Effects of L-carnitine and palmitoylcarnitine on membrane fluidity of human erythrocytes

Attenuation of Oxidative Stress in Erythrocytes Stored with Vitamin C and l-Carnitine in Additive Solution-7

Background: Blood transfusion has advanced toward component therapy for specific requirements during trauma and surgery. Oxidative stress is induced in erythrocytes during storage. Hence, antioxidants as additives can be employed to counteract oxidative stress and enhance antioxidant defenses. Therefore, this study investigates the combinatorial effects of vitamin C and l-carnitine on erythrocytes during storage. Methodology: Erythrocyte samples were categorized into control and experimental groups-vitamin C (10 mM) and l-carnitine (10 mM) and stored under blood bank conditions (at 4°C) for 35 days. Hemoglobin (Hb), antioxidant enzymes (superoxide dismutase [SOD], catalase [CAT] and glutathione peroxidase [GPX]), lipid peroxidation products (conjugate dienes and thiobarbituric acid reactive substances [TBARSs]), protein oxidation products, metabolic markers (glucose, lactate dehydrogenase), glutathione (GSH), superoxides, and hemolysis were assessed at weekly intervals. Results: SOD activity increased on day 7 in the controls, whereas it increased on days 7 and 14 in the experimental groups. CAT activity increased on day 35 in both the groups. GPX activity increased on day 7 in the controls. Hb levels decreased on days 14 and 35 in the controls and on day 35 in the experimental groups. Hemolysis increased from day 7 onward in both the groups. Protein oxidation products were maintained throughout the storage. GSH levels increased on day 21 in the controls and on days 14 and 21 in the experimental groups. Superoxides and conjugate dienes decreased from day 14 in both the groups. TBARSs decreased on day 7 in the experimental groups. Conclusion: Vitamin C and l-carnitine have synergistically enhanced the efficacy of stored erythrocytes in terms of Hb, antioxidant enzymes, and lipid peroxidation.

Metabolomic Profiling of Recombinant Erythropoietin (rHuEpo) in Trained Caucasian Athletes

OBJECTIVE

Recombinant human erythropoietin (rHuEpo) is prohibited by the World Anti-Doping Agency but remains the drug of choice for many cheating athletes wishing to evade detection using current methods. The aim of this study was to identify a robust metabolomics signature of rHuEpo using an untargeted approach in blood (plasma and serum) and urine.

DESIGN

Longitudinal study.

SETTING

University of Glasgow.

PARTICIPANTS

Eighteen male participants regularly engaged in predominantly endurance-based activities, such as running, cycling, swimming, triathlon, and team sports, were recruited.

INTERVENTIONS

Each participant received 50 IU·kg -1 body mass of rHuEpo subcutaneously every 2 days for 4 weeks. Samples were collected at baseline, during rHuEpo administration (over 4 weeks) and after rHuEpo administration (week 7-10). The samples were analyzed using hydrophilic interaction liquid chromatography mass spectrometry.

MAIN OUTCOME MEASURES

Significant metabolic signatures of rHuEpo administration were identified in all biofluids tested in this study.

RESULTS

Regarding metabolomics data, 488 plasma metabolites, 694 serum metabolites, and 1628 urinary metabolites were identified. Reproducible signatures of rHuEpo administration across all biofluids included alterations of pyrimidine metabolism (orotate and dihydroorotate) and acyl-carnitines (palmitoyl-carnitine and elaidic carnitine), metabolic pathways that are associated with erythropoiesis or erythrocyte membrane function, respectively.

CONCLUSIONS

Preliminary metabolic signatures of rHuEpo administration were identified. Future studies will be required to validate these encouraging results in independent cohorts and with orthogonal techniques, such as integration of our data with signatures derived from other "omics" analyses of rHuEpo administration (eg, transcriptomics).

