The Role of the Endocrine System in the Regulation of Acid-Base Balance by the Kidney and the Progression of Chronic Kidney Disease

Systemic acid-base status is primarily determined by the interplay of net acid production (NEAP) arising from metabolism of ingested food stuffs, buffering of NEAP in tissues, generation of bicarbonate by the kidney, and capture of any bicarbonate filtered by the kidney. In chronic kidney disease (CKD), acid retention may occur when dietary acid production is not balanced by bicarbonate generation by the diseased kidney. Hormones including aldosterone, angiotensin II, endothelin, PTH, glucocorticoids, insulin, thyroid hormone, and growth hormone can affect acid-base balance in different ways. The levels of some hormones such as aldosterone, angiotensin II and endothelin are increased with acid accumulation and contribute to an adaptive increase in renal acid excretion and bicarbonate generation. However, the persistent elevated levels of these hormones can damage the kidney and accelerate progression of CKD. Measures to slow the progression of CKD have included administration of medications which inhibit the production or action of deleterious hormones. However, since metabolic acidosis accompanying CKD stimulates the secretion of several of these hormones, treatment of CKD should also include administration of base to correct the metabolic acidosis.

Proteomic analysis of murine kidney proximal tubule sub-segment derived cell lines reveals preferences in mitochondrial pathway activity

The proximal tubule (PT) is a nephron segment that is responsible for the majority of solute and water reabsorption in the kidney. Each of its sub-segments have specialized functions; however, little is known about the genes and proteins that determine the oxidative phosphorylation capacity of the PT sub-segments. This information is critical to understanding kidney function and will provide a comprehensive landscape of renal cell adaptations to injury, physiologic stressors, and development. This study analyzed three immortalized murine renal cell lines (PT S1, S2, and S3 segments) for protein content and compared them to a murine fibroblast cell line. All three proximal tubule cell lines generate ATP predominantly by oxidative phosphorylation while the fibroblast cell line is glycolytic. The proteomic data demonstrates that the most significant difference in proteomic signatures between the cell lines are proteins known to be localized in the nucleus followed by mitochondrial proteins. Mitochondrial metabolic substrate utilization assays were performed using the proximal tubule cell lines to determine substrate utilization kinetics thereby providing a physiologic context to the proteomic dataset. This data will allow researchers to study differences in nephron-specific cell lines, between epithelial and fibroblast cells, and between actively respiring cells and glycolytic cells. SIGNIFICANCE: Proteomic analysis of proteins expressed in immortalized murine renal proximal tubule cells was compared to a murine fibroblast cell line proteome. The proximal tubule segment specific cell lines: S1, S2 and S3 are all grown under conditions whereby the cells generate ATP by oxidative phosphorylation while the fibroblast cell line utilizes anaerobic glycolysis for ATP generation. The proteomic studies allow for the following queries: 1) comparisons between the proximal tubule segment specific cell lines, 2) comparisons between polarized epithelia and fibroblasts, 3) comparison between cells employing oxidative phosphorylation versus anaerobic glycolysis and 4) comparisons between cells grown on clear versus opaque membrane supports. The data finds major differences in nuclear protein expression and mitochondrial proteins. This proteomic data set will be an important baseline dataset for investigators who need immortalized renal proximal tubule epithelial cells for their research.

Regulation of Acid-Base Balance in Patients With Chronic Kidney Disease

Normallly the kidneys handle the daily acid load arising from net endogenous acid production from the metabolism of ingested animal protein (acid) and vegetables (base). With chronic kidney disease, reduced acid excretion by the kidneys is primarily due to reduced ammonium excretion such that when acid excertion falls below acid porduction, acid accumulation occurs. With even mild reductions in glomerular filtration rate (60 to 90 ml/min), net acid excretion may fall below net acid production resulting in acid retention which may be initially sequestered in interstitial compartments in the kidneys, bones, and muscles resulting in no fall in measured systemic bicarbonate levels (eubicarbonatemic metabolic acidosis). With greater reductions in kidney function, the greater quantities of acid retained spillover systemically resulting in low pH (overt metabolic acidosis). The evaluation of acid-base balance in patients with CKD is complicated by the heterogeneity of clinical acid-base disorders and by the eubicarbonatemic nature of the early phase of acid retention. If supported by more extensive studies, blood gas analyses to confirm the acid-base disorder and newer ways for assessing the presence of acidosis such as urinary citrate measurements may become routine tools to evaluate and treat acid-base disorders in individuals with CKD.

Effects of Prebiotic Fiber Xylooligosaccharide in Adenine-Induced Nephropathy in Mice

SCOPE

This study evaluates the effect of the prebiotic fiber xylooligosaccharide (XOS) on kidney function and gut microbiome in mice with adenine-induced chronic kidney disease (CKD).

METHOD AND RESULTS

Mice are fed the control diet containing adenine for 3 weeks to induce CKD and are switched to XOS supplemented (2 or 7%) or control diets for another 3 weeks. Mice with CKD exhibit increased blood urea nitrogen (BUN), creatinine, and kidney histopathology. XOS significantly reverses kidney injuries in CKD mice. Analysis of cecum microbiota reveales that adenine-induced CKD does not change alpha diversity, and XOS induces a decrease of alpha diversity in control mice and mice with CKD. Beta diversity analysis shows significant clustering according to experimental groups. Six out of the nine bacterial genera enriched in CKD are significantly reduced with XOS intervention. Furthermore, XOS increases cecal short-chain fatty acid (SCFA) production in both control and CKD mice. Cecal SCFAs and blood propionate are negatively correlated with BUN. XOS also decreases blood p-cresol sulfate in CKD mice, likely resulting from altered microbial tyrosine metabolism.

CONCLUSION

These results show that XOS intervention improves kidney function in mice with CKD, and is associated with profound changes in microbial composition and metabolism.

