L-carnitine and acetyl-L-carnitine status during hemodialysis with acetate in humans: a kinetic analysis

Effect of l-carnitine on the kinetics of carnitine, acylcarnitines and butyrobetaine in long-term haemodialysis

BACKGROUND

The current study was performed to investigate the kinetics of carnitine, individual acylcarnitines and butyrobetaine in patients on haemodialysis.

METHODS

Eight stable long-term haemodialysis patients were studied under basal conditions (no carnitine supplementation) and 3 weeks after intravenous supplementation with l-carnitine (10 or 20 mg/kg body weight) after each haemodialysis session. The kinetic studies included serial determinations of carnitine and metabolites just before, during or between haemodialysis sessions. Analysis was performed by liquid chromatography-tandem mass spectrometry.

RESULTS

Before haemodialysis, the plasma concentrations were (micromol/l) 15.1+/-0.6 (mean+/-SEM) for carnitine, 5.9+/-0.7 for acetylcarnitine, 0.66+/-0.04 for propionylcarnitine and 0.98+/-0.08 for butyrobetaine (basal conditions) or 142+/-23 for carnitine, 69+/-12 for acetylcarnitine, 6.0+/-1.1 for propionylcarnitine and 2.6+/-0.3 for butyrobetaine (carnitine 20 mg/kg). During haemodialysis, the plasma concentrations dropped by approximately 80% for all compounds determined, with extraction coefficients ranging from 0.65 to 0.86. In patients supplemented with 20 mg/kg carnitine, the amount of carnitine removed by haemodialysis equalled 42% of the dose administered, consisting of 2.08 mmol carnitine, 1.03 mmol acetylcarnitine and 0.051 mmol propionylcarnitine. Between the haemodialysis sessions, carnitine, acylcarnitines and butyrobetaine reached apparent steady-state concentrations within 1 day both under basal conditions and after supplementation.

CONCLUSIONS

Patients on haemodialysis have reduced carnitine, acylcarnitine and butyrobetaine plasma levels, which can be increased by supplementing carnitine. Propionylcarnitine, an important constituent of the acylcarnitine pool, can be removed by haemodialysis. Removal of potentially toxic acyl-groups may represent a mechanism for a beneficial effect of carnitine in these patients.

Pharmacokinetics of L-carnitine

L-Carnitine is a naturally occurring compound that facilitates the transport of fatty acids into mitochondria for beta-oxidation. Exogenous L-carnitine is used clinically for the treatment of carnitine deficiency disorders and a range of other conditions. In humans, the endogenous carnitine pool, which comprises free L-carnitine and a range of short-, medium- and long-chain esters, is maintained by absorption of L-carnitine from dietary sources, biosynthesis within the body and extensive renal tubular reabsorption from glomerular filtrate. In addition, carrier-mediated transport ensures high tissue-to-plasma concentration ratios in tissues that depend critically on fatty acid oxidation. The absorption of L-carnitine after oral administration occurs partly via carrier-mediated transport and partly by passive diffusion. After oral doses of 1-6g, the absolute bioavailability is 5-18%. In contrast, the bioavailability of dietary L-carnitine may be as high as 75%. Therefore, pharmacological or supplemental doses of L-carnitine are absorbed less efficiently than the relatively smaller amounts present within a normal diet.L-Carnitine and its short-chain esters do not bind to plasma proteins and, although blood cells contain L-carnitine, the rate of distribution between erythrocytes and plasma is extremely slow in whole blood. After intravenous administration, the initial distribution volume of L-carnitine is typically about 0.2-0.3 L/kg, which corresponds to extracellular fluid volume. There are at least three distinct pharmacokinetic compartments for L-carnitine, with the slowest equilibrating pool comprising skeletal and cardiac muscle.L-Carnitine is eliminated from the body mainly via urinary excretion. Under baseline conditions, the renal clearance of L-carnitine (1-3 mL/min) is substantially less than glomerular filtration rate (GFR), indicating extensive (98-99%) tubular reabsorption. The threshold concentration for tubular reabsorption (above which the fractional reabsorption begins to decline) is about 40-60 micromol/L, which is similar to the endogenous plasma L-carnitine level. Therefore, the renal clearance of L-carnitine increases after exogenous administration, approaching GFR after high intravenous doses. Patients with primary carnitine deficiency display alterations in the renal handling of L-carnitine and/or the transport of the compound into muscle tissue. Similarly, many forms of secondary carnitine deficiency, including some drug-induced disorders, arise from impaired renal tubular reabsorption. Patients with end-stage renal disease undergoing dialysis can develop a secondary carnitine deficiency due to the unrestricted loss of L-carnitine through the dialyser, and L-carnitine has been used for treatment of some patients during long-term haemodialysis. Recent studies have started to shed light on the pharmacokinetics of L-carnitine when used in haemodialysis patients.