Inhibition of Renal Metabolism. Relative effects of arsenate on sodium, phosphate, and glucose transport by the rabbit proximal tubule

Protective Effects of Xanthine Derivatives Against Arsenic Trioxide-Induced Oxidative Stress in Mouse Hepatic and Renal Tissues

In this study, the protective efficacy of pentoxifylline (PTX) as a xanthine derivative against arsenic trioxide (ATO)-induced kidney and liver damage in mice was investigated. Thirty-six mice were divided into six groups, receiving intraperitoneal injections of saline, ATO, PTX, or a combination for four weeks. Blood samples were analyzed for serum biochemistry, while hepatic tissue underwent examination for histopathological changes and assessment of oxidative stress markers and antioxidant gene expression through Real-Time PCR. ATO exposure significantly increased serum markers (creatinine, ALT, BUN, ALP, AST) and induced histopathological changes in the liver. Moreover, it elevated renal and hepatic nitric oxide (NO) and lipid peroxidation (LPO) levels, and reduced antioxidant enzyme expression (CAT, GSR, GPx, MPO, SOD), total thiol groups (TTGs), and total antioxidant capacity (TAC). Conversely, PTX treatment effectively lowered serum hepatic and renal markers, improved antioxidant markers, and induced histopathological alterations. Notably, PTX did not significantly affect renal and hepatic NO levels. These findings suggest that PTX offers therapeutic potential in mitigating liver and acute kidney injuries induced by various insults, including exposure to ATO.

Biochemical Investigation of Therapeutic Potential of Resveratrol Against Arsenic Intoxication

Arsenic has been reported to cause damaging effects on different body organs. This study was designed to evaluate the protective effect of resveratrol (RSV) against arsenic trioxide (ATO)–induced intoxication in experimental animals. Twenty-four Wistar rats were allocated in 4 groups: group 1: control group, received normal diet; group 2: received ATO (3 mg/kg); group 3: received RSV (8 mg/kg) 30 minutes before administration of ATO; and group 4: received ascorbic acid (25 mg/kg) 30 minutes before administration of ATO. Treatments were given to experimental rats daily for consecutive 8 days. At the end of experimental period, bioaccumulation of arsenic in liver and kidney was assessed by hydride generation-atomic absorption spectrophotometer to investigate the association of arsenic accumulation with histological aberrations. Following parameters were also investigated: serum biochemical profile (alanine aminotransferase, aspartate transaminase, alkaline phosphatase, blood urea nitrogen, and creatinine) for evaluation of liver and kidney functions and lipid peroxidation and oxidative stress (malondialdehyde, glutathione, superoxide dismutase, catalase, and glutathione peroxidase) in tissue homogenates of liver and kidney for estimation of oxidative status. The findings of this study indicate that RSV remarkably ameliorated the hepatic and renal toxicity in arsenic-exposed rat model due to its strong antioxidant potential.

Arsenic exposure and non-carcinogenic health effects

Inorganic arsenic (iAs) exposure is a serious health problem that affects more than 140 million individuals worldwide, mainly, through contaminated drinking water. Acute iAs poisoning produces several symptoms such as nausea, vomiting, abdominal pain, and severe diarrhea, whereas prolonged iAs exposure increased the risk of several malignant disorders such as lung, urinary tract, and skin tumors. Another sensitive endpoint less described of chronic iAs exposure are the non-malignant health effects in hepatic, endocrine, renal, neurological, hematological, immune, and cardiovascular systems. The present review outlines epidemiology evidence and possible molecular mechanisms associated with iAs-toxicity in several non-carcinogenic disorders.

3,4-Dihydroxybenzaldehyde lowers ROS generation and protects human red blood cells from arsenic(III) induced oxidative damage

Arsenic (As) is a potent environmental toxicant and chronic exposure to it results in various malignancies in humans. Oxidative stress has been implicated in the etiopathogenesis of As-induced toxicity. This investigated the protective effect of plant antioxidant 3,4-dihydroxybenzaldehyde (DHB) on sodium meta-arsenite (SA), an As-(III) compound, induced oxidative damage in human red blood cells (RBC). The RBC were first incubated with different concentrations of DHB and then treated with SA at 37°C. Hemolysates were prepared and assayed for various biochemical parameters. Treatment of RBC with SA alone enhanced the generation of reactive oxygen species and increased lipid and protein oxidation. Reduced glutathione levels, total sulfhydryl content and cellular antioxidant power were significantly decreased in SA alone treated RBC, compared to the untreated control cells. This was accompanied by membrane damage, alterations in activities of antioxidant enzymes and deranged glucose metabolism. Incubation of RBC with DHB, prior to treatment with SA, significantly and dose-dependently attenuated the SA-induced changes in all these parameters. Scanning electron microscopy of RBC confirmed these biochemical results. Treatment of RBC with SA alone converted the biconcave discoids to echinocytes but the presence of DHB inhibited this conversion and the RBC retained their normal shape. These results show that DHB protects human RBC from SA-induced oxidative damage, most probably due to its antioxidant character.

