Carnitine Reabsorption Research Articles

Multiomics reveals multilevel control of renal and systemic metabolism by the renal tubular circadian clock.

Circadian rhythmicity in renal function suggests rhythmic adaptations in renal metabolism. To decipher the role of the circadian clock in renal metabolism, we studied diurnal changes in renal metabolic pathways using integrated transcriptomic, proteomic, and metabolomic analysis performed on control mice and mice with an inducible deletion of the circadian clock regulator Bmal1 in the renal tubule (cKOt). With this unique resource, we demonstrated that approximately 30% of RNAs, approximately 20% of proteins, and approximately 20% of metabolites are rhythmic in the kidneys of control mice. Several key metabolic pathways, including NAD+ biosynthesis, fatty acid transport, carnitine shuttle, and β-oxidation, displayed impairments in kidneys of cKOt mice, resulting in perturbed mitochondrial activity. Carnitine reabsorption from primary urine was one of the most affected processes with an approximately 50% reduction in plasma carnitine levels and a parallel systemic decrease in tissue carnitine content. This suggests that the circadian clock in the renal tubule controls both kidney and systemic physiology.

Carnitine Inborn Errors of Metabolism.

Carnitine plays essential roles in intermediary metabolism. In non-vegetarians, most of carnitine sources (~75%) are obtained from diet whereas endogenous synthesis accounts for around 25%. Renal carnitine reabsorption along with dietary intake and endogenous production maintain carnitine homeostasis. The precursors for carnitine biosynthesis are lysine and methionine. The biosynthetic pathway involves four enzymes: 6-N-trimethyllysine dioxygenase (TMLD), 3-hydroxy-6-N-trimethyllysine aldolase (HTMLA), 4-N-trimethylaminobutyraldehyde dehydrogenase (TMABADH), and γ-butyrobetaine dioxygenase (BBD). OCTN2 (organic cation/carnitine transporter novel type 2) transports carnitine into the cells. One of the major functions of carnitine is shuttling long-chain fatty acids across the mitochondrial membrane from the cytosol into the mitochondrial matrix for β-oxidation. This transport is achieved by mitochondrial carnitine–acylcarnitine cycle, which consists of three enzymes: carnitine palmitoyltransferase I (CPT I), carnitine-acylcarnitine translocase (CACT), and carnitine palmitoyltransferase II (CPT II). Carnitine inborn errors of metabolism could result from defects in carnitine biosynthesis, carnitine transport, or mitochondrial carnitine–acylcarnitine cycle. The presentation of these disorders is variable but common findings include hypoketotic hypoglycemia, cardio(myopathy), and liver disease. In this review, the metabolism and homeostasis of carnitine are discussed. Then we present details of different inborn errors of carnitine metabolism, including clinical presentation, diagnosis, and treatment options. At the end, we discuss some of the causes of secondary carnitine deficiency.

Repeated Combined Chemotherapy with Cisplatin Lowers Carnitine Levels in Gastric Cancer Patients

Background/Aims: Carnitine plays an important role in the metabolism of fatty acids. It has also been reported that the administration of anticancer drugs may lead to reductions in serum carnitine levels due to decreased activity of organic cation transporter novel 2, which plays a role in the reabsorption of carnitine in the tubules of the kidney. We therefore studied the change in carnitine levels when chemotherapy was administered repeatedly to patients with gastric cancer. Methods: Ten patients with upper gastrointestinal cancer were enrolled in this study between December 2014 and August 2015. All patients were administered chemotherapy consisting of TS-1 and cisplatin every 3 weeks: 3 received it as adjuvant therapy post resection, the remaining 7 received it as treatment for unresectable tumors. Before the start of each chemotherapy cycle, serum was collected. Results: The mean total carnitine level was 54.5 ± 13.7 μmol/L prior to commencing chemotherapy; it was 46.7 ± 13.5 and 41.4 ± 14.8 μmol/L at the second and third cycles respectively. The total carnitine level was decreased in a statistically significant manner (p = 0.0039). The serum level of total protein and cholinesterase was also decreased significantly (p = 0.0218 and p = 0.0418). Conclusion: Carnitine levels decreased during repeated chemotherapy in patients with gastric cancer, and they are associated with the nutritional status.