Surface adsorption and solution aggregation of a novel lauroyl-l-carnitine surfactant

HYPOTHESIS

l-carnitine plays a crucial role in the cellular production of energy by transporting fatty acids into mitochondria. Acylated l-carnitines are amphiphilic and if appropriate physical properties were demonstrated, they could replace many currently used surfactants with improved biocompatibility and health benefits.

EXPERIMENTS

This work evaluated the surface adsorption of lauroyl-l-carnitine (C₁₂LC) and its aggregation behavior. The size and shape of the aggregates of C₁₂LC surfactant were studied at different temperatures, concentrations, pH and ionic strength by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). Surface tension measurements were carried out to determine the critical micellar concentration (CMC) of C₁₂LC. Combining with the Gibbs equation, the surface excess at different concentrations could be determined. Neutron reflection (NR) was used to determine the structure of the adsorbed layer at the air/water interface with the help of isotopic contrast variations.

FINDINGS

At pH 7, the limiting area per molecule (A_CMC) of the zwitterionic C₁₂LC adsorbed layer at the air/water interface was found to be 46 Å² from surface tension and neutron reflection, smaller than the values of C₁₂PC, C₁₂E₅, DTAB, C₁₂C₄betaine and C₁₂C₈betaine but close to that of SDS. A pronounced surface tension minimum at pH 2 at the low ionic strength was linked to a minimum value of area per molecule of about 30 Å², indicating the competitive adsorption from traces of lauric acid produced by hydrolysis of C₁₂LC. As the concentration increased, area per molecule reached a plateau of 37-39 Å², indicating the dissolution of the more surface-active lauric acid into the micelles of C₁₂LC. DLS and SANS showed that the size and shape of micelles had little response to temperature, concentration, ionic strength or pH. The SANS profiles measured under 3 isotopic contrasts could be well fitted by the core-shell model, giving a spherical core radius of 15.7 Å and a shell thickness of 10.5 Å. The decrease of pH led to more protonated carboxyl groups and more positively charged micelles, but the micellar structures remained unchanged, in spite of their stronger interaction. These features make C₁₂LC potentially attractive as a solubilizing agent.

Acylcarnitines at the Membrane Surface: Insertion Parameters for a Mitochondrial Leaflet Model

Excessive accumulation of acylcarnitines (ACs), often caused by metabolic disorders, has been associated with obesity, arrhythmias, cardiac ischemia, insulin resistance, etc. Mechanisms whereby elevated ACs might contribute to pathophysiological effects remain largely unexplored. We have aimed to gain insight into AC interactions with the mitochondrial inner membrane. To model its outer leaflet, Langmuir monolayers and cushioned supported bilayers were employed. Their interactions with ACs were monitored with epifluorescence microscopy, which revealed a local leaflet expansion upon exposure to elevated concentrations of a long-chain AC, plausibly caused by its insertion. To assess the AC insertion parameters, constant-pressure insertion assays were performed. A value of 21 ± 3 Å² was obtained for the AC insertion area, which is roughly the same as the cross-sectional area of an acyl chain. By contrast, the carnitine moiety was found to require an area of 37 ± 3 Å². The AC insertion has thus been concluded to involve solely the AC acyl chain. This mode of insertion implies that the carnitine moiety, with its nontitratable positive charge, is left dangling at the membrane surface, which is likely to alter the surface electrostatics of the outer leaflet. The extrapolation of these findings has enabled us to hypothesize that, by altering the morphology and surface electrostatics of the outer leaflet, the insertion of ACs, in particular their long-chain counterparts, may trigger a nonspecific activation of signaling pathways in the inner mitochondrial membrane, thereby modulating its function and potentially leading to pathophysiological responses.

l-carnitine as a Potential Additive in Blood Storage Solutions: A Study on Erythrocytes