Arsenic-mediated nephrotoxicity

Chronic kidney disease (CKD) is an important global health problem that affects 8-15% of the population according to epidemiological studies done in different countries. Essential to prevention is the knowledge of the environmental factors associated with this disease, and heavy metals such as lead and cadmium are clearly associated with kidney injury and CKD progression. Arsenic is one of the most abundant contaminants in water and soil, and many epidemiological studies have found an association between arsenic and type 2 diabetes mellitus, hypertension and cancer; however, there is a scarcity of epidemiological studies about its association with kidney disease, and the evidence linking urinary arsenic excretion with CKD, higher urinary excretion of low molecular proteins, albuminuria or other markers of renal in injury is still limited, and more studies are necessary to characterize the role of arsenic on renal injury and CKD progression. Global efforts to reduce arsenic exposure remain important and research is also needed to determine whether specific therapies are beneficial in susceptible populations.

Arsenic-induced abnormalities in glucose metabolism: Biochemical basis and potential therapeutic and nutritional interventions

Health hazards due to the consumption of heavy metals such as arsenic have become a worldwide problem. Metabolism of arsenic produces various intermediates which are more toxic and cause toxicity. Arsenic exposure results in impairment of glucose metabolism, insulin secretion in pancreatic β-cells, altered gene expressions and signal transduction, and affects insulin-stimulated glucose uptake in adipocytes or skeletal muscle cells. Arsenic toxicity causes abnormalities in glucose metabolism through an increase in oxidative stress. Arsenic interferes with the sulfhydryl groups and phosphate groups present in various enzymes involved in glucose metabolism including pyruvate dehydrogenase and α-ketoglutarate dehydrogenase, and contributes to their impairment. Arsenic inhibits glucose transporters present in the cell membrane, alters expression of genes involved in glucose metabolism, transcription factors and inflammatory cytokines which stimulate oxidative stress. Some theories suggest that arsenic exposure under diabetic conditions inhibits hyperglycemia. However, the exact mechanism behind the behavior of arsenic as an antagonist or synergist on glucose homeostasis and insulin secretion is not yet fully understood. The present review delineates the relationship between arsenic and the biochemical basis of its relationship to glucose metabolism. This review also addresses potential therapeutic and nutritional interventions for attenuating arsenic toxicity. Several other potential nutritional supplements are highlighted in the review that could be used to combat arsenic toxicity.

Neuroprotective effect of resveratrol on arsenic trioxide–induced oxidative stress in feline brain

Arsenic trioxide (As2O3) is a known environmental toxicant and potent chemotherapeutic agent. Significant correlation has been reported between arsenic exposure (including consumption of arsenic-contaminated water and clinical use of As2O3) and dysfunction in the nervous system. In this study, we aimed to elucidate the effect of resveratrol with neuroprotective activities on As2O3-induced oxidative damage and cerebral cortex injury. Twenty-four healthy Chinese Dragon Li cats of either sex were randomly divided into four groups: control (1 ml/kg physiological saline), As2O3 (1 mg/kg), resveratrol (3 mg/kg) and As2O3 (1 mg/kg) + resveratrol (3 mg/kg). As2O3+resveratrol-treated group were given resveratrol (3 mg/kg) 1 h before As2O3 (1 mg/kg) administration. Pretreatment with resveratrol upregulated the activities of antioxidant enzymes and attenuated As2O3-induced increases in reactive oxygen species and malondialdehyde production. In addition, resveratrol attenuated the As2O3-induced reduction in the level of reduced glutathione and the ratio of reduced glutathione to oxidised glutathione, and accumulation of arsenic in the cerebral cortex. These findings support neuroprotective effect of resveratrol on As2O3 toxicity in feline brain and provide a better understanding of the mechanism that resveratrol modulates As2O3-induced oxidative damage and a stronger rational for clinical use of resveratrol to protect brain against the toxicity of arsenic.