Impaired Exercise Performance and Skeletal Muscle Mitochondrial Function in Rats with Secondary Carnitine Deficiency.

Purpose: The effects of carnitine depletion upon exercise performance and skeletal muscle mitochondrial function remain largely unexplored. We therefore investigated the effect of N-trimethyl-hydrazine-3-propionate (THP), a carnitine analog inhibiting carnitine biosynthesis and renal carnitine reabsorption, on physical performance and skeletal muscle mitochondrial function in rats.Methods: Male Sprague Dawley rats were treated daily with water (control rats; n = 12) or with 20 mg/100 g body weight THP (n = 12) via oral gavage for 3 weeks. Following treatment, half of the animals of each group performed an exercise test until exhaustion.Results: Distance covered and exercise performance were lower in THP-treated compared to control rats. In the oxidative soleus muscle, carnitine depletion caused atrophy (–24%) and impaired function of complex II and IV of the mitochondrial electron transport chain. The free radical leak (ROS production relative to oxygen consumption) was increased and the cellular glutathione pool decreased. Moreover, mRNA expression of markers of mitochondrial biogenesis and mitochondrial DNA were decreased in THP-treated compared to control rats. In comparison, in the glycolytic gastrocnemius muscle, carnitine depletion was associated with impaired function of complex IV and increased free radical leak, whilst muscle weight and cellular glutathione pool were maintained. Markers of mitochondrial proliferation and mitochondrial DNA were unaffected.Conclusions: Carnitine deficiency is associated with impaired exercise capacity in rats treated with THP. THP-induced carnitine deficiency is associated with impaired function of the electron transport chain in oxidative and glycolytic muscle as well as with atrophy and decreased mitochondrial DNA in oxidative muscle.

Disorders of carnitine biosynthesis and transport.

Carnitine is a hydrophilic quaternary amine that plays a number of essential roles in metabolism with the main function being the transport of long-chain fatty acids from the cytosol to the mitochondrial matrix for β-oxidation. Carnitine can be endogenously synthesized. However, only a small fraction of carnitine is obtained endogenously while the majority is obtained from diet, mainly animal products. Carnitine is not metabolized and is excreted in urine. Carnitine homeostasis is regulated by efficient renal reabsorption that maintains carnitine levels within the normal range despite variabilities in dietary intake. Diseases occurring due to primary defects in carnitine metabolism and homeostasis are comprised in two groups: disorders of carnitine biosynthesis and carnitine transport defect. While the hallmark of carnitine transport defect is profound carnitine depletion, disorders of carnitine biosynthesis do not cause carnitine deficiency due to the fact that both carnitine obtained from diet and efficient renal carnitine reabsorption can maintain normal carnitine levels with the absence of endogenously synthesized carnitine. Carnitine transport defect phenotype encompasses a broad clinical spectrum including metabolic decompensation in infancy, cardiomyopathy in childhood, fatigability in adulthood, or absence of symptoms. The phenotypes associated with the carnitine transport defect result from the unavailability of enough carnitine to perform its functions particularly in fatty acid β-oxidation. Carnitine biosynthetic defects have been recently described and the phenotypic consequences of these defects are still emerging. Although these defects do not result in carnitine deficiency, they still could be associated with pathological phenotypes due to excess or deficiency of intermediate metabolites in the carnitine biosynthetic pathway and potential carnitine deficiency in early stages of life when brain and other organs develop. In addition to these two groups of primary carnitine defects, several metabolic diseases and medical conditions can result in excessive carnitine loss leading to a secondary carnitine deficiency.

Transcriptional regulation of the human, porcine and bovine OCTN2 gene by PPARα via a conserved PPRE located in intron 1.