Erythrocytes undergo various changes during storage (storage lesion) that in turn reduces their functioning and survival. Oxidative stress plays a major role in the storage lesion and antioxidants can be used to combat this stress. This study elucidates the effects of l-carnitine (LC) on erythrocytes of stored blood. Blood was obtained from male Wistar rats and stored (4 °C) for 20 days in CPDA-1 (citrate phosphate dextrose adenine) solution. Samples were divided into-(i) controls (ii) LC 10 (l-carnitine at a concentration of 10 mM) (iii) LC 30 (l-carnitine at a concentration of 30 mM) and (iv) LC 60 (l-carnitine at a concentration of 60 mM). Every fifth day, the biomarkers (haemoglobin, hemolysis, antioxidant enzymes, lipid peroxidation and protein oxidation products) were analysed in erythrocytes. Hemoglobin and protein sulfhydryls were insignificant during storage indicative of the maintenance of hemoglobin and sulfhydryls in all groups. Superoxide dismutase and malondialdehyde levels increased initially and decreased towards the end of storage. The levels of catalase and glutathione peroxidase were lower in experimentals than controls during storage. l-carnitine assisted the enzymes by scavenging the reactive oxygen species produced. Hemolysis increased in all groups with storage, elucidating that l-carnitine could not completely protect lipids and proteins from oxidative stress. Hence, this study opens up new avenues of using l-carnitine as a component of storage solutions with combinations of antioxidants in order to maintain efficacy of erythrocytes.

Carnitine and acylcarnitines: pharmacokinetic, pharmacological and clinical aspects

L-Carnitine (levocarnitine) is a naturally occurring compound found in all mammalian species. The most important biological function of L-carnitine is in the transport of fatty acids into the mitochondria for subsequent β-oxidation, a process which results in the esterification of L-carnitine to form acylcarnitine derivatives. As such, the endogenous carnitine pool is comprised of L-carnitine and various short-, medium- and long-chain acylcarnitines. The physiological importance of L-carnitine and its obligatory role in the mitochondrial metabolism of fatty acids has been clearly established; however, more recently, additional functions of the carnitine system have been described, including the removal of excess acyl groups from the body and the modulation of intracellular coenzyme A (CoA) homeostasis. In light of this, acylcarnitines cannot simply be considered by-products of the enzymatic carnitine transfer system, but provide indirect evidence of altered mitochondrial metabolism. Consequently, examination of the contribution of L-carnitine and acylcarnitines to the endogenous carnitine pool (i.e. carnitine pool composition) is critical in order to adequately characterize metabolic status. The concentrations of L-carnitine and its esters are maintained within relatively narrow limits for normal biological functioning in their pivotal roles in fatty acid oxidation and maintenance of free CoA availability. The homeostasis of carnitine is multifaceted with concentrations achieved and maintained by a combination of oral absorption, de novo biosynthesis, carrier-mediated distribution into tissues and extensive, but saturable, renal tubular reabsorption. Various disorders of carnitine insufficiency have been described but ultimately all result in impaired entry of fatty acids into the mitochondria and consequently disturbed lipid oxidation. Given the sensitivity of acylcarnitine concentrations and the relative carnitine pool composition in reflecting the intramitochondrial acyl-CoA to free CoA ratio (and, hence, any disturbances in mitochondrial metabolism), the relative contribution of L-carnitine and acylcarnitines within the total carnitine pool is therefore considered critical in the identification of mitochondria dysfunction. Although there is considerable research in the literature focused on disorders of carnitine insufficiency, relatively few have examined relative carnitine pool composition in these conditions; consequently, the complexity of these disorders may not be fully understood. Similarly, although important studies have been conducted establishing the pharmacokinetics of exogenous carnitine and short-chain carnitine esters in healthy volunteers, few studies have examined carnitine pharmacokinetics in patient groups. Furthermore, the impact of L-carnitine administration on the kinetics of acylcarnitines has not been established. Given the importance of L-carnitine as well as acylcarnitines in maintaining normal mitochondrial function, this review seeks to examine previous research associated with the homeostasis and pharmacokinetics of L-carnitine and its esters, and highlight potential areas of future research.

