The evolutionary loss of hepatic urate oxidase (uricase) has resulted in humans with elevated serum uric acid (urate). Uricase loss may have been beneficial to early primate survival. However, an elevated serum urate has predisposed man to hyperuricemia, a metabolic disturbance leading to gout, hypertension, and various cardiovascular diseases. Human serum urate levels are largely determined by urate reabsorption and secretion in the kidney. Renal urate reabsorption is controlled via two proximal tubular urate transporters: apical URAT1 (SLC22A12) and basolateral URATv1/GLUT9 (SLC2A9). In contrast, the molecular mechanism(s) for renal urate secretion remain unknown. In this report, we demonstrate that an orphan transporter hNPT4 (human sodium phosphate transporter 4; SLC17A3) was a multispecific organic anion efflux transporter expressed in the kidneys and liver. hNPT4 was localized at the apical side of renal tubules and functioned as a voltage-driven urate transporter. Furthermore, loop diuretics, such as furosemide and bumetanide, substantially interacted with hNPT4. Thus, this protein is likely to act as a common secretion route for both drugs and may play an important role in diuretics-induced hyperuricemia. The in vivo role of hNPT4 was suggested by two hyperuricemia patients with missense mutations in SLC17A3. These mutated versions of hNPT4 exhibited reduced urate efflux when they were expressed in Xenopus oocytes. Our findings will complete a model of urate secretion in the renal tubular cell, where intracellular urate taken up via OAT1 and/or OAT3 from the blood exits from the cell into the lumen via hNPT4.Urate is the end product of purine metabolism in humans and certain primates as a result of uricase genetic loss (urate oxidase degrades urate to allantoin) (1). Two independent nonsense mutations in this gene, found in human, chimpanzee, and orangutan but not in the gibbon, indicate that this loss had evolutionary advantages for early primates (2). Because urate has powerful antioxidant properties, uricase loss resulting in elevated serum urate may have been beneficial to early primate survival (1). In addition, Watanabe et al. (3) hypothesized that elevated serum urate levels provided a survival advantage by helping to maintain blood pressure under the low salt dietary conditions that prevailed during the middle to late Miocene period. Despite its beneficial role and given the fact that more than half of uricase-deficient mice die from urate nephropathy within 4 weeks of age, elevation in serum urate level produces a burden on the body (4). To circumvent this problem, the human body had to develop a urate excretion system.The kidney plays a dominant role in maintaining serum urate levels (1, 5). Renal urate excretion is a function of the balance between reabsorption and secretion. Recently it was demonstrated that luminal urate is taken up by a urate-anion exchanger (URAT1; SLC22A12) 3 (6) into the renal proximal tubular cell and that intracellular urate exits the cell into the interstitium/blood ...

In chronic renal failure, substances that are effectively excreted in healthy subjects accumulate in serum. These substances, uremic toxins, include a variety of organic acids. It has been reported that a decrease in the bilirubin (BR) binding capacity occurs in the serum of renal failure patients. 3-Carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) has a high affinity for human serum albumin (HSA) and is a potent inhibitor of the serum protein binding of many drugs. We recently reported that CMPF and BR share the binding site for dicarboxylate molecules on the HSA molecule [Pharm Res 1999;16:916–923]. In this study, in order to confirm whether CMPF is involved in the decrease of BR serum binding capacity in chronic renal failure patients, the total concentrations of uremic toxins, CMPF, and indoxyl sulfate (IS) and the free BR concentration in serum from healthy volunteers and renal failure patients were determined. Both total CMPF and IS concentrations correlate with the free BR concentration. However, results from the peroxidase method reveal that IS cannot displace BR under the physiological condition [IS]/[HSA] <1. We, therefore, conclude that CMPF is one of the substances which contribute to the decreased binding capacity of BR in uremic serum.