BackgroundThe novel organic cation transporter 2 (OCTN2) is the physiologically most important carnitine transporter in tissues and is responsible for carnitine absorption in the intestine, carnitine reabsorption in the kidney and distribution of carnitine between tissues. Genetic studies clearly demonstrated that the mouse OCTN2 gene is directly regulated by peroxisome proliferator-activated receptor α (PPARα). Despite its well conserved role as an important regulator of lipid catabolism in general, the specific genes under control of PPARα within each lipid metabolic pathway were shown to differ between species and it is currently unknown whether the OCTN2 gene is also a PPARα target gene in pig, cattle, and human. In the present study we examined the hypothesis that the porcine, bovine, and human OCTN2 gene are also PPARα target genes.ResultsUsing positional cloning and reporter gene assays we identified a functional PPRE, each in the intron 1 of the porcine, bovine, and human OCTN2 gene. Gel shift assay confirmed binding of PPARα to this PPRE in the porcine, bovine, and the human OCTN2 gene.ConclusionsThe results of the present study show that the porcine, bovine, and human OCTN2 gene, like the mouse OCTN2 gene, is directly regulated by PPARα. This suggests that regulation of genes involved in carnitine uptake by PPARα is highly conserved across species.

A Potential Biomarker for Acute Kidney Injury in Preterm Infants from Metabolic Profiling.

BackgroundCurrently used biomarkers for acute kidney injury (AKI), namely Ngal, SCr, and BUN, are inadequate for timely detection of AKI in preterm infants.MethodsNuclear magnetic resonance (NMR) spectroscopy-based metabolic profiling was conducted on urines from 20 preterm infants to determine if novel metabolic biomarkers could be identified for early detection of AKI. Urines were collected from every patient each day for the first 14 days of life. NMR spectra were measured for all urines and metabolic profiling analysis conducted.ResultsOne metabolite, carnitine, increased significantly in urines of three extremely low birth weight (ELBW) infants starting on day five of life. The three affected infants either received prolonged antibiotic treatment, extended treatment with indomethacin, or both. One ELBW patient who received both treatments and reached the highest urinary carnitine level died on day 10 of life due to localized gut perforation complicated by suspected AKI.ConclusionsIt was concluded that carnitine increased in the three neonates in part due to antibiotic- and/or indomethacin-induced AKI. It is hypothesized that combined antibiotic and indomethacin treatment promoted AKI resulting in reduced proximal renal tubule reabsorption of carnitine and that β-lactam antibiotics blocked renal carnitine uptake by human organic cation transporter, hOCTN2.

The effects of exercise training on γ-butyrobetaine hydroxylase and novel organic cation transporter-2 gene expression in the rat

The concentration of carnitine in plasma is generally increased with exercise training, suggesting that either carnitine biosynthesis is stimulated or renal reabsorption of carnitine is enhanced, or both. Carnitine, an essential cofactor in the oxidation of fatty acids, is released into the plasma following hydroxylation by γ-butyrobetaine hydroxylase (BBH), the final enzyme in the biosynthetic pathway found primarily in the liver. The organic cation transporter (OCTN2), the carnitine transporter found in kidney, is important in the distribution of carnitine by facilitating its renal reabsorption from urine. In this study, we tested the hypothesis that exercise training increases gene and protein expression of BBH and OCTN2, resulting in enhanced plasma carnitine levels. Male Wistar rats were subjected to 2 daily exercise sessions of treadmill running, 5days per week, for a 10-week period. The concentration of total carnitine in plasma was significantly increased in trained rats compared with sedentary rats. In trained rats, mRNA and protein expression of BBH were increased in liver, whereas only BBH mRNA expression was increased in kidney. Liver of trained rats demonstrated increased mRNA and protein expression of OCTN2 compared with sedentary rats. In kidney of trained rats, however, only an increase in mRNA expression of OCTN2 was observed. Our results suggest that the improved plasma carnitine status in the trained rat is associated with increased carnitine biosynthesis in liver and kidney. The observation that OCTN2 expression was increased in kidney suggests a potential role of the kidney in the reabsorption of carnitine from the urine.