Endogenous plasma carnitine pool composition and response to erythropoietin treatment in chronic haemodialysis patients

BACKGROUND

Anaemia is a common complication associated with haemodialysis and is usually managed by treatment with recombinant human erythropoietin (rHuEPO). However, many patients remain hyporesponsive to rHuEPO treatment despite adequate iron therapy. The effect of L-carnitine administration on rHuEPO dose and/or haematocrit in haemodialysis patients has been previously reported with equivocal results. This study examined the relationship between endogenous carnitine pool composition and rHuEPO requirements in long-term haemodialysis patients.

METHODS

Pre-dialysis blood samples were collected from 87 patients and analysed for plasma L-carnitine and individual acylcarnitine levels by LCMS/MS. As an indication of rHuEPO responsiveness, erythropoietin resistance index (ERI) was calculated as rHuEPO dose/kg/week normalized for haemoglobin levels.

RESULTS

A significant negative correlation between L-carnitine levels and ERI was found (P = 0.0421). All patients categorized as high ERI (>0.02 microg/kg/week/gHb) exhibited subnormal L-carnitine levels (<30 microM); conversely, patients with normal L-carnitine levels (>30 microM) displayed low ERI values (<0.02 microg/kg/week/gHb). More importantly, the ratio of non-acetyl acylcarnitines/total carnitine was significantly positively correlated with ERI (P = 0.0062).

CONCLUSIONS

These data illustrate the relationship between carnitine levels and response to rHuEPO treatment in haemodialysis patients, in particular, the importance of the proportion of long-chain acylcarnitines within the plasma carnitine pool. This proportion may be more indicative of the response to L-carnitine supplementation than absolute L-carnitine levels alone.

L-carnitine supplementation in the dialysis population: are Australian patients missing out?

It has been widely established that patients with end-stage renal disease undergoing chronic haemodialysis therapy exhibit low endogenous levels of L-carnitine and elevated acylcarnitine levels; however, the clinical implication of this altered carnitine profile is not as clear. It has been suggested that these disturbances in carnitine homeostasis may be associated with a number of clinical problems common in this patient population, including erythropoietin-resistant anaemia, cardiac dysfunction, and dialytic complications such as hypotension, cramps and fatigue. In January 2003, the Centers for Medicare and Medicaid Services (USA) implemented coverage of intravenous L-carnitine for the treatment of erythropoietin-resistant anaemia and/or intradialytic hypotension in patients with low endogenous L-carnitine concentrations. It has been estimated that in the period of 1998-2003, 3.8-7.2% of all haemodialysis patients in the USA received at least one dose of L-carnitine, with 2.7-5.2% of patients receiving at least 3 months of supplementation for one or both of these conditions. The use of L-carnitine within Australia is virtually non-existent, which leads us to the question: Are Australian haemodialysis patients missing out? This review examines the previous research associated with L-carnitine administration to chronic dialysis patients for the treatment of anaemia, cardiac dysfunction, dyslipidaemia and/or dialytic symptoms, and discusses whether supplementation is warranted within the Australian setting.

Cellular properties of human erythrocytes preserved in saline-adenine-glucose-mannitol in the presence of L-carnitine