Molecular mechanisms of the diabetogenic effects of arsenic: inhibition of insulin signaling by arsenite and methylarsonous acid

BACKGROUND

Increased prevalences of diabetes mellitus have been reported among individuals chronically exposed to inorganic arsenic (iAs). However, the mechanisms underlying the diabetogenic effects of iAs have not been characterized. We have previously shown that trivalent metabolites of iAs, arsenite (iAs(III)) and methylarsonous acid (MAs(III)) inhibit insulin-stimulated glucose uptake (ISGU) in 3T3-L1 adipocytes by suppressing the insulin-dependent phosphorylation of protein kinase B (PKB/Akt).

OBJECTIVES

Our goal was to identify the molecular mechanisms responsible for the suppression of PKB/Akt phosphorylation by iAs(III) and MAs(III).

METHODS

The effects of iAs(III) and MAs(III) on components of the insulin-activated signal transduction pathway that regulate PKB/Akt phosphorylation were examined in 3T3-L1 adipocytes.

RESULTS

Subtoxic concentrations of iAs(III) or MAs(III) had little or no effect on the activity of phosphatidylinositol 3-kinase (PI-3K), which synthesizes phosphatidylinositol-3,4,5-triphosphate (PIP(3)), or on phosphorylation of PTEN (phosphatase and tensin homolog deleted on chromosome ten), a PIP(3) phosphatase. Neither iAs(III) nor MAs(III) interfered with the phosphorylation of 3-phosphoinositide-dependent kinase-1 (PDK-1) located downstream from PI-3K. However, PDK-1 activity was inhibited by both iAs(III) and MAs(III). Consistent with these findings, PDK-1-catalyzed phosphorylation of PKB/Akt(Thr308) and PKB/Akt activity were suppressed in exposed cells. In addition, PKB/Akt(Ser473) phosphorylation, which is catalyzed by a putative PDK-2, was also suppressed. Notably, expression of constitutively active PKB/Akt restored the normal ISGU pattern in adipocytes treated with either iAs(III) or MAs(III).

CONCLUSIONS

These results suggest that inhibition of the PDK-1/PKB/Akt-mediated transduction step is the key mechanism for the inhibition of ISGU in adipocytes exposed to iAs(III) or MAs(III), and possibly for impaired glucose tolerance associated with human exposures to iAs.

Arsenic exposure and type 2 diabetes: a systematic review of the experimental and epidemiological evidence

Chronic arsenic exposure has been suggested to contribute to diabetes development. We performed a systematic review of the experimental and epidemiologic evidence on the association of arsenic and type 2 diabetes. We identified 19 in vitro studies of arsenic and glucose metabolism. Five studies reported that arsenic interfered with transcription factors involved in insulin-related gene expression: upstream factor 1 in pancreatic beta-cells and peroxisome proliferative-activated receptor gamma in preadipocytes. Other in vitro studies assessed the effect of arsenic on glucose uptake, typically using very high concentrations of arsenite or arsenate. These studies provide limited insight on potential mechanisms. We identified 10 in vivo studies in animals. These studies showed inconsistent effects of arsenic on glucose metabolism. Finally, we identified 19 epidemiologic studies (6 in high-arsenic areas in Taiwan and Bangladesh, 9 in occupational populations, and 4 in other populations). In studies from Taiwan and Bangladesh, the pooled relative risk estimate for diabetes comparing extreme arsenic exposure categories was 2.52 (95% confidence interval, 1.69-3.75), although methodologic problems limit the interpretation of the association. The evidence from occupational studies and from general populations other than Taiwan or Bangladesh was inconsistent. In summary, the current available evidence is inadequate to establish a causal role of arsenic in diabetes. Because arsenic exposure is widespread and diabetes prevalence is reaching epidemic proportions, experimental studies using arsenic concentrations relevant to human exposure and prospective epidemiologic studies measuring arsenic biomarkers and appropriately assessing diabetes should be a research priority.

Oxidative mechanism of arsenic toxicity and carcinogenesis

Arsenic is a known toxin and carcinogen that is present in industrial settings and in the environment. The mechanisms of disease initiation and progression are not fully understood. In the last a few years, there has been increasing evidence of the correlation between the generation of reactive oxygen species (ROS), DNA damage, tumor promotion, and arsenic exposure. This article summarizes the current literature on the arsenic mediated generation of ROS and reactive nitrogen species (RNS) in various biological systems. This article also discusses the role of ROS and RNS in arsenic-induced DNA damage and activation of oxidative sensitive gene expression.