The Carnitine Transporter Network: Interactions with Drugs

Carnitine homeostasis has a pivotal role in the life of mammals. It is realized by the carnitine system, which consists of networks of enzymes and membrane transporters and plays an essential role in functions such as the regulation of the CoA/acyl-CoA ratio, the supply of substrates for the β-oxidation to mitochondria and peroxisomes and of acyl units for VLDL assembly to the ER, the efflux of acetyl groups from mitochondria during glucose metabolism and the detoxification of the organism. The network of the transporters plays a crucial role in maintaining homeostasis since it allows the absorption, excretion and re-absorption of carnitine and carnitine derivatives as well as the flux of these metabolites through different tissues and within sub-cellular compartments. Several transport systems which were thought to be involved in the network have been identified and characterized to a certain extent. These are the plasma membrane transporters OCTN1, 2 and 3, the mitochondrial CACT and the carnitine transport system of endoplasmic reticulum (ERCT). These transporters have been functionally characterized by studies in eukaryotic cell systems and/or in reconstituted liposomes. Interestingly, it was found that some commonly used drugs interact with different carnitine transporters, causing alterations of the transport function by displacing the substrate from the binding site or by irreversibly inactivating the transporters. These interactions will cause derangements of the carnitine homeostasis. The current knowledge of the characterization of the carnitine transporter network and the interaction with drugs are reviewed with emphasis to the most recent data obtained using the proteoliposome reconstituted systems. Keywords: Carnitine, transporter, homeostasis, drugs

IMPROVED CARNITINE STATUS IN THE TRAINED RAT IS ASSOCIATED WITH INCREASED mRNA EXPRESSION OF LIVER γ‐BBH AND KIDNEY OCTN2

Endurance training alters the oxidative capacity of tissues to favor the oxidation of fatty acids. In response to training, carnitine concentrations in plasma are increased suggesting that either carnitine biosynthesis is stimulated or renal uptake of carnitine from the urine is enhanced, or both. Carnitine is essential in the oxidation of fatty acids by permitting their transport into the mitochondria for oxidation. Carnitine is released into the circulation following its hydroxylation by γ‐butyrobetaine hydroxylase (BBH), the final enzyme in the biosynthetic pathway in liver. The organic cation transporter (OCTN2) is also important in the distribution of carnitine by facilitating renal reabsorption of carnitine from the urine. To investigate whether these proteins are implicated in carnitine homeostasis in response to exercise, BBH and OCTN2 expression were determined following 10 weeks of treadmill exercise training in male Wistar rats. Exercise training significantly increased the level of total carnitine in plasma, whereas in liver the level of total carnitine (sum of free, short‐chain acylcarnitine and long‐chain acylcarnitine) was not modified by exercise. In exercise‐trained rats, BBH expression in liver was significantly increased compared to sedentary rats, whereas in kidney a significant decrease was observed. OCNT2 expression in both liver and kidney were significant increased in exercise‐trained rats. Our results indicate that the improved plasma carnitine status in the trained rat can be explained, in part, by increases in both carnitine biosynthesis and renal reabsorption of carnitine.

The plasma carnitine concentration regulates renal OCTN2 expression and carnitine transport in rats

Previous findings in rats and in human vegetarians suggest that the plasma carnitine concentration and/or carnitine ingestion may influence the renal reabsorption of carnitine. We tested this hypothesis in rats with secondary carnitine deficiency following treatment with N-trimethyl-hydrazine-3-propionate (THP) for 2 weeks and rats treated with excess l-carnitine for 2 weeks. Compared to untreated control rats, treatment with THP was associated with an approximately 70% decrease in plasma carnitine and with a 74% decrease in the skeletal muscle carnitine content. In contrast, treatment with l-carnitine increased plasma carnitine levels by 80% and the skeletal muscle carnitine content by 50%. Treatment with l-carnitine affected neither the activity of carnitine transport into isolated renal brush border membrane vesicles, nor renal mRNA expression of the carnitine transporter OCTN2. In contrast, in carnitine deficient rats, carnitine transport into isolated brush border membrane vesicles was increased 1.9-fold compared to untreated control rats. Similarly, renal mRNA expression of OCTN2 increased by a factor of 1.7 in carnitine deficient rats, whereas OCTN2 mRNA expression remained unchanged in gut, liver or skeletal muscle. Our study supports the hypothesis that a decrease in the carnitine plasma and/or glomerular filtrate concentration increases renal expression and activity of OCTN2.