L-Carnitine (LC) in the preservation medium during storage of red blood cells (RBC) can improve the mean 24-hr percent recovery in vivo and increase RBC life-span after reinfusion. The purpose of the study was to investigate the differences in the biochemical properties of RBCs stored in the presence or absence of LC, and the cell-age related responses to storage conditions and to LC. RBC concentrates in saline-adenine-glucose-mannitol (SAG-M) were stored in the presence or absence of 5 mM LC at 4 degrees C for up to 8 weeks. RBC subpopulations of different densities were prepared by centrifugation on Stractan density gradient. Cells were sampled at 0, 3, 6, and 8 weeks, and hematological and cellular properties analyzed (MCV, MCHC, 4.1a/4.1b ratio as a cell age parameter, intracellular Na(+) and K(+)). After 6 weeks, MCV of RBC stored in the presence of LC was lower than that of controls (6 weeks MCV: controls 95.4 +/- 1.8 fl; LC 91.5 +/- 2.0 fl; n = 6; P < 0.005). This was due to swelling of control cells, and affected mainly older RBCs. LC appeared to reduce or retard cell swelling. Among the osmotically active substances whose changes during storage could contribute to cell swelling, only intracellular Na(+) and K(+) differed between stored control RBCs and LC-treated cells. LC reduces the swelling of older cells during storage at 4 degrees C in SAG-M, possibly by acting on the permeability of cell membrane to monovalent cations.

Long-chain acylcarnitine induces Ca2+ efflux from the sarcoplasmic reticulum

Long-chain acylcarnitines increase intracellular Ca2+ (Ca2+i) and induce electrophysiologic alterations that likely contribute to the genesis of malignant ventricular arrhythmias induced during myocardial ischemia. The mechanisms by which long-chain acylcarnitines increase Ca2+i are not known, although it occurs in the presence of Ca2+ channel blockade and inhibition of Na+/Ca2+ exchange. Long-chain acylcarnitines activate Ca2+ release channels from skeletal muscle sarcoplasmic reticulum (SR), but their effect on cardiac SR is unclear. To test the hypothesis that long-chain acylcarnitines increase Ca2+i from the SR, SR-enriched membrane fractions were prepared from rabbit left ventricular myocardium using sucrose density-gradient centrifugation and characterized by marker enzyme analysis. 45Ca2+ efflux was assessed in the presence or absence of long-chain acylcarnitines. Palmitoylcarnitine and stearoylcarnitine produced concentration-dependent efflux of 45Ca2+, whereas shorter chain acylcarnitines, palmitate, and palmitoyl-coenzyme A did not. Pretreatment of cardiac SR vesicles with ryanodine did not prevent palmitoylcarnitine-induced Ca2+ release. In addition, palmitoylcarnitine did not influence specific [3H]ryanodine binding, suggesting a mechanism independent of alterations in ryanodine receptor/Ca2+ release channel binding. In summary, long-chain acylcarnitines enhance Ca2+ release from cardiac SR vesicles and may thereby mobilize Ca2+i to induce electrophysiologic derangements under conditions, such as ischemia, in which these amphiphiles accumulate.

The effect of chronic L-carnitine treatment on blood pressure and plasma lipids in spontaneously hypertensive rats

The effect of chronic L-carnitine treatment on blood pressure and plasma lipids was studied in spontaneously hypertensive rats (SHR). L-Carnitine treatment for 6 weeks lowered significantly both the systolic and mean arterial pressure of SHR but its influence on diastolic and pulse pressure was only modest. L-Carnitine did not influence the relative heart and kidney weight of SHR. However, L-carnitine completely abolished the age-dependent rise of plasma total cholesterol, triglycerides and uric acid seen in untreated SHR. On the other hand, L-carnitine treatment had no significant effects on blood pressure, relative organ weight and plasma lipids in normotensive Wistar-Kyoto rats. Our results suggest that L-carnitine might prevent some cardiovascular alterations by its influence on lipid metabolism.

Carnitine palmitoyl transferase-I activity in the aging mouse heart

We investigated the influence of age on carnitine palmitoyl transferase-I (CPT-I, EC 2.3.1.21) activity in the mouse heart. There was an age-associated decrease in CPT-I activity from 2 to 26 months (P = 0.006). We studied the effect of oxygen-derived radicals on CPT-I activity. Mitochondria from 2-month-old mouse hearts exposed to different concentrations of hydrogen peroxide (H2O2) showed a dose-related decrease in CPT-I activity (P < 0.002). To determine the possible reversibility of the age change in CPT-I activity, we studied the effect of oral administration of propionyl-L-carnitine (PLC). Oral pretreatment of middle-aged (18-month-old) mice with PLC resulted in a 37% increase of basal CPT-I activity (P < 0.05) compared to age-matched untreated animals, and restored it to a level similar to that of 2-month-old mice. Pretreatment of senescent (26-month-old) mice with PLC, however, showed no significant change in basal CPT-I activity. It is possible that the age-related decrease in CPT-I activity may result from an in vivo accumulation of oxygen-derived radical damage. It appears that the age change in CPT-I activity in 18- but not in the 26-month-old mice is reversible with PLC.

