Pathways of urea transport in the mammalian kidney

Urea is the chief end product of nitrogen metabolism in mammals. Most of the urea produced is excreted by the kidneys. Excretion of a large fraction of the urea filtered by the glomerulus is required to keep pace with the rate of hepatic urea production. Consequently, the maintenance of nitrogen balance depends on a sustained high rate of renal urea excretion. One factor that might challenge the ability of the kidney to maintain a high rate of urea excretion is the need to limit water excretion. Theoretically, an increase in water absorption along the nephron would be expected to increase luminal urea concentration and consequently increase the driving force for passive urea reabsorption. Indeed, as demonstrated originally by Shannon [1] and confirmed in many subsequent studies, the rate of urea excretion decreases when water excretion decreases. However, the decline in urea excretion is relatively modest, even when the rate of water excretion is very low. In Shannon's studies in dogs [1], when water diuresis was established (water excretion 5–10 percent of filtered load), urea excretion was about 60 percent of the filtered load. During antidiuresis, when water excretion was reduced by 95% or more (to less than 0.5 percent of the glomerular filtration rate), urea excretion fell only by 25 to 30% (to 40 to 45 percent of the filtered load)

Mutations in the SLC2A9 Gene Cause Hyperuricosuria and Hyperuricemia in the Dog

Allantoin is the end product of purine catabolism in all mammals except humans, great apes, and one breed of dog, the Dalmatian. Humans and Dalmatian dogs produce uric acid during purine degradation, which leads to elevated levels of uric acid in blood and urine and can result in significant diseases in both species. The defect in Dalmatians results from inefficient transport of uric acid in both the liver and renal proximal tubules. Hyperuricosuria and hyperuricemia (huu) is a simple autosomal recessive trait for which all Dalmatian dogs are homozygous. Therefore, in order to map the locus, an interbreed backcross was used. Linkage mapping localized the huu trait to CFA03, which excluded the obvious urate transporter 1 gene, SLC22A12. Positional cloning placed the locus in a minimal interval of 2.5 Mb with a LOD score of 17.45. A critical interval of 333 kb containing only four genes was homozygous in all Dalmatians. Sequence and expression analyses of the SLC2A9 gene indicated three possible mutations, a missense mutation (G616T;C188F) and two promoter mutations that together appear to reduce the expression levels of one of the isoforms. The missense mutation is associated with hyperuricosuria in the Dalmatian, while the promoter SNPs occur in other unaffected breeds of dog. Verification of the causative nature of these changes was obtained when hyperuricosuric dogs from several other breeds were found to possess the same combination of mutations as found in the Dalmatian. The Dalmatian dog model of hyperuricosuria and hyperuricemia underscores the importance of SLC2A9 for uric acid transport in mammals

Probenecid Blocks Human P2X7 Receptor-Induced Dye Uptake via a Pannexin-1 Independent Mechanism

P2X7 is a ligand-gated ion channel which is activated by ATP and displays secondary permeability characteristics. The mechanism of development of the secondary permeability pathway is currently unclear, although a role for the hemichannel protein pannexin-1 has been suggested. In this study we investigated the role of pannexin-1 in P2X7-induced dye uptake and ATP-induced IL-1β secretion from human monocytes. We found no pharmacological evidence for involvement of pannexin-1 in P2X7-mediated dye uptake in transfected HEK-293 cells with no inhibition seen for carbenoxolone and the pannexin-1 mimetic inhibitory peptide, 10Panx1. However, we found that probenecid inhibited P2X7-induced cationic and anionic dye uptake in stably transfected human P2X7 HEK-293 cells. An IC50 value of 203 μM was calculated for blockade of ATP-induced responses at human P2X7. Probenecid also reduced dye uptake and IL-1β secretion from human CD14+ monocytes whereas carbenoxolone and 10Panx1 showed no inhibitory effect. Patch clamp and calcium indicator experiments revealed that probenecid directly blocks the human P2X7 receptor

High plasma uric acid concentration: causes and consequences

High plasma uric acid (UA) is a precipitating factor for gout and renal calculi as well as a strong risk factor for Metabolic Syndrome and cardiovascular disease. The main causes for higher plasma UA are either lower excretion, higher synthesis or both. Higher waist circumference and the BMI are associated with higher insulin resistance and leptin production, and both reduce uric acid excretion. The synthesis of fatty acids (tryglicerides) in the liver is associated with the de novo synthesis of purine, accelerating UA production. The role played by diet on hyperuricemia has not yet been fully clarified, but high intake of fructose-rich industrialized food and high alcohol intake (particularly beer) seem to influence uricemia. It is not known whether UA would be a causal factor or an antioxidant protective response. Most authors do not consider the UA as a risk factor, but presenting antioxidant function. UA contributes to > 50% of the antioxidant capacity of the blood. There is still no consensus if UA is a protective or a risk factor, however, it seems that acute elevation is a protective factor, whereas chronic elevation a risk for disease

Combination of probenecid-sulphadoxine-pyrimethamine for intermittent preventive treatment in pregnancy

The antifolate sulphadoxine-pyrimethamine (SP) has been used in the intermittent prevention of malaria in pregnancy (IPTp). SP is an ideal choice for IPTp, however, as resistance of Plasmodium falciparum to SP increases, data are accumulating that SP may no longer provide benefit in areas of high-level resistance. Probenecid was initially used as an adjunctive therapy to increase the blood concentration of penicillin; it has since been used to augment concentrations of other drugs, including antifolates. The addition of probenecid has been shown to increase the treatment efficacy of SP against malaria, suggesting that the combination of probenecid plus SP may prolong the useful lifespan of SP as an effective agent for IPTp. Here, the literature on the pharmacokinetics, adverse reactions, interactions and available data on the use of these drugs in pregnancy is reviewed, and the possible utility of an SP-probenecid combination is discussed. This article concludes by calling for further research into this potentially useful combination

Transport of salicylate in proximal tubule (S2 segment) isolated from rabbit kidney

The secretory and reabsorptive transport of salicylate was studied in the isolated and perfused rabbit proximal tubule (S2 segment). Salicylate secretion (Jb----lsal) fulfilled the criteria for a carrier-mediated transport system: Jb----lsal was saturable, was reversibly inhibited by probenecid, and occurred against a concentration gradient. The Km and Vmax for this secretory transport were 80 microM and 3,200 fmol.min-1.mm-1, respectively. At luminal pH of 7.4 and 6.6, salicylate reabsorption (Jl----bsal) was low (100 fmol.min-1.mm-1). Jl----bsal was stimulated by increasing the bath PCO2 or by removing basolateral HCO3-; Jl----bsal was inhibited by ethoxyzolamide and by SITS in the bath. Our results indicate that salicylate reabsorption depends on H+ secretion, consistent with reabsorption by simple nonionic diffusion. When salicylate was present in the lumen only, Jl----bsal increased after inhibition of the secretory transport by adding ouabain or probenecid in the bath or by lowering the bath temperature. These results are compatible with luminal recycling of salicylate, and suggest the presence of a mediated secretory transporter located at the luminal membrane