Exogenous Gene Transmission of Isocitrate Dehydrogenase 2 Mimics Ischemic Preconditioning Protection

Ischemic preconditioning confers organ-wide protection against subsequent ischemic stress. A substantial body of evidence underscores the importance of mitochondria adaptation as a critical component of cell protection from ischemia. To identify changes in mitochondria protein expression in response to ischemic preconditioning, we isolated mitochondria from ischemic preconditioned kidneys and sham-treated kidneys as a basis for comparison. The proteomic screen identified highly upregulated proteins, including NADP+-dependent isocitrate dehydrogenase 2 (IDH2), and we confirmed the ability of this protein to confer cellular protection from injury in murine S3 proximal tubule cells subjected to hypoxia. To further evaluate the role of IDH2 in cell protection, we performed detailed analysis of the effects of Idh2 gene delivery on kidney susceptibility to ischemia-reperfusion injury. Gene delivery of IDH2 before injury attenuated the injury-induced rise in serum creatinine (P<0.05) observed in controls and increased the mitochondria membrane potential (P<0.05), maximal respiratory capacity (P<0.05), and intracellular ATP levels (P<0.05) above those in controls. This communication shows that gene delivery of Idh2 can confer organ-wide protection against subsequent ischemia-reperfusion injury and mimics ischemic preconditioning.

Regulation of Acid-Base Balance in Chronic Kidney Disease

The kidneys play a major role in the regulation of acid-base balance by reabsorbing bicarbonate filtered by the glomeruli and excreting titratable acids and ammonia into the urine. In CKD, with declining kidney function, acid retention and metabolic acidosis occur, but the extent of acid retention depends not only on the degree of kidney impairment but also on the dietary acid load. Acid retention can occur even when the serum bicarbonate level is apparently normal. With reduced kidney function, acid transport processes in the surviving nephrons are augmented but as disease progresses ammonia excretion and, in some individuals, the ability to reabsorb bicarbonate falls, whereas titratable acid excretion is preserved until kidney function is severely impaired. Urinary ammonia levels are used to gauge the renal response to acid loads and are best assessed by direct measurement of urinary ammonia levels rather than by indirect assessments. In individuals with acidosis from CKD, an inappropriately low degree of ammonia excretion points to the pathogenic role of impaired urinary acid excretion. The presence of a normal bicarbonate level in CKD complicates the interpretation of the urinary ammonia excretion as such individuals could be in acid-base balance or could be retaining acid without manifesting a low bicarbonate level. At this time, the decision to give bicarbonate supplementation in CKD is reserved for those with a bicarbonate level of 22 mEq/L, but because of potential harm of overtreatment, supplementation should be adjusted to maintain a bicarbonate level of <26 mEq/L.

Non-Anion Gap Metabolic Acidosis: A Clinical Approach to Evaluation

Acid-base disturbances can result from kidney or nonkidney disorders. We present a case of high-volume ileostomy output causing large bicarbonate losses and resulting in a non-anion gap metabolic acidosis. Non-anion gap metabolic acidosis can present as a form of either acute or chronic metabolic acidosis. A complete clinical history and physical examination are critical initial steps to begin the evaluation process, followed by measuring serum electrolytes with a focus on potassium level, blood gas, urine pH, and either direct or indirect urine ammonium concentration. The present case was selected to highlight the differential diagnosis of a non-anion gap metabolic acidosis and illustrate a systematic approach to this problem.

Effects of acid challenges on type 2 angiotensin II receptor-sensitive ammonia production by the proximal tubule

Angiotensin II (ANG II) acting through its type 1 (AT1) receptor stimulates total ammonia (tNH3) production by the proximal tubule. The present studies explored the role of ANG II type 2 (AT2) receptors in modulating the stimulatory effects of ANG II on tNH3 production. Mouse S2 proximal tubule segments derived from 18-h and 7-day acid-loaded mice, and non-acid-loaded controls were dissected and microperfused in vitro. Adding ANG II to the luminal perfusion solution resulted in different increments in tNH3 production rates in tubules derived from 18-h vs. 7-day acid-loaded mice such that the increase in tNH3 production with ANG II was higher in tubules derived from 18-h acid-loaded mice compared with those derived from control and 7-day acid-loaded mice. Adding the AT2 receptor blocker PD123319 with ANG II increased ANG II-stimulated tNH3 production in S2 segments from control and 7-day acid-loaded mice but not in those from 18-h acid-loaded mice, and this increased effect of PD123319 was associated with higher AT2 receptor protein levels in brush-border membranes. Studies in cultured proximal tubule cells demonstrated that 2-h exposure to pH 7.0 reduced the modulating effect of PD123319 on ANG II-simulated tNH3 production and reduced cell surface AT2 receptor levels. We concluded that AT2 receptors reduce the stimulatory effect of ANG II on proximal tubule tNH3 production and that the time-dependent impact of AT2 receptor blockade on the ANG II-stimulated tNH3 production corresponded to time-dependent changes in AT2 receptor cell surface expression in the proximal tubule.

The serum anion gap in the evaluation of acid-base disorders: what are its limitations and can its effectiveness be improved?

The serum anion gap has been utilized to identify errors in the measurement of electrolytes, to detect paraproteins, and, most relevant to the nephrologist, to evaluate patients with suspected acid-base disorders. In regard to the latter purpose, traditionally an increased anion gap is identified when it exceeds the upper limit of normal for a particular clinical laboratory measurement. However, because there is a wide range of normal values (often 8-10 mEq/L), an increase in anion concentration can be present in the absence of an increased anion gap. In addition, the type of retained anion can affect the magnitude of the increase in anion gap relative to change in serum [HCO3(-)] being greater with lactic acidosis compared with ketoacidosis. This review examines the methods of calculation of the serum anion gap in textbooks and published literature, the effect of perturbations other than changes in acid-base balance, and its effectiveness in identifying mild and more severe disturbances in acid-base balance. Limitations of the present methods of determining the normal anion gap and change in the anion gap are highlighted. The possibility of identifying the baseline value for individuals to optimize the use of the calculation in the detection of metabolic acidosis is suggested.