Effects of L-carnitine and its acetate and propionate esters on the molecular dynamics of human erythrocyte membrane

EPR and fluorescence probes were used in this study to define the effects of L-carnitine and its short-chain esters, acetyl-L-carnitine and propionyl-L-carnitine, on the natural fluidity gradient and molecular packing of phospholipid headgroups of erythrocyte membrane in intact cells. Purified erythrocyte suspensions, labeled with different stearic acid derivatives containing a stable doxyl radical ring at the C-5, C-7, C-12 and C-16, were incubated with 0.5-5 mM L-carnitine and its esters for 60 min at 37 degrees C and washed twice with an isosmotic buffer. A decrease in the order parameter, calculated from the EPR spectra of the 5-doxylstearic acid derivative, was observed at all the concentrations of propionyl-L-carnitine and the extent of the decrease was dose and temperature dependent. An increase of the chain length between the doxyl ring and the carboxylic group of the spin label, resulted in a much lower efficacy of propionyl-L-carnitine in decreasing the order parameter. Acetyl-L-carnitine also showed a significant effect of decreasing the molecular order but only at the lower temperatures of red cells labeled with 5-doxyl and treated with the highest concentration of the drug. L-Carnitine did not modify the molecular dynamics at all the temperatures and concentrations used in this study. L-Carnitine and its short-chain derivatives did not alter significantly membrane fluidity of deeper regions of the erythrocyte membrane, measured by means of the excimer/monomer fluorescence intensity ratio of pyrene incorporated into the membrane of intact erythrocytes. However, these compounds were all capable of loosening the molecular packing of the polar head of erythrocyte membrane phospholipids evaluated by the membrane binding fluorescence properties of merocyanine-540. The binding of the fluorescent probe decreased in the order propionyl-L-carnitine > acetyl-L-carnitine > L-carnitine. Our findings suggest that this category of compounds affect the molecular dynamics of a membrane bilayer region close to the glycerol backbone of phospholipids, which might be relevant for the expression of membrane functions.

Acetyl-L-carnitine influences the fluidity of brain microsomes and of liposomes made of rat brain microsomal lipid extracts

The fluorescence anisotropy (r) of diphenylhexatriene (DPH) was measured in different preparations (bovine spinal cord phosphatidylserine liposomes, rat brain microsomes, liposomes made with rat brain microsomal lipid having different phospholipid:cholesterol ratios) at temperatures ranging from 10 degrees to 55 degrees C. Phosphatidylserine liposomes exhibited an exponential relationship of r versus temperature, whereas the relationship shown by microsomes and liposomes prepared with microsomal lipid extracts was a linear one. The removal of protein and high phospholipid:cholesterol ratios decreased the slope of the lines (fluidity increased), although the intercept was unaffected. This means that differences were better appreciated at high temperatures and were well evident at 37 degrees C. Acetyl-L-carnitine decreased r in rat brain microsomes and in liposomes made with microsomal lipids with different phospholipid:cholesterol ratios. The fluidifying effect of acetyl-L-carnitine was mild but statistically significant and could explain, at least in part, the data reported in the literature of acetyl-L-carnitine acting on some parameters affected by ageing. Besides, acetyl-L-carnitine seemed to oppose the changes of viscosity due to lipid peroxidation, which has been reported to increase in ageing and dementia. L-carnitine shares the properties of its acetyl ester, but only in part.

