit) with continuation of BP reduction, 30 months after study entry. *P<0.05 vs No-NaCit at the indicated time point; +P<0.05 vs respective 6-month value within groups.View Large Image Figure ViewerDownload (PPT) Comparisons using linear mixed models showed that the yearly rate of eGFR decline was significantly slower in NaCit than No-NaCit for eGFRcr (-1.60±0.13 vs -3.79±0.30 ml/min per year, P<0.0001) and eGFRcys (-1.82±0.08 vs -4.38±0.98 ml/min per year, P<0.0001). We tested the hypothesis that the recommended alkali treatment of metabolic acidosis associated with low GFR2.National Kidney Foundation K/DOQI clinical practice guidelines for nutrition in chronic renal failure.Am J Kid Dis. 2000; 35: S1-S140Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar reduces kidney endothelin production, reduces urine parameters of tubulointerstitial injury, and slows GFR decline in subjects with low GFR due to hypertensive nephropathy. Alkali prescribed as Na+ citrate was given after pharmacologic SBP reduction because BP reduction was necessary to show the ameliorating effects of alkali therapy on kidney injury in the five-sixths model of low GFR.5.Phisitkul S. Hacker C. Simoni J. et al.Dietary protein causes a decline in the glomerular filtration rate of the remnant kidney mediated by metabolic acidosis and endothelin receptors.Kid Int. 2008; 73: 192-199Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar In addition, the antihypertensive regimens of these hypertensive nephropathy subjects with higher than normal UalbV included ACE inhibition as recommended for hypertensive nephropathy subjects with albuminuria16.Chobanion A.V. Bakris G.L. Black H.R. et al.The seventh report of the Joint National Commission on Detection, Evaluation, and Treatment of High Blood Pressure: The JNC 7 Report.JAMA. 2003; 289: 2560-2572Crossref PubMed Scopus (16537) Google Scholar and because of the suggestion that ACE inhibition reduces hypertensive nephropathy progression to chronic kidney disease (CKD) stage 5.17.Wright J.T. Bakris G. Greene T. et al.Effect of blood pressure lowering and antihypertensive drug class on progression of hypertensive kidney disease.JAMA. 2002; 288: 2421-2431Crossref PubMed Scopus (1629) Google Scholar The data support that Na+ citrate reduces kidney ET-1 production as measured by UET-1V, reduces UNAGV, the marker of kidney tubulointerstitial injury, and slows eGFR decline. By contrast, subjects not prescribed Na+ citrate had increases in UET-1V and UNAGV as well as had faster eGFR decline, supporting that this therapy also prevented what otherwise would have been progressive kidney injury with faster eGFR decline. This reduction in kidney injury and better eGFR preservation was in addition to any provided by the conventional kidney protection strategies of SBP reduction and ACE inhibition. These studies suggest that treating metabolic acidosis associated with low GFR due to hypertensive nephropathy ameliorates progressive kidney injury, some of which might be induced by endothelins, and is an effective adjunct to SBP reduction and ACE inhibition as a kidney protection strategy. Earlier studies suggest mechanisms by which amelioration of metabolic acidosis associated with low GFR reduces UET-1V. Kidney ET-1 production is increased by dietary acid in animals with intact nephron mass18.Wesson D.E. Endogenous endothelins mediate increased distal tubule acidification induced by dietary acid in rats.J Clin Invest. 1997; 99: 2203-2211Crossref PubMed Google Scholar,19.Wesson D.E. Nathan T. Rose T. et al.Dietary protein induces endothelin-mediated kidney injury through enhanced intrinsic acid production.Kid Int. 2007; 71: 210-217Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar and by metabolic acidosis in animals with reduced nephron mass.5.Phisitkul S. Hacker C. Simoni J. et al.Dietary protein causes a decline in the glomerular filtration rate of the remnant kidney mediated by metabolic acidosis and endothelin receptors.Kid Int. 2008; 73: 192-199Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar,13.Benigni A. Perico N. Gaspari F. et al.Increased renal endothelin production in rats with reduced renal mass.Am J Physiol. 1991; 260: F331-F339PubMed Google Scholar This increase includes higher kidney cortical interstitial ET-1 in acid-ingesting animals with intact nephron mass18.Wesson D.E. Endogenous endothelins mediate increased distal tubule acidification induced by dietary acid in rats.J Clin Invest. 1997; 99: 2203-2211Crossref PubMed Google Scholar and those with reduced nephron mass and metabolic acidosis.14.Wesson D.E. Endogenous endothelins mediate augmented acidification in remnant kidneys.J Am Soc Nephrol. 