The potential biological mechanisms of arsenic-induced diabetes mellitus

Although epidemiologic studies carried out in Taiwan, Bangladesh, and Sweden have demonstrated a diabetogenic effect of arsenic, the mechanisms remain unclear and require further investigation. This paper reviewed the potential biological mechanisms of arsenic-induced diabetes mellitus based on the current knowledge of the biochemical properties of arsenic. Arsenate can substitute phosphate in the formation of adenosine triphosphate (ATP) and other phosphate intermediates involved in glucose metabolism, which could theoretically slow down the normal metabolism of glucose, interrupt the production of energy, and interfere with the ATP-dependent insulin secretion. However, the concentration of arsenate required for such reaction is high and not physiologically relevant, and these effects may only happen in acute intoxication and may not be effective in subjects chronically exposed to low-dose arsenic. On the other hand, arsenite has high affinity for sulfhydryl groups and thus can form covalent bonds with the disulfide bridges in the molecules of insulin, insulin receptors, glucose transporters (GLUTs), and enzymes involved in glucose metabolism (e.g., pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase). As a result, the normal functions of these molecules can be hampered. However, a direct effect on these molecules caused by arsenite at physiologically relevant concentrations seems unlikely. Recent evidence has shown that treatment of arsenite at lower and physiologically relevant concentrations can stimulate glucose transport, in contrary to an inhibitory effect exerted by phenylarsine oxide (PAO) or by higher doses of arsenite. Induction of oxidative stress and interferences in signal transduction or gene expression by arsenic or by its methylated metabolites are the most possible causes to arsenic-induced diabetes mellitus through mechanisms of induction of insulin resistance and beta cell dysfunction. Recent studies have shown that, in subjects with chronic arsenic exposure, oxidative stress is increased and the expression of tumor necrosis factor alpha (TNFalpha) and interleukin-6 (IL-6) is upregulated. Both of these two cytokines have been well known for their effect on the induction of insulin resistance. Arsenite at physiologically relevant concentration also shows inhibitory effect on the expression of peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear hormone receptor important for activating insulin action. Oxidative stress has been suggested as a major pathogenic link to both insulin resistance and beta cell dysfunction through mechanisms involving activation of nuclear factor-kappaB (NF-kappaB), which is also activated by low levels of arsenic. Although without supportive data, superoxide production induced by arsenic exposure can theoretically impair insulin secretion by interaction with uncoupling protein 2 (UCP2), and oxidative stress can also cause amyloid formation in the pancreas, which could progressively destroy the insulin-secreting beta cells. Individual susceptibility with respect to genetics, nutritional status, health status, detoxification capability, interactions with other trace elements, and the existence of other well-recognized risk factors of diabetes mellitus can influence the toxicity of arsenic on organs involved in glucose metabolism and determine the progression of insulin resistance and impaired insulin secretion to a status of persistent hyperglycemia or diabetes mellitus. In conclusions, insulin resistance and beta cell dysfunction can be induced by chronic arsenic exposure. These defects may be responsible for arsenic-induced diabetes mellitus, but investigations are required to test this hypothesis.

Functional characterization of the human intestinal NaPi-IIb cotransporter in hamster fibroblasts and Xenopus oocytes

The recently cloned NaPi-IIb cotransporter is an apical membrane protein that is involved in the absorption of phosphate in the intestine. To expedite functional and structural studies, the human intestinal NaPi-IIb cotransporter was stably expressed in hamster fibroblast (PS120) cells. The hNaPi-IIb cDNA stably transfected cells exhibited a 1.8-fold higher sodium-dependent phosphate uptake than vector DNA transfected cells, and had a K(m) for Pi of approximately 106 microM and a K(m) for Na(+) of approximately 34 mM. The hNaPi-IIb cotransporter was also expressed in Xenopus oocytes and it exhibited a K(m) for Pi of approximately 113 microM and a K(m) for Na(+) of approximately 65 mM. The hNaPi-IIb cotransporter expressed in both PS120 cells and oocytes was inhibited by high external pH. Furthermore, the phosphate uptake mediated by the hNaPi-IIb cotransporter was inhibited by 5 mM phosphonoformic acid (PFA), 1 mM arsenate and 100 nM phorbol myristate acetate (PMA). These results demonstrate that the human intestinal NaPi-IIb cotransporter is functional when expressed in hamster fibroblasts, and that this model system may be useful in the future to identify NaPi-IIb cotransporter-specific inhibitors.