Reactive oxygen species and IRF1 stimulate IFNα production by proximal tubules during ischemic AKI

We previously reported that expression of the transcription factor interferon regulatory factor 1 (IRF1) is an early, critical maladaptive signal expressed by renal tubules during murine ischemic acute kidney injury (AKI). We now show that IRF1 mediates signals from reactive oxygen species (ROS) generated during ischemic AKI and that these signals ultimately result in production of α-subtypes of type I interferons (IFNαs). We found that genetic knockout of the common type I IFN receptor (IFNARI-/-) improved kidney function and histology during AKI. There are major differences in the spatial-temporal production of the two major IFN subtypes, IFNβ and IFNαs: IFNβ expression peaks at 4 h, earlier than IFNαs, and continues at the same level at 24 h; expression of IFNαs also increases at 4 h but continues to increase through 24 h. The magnitude of the increase in IFNαs relative to baseline is much greater than that of IFNβ. We show by immunohistology and study of isolated cells that IFNβ is produced by renal leukocytes and IFNαs are produced by renal tubules. IRF1, IFNαs, and IFNARI were found on the same renal tubules during ischemic AKI. Furthermore, we found that ROS induced IFNα expression by renal tubules in vitro. This expression was inhibited by small interfering RNA knockdown of IRF1. Overexpression of IRF1 resulted in the production of IFNαs. Furthermore, we found that IFNα stimulated production of maladaptive proinflammatory CXCL2 by renal tubular cells. Altogether our data support the following autocrine pathway in renal tubular cells: ROS > IRF1 > IFNα > IFNARI > CXCL2.

The effects of varying acidity on Helicobacter pylori growth and the bactericidal efficacy of ampicillin

BACKGROUND

Penicillins inhibit cell wall synthesis; therefore, Helicobacter pylori must be dividing for this class of antibiotics to be effective in eradication therapy. Identifying growth responses to varying medium pH may allow design of more effective treatment regimens.

AIM

To determine the effects of acidity on bacterial growth and the bactericidal efficacy of ampicillin.

METHODS

H. pylori were incubated in dialysis chambers suspended in 1.5-L of media at various pHs with 5 mM urea, with or without ampicillin, for 4, 8 or 16 h, thus mimicking unbuffered gastric juice. Changes in gene expression, viability and survival were determined.

RESULTS

At pH 3.0, but not at pH 4.5 or 7.4, there was decreased expression of ~400 genes, including many cell envelope biosynthesis, cell division and penicillin-binding protein genes. Ampicillin was bactericidal at pH 4.5 and 7.4, but not at pH 3.0.

CONCLUSIONS

Ampicillin is bactericidal at pH 4.5 and 7.4, but not at pH 3.0, due to decreased expression of cell envelope and division genes with loss of cell division at pH 3.0. Therefore, at pH 3.0, the likely pH at the gastric surface, the bacteria are nondividing and persist with ampicillin treatment. A more effective inhibitor of acid secretion that maintains gastric pH near neutrality for 24 h/day should enhance the efficacy of amoxicillin, improving triple therapy and likely even allowing dual amoxicillin-based therapy for H. pylori eradication.

IRF-1 promotes inflammation early after ischemic acute kidney injury

Acute renal ischemia elicits an inflammatory response that may exacerbate acute kidney injury, but the regulation of the initial signals that recruit leukocytes is not well understood. Here, we found that IFN regulatory factor 1 (IRF-1) was a critical, early proinflammatory signal released during ischemic injury in vitro and in vivo. Within 15 min of reperfusion, proximal tubular cells of the S3 segment produced IRF-1, which is a transcription factor that activates proinflammatory genes. Transgenic knockout of IRF-1 ameliorated the impairment of renal function, morphologic injury, and inflammation after acute ischemia. Bone marrow chimera experiments determined that maximal ischemic injury required IRF-1 expression by both leukocytes and radioresistant renal cells, the latter identified as S3 proximal tubule cells in the outer medulla by in situ hybridization and immunohistochemistry. In vitro, reactive oxygen species, generated during ischemia/reperfusion injury, stimulated expression of IRF-1 in an S3 proximal tubular cell line. Taken together, these data suggest that IRF-1 gene activation by reactive oxygen species is an early signal that promotes inflammation after ischemic renal injury.

The top 10 things nephrologists wish every primary care physician knew

Renal disease is commonly encountered by primary care physicians during their day-to-day visits with patients. Common renal disorders include hypertension, proteinuria, kidney stones, and chronic kidney disease. Despite their prevalence, many physicians may be unfamiliar with the diagnosis and initial treatment of these common renal disorders. Early recognition and intervention are important in slowing the progression of chronic kidney disease and preventing its complications. The evidence-based pearls in this article will help primary care physicians avoid common pitfalls in the recognition and treatment of such disorders and guide their decision to refer their patients to a specialist.

The top 10 things nephrologists wish every primary care physician knew

Renal disease is commonly encountered by primary care physicians during their day-to-day visits with patients. Common renal disorders include hypertension, proteinuria, kidney stones, and chronic kidney disease. Despite their prevalence, many physicians may be unfamiliar with the diagnosis and initial treatment of these common renal disorders. Early recognition and intervention are important in slowing the progression of chronic kidney disease and preventing its complications. The evidence-based pearls in this article will help primary care physicians avoid common pitfalls in the recognition and treatment of such disorders and guide their decision to refer their patients to a specialist.

Acid loading in vivo and low pH in culture increase angiotensin receptor expression: enhanced ammoniagenic response to angiotensin II

The proximal tubule defends the body against acid challenges by enhancing its production and secretion of ammonia. Our previous studies demonstrated an enhanced ammoniagenic response of the proximal tubule to ANG II added to the lumen in vitro after an in vivo acid challenge. The present study examined the effect of NH4Cl acid loading in vivo on renal cortical type 1 ANG II (AT1) receptor expression, the effect of low pH on AT1receptor expression in a proximal tubule cells in culture, and their response to ANG II. A short-term (18 h) NH4Cl load in vivo resulted in increased renal cortical AT1receptor mRNA expression and increased brush-border membrane AT1receptor protein expression levels. Changing the cell culture pH from 7.4 to 7.0 for at least 2 h increased cell surface expression of AT1receptors and enhanced the stimulatory effect of ANG II on ammonia production rates. This increased ammoniagenic response to ANG II and the early enhancement of cell surface expression induced by exposure of the cultured proximal tubule cells to pH 7.0 were prevented by treatment with colchicine. These results suggest that, after acid challenges, the enhanced ammoniagenic response of the proximal tubule to ANG II is, in part, mediated by increased AT1receptor cell surface expression and that the enhancement of receptor expression plays an important role in the early response of the proximal tubule to acid challenges.

