[Advantage of a high-sodium hemodialysis solution: theoretical bases]. / Intérêt d'un bain d'hémodialyse riche en sodium: fondements théoriques.

In hemodialysis, an excessive oversimplification leads to confuse the transfer direction of a given solute (patient towards dialysate or dialysate towards patient) and the direction of transmembranar concentration gradient. Applied to sodium, this confusion leads to the usual iso- or hypotonic dialysate use, in order to eliminate the patient's sodium overload. We show here how the use of a high sodium dialysate allows the elimination of the sodium overload and makes the free water clearance (approached by the Na-free water clearance) positive, which is the necessary condition to restore an hydro-electrolytic equilibrium near from the normal. That allows a disappearance of hypovolemic symptoms during hemodialysis, a better general condition and a possible increase of water intake between sessions.

Characterization of sub-peak b4.2, middle molecule.

The Middle Molecules (MM) within the molecular weight (MW) range of vitamin B12 (1355 daltons) are assumed to be partly responsible for uremic toxicity. We have isolated a solute, b4.2, the purity of which is controlled by thin layer chromatography on silica gel. It correlates with active clinical polyneuropathy. The Stockholm group is dealing with a MM they call peak 7c. After exchange of purified solutes between the Stockholm group and us, comparative analyses demonstrate that 7c and b4.2 are different. The b4.2 solute is a glucuronide but it is impossible to obtain the aglycon moiety after enzymatic or acidic hydrolysis. Desorption chemical ionization and electron-impact ionization mass spectrometry results of b4.2 after transformation in methyl ester trimethylsilyl derivative are compatible with a b4.2 MW of 568 daltons (or 526 in native form) corresponding to a glucuronoconjugate of an aglycon with a MW 392 daltons (or 350 in native form). Moreover mass spectrometry confirms that b4.2 isolated from normal human urine and from uremic RP6 hemofiltrate fluid are identical.

Technical aspects on middle molecules: separation, isolation, and identification.

For the last few years we have attempted to separate and isolate from biological fluids the uremic Middle Molecules (MM). The first step in the treatment of plasma is an ultrafiltration through AN69 membrane. The samples are fractionated by Sephadex G-15 chromatography into 9 peaks "a" to "i' under precise conditions that have been selected in order to exhibit different patterns in the MM range (peak b and c) between uremic and uremic polyneuropathic patients. Further separation of peak b is performed by ion exchange chromatography with Sephadex DEAE A-25 into 7 sub-peaks b; sub-peak b 4-2 is the only one to correlate with neuropathy. Prior to b 4-2 identification studies, specific large scale isolation procedures are necessary. The purity of the product is monitored at each step by thin-layer chromatography on silica gel.

Uremic neurotoxin in the middle molecular weight range.

Among the middle molecule fractions obtained by high performance gel chromatography on Sephadex G-15 combined with gradient ion exchange chromatography on DEAE Sephadex A-25 from plasma ultrafiltrates of six polyneuropathic patients, only peak b4-2 was at a significantly higher concentration than that obtained from uremic patients with neuropathy. Purification of the b4-2 solute allows its quantitative determination in biological fluids. The b4-2 plasma concentrations are 1 mg/L in healthy subjects (n=30), 4.6 +/- 0.2 mg/L in uremic patients (n=67) and 13-19 mg/L in six polyneuropathic patients. The 24-hr urinary excretion is 13 +/- 1 mg. The weekly removal rates in hemofiltration or in hemodialysis, using high permeability membrane three times a week, are 38-40 mg. In an in vitro sural nerve test for the evalution of middle molecule neurotoxicity, the b4-2 solute exhibits a neurotoxic effect at concentrations similar to those found in plasma of neuropathic patients. Preliminary results of an attempt to identify the neurotoxin indicate that it is an acid-polyol derivative.

Evaluation of plasma neurotoxin concentration in uraemic polyneuropathic patients.

Purification of b4-2 sub-peak obtained on DEAE Sephadex A25 chromatography gave us the possibility of quantifying the plasma concentration of the neurotoxin present in uraemic patients with active polyneuropathy. From the purified neurotoxin isolated by kieselguhr and cellulose chromatography we calibrated analytic columns for b4-2 analysis. Plasma concentration, measured in 6 uraemic neuropathic patients, is between 13 and 19 mg/litre. In 52 uraemic patients without neuropathy, the plasma concentration is between 3 and 9 mg/litre. In 20 healthy subjects the plasma concentration is less than 1 mg/litre. The weekly neurotoxin removal in uraemic patients without neuropathy, treated by a five hours RP6 session 3 times a week, is of the same order of magnitude as the weekly urinary excretion in healthy subjects. Preliminary results of a tentative identification of this purified product indicate that it is not a polypeptide but an acid-polyol with carbohydrate structure.