Arsenic intoxication associated with tubulointerstitial nephritis

Arsenic poisoning is an often unrecognized cause of renal insufficiency. We report a case of tubulointerstitial nephritis associated with an elevated urinary arsenic concentration. Removal of the putative source of arsenic resulted in symptomatic improvement, resolution of abnormal abdominal radiographs, and stabilization of renal function. This case emphasizes the importance of heavy metal screening in patients with multisystem complaints and tubulointerstitial nephritis.

Acute phosphate depletion and in vitro rat proximal tubule injury: protection by glycine and acidosis

The effects of phosphate (PO4) removal from Krebs Henseleit buffer on freshly isolated rat proximal tubules (rPT) were assessed by measuring Ca2+ uptake (nmol/mg protein), cellular adenosine triphosphate (ATP) (nmol/mg), tissue K+ content (nmol/mg) and lactate dehydrogenase (LDH) as an index of cell integrity. Ca2+ uptake increased by 50% in rPT incubated in zero PO4 medium as compared to control (2.6 +/- 0.1 vs. 3.9 +/- 0.19, P less than 0.001) and LDH release increased 2.5-fold from 14.2 +/- 0.6 to 31.6 +/- 1.6%, P less than 0.001. Neither verapamil (200 microM) nor mepacrine (50 microM) reduced Ca2+ uptake or decreased LDH release suggesting that the increased Ca2+ uptake was not occurring through potential operated channels and that phospholipase-induced cell injury was not the cause of increased LDH release. Either glycine (2 mM) or extracellular fluid acidosis (pH 7.06), however, significantly diminished rPT injury and Ca2+ uptake. Specifically, as compared to the increased LDH released in untreated. PO4-depleted rPT, LDH release was diminished significantly by glycine treatment (31.0 +/- 0.9 vs. 15.5 +/- 1.6%, P less than 0.001) or acidosis (30.3 +/- 0.04 vs. 19.2 +/- 0.9%, P less than 0.01). Ca2+ uptake did not increase in glycine treated tubules (2.6 +/- 0.1 vs. 2.8 +/- 0.2 nmol/mg, NS) or in the presence of acidosis (2.6 +/- 0.1 vs. 2.97 +/- 0.17 nmol/mg, NS). ATP concentrations were markedly reduced by PO4 depletion (2.8 +/- 0.2 vs. 4.8 +/- 0.3 nmol/mg, P less than 0.001) and remained at low levels during either acidosis or glycine-induced protection.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphate transport in isolated rat inner medullary collecting duct

Phosphate transport by the inner medullary collecting duct of normal rats was studied using an in vitro microperfusion technique. Net (Jnet), lumen-to-bath (Jlb) and bath-to-lumen (Jbl) phosphate fluxes were measured using 32PO4 as tracer, in the absence of net water absorption. A net absorption of phosphate (22.3 +/- 3.3 pmol cm-2 s-1) was observed by direct determination, and was similar to the difference between the Jlb and Jbl (57.7 +/- 8.2 and 32.2 +/- 1.5 pmol cm-2 s-1 respectively). The addition of amiloride (10 microM) to the perfusate did not change the Jlb of phosphate but blocked the efflux of sodium. Also, the withdrawal of sodium from the bath and perfusion solution did not change the Jlb of phosphate. In parallel, the addition of ouabain (10 mM) to the bath fluid decreased the Jlb of sodium more (37%) than the Jlb of phosphate (12%) and did not change the Jbl of phosphate. The addition of arsenate (10 microM) to the perfusate both in the presence and in the absence of sodium caused a decrease in Jlb, but Jbl remained unchanged, and parathyroid hormone (10 U) added to the bath did not change the Jlb. The increase in pH of the bath and perfusion fluid was associated with an increase in the Jlb of phosphate, and the decrease in pH was similarly followed by a decrease in phosphate efflux. The Jbl did not change with the pH alterations. These data demonstrate that a net phosphate absorption takes place in rat inner medullary collecting duct perfused in vitro and that this transport appears to be independent of sodium absorption and the action of parathyroid hormone. Moreover, a decrease in luminal and bath pH induces a decrease in phosphate efflux.