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We examined different fluorescent probes suitable for fluorometric determination of alpha 1-acid glycoprotein (AGP) in serum. Quinaldine red (QR) was shown to bind strongly and selectively to AGP. Taking advantage of the enhanced fluorescence of QR in the presence of AGP, we developed a direct method for the determination of serum AGP without removal of other serum proteins such as albumin. AGP concentrations in serum of healthy volunteers and patients correlated well with results from the conventional single radial immunodiffusion (SRID) method (r = 0.93, slope = 1). The newly developed method is faster and has a larger analytical concentration range than the SRID method. This method can also be used to determine AGP in serum of experimental animals, and it can serve to monitor AGP serum concentrations for pharmacokinetic evaluation of basic drugs.

A surface electromyogram model has been implemented with both electrical and mechanical outputs. The model has been populated with parameter values based on random distributions about experimentally observed averages. Motor unit firing rates and recruitment threshold have been defined independently for type I and type II muscle fibres. This has led to non-linear inputs to the sEMG model. The observed model outputs have been compared with experimental data. Both the electrical and mechanical outputs are shown to increase linearly with force. The rates of increase are comparable between the simulated and experimental data sets. This model is generating life-like sEMG signals and may be used to study the relationship between muscle physiology and the sEMG signal.

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Spermine is the end-product in the polyamine biosynthetic pathway, and its excess accumulation induces neuroexcitatory responses and neurotoxicity. The purpose of this study was to elucidate the involvement of transport systems at the brain barriers in the clearance of spermine. In vivo rat spermine elimination from brain parenchyma across the blood-brain barrier (BBB) and blood-cerebrospinal fluid (CSF) barrier (BCSFB) was assessed by intracerebral and intracerebroventricular administration techniques, respectively. To characterize spermine transport at the BCSFB, a transport study using rat choroid plexus was performed. After the intracerebral microinjection of [ Spermine is a natural polyamine, and the end-product in the polyamine biosynthetic pathway.1) In the brain, spermine binds to N-methyl-D-aspartate receptors, and is involved in the modulation of learning and memory.2,3) In addition, excess accumulation of spermine induces neuroexcitatory responses and, thus, neurotoxicity. 4) Therefore, it is considered that some mechanisms for control of the cerebral spermine concentration are present in the brain to maintain homeostasis of cerebral function via spermine-related neuro-responses. The precursor of spermine is spermidine, 5) which is synthesized from putrescine and it has been reported that the biosynthesis of spermine from spermidine is superior to that of spermidine from spermine.6-8) Hence, neural spermine transport system(s) could be important for the removal of spermine from the brain, and the modulation of cerebral spermine concentration.Brain parenchyma and cerebrospinal fluid (CSF) are separated from the circulating blood by the blood-brain barrier (BBB) and blood-CSF barrier (BCSFB), respectively. The BBB and BCSFB are formed by brain capillary endothelial and choroid plexus epithelial cells, respectively, and express some transporting molecules which are involved in active elimination of compound from the brain or CSF.9) Regarding the cationic compound elimination system(s) at these barriers, mRNAs of plasma membrane monoamine transporter (PMAT/ solute carrier (SLC)29A4), organic cation/carnitine transporter 1-2 (OCTN1-2/SLC22A4-5), organic cation transporter 1-3 (OCT1-3/SLC22A1-3), and multidrug and toxin extrusion 1-2 (MATE1-2/SLC47A1-2) are expressed in rat BBB and BCSFB cell lines. 10) Among these transporters, PMAT is, at least in part, involved in brain/CSF-to-blood transport of 1-methyl-4-phenylpyridinium (MPP + ) 10) and CSF-to-blood transport of histamine.11) In addition, OCT3 takes part in the efflux transport of creatinine, a cationic guanidino compound.12) Therefore, it is possible that cerebral spermine is eliminated across these brain barriers.Some previous studies have reported that polyamines including spermine are transported in a carrier-mediated manner in some organs and it has been reported that carrier-mediated transport of polyamines is present in mammalian intestine, liver, and kidney. [13][14][15] In addition, we have demonstrated that the rat inner blood-retinal barr...