Interaction between pivaloylcarnitine and l-carnitine transport into L6 cells overexpressing hOCTN2

Patients ingesting pivalic acid containing prodrugs develop hypocarnitinemia. Pivalic acid is cleaved from such drugs and excreted renally as pivaloylcarnitine. Plasma concentrations (reflecting the concentration in the glomerular filtrate entering the proxinmal tubule) in patients treated with cefditoren pivoxil are approximately 5 μM for pivaloylcarnitine and 10 μM for carnitine. Kinetic studies were performed using L6 cells overexpressing the human kidney carnitine transporter (hOCTN2) to assess the mechanisms leading to hypocarnitinemia in such patients. l-carnitine transport showed saturation kinetics ( K m 6.3 μM) and could be inhibited competitively by pivaloylcarnitine ( K i 70 μM). Pivaloylcarnitine was also transported by OCTN2 ( K m 212 μM) and its transport could be inhibited competitively by l-carnitine ( K i 7.8 μM). Haldane and Eadie-Hofstee plots were linear for both carnitine and pivaloylcarnitine. Our data indicate that both carnitine and pivaloylcarnitine bind to OCTN2 at a single, identical site. Considering the low plasma and tubular pivaloylcarnitine concentration, the high K m of pivaloylcarnitine regarding OCTN2 and the inhibition of pivaloylcarnitine transport by carnitine, pivaloylcarnitine is unlikely to be reabsorbed under these conditions. On the other hand, our data indicate that the renal reabsorption of carnitine is not impaired in patients treated with pivalic acid containing prodrugs. Hypocarnitinemia in such patients therefore develops due to massive renal losses of pivaloylcarnitine and not due to inhibition of carnitine reabsorption by pivaloylcarnitine.

Carnitine/Organic Cation Transporter OCTN2 (Slc22a5) Is Responsible for Renal Secretion of Cephaloridine in Mice

Carnitine/organic cation transporter (OCTN) 2 (SLC22A5) plays a pivotal role in renal tubular reabsorption of carnitine, a vitamin-like compound, on apical membranes of proximal tubules, but its role in relation to therapeutic drugs remains to be clarified. The purpose of the present study was to elucidate the involvement of OCTN2 in renal disposition of a beta-lactam antibiotic, cephaloridine (CER), based on experiments with juvenile visceral steatosis (jvs) mice, which have a functional deficiency of the octn2 gene. Renal clearance of CER during constant intravenous infusion in wild-type mice was much higher than could be accounted for by glomerular filtration, but was decreased by increasing the infusion rate with minimal change in kidney-to-plasma concentration ratio, suggesting the existence of saturable transport mechanism(s) across the apical membranes. The plasma concentration profile and kidney-to-plasma concentration ratio after intravenous injection in jvs mice were higher than those in wild-type mice, whereas renal clearance in jvs mice was much lower than that in wild-type mice and could be accounted for by glomerular filtration. Uptake of CER by mouse OCTN2 was shown in Xenopus laevis oocytes expressing mouse OCTN2. The CER transport by OCTN2 exhibited saturation with K(m) of approximately 3 mM, which is similar to the renal CER concentration exhibiting saturation in renal clearance in vivo. The OCTN2-mediated CER transport was inhibited by carnitine and independent of Na(+) replacement in the medium. These results show OCTN2 on apical membranes of proximal tubules plays a major role in renal secretion of CER in mice.

Toxicity of Valproic Acid in Mice with Decreased Plasma and Tissue Carnitine Stores

The aim of this study was to investigate whether a decrease in carnitine body stores is a risk factor for valproic acid (VPA)-associated hepatotoxicity and to explore the effects of VPA on carnitine homeostasis in mice with decreased carnitine body stores. Therefore, heterozygous juvenile visceral steatosis (jvs)(+/-) mice, an animal model with decreased carnitine stores caused by impaired renal reabsorption of carnitine, and the corresponding wild-type mice were treated with subtoxic oral doses of VPA (0.1 g/g b.wt./day) for 2 weeks. In jvs(+/-) mice, but not in wild-type mice, treatment with VPA was associated with the increased plasma activity of aspartate aminotransferase and alkaline phosphatase. Furthermore, jvs(+/-) mice revealed reduced palmitate metabolism assessed in vivo and microvesicular steatosis of the liver. The creatine kinase activity was not affected by treatment with VPA. In liver mitochondria isolated from mice that were treated with VPA, oxidative metabolism of l-glutamate, succinate, and palmitate, as well as beta-oxidation of palmitate, were decreased compared to vehicle-treated wild-type mice or jvs(+/-) mice. In comparison to vehicle-treated wild-type mice, vehicle-treated jvs(+/-) mice had decreased carnitine plasma and tissue levels. Treatment with VPA was associated with an additional decrease in carnitine plasma (wild-type mice and jvs(+/-) mice) and tissue levels (jvs(+/-) mice) and a shift of the carnitine pools toward short-chain acylcarnitines. We conclude that jvs(+/-) mice reveal a more accentuated hepatic toxicity by VPA than the corresponding wild-type mice. Therefore, decreased carnitine body stores can be regarded as a risk factor for hepatotoxicity associated with VPA.