The metabolic syndrome and signal transduction of gene expression

Epidemiological studies have clearly shown that the so-called metabolic syndrome which is linked to insulin resistance and a reduced glucose utilization of muscle represents an important risk factor for cardiovascular disease. However, only little is known of the intracellular consequences of insulin resistance. An important feature of an altered substrate utilization is related to signal transduction of gene expression. For the example of myosin heavy chain expression, it is shown that metabolic signals exist which reflect the fuel flux and substrate utilization of the heart muscle cell. The signals were characterized in functional states of the heart associated with altered metabolic influences (fasting, diabetes, sucrose feeding, increased calorie intake, carnitine palmitoyltransferase inhibition). In the pressure-overloaded heart, metabolic interventions which are expected to increase glucose utilization (sucrose feeding, captopril treatment) have a pronounced effect. Although a link with gene expression remains to be established, it should be noted that the sarcoplasmic reticulum Ca(2+)-pump activity seems to be affected in a functionally comparable manner. It is concluded that metabolic signals alter the protein phenotype of heart muscle and it is expected that a deranged signal transduction affects, not only the heart, but also vascular muscle.

Shape changes of the human red cell studied by aqueous two-phase partition

Aqueous two-phase systems have been used to study the human red cell during metabolically induced shape changes. When the discoid character of the cells was lost in favour of echinocytic forms, the partition increased both in charge-sensitive and in charge-insensitive two-phase systems. Reversal of the shape transformation by ATP repletion not only led to shape recovery but also restored the initial partition. Therefore it is apparent that red cells exhibit a shape-dependent partition behaviour. As the partition is dependent on surface properties (such as charge and hydrophobicity) of the partitioned material, the results show that the shape changes caused rearrangement of the membrane and thereby exposure of or greater accessibility of binding groups on the cell surface. The similar partition behaviour in the charge-sensitive and charge-insensitive phase systems show that the increased partition was caused mainly by increased hydrophobic interactions between the cells and the upper phase. The observed partition behaviour therefore suggests that the echinocytic cells acquire a higher affinity for the upper phase by repacking the lipid bilayer or at least the outer leaflet into a less efficient packed and thus more fluid structure.

Effect of L-carnitine and acetyl-L-carnitine on the human erythrocyte membrane stability and deformability

In this study we examined the effect of carnitine and acetylcarnitine on the human erythrocyte membrane stability and membrane deformability. Since erythrocyte membranes are impermeable to these compounds, we resealed erythrocyte ghosts in the presence of different concentrations of carnitine or acetylcarnitine. Resealed ghosts can be adequately studied in their cellular deformability and membrane stability properties by means of ektacytometry. Both carnitine and acetylcarnitine alter the membrane stability but not membrane deformability of the red cell membrane. Resealed ghosts containing 20, 50, 150, and 300 microM carnitine had 1.1, 1.6, 0.9, and 0.7 times the normal stability. While resealed ghosts containing 20, 50, 150, and 300 microM acetylcarnitine had 1.1, 1.5, 1.3, and 1.2 times the normal stability. Such changes were found to be reversible. We also conducted SDS PAGE of cytoskeletal membrane proteins from membrane fragments and residual membranes produced during membrane stability analysis, and unsheared resealed membranes in those samples where we observed an increase or a decrease of membrane stability. No changes in the cytoskeletal membrane proteins were noticed, even when the samples, prior SDS PAGE analysis, were treated with or without dithiothreitol. In addition, fluorescence steady state anisotropy of DPH in the erythrocyte membrane treated with carnitine or acetylcarnitine shows no modification of the lipid order parameter. Our results would suggest that both carnitine and its acetyl-ester, at physiological concentrations, may increase membrane stability in mature erythrocytes, most likely via a specific interaction with one or more cytoskeletal proteins, and that this effect would manifest when the erythrocytes are subjected to high shear stress.