Early effects of uranyl nitrate on respiration and K+ transport in rabbit proximal tubule

The mechanisms by which uranyl nitrate (UN) is toxic to the proximal tubule are incompletely understood. To define these further we studied potassium (K+) transport and oxygen consumption (QO2) in rabbit proximal tubule suspensions in vitro immediately after exposure to UN using extracellular O2- and K+-sensitive electrodes. UN caused a cumulative dose-dependent inhibition of proximal tubule QO2, with a threshold concentration of 5 x 10(-5) M. Kinetic analysis suggested two patterns of cell injury: a higher affinity inhibition of QO2 with a Ki of 5 x 10(-4) M, and a lower affinity inhibition of QO2 with a Ki of 10 mM. QO2 was studied in detail in the presence of these Ki concentrations of UN to define the initial cellular events. The results indicated that different cellular processes displayed different sensitivities to UN. At submillimolar concentrations UN caused progressive selective inhibition of ouabain-insensitive QO2 (15% inhibition at 2 minutes). Ouabain-sensitive QO2 and nystatin-stimulated QO2 were not affected, suggesting that Na+,K+-ATPase activity and its coupling to mitochondrial ATP synthesis were intact. Direct measurement of proximal tubule net K+ flux confirmed that Na+,K+-ATPase activity was unchanged. Similarly, UN did not inhibit basal (state 4) or ADP-stimulated (state 3) mitochondrial QO2 in digitonin-permeabilized tubules, confirming that the mitochondria were intact. In contrast, higher concentrations of UN (greater than or equal to 1 mM) caused rapid inhibition of QO2 and net K+ efflux, due to inhibition of Na+,K+-ATPase activity and mitochondrial injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of Na+-dependent phosphate transport by group-specific covalent reagents in rat kidney brush border membrane vesicles. Evidence for the involvement of tyrosine and sulfhydryl groups on the interior of the membrane

The effects of tyrosine- and sulfhydryl-specific reagents on the Na+-dependent transport of phosphate in brush border membrane vesicles prepared from rat renal cortex were investigated. This study is the first to show that the tyrosine-specific reagents 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole and tetranitromethane inactivate the transporter in a concentration- and time-dependent fashion while the membrane impermeant tyrosine reagent, N-acetylimidazole, has no effect on phosphate uptake. The membrane permeant sulfhydryl reagent N-ethylmaleimide also caused a time- and concentration-dependent inactivation of this transport process but the membrane impermeant reagents 7-chloro-4-sulfobenzo-2-oxa-1,3-diazole and eosin-5-maleimide had little effect on phosphate uptake. The inhibitory effects of both tyrosine- and sulfhydryl-specific reagents were additive, but no protection from inactivation by tyrosine-specific reagents could be achieved by preincubation of the vesicles with the substrates of the transporter or with competitive inhibitors of the transport process. These results suggest that the amino acids modified by these agents are located either within the membrane or on the cytosolic surface of the transporter. These residues may not participate in substrate binding, but may be important for the conformational change of the transporter necessary for the translocation of phosphate across these membranes. This study also shows that Na+-dependent phosphate transport can be inactivated by other reagents which covalently modify histidine, carboxyl, and amino groups on proteins.

Inhibition of human renal epithelial Na+/Pi cotransport by phosphonoformic acid

Phosphonoformic acid was a potent competitive inhibitor of Na+/Pi cotransport by brush border membrane vesicles isolated from human renal cortex (Ki 31 microM). The Na+/Pi cotransport system in rat renal brush border membrane vesicles was much less sensitive to inhibition (Ki 210 microM). Na+/Pi cotransport by cultured cells derived from human renal cortex also was inhibited by phosphonoformic acid. Both in isolated membranes and cells the inhibition was dose-dependent, competitive, and specific for Pi. The effectiveness of phosphonoformic acid on Na+/Pi cotransport in human kidney suggests that its administration may be a useful maneuver to increase renal Pi excretion.

In vivo 31P nuclear magnetic resonance studies of arsenite induced changes in hepatic phosphate levels

Hepatic phosphate resonances were evaluated in vivo by 31P nuclear magnetic resonance (31P-NMR) following a single intravenous dose of sodium arsenite (10 mg/kg). Acute in vivo administration of arsenite rapidly decreased intracellular pools of all ATP phosphate with concomitant increases in inorganic phosphate and phosphomonoesters. In the phosphodiester resonance region, glycerolphosphorylcholine was also increased. The data suggest that liver cannot compensate for the rapid loss of NAD-linked substrate oxidation via other metabolic pathways, such as glycolysis for the production of ATP, and also demonstrate that 31P-NMR spectroscopy can disclose time-dependent metabolic changes of the liver in vivo.