Pressure Overload-Induced Cardiomyopathy in Heterozygous Carrier Mice of Carnitine Transporter Gene Mutation

Primary systemic carnitine deficiency is an autosomal recessive disorder caused by a decreased renal reabsorption of carnitine because of mutations of the carnitine transporter OCTN2 gene, and hypertrophic cardiomyopathy is a common clinical feature of homozygotes. Although heterozygotes for OCTN2 mutations are generally healthy with normal cardiac performance, heterozygotes may be at risk for cardiomyopathy in the presence of additional risk factors, such as hypertension. To test this hypothesis, we investigated the effects of surgically induced pressure overload on the hearts of heterozygous mutants of a murine model of OCTN2 mutation, juvenile visceral steatosis mouse (jvs/+). Eleven-week-old jvs/+ mice and age-matched wild-type mice were used. At baseline, there were no differences in physical characteristics between wild-type and jvs/+ mice. However, plasma and myocardial total carnitine levels in jvs/+ mice were lower than in wild-type mice. Both wild-type and jvs/+ mice were subjected to ascending aortic constriction with or without 1% l-carnitine supplementation for 4 weeks. At 4 weeks after ascending aortic constriction, jvs/+ mice showed an exaggeration of cardiac hypertrophy and pulmonary congestion, further increased gene expression of atrial natriuretic peptide in the left ventricles, further deterioration of left ventricular fractional shortening, reduced myocardial phosphocreatine:adenosine triphosphate ratio, and increased mortality compared with wild-type mice; l-carnitine supplementation prevented these changes in jvs/+ mice subjected to ascending aortic constriction. In conclusion, cardiomyopathy and heart failure with energy depletion may be induced by pressure overload in heterozygotes for OCTN2 mutations and could be prevented by l-carnitine supplementation.

Increased susceptibility to heart failure in heterozygous carrier mice of carnitine transporter gene mutation

Systemic carnitine deficiency (SCD) is an autosomal recessive disorder caused by a decreased renal re-absorption of carnitine due to mutations of the carnitine transporter OCTN2 gene, and hypertrophic cardiomyopathy is a common clinical feature of homozygotes. Although heterozygotes of SCD are generally healthy with normal cardiac performance, heterozygotes may be at risk for cardiomyopathy in the presence of additional risk factors such as hypertension. Eleven-week-old heterozygous mutant (jvs/+) mice and age-matched wild-type (WT) mice were used.

OCTN2-Mediated Carnitine Uptake in a Newly Discovered Human Proximal Tubule Cell Line (Caki-1)

The proximal tubular reabsorption of carnitine in the human kidney is significant because more than 95% of the carnitine filtered in the kidney is reabsorbed by the proximal tubules therefore maintaining the homeostatic balance of carnitine in the body. Objectives of this study include the characterization of OCTN2 function in the Caki-1 cell line and the potential interactions of carnitine uptake with renally secreted drugs, including drugs of quaternary ammonium structure. Caki-1 cells were additionally characterized to be of proximal tubule nature, and an apical membrane expression pattern of OCTN2 in Caki-1 cells was discovered. Uptake studies with radiolabeled L-carnitine in Caki-1 cells revealed a Na+- and temperature-dependent carrier-mediated process (K(m) = 15.90 microM) which was unaffected by pH in a range from 6.5 to 8.5. All drugs tested were able to inhibit the carnitine uptake process to different degrees. The quaternary ammonium compounds ciclotropium bromide and ipratropium bromide were strong inhibitors with IC(50) values of 30 microM and 95 microM, respectively. The observed kinetics, immunohistolocalization, and inhibition studies indicate that the high-affinity uptake of carnitine in the Caki-1 cell line is most likely mediated by OCTN2. The interaction of drugs at the renal level with OCTN2 indicates a possible pathway for the final step of cationic secretion into the urine.