2001; 12: 1826-1835PubMed Google Scholar Metabolic acidosis in subjects with low GFR might increase kidney ET-1 through acid-induced ET-1 release from kidney microvascular endothelium because an acid extracellular environment within the physiologic range increases ET-1 release from human kidney microvascular endothelium in vitro.20.Wesson D.E. Simoni J. Green D.F. Reduced extracellular pH increases endothelin-1 secretion by human renal microvascular cells.J Clin Invest. 1998; 101: 578-583Crossref PubMed Scopus (59) Google Scholar ET-1 release from vascular endothelium is predominantly basolateral21.Wagner O.F. Christ G. Wojta J. et al.Polar secretion of endothelin-1 by cultured endothelial cells.J Biol Chem. 1992; 267: 16066-16068Abstract Full Text PDF PubMed Google Scholar and so microvascular endothelium might add ET-1 to cortical interstitium adjacent to the interstitial space.22.Kriz W. Kaissling B. Structural organization of the mammalian kidney.in: Seldin D. Giebisch G. The Kidney. Physiology and Pathophysiology. Raven, New York1992: 207Google Scholar Cortical epithelium secretes and releases basolateral ET-123.Zoja C. Morigi M. Figliuzzi M. et al.Proximal tubular cell synthesis and secretion of endothelin-1 on challenge with albumin and other proteins.Am J Kid Dis. 1995; 26: 934-941Abstract Full Text PDF PubMed Scopus (228) Google Scholar and might also be a source of kidney cortical interstitial ET-1. The present studies specifically explored tubulointerstitial injury as a mechanism for metabolic acidosis-induced kidney injury because it is a consistent and important feature of hypertensive nephropathy.9.Olsen J.L. Renal disease in hypertension.in: Jennette C.J. Olsen J.L. Schwartz M.M. Silva F.G. Pathology of the Kidney. 6th edn. Lippincott Williams and Wilkins, Baltimore, MD2007: 947Google Scholar The net increase in UNAGV in No-NaCit and its net reduction in NaCit supports that metabolic acidosis induces and its amelioration with Na+ citrate reduces tubulointerstitial injury. Nevertheless, Na+ citrate therapy was also associated with reduced UalbV and reduced UTGFV and its absence was associated with increases in each. Because tubulointerstitial injury contributes to UalbV in some nephropathies,24.Comper W.D. Hilliard L.M. Nikolic-Paterson D.J. et al.Disease-dependent mechanisms of albuminuria.Am J Physiol. 2008; 295: F1589-F1600Google Scholar the suggested increase in tubulointerstitial injury might contribute to the increase in UalbV. Tubulointerstitial injury mediates GFR decline in five-sixths nephrectomy3.Nath K.A. Hostetter M.K. Hostetter T.H. Pathophysiology of chronic tubulo-interstitial disease in rats—interactions of dietary acid load, ammonia, and complement component C3.J Clin Invest. 1985; 76: 667-675Crossref PubMed Scopus (415) Google Scholar,5.Phisitkul S. Hacker C. Simoni J. et al.Dietary protein causes a decline in the glomerular filtration rate of the remnant kidney mediated by metabolic acidosis and endothelin receptors.Kid Int. 2008; 73: 192-199Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar and the present data support that Na+ citrate reduces eGFR decline rate and leads to higher eGFRcys after 24 months of therapy. Because eGFR calculations with Pcys more accurately reflect GFR than those carried out with Pcr,25.Uhlmann E.J. Hook K.G. Issitt C. et al.Reference intervals for plasma cystatin C in healthy volunteers and renal patients, as measured by the Dade Behring BN II System, and correlation with creatinine.Clin Chem. 2001; 47: 2031-2033PubMed Google Scholar we consider that the statistically higher eGFRcys in NaCit than No-NaCit supports better GFR preservation with Na+ citrate, just as 2 years of NaHCO3 led to better preservation of creatinine clearance in subjects with low GFR for various causes.7.de Brito-Ashurst I. Varagunam M. Raferty M.J. et al.Bicarbonate supplementation slows progression of CKD and improves nutritional status.J Am Soc Nephrol. 2009; 20: 2075-2084Crossref PubMed Scopus (630) Google Scholar There are some limitations to be pointed out for this study. The choice of treatment and not treatment with Na+ citrate was not determined in a randomized manner for the reasons stated in Materials and Methods. Nevertheless, the two groups were very similar regarding entry characteristics as detailed. In addition, the group numbers are small although the indicated differences between them were highly significant. It remains possible, however, that greater subject numbers would yield different outcomes. This illustrates the need for a larger-scale study to explore the efficacy of this potentially effective and comparatively inexpensive adjunctive