Abstract
BackgroundThere is growing evidence that the accumulation of protein- bound uremic retention solutes, such as indoxyl sulfate, p-cresyl sulfate and kynurenic acid, play a role in the accelerated cardiovascular disease seen in patients undergoing chronic hemodialysis. Protein-binding, presumably to albumin, renders these solutes poor-dialyzable.We previously observed that the free fraction of indoxyl sulfate was markedly reduced at the end of hemodialysis. We hypothesized that solute binding might be pH-dependent and attributed the changes in free solute concentration to the higher serum pH observed at the end of standard hemodialysis with dialysis buffer bicarbonate concentration greater than 35 mmol/L. We observed that acidification of uremic plasma to pH 6 in vitro greatly increased the proportion of freeIS.MethodsWe tested our hypothesis by reducing the dialysate bicarbonate buffer concentration to 25 mmol/L for the initial half of the hemodialysis treatment (“isohydric dialysis”). Eight stable hemodialysis patients underwent “isohydric dialysis” for 90 minutes and then were switched to standard buffer (bicarbonate = 37mmol/L). A second dialysis, 2 days later, employed standard buffer throughout.ResultsWe found a clearcut separation of blood pH and bicarbonate concentrations after 90 minutes of “isohydric dialysis” (pH = 7.37, bicarbonate = 22.4 mmol/L) and standard dialysis (pH = 7.49, bicarbonate = 29.0 mmol/L). Binding affinity varied widely among the 10 uremic retention solutes analyzed. Kynurenic acid (0.05 free), p-cresyl sulfate (0.12 free) and indoxyl sulfate (0.13 free) demonstrated the greatest degree of binding. Three solutes (indoxyl glucuronide, p-cresyl glucuronide, and phenyl glucuronide) were virtually unbound. Analysis of free and bound concentrations of uremic retention solutes confirmed our prediction that binding of solute is affected by pH. However, in a mixed models analysis, we found that the reduction in total uremic solute concentration during dialysis accounted for a greater proportion of the variation in free concentration, presumably an effect of saturation binding to albumin, than did the relatively small change in pH produced by isohydric dialysis. The effect of pH on binding appeared to be restricted to those solutes most highly protein-bound.The solutes most tightly bound exhibited the lowest dialyzer clearances. An increase in dialyzer clearance during isohydric and standard dialyses was statistically significant only for kynurenic acid.ConclusionThese findings provide evidence that the binding of uremic retention solutes is influenced by pH. The effect of reducing buffer bicarbonate concentration (“isohydric dialysis:”), though significant, was small but may be taken to suggest that further modification of dialysis technique that would expose blood to a greater decrease in pH would lead to a greater increase the free fraction of solute and enhance the efficacy of hemodialysis in the removal of highly protein-bound uremic retention solutes.
Highlights
We found a clearcut separation of blood pH and bicarbonate concentrations after 90 minutes of “isohydric dialysis” and standard dialysis
In a mixed models analysis, we found that the reduction in total uremic solute concentration during dialysis accounted for a greater proportion of the variation in free concentration, presumably an effect of saturation binding to albumin, than did the relatively small change in pH produced by isohydric dialysis
An increase in dialyzer clearance during isohydric and standard dialyses was statistically significant only for kynurenic acid. These findings provide evidence that the binding of uremic retention solutes is influenced by pH
Summary
Hemodialysis was established as an effective technique for maintaining patients with end-stage renal disease by Belding Scribner and his associates [1], but systematic investigation of the nature of uremic toxins is usually dated to the report of the European Uremic Toxin Workshop (EUTox) group which surveyed a wide range of published studies and defined uremic toxins as substances whose concentration in blood was significantly higher in patients with evidence of reduced glomerular filtration [2]. Techniques, generally employing high pressure liquid chromatography (HPLC), coupled with mass spectroscopy (MS), have identified a great number of solutes that are at higher plasma concentration in patients with evidence of decreased glomerular filtration (estimated by a variety of techniques) These direct measurements have generally confirmed the conclusion of the original EUTox report that these solutes fall into three classes (1) small dialyzable water soluble molecules such as urea or creatinine, (2) larger molecules, from a historical perspective often referred to as “middle molecules”, such as beta-2 microglobulin and complement, defined as being too large to pass across conventional dialysis membranes, and (3) small protein-bound solutes that do not readily diffuse across conventional dialysis membranes. We observed that acidification of uremic plasma to pH 6 in vitro greatly increased the proportion of freeIS
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