Urinary excretion of l-carnitine and its short-chain acetyl-l-carnitine in patients undergoing carboplatin treatment

To evaluate the effect of the anti-cancer drug carboplatin on plasma concentrations and urinary excretion of L-carnitine (LC) and its main ester, acetyl-L-carnitine (ALC), in cancer patients. Plasma and urine concentrations of LC and ALC from 11 patients on carboplatin therapy (1 h intravenous infusion; AUC dose 4.8 +/- 1.1 mg/ml min) in combination with docetaxel, paclitaxel or vinorelbine, were determined by high-performance liquid chromatography with fluorimetric detection. Before carboplatin therapy, the mean +/- SD plasma concentrations of LC and ALC were 47.8 +/- 10.9 and 7.04 +/- 1.04 nmoles/ml, respectively, and remained constant throughout the entire study period. In contrast, urinary excretion of LC and ALC, increased significantly during the chemotherapy from 115 +/- 105 to 480 +/- 348 micromoles/day (P < 0.01) and from 41 +/- 41 to 89 +/- 52 micromoles/day (P < 0.05) for LC and ALC, respectively, subsequently reverting to normal 6 days after the end of chemotherapy. Similarly, the renal clearance of LC and ALC increased substantially during chemotherapy from 1.67 +/- 1.43 to 9.05 +/- 9.52 ml/min (P < 0.05) and from 4.02 +/- 4.51 to 7.97 +/- 5.05 ml/min (P = not significant) for LC and ALC, respectively, reverting to normal 6 days after the end of chemotherapy. Plasma concentrations and urinary excretion of glucose, phosphate and urea nitrogen and creatinine clearance, however, were not affected by carboplatin therapy, indicating no impaired kidney function. Treatment with carboplatin was associated with a marked urinary loss of LC and ALC, most likely due to inhibition of carnitine reabsorption in the kidney.

Carnitine and Fatty Acid Oxidation in Sepsis

Fatty acid oxidation is usually thought of as being a preferred ATP source during sepsis, as shown by the decrease in respiratory quotient in septic patients and animals. However, fatty acid oxidation may be impaired relative to circulating triglyceride levels, resulting in increased cycling between triglycerides and non-esterified fatty acids. The rate of fatty acid oxidation is critically dependent on the stage of sepsis. Carnitine may be depleted during sepsis by a combination of muscle wasting, erythrocyte haemolysis, decreased tissue uptake, reduced kidney reabsorption of free carnitine, and increased excretion of acyl-carnitines. Whether there may be any beneficial effects of carnitine supplementation during sepsis, is, as yet, uncertain.

Genetic Polymorphism of Organic Anion and Cation Transporters: Pharmacokinetic and Pharmacodynamic Consequences in Pharmacotherapy

It has been suggested that genetic polymorphisms in drug transporters as well as drug-metabolizing enzymes are associated with interindividual differences in drug disposition, efficacy, and toxicity and in disease. Organic anion and cation transporters are expressed in the selective or multiple tissues such as small intestine, liver and kidney, and mediate the transport of many clinically useful drugs. Polymorphisms of drug transporter genes have recently been identified and demonstrated to have functional significance for transporter activity and expression. For example, genetic variants in the OATP-C (SLC21A6) gene are associated with alterations in pravastatin and rifampin uptake into liver. In addition, homozygotes for OCTN2 (SLC22A5) mutant alleles cause systemic carnitine deficiency because of a disruption of carnitine reabsorption in the kidney. Since a growing number of preclinical and clinical studies have demonstrated that the polymorphisms of various drug transporter genes may be responsible for overall outcomes of pharmacokinetics and pharmacotherapy of certain drugs, further understanding of the physiology and biochemistry of drug transporters with respect to genetic variations will be important to establish individualized pharmacotherapy with clinically used drugs. Keywords: anion and cation transporters, genetic polymorphisms, clinically useful drugs, pharmacokinetics, pharmacodynamics