kidney protection strategy. In summary, these studies show that Na+ citrate, as recommended for subjects with low GFR and VTCO2 <22 mM, reduces kidney ET-1 production, reduces urine parameters of kidney injury including tubulointerstitial injury, and slows eGFR decline with better GFR preservation in subjects with low GFR due to hypertensive nephropathy. These studies support that Na+ citrate treatment of subjects with low GFR due to hypertensive nephropathy be further explored as an adjunctive kidney protection strategy to SBP reduction and ACE inhibition. This prospective interventional study hypothesized that oral Na+ citrate, 1 meq of HCO3 equivalent/kg body weight per day in three divided doses as recommended for subjects with low GFR and VTCO2 <22 mM,2.National Kidney Foundation K/DOQI clinical practice guidelines for nutrition in chronic renal failure.Am J Kid Dis. 2000; 35: S1-S140Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar reduces kidney endothelin production and tubulointerstitial injury in subjects with low GFR due to hypertensive nephropathy. Primary outcome was urine ET-1 excretion (UET-1V), a surrogate of kidney endothelin production18.Wesson D.E. Endogenous endothelins mediate increased distal tubule acidification induced by dietary acid in rats.J Clin Invest. 1997; 99: 2203-2211Crossref PubMed Google Scholar that mediates progressive kidney injury in experimental CKD models.5.Phisitkul S. Hacker C. Simoni J. et al.Dietary protein causes a decline in the glomerular filtration rate of the remnant kidney mediated by metabolic acidosis and endothelin receptors.Kid Int. 2008; 73: 192-199Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar, 12.Feldman D.L. Mogelesky T.C. Chou M. et al.Enhanced expression of renal endothelin-converting enzyme-1 and endothelin-A-receptor mRNA in rats with interstitial fibrosis following ureter ligation.J Cardiovasc Pharmacol. 2000; 36: S255-S259Crossref PubMed Google Scholar, 19.Wesson D.E. Nathan T. Rose T. et al.Dietary protein induces endothelin-mediated kidney injury through enhanced intrinsic acid production.Kid Int. 2007; 71: 210-217Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar Secondary outcomes included urine excretion of parameters of kidney injury and eGFR by MDRDS formula using Pcr.26.Klahr S. Levey A.S. Beck G.J. et al.The effects of dietary protein restriction and blood-pressure control on the progression of chronic renal disease. Modification of Diet in Renal Disease Study Group.N Engl J Med. 1994; 330: 877-884Crossref PubMed Scopus (2051) Google Scholar Subsequent to protocol completion, cystatin C-derived eGFR (eGFRcys) was calculated using the CKD-EPI equation.27.Stevens L.A. Coresh J. Schmid C.H. et al.Estimating GFR using serum cystatin C alone and in combination with serum creatinine: a pooled analysis of 3,418 individuals with CKD.Am J Kid Dis. 2008; 51: 395-406Abstract Full Text Full Text PDF PubMed Scopus (876) Google Scholar Urine NAG excretion (UNAGV) was measured as a marker of kidney tubulointerstitial injury,28.Costigan M. Rustom R. Shenkin A. et al.Origin and significance of urinary N-acetyl β-D-glucosaminidase (NAG), in renal patients with variable function, pathology and proteinuria.Clinica Chimica Acta. 1996; 255: 133-344Crossref PubMed Scopus (32) Google Scholar albumin excretion (UalbV) was measured as a general marker of progressive kidney injury,29.Remuzzi G. Bertani T. Pathophysiology of progressive nephropathies.N Engl J Med. 1998; 339: 1448-1456Crossref PubMed Scopus (1150) Google Scholar and transforming growth factor-β1 excretion (UTGFV) because it reflected kidney injury induced by dietary acid in an experimental model of CKD5.Phisitkul S. Hacker C. Simoni J. et al.Dietary protein causes a decline in the glomerular filtration rate of the remnant kidney mediated by metabolic acidosis and endothelin receptors.Kid Int. 2008; 73: 192-199Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar and because it might be a mediator of hypertensive nephropathy.30.Suthanthiran M. Li B. Song J.O. et al.Transforming growth factor β1 hyperexpression in African-American hypertensives: a novel mediator of hypertension and/or target organ damage.Proc Natl Acad Sci. 2000; 97: 3479-3484PubMed Scopus (0) Google Scholar Urine net acid excretion was calculated by measuring urine titratable acidity (TA), ammonium (NH4+), and HCO3 ([NH4+]+[TA]-[HCO3]). These parameters were followed in hypertensive nephropathy subjects after lowering SBP toward recommended levels16.Chobanion A.V. Bakris G.L. Black H.R. et al.The seventh report of the Joint National Commission on Detection, Evaluation, and Treatment of High Blood Pressure: The