Primary active sodium transport, oxygen consumption, and ATP: coupling and regulation

Several metabolic aspects of primary active transport have been explored in this communication. One emphasized theme entailed the need to investigate the properties of the mitochondria and the active transport systems within the intact cell. Several methodological and conceptual approaches were described that permitted such an analysis. The answers provided were sometimes qualitative or quantitative. Qualitative information was provided regarding the cytosolic signal linking active transport with respiration, suggesting that the cytosolic ADP concentration was an important element in that link. The intact renal cell was found to work normally at 50 to 60% of its maximal respiratory capacity, indicating that sufficient reserve capacity was present for increased metabolic demands. Several examples were described in which a combination of qO2 measurements and/or optical techniques were used to differentiate between effects of agents which act primarily on transport or metabolic events. Finally, the control of transport by metabolism was discussed, primarily emphasizing the role of ATP and Pi. One of the overall conclusions from these studies is that, in general, the mitochondria and the transport systems seem to display similar properties in the intact cell as they do in isolated form. However, uncertainties concerning the cellular microenvironment surrounding the mitochondria and the plasma membrane transporters have produced some interesting surprises concerning their function in the intact cell. More quantitative information on the energy compartmentation of the renal cell would be helpful to clarify numerous aspects of metabolic function.

Sodium-dependent phosphate transport by apical membrane vesicles from a cultured renal epithelial cell line (LLC-PK1)

Apical membrane vesicles were prepared from confluent monolayers of LLC-PK1 cells grown upon microcarrier beads. The final membrane preparation, obtained by a modified divalent cation precipitation technique, was enriched in alkaline phosphatase, leucine aminopeptidase and trehalase (8-fold compared to the initial homogenate). Analysis of phosphate uptake into the vesicles identified a specific sodium-dependent pathway. Lithium and other cations were unable to replace sodium. At 100 mmol/l sodium and pH 7.4, an apparent Km for phosphate of 99 +/- 19 mumol/l and an apparent Ki for arsenate of 1.9 mmol/l were found. Analysis of the sodium activation of phosphate uptake gave an apparent Km for sodium of 32 +/- 12 mmol/l and suggested the involvement of two sodium ions in the transport mechanism. Sodium modified the apparent Km of the transport system for phosphate. The rate of sodium-dependent phosphate uptake was higher at pH 6.4 than at pH 7.4. At both pH values, an inside negative membrane potential (potassium gradient plus valinomycin) had no stimulatory effect on the rate of the sodium-dependent component of phosphate uptake. It is concluded that the apical membrane of LLC-PK1 cells contains a sodium-phosphate cotransport system with a stoichiometry of 2 sodium ions: 1 phosphate anion.

Sodium-dependent transport of phosphate in LLC-PK1 cells

Transport of phosphate has been studied in subconfluent monolayers of LLC-PK1 cells. It was found that this transport system shows similar characteristics to those observed in the kidney. Uptake of phosphate is mediated by a Na+-dependent, substrate-saturable process with an apparent Km value for phosphate of 96 +/- 15 mumol/l. Kinetic analysis of the effect of Na+ indicated that at (pH 7.4) two sodium ions are cotransported with one HOP4(2-) ion (Hill coefficient 1.5) with an apparent Km value for sodium of 56 mmol/l. Pi uptake is inhibited by metabolic inhibitors (ouabain and FCCP). In the pH range of 6.6 of 7.4 Pi uptake rate does not change significantly, indicating that both the monovalent and the divalent form of phosphate are accepted by the transport system. It is suggested that phosphate is transported by LLC-PK1 cells together with sodium (2 Na+:1 HPO4(2-) in an electroneutral manner down a favourable sodium gradient.