Role of angiotensin II in the enhancement of ammonia production and secretion by the proximal tubule in metabolic acidosis

Acidosis and angiotensin II stimulate ammonia production and transport by the proximal tubule. We examined the modulatory effect of the type 1 angiotensin II receptor blocker losartan on the ability of metabolic acidosis to stimulate ammonia production and secretion by mouse S2 proximal tubule segments. Mice given NH4Cl for 7 days developed metabolic acidosis (low serum bicarbonate concentration) and increased urinary excretion of ammonia. S2 tubule segments from acidotic mice displayed higher rates of ammonia production and secretion compared with those from control mice. However, when losartan was coadministered in vivo with NH4Cl, both the acidosis-induced increase in urinary ammonia excretion and the adaptive increase in ammonia production and secretion of microperfused S2 segments were largely blocked. In renal cortical tissue, losartan blocked the acid-induced increase in brush-border membrane NHE3 expression but had no effect on the acid-induced upregulation of phosphate-dependent glutaminase or phosphoenolpyruvate carboxykinase 1 in cortical homogenates. Addition of angiotensin II to the microperfusion solution enhanced ammonia secretion and production rates in tubules from NH4Cl-treated and control mice in a losartan-inhibitable manner. These results demonstrate that a 7-day acid challenge induces an adaptive increase in ammonia production and secretion by the proximal tubule and suggest that during metabolic acidosis, angiotensin II signaling is necessary for adaptive enhancements of ammonia excretion by the kidney and ammonia production and secretion by S2 proximal tubule segments, as mediated, in part, by angiotensin receptor-dependent enhancement of NHE3 expression.

Maladaptive role of IL-6 in ischemic acute renal failure

The role of IL-6 was investigated in murine ischemic acute renal failure. The renal pedicles were clamped for 17 min, and the mice were studied at various times after reperfusion. We found that serum IL-6 increased after murine ischemic renal injury. This increase was associated with increased IL-6 mRNA in the ischemic kidney but not in the contralateral kidney or the liver. Maximal IL-6 production occurred at 4 to 8 h and decreased to baseline by 24 h. Reperfusion of the kidney was required for IL-6 production. In situ hybridization and immunohistochemistry showed that macrophages infiltrated areas adjacent to the vascular bundles in the outer medulla within hours of reperfusion and showed that these macrophages produced IL-6 mRNA. For understanding how macrophages were stimulated to produce IL-6, an in vitro model in which S3 proximal tubular cells were injured by reactive oxygen species was set up. These injured cells released molecules that activated macrophages to produce IL-6 in vitro. IL-6 that was produced in response to renal ischemia was maladaptive because transgenic knockout of IL-6 ameliorated renal injury as measured by serum creatinine and histology. IL-6 transgenic knockout mice were lethally irradiated, and their bone marrow was reconstituted with wild-type IL-6 cells. Such bone marrow transfers abolished the protective effects of transgenic IL-6 knockout. It is concluded that macrophages infiltrate the area of the vascular bundles of the outer medulla, these macrophages produce IL-6, and this IL-6 exacerbates ischemic murine acute renal failure.

Ammonia production and secretion by S3 proximal tubule segments from acidotic mice: role of ANG II

ANG II has potent effects on ammonia production and secretion rates by the proximal tubule and is found in substantial concentrations in the lumen of the proximal tubule in vivo. Because our previous studies demonstrated that acid loading enhanced the stimulatory effects of ANG II on ammonia production and secretion by S2 proximal tubule segments, we examined the effect of ANG II on ammonia production and secretion by isolated, perfused S3 segments from nonacidotic control mice and acidotic mice given NH4Cl for 7 days. In the absence of ANG II, ammonia production and secretion rates were no different in S3 segments from acidotic and control mice. By contrast, when ANG II was present in the luminal perfusion solution, ammonia production and secretion rates were stimulated, in a losartan-inhibitable manner, to a greater extent in S3 segments from acidotic mice. Ammonia secretion rates in S3 segments were largely inhibited by perfusion with a low-sodium solution containing amiloride in the presence or absence of ANG II. These results demonstrated that isolated, perfused mouse S3 proximal tubule segments produce and secrete ammonia, that NH4Cl-induced acidosis does not affect the basal rates of ammonia production and secretion, and that ANG II, added to the luminal fluid, stimulates ammonia production and secretion to a greater extent in S3 segments from acidotic mice. These findings suggest that S3 segments, in the presence of ANG II, can contribute to the enhanced renal excretion that occurs with acid loading.

Enhanced ammonia secretion by proximal tubules from mice receiving NH(4)Cl: role of angiotensin II

Acidosis and angiotensin II (ANG II) stimulate ammonia production and transport by the proximal tubule. We examined the effect of short-term (18 h) in vivo acid loading with NH(4)Cl on ammonia production and secretion rates by mouse S2 proximal tubule segments microperfused in vitro with or without ANG II in the luminal microperfusion solution. S2 tubules from NH(4)Cl-treated mice displayed higher rates of luminal ammonia secretion compared with those from control mice. The adaptive increase in ammonia secretion in NH(4)Cl-treated mice was eliminated when losartan was coadministered in vivo with NH(4)Cl. Ammonia secretion rates from both NH(4)Cl-treated and control mice were largely inhibited by amiloride. Addition of ANG II to the microperfusion solution enhanced ammonia secretion and production rates to a greater extent in tubules from NH(4)Cl-treated mice compared with those from controls, and the stimulatory effects of ANG II were blocked by losartan. These results demonstrate that a short-term acid challenge induces an adaptive increase in ammonia secretion by the proximal tubule and suggest that ANG II plays an important role in the adaptive enhancement of ammonia secretion that is observed with short-term acid challenges.