JNC 7 Report.JAMA. 2003; 289: 2560-2572Crossref PubMed Scopus (16537) Google Scholar over 6 months and maintaining the reduced SBP thereafter. Subjects with hypertensive nephropathy, eGFR ≥20 but <60 ml/min, and VTCO2 <22 mM were recommended Na+ citrate2.National Kidney Foundation K/DOQI clinical practice guidelines for nutrition in chronic renal failure.Am J Kid Dis. 2000; 35: S1-S140Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar but some either refused it because of its bad taste and/or because they could not afford it (it is not covered by the local indigent health plan). Those who refused or could not afford Na+ citrate were offered NaHCO3 tablets. Those who accepted, could afford, and tolerated NaHCO3 were treated as such but were not followed in this study. Those who did not tolerate NaHCO3 (most commonly due to bloating), refused it and Na+ citrate outright, and agreed to participate in the study were enrolled and followed as controls to the NaCit subjects and followed an additional 24 months. They were instructed to take no over-the-counter medications. Follow-up data were available for all recruited subjects. Our local institutional review board approved the protocol. The study sought subjects whose exclusive cause for low GFR was hypertensive nephropathy. Candidates had been referred for control of ‘resistant’ hypertension. Such referred subjects at our clinic undergo duplex Doppler ultrasonography and serum aldosterone/renin ratios to help rule out renal artery stenosis31.Helenon O. el Rody F. Correas J.M. et al.Color Doppler US of renovascular disease in native kidneys.Radiographics. 1995; 15: 833-854Crossref PubMed Scopus (86) Google Scholar and hyperaldosteronism,32.Gordon R.D. Stowasser M. Tunny T.J. et al.High incidence of primary aldosteronism in 199 patients referred with hypertension.Clin Exp Pharmacol Physiol. 1994; 21: 315-318Crossref PubMed Scopus (442) Google Scholar as contributors to ‘resistant’ hypertension. Inclusion criteria were (1) age ≥18 years and able to give consent, (2) ≥2 visits with their primary care providers showing compliance with clinic visits, and (3) 20≤ eGFR <60 ml/min. Exclusion criteria were (1) known primary kidney disease or findings consistent thereof such as ≥3 red blood cells per high-powered field of urine or urine cellular casts; (2) history of diabetes or fasting blood glucose ≥110 mg/dl; (3) history of malignances, chronic infections, pregnancy, or clinical evidence of cardiovascular disease; (4) peripheral edema or diagnoses associated with edema such as heart/liver failure or nephrotic syndrome; (5) history of taking Al++-containing products; (6) and Doppler studies and/or serum aldosterone/renin ratios consistent with renal artery stenosis and/or primary hyperaldosteronism, respectively; or (7) history of medication noncompliance. Improved BP control reduces the rate of GFR decline in subjects with hypertensive nephropathy33.Toto R.D. Mitchell H.C. Smith R.D. et al.‘Strict’ blood pressure control and progression of renal disease in hypertensive nephrosclerosis.Kid Int. 1995; 48: 851-859Abstract Full Text PDF PubMed Scopus (158) Google Scholar and guidelines recommend that ACE inhibitors be included in the drug regimens of such subjects.16.Chobanion A.V. Bakris G.L. Black H.R. et al.The seventh report of the Joint National Commission on Detection, Evaluation, and Treatment of High Blood Pressure: The JNC 7 Report.JAMA. 2003; 289: 2560-2572Crossref PubMed Scopus (16537) Google Scholar Consequently, all subjects underwent a BP reduction protocol34.Regalado M. Yang S. Wesson D.E. Cigarette smoking is associated with augmented progression of renal insufficiency in severe essential hypertension.Am J Kid Dis. 2000; 35: 687-694Abstract Full Text Full Text PDF PubMed Scopus (140) Google Scholar including ACE inhibition as tolerated for 6 months that was maintained through 30 months. Those unable to tolerate ACE inhibition were excluded. Subjects had visits at least every 6 months with measurements of the indicated parameters and were followed for 30 months. Serum and urine creatinine and urine albumin were measured using the Sigma Diagnostics Creatinine Kit (Procedure No. 555; Sigma Diagnostics, St Louis, MO, USA).35.Chuahirun T. Wesson D.E. Cigarette smoking predicts faster progression of type 2 established diabetic nephropathy despite angiotensin converting enzyme inhibition.Am J Kid Dis. 2002; 39: 376-382Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar Cystatin C was measured using a particle-enhanced immunonephelometric assay (N Latex Cystatin C; Dade Behring, Somerville, NJ) with a nephelometer (BNII; Dade Bering).25.Uhlmann E.J. Hook K.G. Issitt C. et al.Reference intervals for plasma cystatin C in healthy volunteers and renal patients, as measured by the Dade Behring BN II System, and correlation with creatinine.Clin Chem. 2001; 47: 2031-2033PubMed Google Scholar,36.Erlandsen D.J. Randers E. Kristensen J.H. Evaluation of the Dade Behring N Latex Cystatin C assay on the Dade Behring Nephelometer II System.Scand J Clin Invest. 