Lack of effect of peritubular protein on passive NaCl transport in the rabbit proximal tubule

The effect of peritubular protein removal on passive NaCl transport was examined in the isolated rabbit proximal convoluted tubule (PCT). Three modes of passive NaCl transport were tested: (a) paracellular backflux of NaCl, (b) convective flow of NaCl through junctional complexes, and (c) anion gradient-dependent NaCl transport. The effect of peritubular protein removal on the paracellular permeability to NaCl was examined using transepithelial specific resistance. Eight PCT were perfused with ultrafiltrate (UF) and bathed in either serum or UF. Transepithelial specific resistance averaged 14.5 +/- 1.9 in the presence and 13.7 +/- 1.7 omega cm2 in the absence of peritubular protein. The effect of peritubular protein removal on the convective flow of a NaCl solution across functional complexes was examined in the absence of active transport by using colloid osmotic pressure (COP) gradients. 12 PCT were perfused with simple salt solutions in Donnan equilibrium with and without protein at 20 degrees C. A COP gradient of 60.1 and -60.1 mmHg drove only 0.06 and -0.23 nl/min, respectively. These values are approximately 10% of the value predicted for an effect of peritubular protein on NaCl solution flow (1.98 nl/min) and are approximately equal to the value predicted for pure water equilibration for the small osmotic pressure difference between solutions in Donnan equilibrium (0.17-0.18 nl/min). The effect of peritubular protein removal on the passive absorption of NaCl driven by anion concentration gradients was examined in seven PCT perfused with a high chloride solution simulating late proximal tubular fluid and bathed in either serum or UF at 20 degrees C. Volume absorption averaged 0.34 +/- 0.20 in the presence and 0.39 +/- 0.20 nl/mm min in the absence of peritubular protein. In conclusion, peritubular protein removal did not significantly affect any of the three distinct modes of passive NaCl transport tested. The lack of effect of peritubular protein removal on passive paracellular NaCl transport suggests that protein modulates an active transcellular NaCl transport process.

Metabolic requirement for inorganic phosphate by the rabbit proximal tubule

These studies examine the effects of acute changes in the availability of inorganic phosphate on the function of isolated proximal renal tubules from rabbit kidney. We removed phosphate from the extracellular fluids and measured fluid absorption rates in isolated perfused tubules and oxygen consumption rates in suspensions of cortical tubules. In proximal convoluted tubules, the selective removal of phosphate from the luminal fluid reduced fluid absorption rates from 1.11+/-0.12 to -0.01+/-0.08 nl/mm . min. This effect on fluid absorption was dependent on the presence of glucose transport and metabolism. The addition of phlorizin to the phosphate-free luminal fluid preserved fluid absorption rates (1.12+/-0.12 nl/mm . min) as did the substitution of nonmetabolized alpha-methyl d-glucopyranoside for glucose (1.05+/-0.21 nl/mm . min) or the addition of 2-deoxyglucose, an inhibitor of glycolysis, to the bathing medium (1.01+/-0.15 nl/mm . min). There was no effect on fluid absorption if phosphate was removed from the bath only. Additionally, removal of phosphate from the luminal fluid of proximal straight rather than convoluted tubules had no effect on fluid absorption rates. Oxygen consumption rates in suspensions of cortical tubules were reduced from 18.9+/-0.6 to 10.6+/-0.6 nmol O(2)/mg tubular protein . min by the removal of phosphate from the medium. This inhibition was prevented by the substitution of alpha-methyl d-glucopyranoside for glucose in the phosphate-free medium. The data indicate that under certain conditions, proximal convoluted tubules require the presence of phosphate in the luminal fluid to preserve tubular function. In the absence of intraluminal phosphate, glucose metabolism causes a reduction in both oxidative metabolism and fluid absorption. This response is analogous to the Crabtree effect and suggests limitations on the intracellular availability of inorganic phosphate.

Effect of calcium and phosphate on bicarbonate and fluid transport by proximal tubules in vitro

Previous studies in living animals have suggested that variations in the serum concentrations of calcium and phosphate directly affect renal tubular bicarbonate transport. To examine this area more directly, we measured the bicarbonate and the fluid transport in isolated rabbit proximal convoluted and straight tubules perfused in vitro. The concentration of calcium in the perfusate was varied between 0 and 5 mM and in the bath between 0.5 and 5 mM. In proximal convoluted tubules, 5 mM calcium in the perfusate and bath caused a significant increase in the rate of bicarbonate absorption, and removal of calcium from the perfusate inhibited both bicarbonate and fluid absorption. The concentration of phosphate in the perfusate and the bath was varied from 0.25 to 5 mM. In both convoluted and straight tubules, there was no significant change in bicarbonate or fluid absorption with changes in the ambient phosphate concentration. We conclude that changes in calcium concentration have significant effects on bicarbonate transport by proximal convoluted tubules but that wide variations in the concentration of phosphate have no significant effect on bicarbonate, sodium, or fluid transport by proximal tubules in vitro.