Sulfur-containing amino acids decrease cisplatin cytotoxicity and uptake in renal tubule epithelial cell lines

PURPOSE

Nephrotoxicity is one of the major dose-limiting side-effects of cisplatin (DDP). The disproportionate accumulation of cisplatin in kidney tissue may play an important role, however, therapeutic measures to prevent this prime cause of nephrotoxicity are not available. Because certain amino acids (AAs) have been reported to modulate DDP nephrotoxicity in vivo, we explored the potential of all 20 protein AAs, N-acetylcysteine and DL-homocysteine to reduce DDP cytotoxicity and uptake in S1, S3 (proximal tubule), and DCT (distal convoluted tubule) cell lines.

METHODS

Immortalized but non-transformed renal tubule epithelial cell lines, derived from specific portions of the nephron of an SV40 transgenic mouse. were grown to confluency and exposed to various concentrations of DDP for 1 h with or without concurrent exposure to AAs in an otherwise AA-free Krebs-Ringer buffer (KRB). After 1 h, cell layers were washed and replenished with medium for cytotoxicity assays, or processed immediately for the determination of DDP accumulation. Cytotoxicity was assessed 48 h later by an MTT assay, and DDP uptake after 1 h was determined by atomic absorption spectroscopy.

RESULTS

In an initial screening where the cells were concurrently incubated with 0.25 mM DDP and 1 mM AA for 1 h in KRB, only cysteine (Cys), methionine (Met), N-acetylcysteine and DL-homocysteine reduced DDP toxicity. This effect was enhanced at 5 mM AA and most potent for Cys, which reduced DDP cytotoxicity by 79 +/- 3% in S3 cells, by 78 +/- 12.2% in DCT cells, and by 19 +/- 3.6% in S1 cells (P < 0.05). Reduction of cytotoxicity was less for Met, DL-homocysteine, and N-acetylcysteine, in decreasing order. All four AAs also inhibited DDP uptake in renal cells, with Cys as the strongest inhibitor. Inhibition of DDP accumulation by 1 mM Cys after 1 h was 39% in S3 cells, 38% in DCT cells, and 28% in S1 cells. Again, reduction of uptake was less for the three other AAs. Pre-complexing of DDP with Cys for 16 h increased its uptake by 8- to 30-fold compared with native DDP, but markedly inhibited its toxicity. Thus, pre-complexing of DDP with Cys could not explain the reduced uptake of DDP, but could partly account for the reduction in cytotoxicity. Double-reciprocal Lineweaver-Burk plots of DDP concentration-versus-uptake rates at a constant concentration of Cys suggested that Cys competitively inhibited DDP uptake in S1 and DCT cells, and in a more complex fashion in S3 cells.

CONCLUSIONS

We conclude that Cys, Met, N-acetylcysteine, and DL-homocysteine differentially inhibit DDP toxicity and uptake in cultured S1, S3, and DCT cells, and that the inhibition of uptake, as well as the complexation of DDP with Cys within the cell, may prevent toxicity. The structural element R-CH(NH2)-[CH2]1 2-S-R, which is common to all four molecules, may play a crucial role in blocking the transport of DDP, and could have future clinical applications.

Differential effects of cisplatin in proximal and distal renal tubule epithelial cell lines

Pathological studies suggest that cisplatin injures different portions of the nephron to different extents. To investigate this issue further, we examined the cytotoxicity and uptake of cisplatin in cell lines derived from S1 and S3 proximal tubule and distal convoluted tubule segments isolated from a mouse carrying the SV40 large T-antigen transgene. S1 cells displayed the highest sensitivity to cisplatin cytotoxicity, followed by S3 and distal convoluted tubule (DCT) cells. These differences in cytotoxicity did not correlate with differences in cisplatin uptake. Cytotoxic concentrations of cisplatin triggered apoptosis in all three cell lines. Although BAX and BCL-2 expression was similar among the three cell lines, the expression of the anti-apoptotic protein, BCL-X(L), was significantly lower in S1 cells than in S3 and DCT cells, and this may have contributed to the heightened sensitivity of S1 cells. Cisplatin transport characteristics demonstrated a saturable component of cisplatin uptake and differences in apparent K(M) and Vmax values among the three cell lines. The three cell lines were 43- to 176-fold more sensitive to cisplatin than to carboplatin. This distinction between the two drugs could not be fully explained by differences in the uptake rates of carboplatin and cisplatin. We conclude that cells from different portions of the nephron display different sensitivities to cisplatin, different transport characteristics for cisplatin and different levels of expression of BCL-X(L). In addition, the relative resistance of renal cells to carboplatin vs cisplatin is mostly due to the differential effects that follow internalization.

Molecular cloning of the chicken oviduct ecto-ATP-diphosphohydrolase

The chicken oviduct ecto-ATP diphosphohydrolase (ATPDase), a member of the ecto-ATPase family, was purified to homogeneity previously (Strobel, R. S., Nagy, A. K., Knowles, A. F., Buegel, J., and Rosenberg, M. O. (1996) J. Biol. Chem. 271, 16323-16331). It is an 80-kDa glycoprotein with high specific activity (approximately 1,000 micromol/min/mg with MgATP as the substrate) and hydrolyzes both nucleoside triphosphates and diphosphates. Using amino acid sequence information obtained from the purified enzyme, two partial cDNA clones were obtained using reverse transcriptase-polymerase chain reaction and library screening. This is the second ecto-ATPase family member and the first ecto-ATPDase to be cloned from information derived from purified proteins. The deduced primary sequence of the chicken oviduct ecto-ATPDase indicates a protein of 493 amino acid residues with a molecular mass of 54 kDa. The predicted orientation shows it to be anchored to the membrane by two transmembranous segments near the NH2 and COOH termini with very short intracytoplasmic peptides at either end. The bulk of the protein is extracellular and contains 12 potential N-glycosylation sites, several potential phosphorylation sites, and five sequences that are conserved in seven other related membrane proteins. Four of the conserved sequences, designated as apyrase conserved regions, are present in both ecto-ATPases and soluble E-type ATPases. The fifth conserved region, which occurs near the COOH terminus of the eight proteins, is observed only in the membrane-bound ecto-ATPases. Unexpectedly, sequence comparison revealed that the chicken oviduct ecto-ATPDase is equally distant from the two ecto-ATPases, which exhibit low activity toward ADP, and the four putative ecto-ATPDases, which are closely related to CD39.