1999; 59: 1-8PubMed Google Scholar Urine ET-1 was measured using a RIA kit (Peninsula Laboratories, Belmont, CA) after extraction using Bound Elut c/8 columns (Varian, Harbor City, CA, USA) preconditioned with methanol, H2O, and acetic acid as carried out previously.37.Wesson D.E. Endogenous endothelins mediate increased distal tubule acidification induced by dietary acid in rats.J Clin Invest. 1997; 99: 2203-2211Crossref PubMed Scopus (68) Google Scholar Urine TGF-β was measured using quantitative sandwich enzyme immunoassay.38.Chuahirun T. Jan Simoni J. Hudson C. et al.Cigarette smoking exacerbates and its cessation ameliorates renal injury in type 2 diabetes.Am J Med Sci. 2004; 327: 57-67Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar Urine NAG was measured using a colorimetric assay (Boehringer Mannheim, Mannheim, Germany).3.Nath K.A. Hostetter M.K. Hostetter T.H. Pathophysiology of chronic tubulo-interstitial disease in rats—interactions of dietary acid load, ammonia, and complement component C3.J Clin Invest. 1985; 76: 667-675Crossref PubMed Scopus (415) Google Scholar The IRMA SL Series 2000 blood analysis system (Edison, NJ) measured venous plasma/blood pH, pCO2, and ionized calcium. This system calculated TCO2. Serum phosphate was measured with the Hitachi autoanalyzer (Norcross, GA, USA). Urine TA was measured by correction to the ambient serum pH by NaOH addition, NH4+ by the formalin titrimetric (to ambient serum pH) method,39.Cunarro J.A. Weiner M.W. A comparison of methods for measuring urinary ammonium.Kid Int. 1974; 5: 303-305Abstract Full Text PDF PubMed Scopus (56) Google Scholar and urine HCO3 (as TCO2) was measured by ultrafluorometry.40.Wesson D.E. Dietary HCO3 reduces distal tubule acidification by increasing cellular HCO3 secretion.Am J Physiol. 1996; 271: F132-F140PubMed Google Scholar All urine parameters were expressed per gram creatinine of a spot a.m. specimen. Patient characteristics at the time of study enrollment were tabulated or described by mean and standard deviation (s.d.) as appropriate. At this time point, categorical variables were compared between the treatment and control group with the χ2-test and continuous measures were compared between the groups with the two-sample t-test. Similarly, measurements of markers at baseline, 6, and 30 months for each group were plotted and described by mean and s.d. The change from 6 to 30 months within each group was considered with a one-sample t-test. The differences between the two groups at each time point were considered with a two-sample t-test. We considered whether the rate of change between 6 and 30 months was different between the two groups. We fit linear mixed models with terms for treatment (NaCit vs No-NaCit), time (6 vs 30 months), and the interaction between the two. Linear mixed models are used in situations in which data are correlated over more than two time points, as with this study. An important choice in a linear mixed model, which affects the validity of hypothesis tests, is the error/correlation structure within each subject. For this study, the measurements are not equally spaced in time and so the error/correlation structure within each patient over time was modeled using a general symmetric structure. Because ‘practical experience based on many longitudinal studies has led to the empirical observation that variances are rarely constant over time’,41.Fitzmaurice G.M. Laird N.M. Ware J.H. Applied Longitudinal Analysis. Wiley, New York2004: 169Google Scholar account was also taken of the possibility that the variance changed over time. We used restricted maximum likelihood estimation to fit each model. Analyses were performed using R 2.4.1 (R Core Development Team, 2006). All statistical tests were two sided and P-values less than 0.05 were considered statistically significant. No adjustments were made for multiple statistical tests. We thank the nursing and clerical staff of the Internal Medicine Clinic of the Department of Internal Medicine at Texas Tech University Health Sciences Center for their assistance. This work was supported by funds from the Larry and Jane Woirhaye Memorial Endowment in Renal Research at the Texas Tech University Health Sciences Center.
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