Expression of a cut-related homeobox gene in developing and polycystic mouse kidney

Cut is a diverged homeobox gene that is essential for normal development of the Malpighian tubules in Drosophila melanogaster. Homologues of Drosophila cut that encode transcriptional repressors have been identified in several mammalian species and cell lineages. We examined the expression of a murine cut homologue (named Cux-1) in the developing mouse using Northern blot analysis and in situ hybridization. At 12.5 d.p.c. and 13.5 d.p.c., Cux-1 was highly expressed in a subset of embryonic tissues, including the developing metanephros. Within the metanephros, Cux-1 was expressed in the nephrogenic zone including both mesenchymal cells (uninduced and condensed mesenchyme) and epithelial cells (ureteric buds, renal vesicles, S-shaped bodies). During later stages of nephrogenesis, Cux-1 was down-regulated such that there was minimal expression in mature glomeruli and tubules. In addition, Cux-1 was detected in the mesonephros, mesonephric duct, and bladder. Expression of Cux-1 was also examined in polycystic kidneys from C57BL/6J-cpk/ cpk mice. At 21 days of age, Cux-1 was highly expressed in cyst epithelium of polycystic kidneys but was minimally expressed in kidneys from phenotypically normal littermates. These results demonstrate that a cut-related homeobox gene is expressed in the developing kidney and urinary tract of the mouse. Expression of Cux-1 in the kidney is inversely related to degree of cellular differentiation. Cux-1 may encode a transcriptional repressor that inhibits terminally differentiated gene expression during early stages of nephrogenesis.

Cloning and kidney cell-specific activity of the promoter of the murine renal Na-K-C1 cotransporter gene

The murine Nkcc2/Slcl2a1 gene encodes a bumetanide-sensitive Na-K-Cl cotransporter that is expressed exclusively in the kidney in the thick ascending limb of the loop of Henle. Nuclear run-off assays demonstrated that kidney-specific expression of Nkcc2 was due, at least in part, to kidney-specific gene transcription. To begin study of the gene promoter, a genomic clone that contained 13.5 kilobases of the 5'-flanking region of Nkcc2 was isolated. A single transcription initiation site was located 1330 base pairs (bp) upstream of the start codon. The sequence of the proximal 5'-flanking region contained typical eukaryotic promoter elements including a TATA box, two CCAAT boxes, and an initiator. A (G-A)28.(C-T)28 microsatellite and consensus binding sites for hepatocyte nuclear factor 1, cAMP-response element binding protein, CCAAT/enhancer-binding proteins, and basic helix-loop-helix proteins, were also identified. To functionally express the promoter, 2255 bp of the proximal 5'-flanking region was ligated to a luciferase reporter gene and transfected into thick ascending limb (TAL) cells, a stable cell line derived from microdissected loops of Henle of the Tg(SV40E)Bri7 mouse. TAL cells exhibited furosemide-sensitive Na-K((NH4)+)-Cl cotransport activity and endogenously expressed the 5.0-kilobase Nkcc2 transcript. Luciferase activity was 130-fold greater following transfection into TAL cells compared with transfection into cells that did not express Nkcc2 (NIH 3T3 fibroblasts). Deletion analysis revealed that promoter activity in TAL cells was similar in constructs extending from the transcription initiation site to -1529 to -469, whereas further deletion to -190 resulted in a 76% decrease in activity. We conclude that the Nkcc2 promoter exhibits kidney cell-specific activity. Regulatory elements required for maximal promoter activity are located in a 280-bp DNA segment that contains consensus binding sites for several transcription factors expressed in the kidney.

Effect of luminal angiotensin II on ammonia production and secretion by mouse proximal tubules

Angiotensin II is an important regulator of acid-base and ammonia metabolism in the proximal tubule. Because angiotensin II receptors exist on the apical membrane and because luminal fluid angiotensin II concentrations may be substantial, the effects of luminal angiotensin II on ammonia production rates and net luminal total ammonia (tNH3) secretion rates were examined in dissected mouse S2 proximal tubule segments. Ammonia production rates reflected the total release of ammonia via the basolateral and luminal aspects of the tubule, whereas net luminal secretion rates reflected the rates at which ammonia left the tubule via the luminal fluid leaving the distal end of the perfused segment. The results demonstrated that 1) luminal angiotensin II affected tNH3 production in a concentration-dependent fashion, 2) luminal angiotensin II at concentrations that stimulated tNH3 production could counteract the effect of inhibitory basolateral concentrations of angiotensin II, 3) the stimulation of tNH3 production and the rise in intracellular calcium concentration induced by 10(-10) M luminal angiotensin II were blocked by the addition of an angiotensin II receptor inhibitor, saralasin, or the calcium channel blocker nifedipine to the luminal perfusion solution, and 4) in contrast to basolateral angiotensin II, which inhibited net luminal tNH3 secretion, luminal angiotensin II stimulated amiloride-sensitive net luminal tNH3 secretion in parallel with stimulation of luminal fluid acidification. Thus luminal angiotensin II at physiological and superphysiological concentrations has important effects on ammonia production and transport in the proximal tubule that in some ways differ from the effects of basolateral angiotensin II.

Inhibition of gentamicin uptake into cultured mouse proximal tubule epithelial cells by L-lysine

Gentamicin uptake and toxicity was studied in a nontransformed cell line obtained from the S1 segment of the proximal tubule epithelium of a transgenic mouse. Cytotoxicity was assayed using the dye 3-(4,-5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Gentamicin uptake was assayed by a fluorescence polarization assay. No differences in toxicity were found among cells incubated for 4 hours in complete culture medium, enriched Kreb's buffer alone, or enriched Krebs' buffer with added 300 micrograms/mL gentamicin, 0.5 mmol/L L-lysine, or gentamicin plus L-lysine. Uptake of 300 micrograms/mL gentamicin was minimal at zero time and increased as a function of time. Uptake of gentamicin at 4 hours was positively correlated with medium gentamicin concentration. Addition of 0.5 mmol/L L-lysine inhibited uptake of 300 micrograms/mL gentamicin 38.9 +/- 10.2%. No other amino acid, including D-lysine or arginine, significantly changed gentamicin uptake. The authors conclude that gentamicin and L-lysine share a specific uptake mechanism located in the apical membrane of renal proximal tubule cells.

Effect of angiotensin II on ammonia production and secretion by mouse proximal tubules perfused in vitro

The effects of angiotensin II on total ammonia (tNH3) production and net secretion were investigated using in vitro microperfused mouse S2 proximal tubule segments incubated in Krebs-Ringer bicarbonate buffer containing 0.5 mM L-glutamine. Basolateral exposure of mouse S2 segments to 10(-11), 10(-10), and 10(-9) M angiotensin II stimulated tNH3 production rates by 23, 52, and 49%, respectively. Addition of 10(-6) M angiotensin II inhibited the tNH3 production rate by 34%. 10(-10) M angiotensin II inhibited net luminal secretion of tNH3 in the presence of enhanced luminal acidification and in the absence of altered luminal tNH3 efflux rates. Measurements of intracellular pH (pHi) and intracellular calcium concentration [( Ca2+]i) suggested that the effects of angiotensin II on tNH3 production were not mediated by changes in pHi but by the stimulatory effect of angiotensin II correlated with increased [Ca2+]i. Inhibition of the calcium-calmodulin-dependent pathway with W-7 blocked the stimulatory effect of 10(-10) M angiotensin II on tNH3 production and luminal acidification. These results indicate that angiotensin II has concentration-dependent effects on tNH3 production; that its action to stimulate tNH3 production may be mediated by rises in [Ca2+]i and the calcium-calmodulin pathway; and that angiotensin II, at concentrations that stimulate tNH3 production, inhibits net luminal ammonia secretion by a mechanism that is not mediated by diminished luminal acidification or by changes in luminal ammonia efflux rates.

Low-density lipoprotein uptake and cholesterol accumulation by cultured renal cells

The uptake of low-density lipoprotein (LDL) and the accumulation of cholesterol were assessed in opossum kidney (OK) and Madin-Darby canine kidney (MDCK) cells. OK and MDCK cells were grown to confluency on Millicell well inserts. The uptake of human LDL across the apical and basolateral surfaces of OK and MDCK cells was assessed by the degradation of internalized (125I)LDL to trichloroacetic acid-soluble products. LDL uptake via the apical surface of OK cells increased linearly with LDL concentration, indicating nonreceptor-mediated uptake. In contrast, LDL uptake via the basolateral surface of OK cells and both apical and basolateral surfaces of MDCK cells followed a saturable pattern. In addition, (125I)LDL bound to the apical membrane of MDCK cells, but not to the apical membrane of OK cells, was displaced by heparin and by excess of unlabeled LDL. Exposure to LDL (100 mg/mL) resulted in an increase in total cholesterol content of OK and MDCK cells (23 and 18%, respectively). Most of the increase in total cholesterol content with LDL exposure resulted from increased free cholesterol content in MDCK cells and esterified cholesterol in OK cells. The differences in cholesteryl ester formation were consistent with the slower rates of (14C) oleate incorporation into cholesteryl ester and lower cholesterol esterifying activity observed in MDCK cells compared with that in OK cells. These results demonstrate that LDL uptake can be receptor or nonreceptor mediated, depending upon the renal cell type and the surface exposed to LDL, and that LDL exposure leads to increased cholesterol content in OK and MDCK cells.

Effect of bath and luminal potassium concentration on ammonia production and secretion by mouse proximal tubules perfused in vitro

To determine the effects of acute changes in K+ concentration in vitro on ammonia production and secretion by the proximal tubule, we studied mouse S2 segments perfused with and bathed in Krebs-Ringer bicarbonate buffers containing various K+ concentrations. All bath solutions contained L-glutamine as the ammoniagenic substrate. High bath and luminal K+ concentrations (8 mM), but not high luminal K+ concentration alone, inhibited total ammonia production rates by 26%, while low bath and luminal K+ concentrations (2 mM), but not low luminal K+ concentration alone, stimulated total ammonia production rates by 33%. The stimulation of ammonia production by low bath K+ concentration was not observed when L-glutamine was added to the luminal perfusion solution. On the other hand, high luminal K+ concentration stimulated, while low luminal K+ concentration inhibited, net luminal secretion of total ammonia in a way that was: (a) independent of total ammonia production rates, (b) independent of Na(+)-H+ exchange activity, and (c) not due to changes in transepithelial fluxes of total ammonia. These results suggest that luminal potassium concentration has a direct effect on cell-to-lumen transport of ammonia.

AVP reduces transepithelial resistance across IMCD cell monolayers

The inner medullary collecting duct (IMCD) is an important site of action for arginine vasopressin (AVP). To examine the mode of action of AVP in this segment, we measured the change in transepithelial resistance of cultured rat IMCD cells grown to confluence on collagen-coated Millicell culture plate inserts in response to AVP. Resistance was measured by use of an EVOM voltage-ohm meter. AVP at 10(-11)-10(-8) M caused a fall in resistance of 6.9 +/- 1.3 to 14.0 +/- 1.4 omega.cm2 (P less than 0.05 to less than 0.01 vs. no AVP), which was reversed by removal of AVP or addition of 10(-6) M amiloride. Pretreating the apical surface of IMCD cells with trypsin had no effect on resistance but totally prevented the antidiuretic hormone-induced fall in resistance. Pretreating the apical surface with trypsin and amiloride did not prevent the fall in resistance to AVP. Addition of 10(-9) M AVP or 10(-6) M forskolin increased 2-min adenosine 3',5'-cyclic monophosphate (cAMP) accumulation by 55 or 96%, respectively. Stimulation of endogenous cAMP accumulation by forskolin or the addition of exogenous 8-bromo-cAMP caused no change in resistance. To examine the relationship between intracellular calcium [( Ca2+]i) and AVP action, the response of [Ca2+]i to AVP was measured by use of fura-2. AVP induced no change in [Ca2+]i in IMCD cells in suspension, on glass cover slips, or on permeable supports. Ionomycin (25 nM) increased [Ca2+]i in IMCD cells and lowered resistance across monolayers, but the fall in resistance was not blocked by amiloride.(ABSTRACT TRUNCATED AT 250 WORDS)

Ammonia production and secretion by the proximal tubule

Ammonia production and secretion by the proximal tubule accounts for most of the ammonia that appears in the urine. Rates of ammonia production and net luminal ammonia secretion were measured in isolated perfused mouse proximal tubule segments. This approach combines the in vitro microperfusion technique with a sensitive bioluminescence assay for total ammonia and permits the determination of ammonia production and secretion rates in specific proximal tubule segments bathed and perfused with defined solutions. Luminal perfusion stimulates ammonia production by proximal tubule segments in a flow-related manner. The effect of perfusion is not dependent on intact Na+-H+ exchange. In contrast, the rate of net luminal secretion of ammonia is largely dependent on Na+-H+ exchange but not markedly dependent on an acid luminal fluid pH. These results suggest an important role of Na+-NH4+ exchange in the mechanism by which the Na+-H+ exchanger facilitates net ammonia secretion.

Effect of luminal perfusion on glucose production by isolated proximal tubules

The effects of luminal perfusion on glucose production by the proximal tubule were examined by use of the technique of in vitro microperfusion with an ultramicroassay for glucose to measure the net glucose production rates in isolated mouse midproximal tubule segments. Tubules bathed in Krebs-Ringer bicarbonate (KRB) buffer containing L-glutamine and acetate and perfused with KRB buffer at a high flow rate produced glucose at a lower rate (0.12 +/- 0.02 pmol.min-1.mm-1) than unperfused segments (0.40 +/- 0.03) or segments perfused at a lower flow rate (0.24 +/- 0.03). In contrast, the estimated rates of glucose utilization were not affected by luminal perfusion. The inhibition of net fluid reabsorption by perfusion with a modified KRB buffer containing amiloride or by addition of ouabain to the bath medium raised glucose production rates to levels equaling or exceeding those observed in unperfused tubules. The inhibition of glucose production by luminal perfusion occurred in the presence of multiple substrates (i.e., glutamine, acetate, lactate, pyruvate, alanine, and valerate) or nonammoniagenic substrates (i.e., lactate and pyruvate) in the bath medium. Thus net glucose production is inhibited by luminal perfusion and the inhibitory effect is dependent on intact fluid reabsorption. The reduction in net glucose production observed with perfusion does not result from increased glucose utilization and is not dependent on the presence of specific substrates.

Luminal secretion of ammonia in the mouse proximal tubule perfused in vitro

A major portion of the total ammonia (tNH3 = NH3 + NH+4) produced by the isolated perfused mouse proximal tubule is secreted into the luminal fluid. To assess the role of Na+-H+ exchange in net tNH3 secretion, rates of net tNH3 secretion and tNH3 production were measured in proximal tubule segments perfused with control pH 7.4 Krebs-Ringer bicarbonate (KRB) buffer or with modified KRB buffers containing 10 mM sodium and 0.1 mM amiloride. Net tNH3 secretion was inhibited by 90% in proximal tubule segments perfused with the pH 7.4 modified KRB buffer while tNH3 production remained unaffected. The inhibition of net tNH3 secretion by perfusion with the modified KRB buffer was only partially reversed by acidifying the modified KRB luminal perfusate from 7.4 to as low as 6.2. These data indicate that the Na+-H+ exchanger facilitates a major portion of net tNH3 secretion by the proximal tubule and that luminal acidification may play only a partial role in the mechanism by which the Na+-H+ exchanger mediates net tNH3 secretion.

Ammonia production by isolated mouse proximal tubules perfused in vitro. Effect of metabolic acidosis

We examined the effects of metabolic acidosis in vivo and reduced bath and luminal pH in vitro on total NH3 (NH3 + NH+4) production rates by isolated mouse proximal tubule segments. Midproximal tubule segments were obtained from mice with NH4Cl-induced metabolic acidosis and from nonacidotic controls. The segments were perfused with modified Krebs-Ringer bicarbonate (KRB) buffer, incubated in KRB buffer containing 0.5 mM L-glutamine and 1.0 mM sodium acetate, and gassed with 95% O2 and 5% CO2. Isolated unperfused and perfused proximal tubules from acidotic mice produced total NH3 at higher rates than corresponding tubules from nonacidotic mice. Perfusion of the tubular lumen stimulated total NH3 production by tubules from both acidotic and nonacidotic mice. In contrast, lowering the bath pH to 7.0 by lowering the HCO3- concentration increased total NH3 production rates by tubules from nonacidotic mice but not by tubules from acidotic mice. Reducing the HCO3- concentration of the bath buffer to 10 mM while maintaining a pH of 7.4 had no significant effect on total NH3 production by tubules from nonacidotic mice. Lowering the luminal fluid pH by reducing the perfusate HCO-3 from 25 mM to 10, 5, or 1.2 mM while maintaining a bath pH of 7.4 lowered collected luminal fluid pH but had no effect on total NH3 production by proximal tubules from nonacidotic mice. These observations demonstrated that metabolic acidosis in vivo stimulated total NH3 production in isolated mouse proximal tubule segments and that low peritubular pH and HCO-3 stimulated total NH3 production by proximal tubule segments from nonacidotic mice in vitro.

Regulation of ammonia production by mouse proximal tubules perfused in vitro. Effect of luminal perfusion

To investigate factors regulating ammonia (NH3) production by isolated defined proximal tubule segments, we examined the rates of total NH3 (NH3 + NH+4) production by individual proximal tubule segments perfused in vitro under a variety of perfusion conditions. Segments consisting of late convoluted and early straight portions of superficial proximal tubules were incubated at 37 degrees C in Krebs-Ringer bicarbonate (KRB) buffer containing 0.5 mM L-glutamine and 1.0 mM sodium acetate, pH 7.4. The rate of total ammonia production was calculated from the rate of accumulation of total NH3 in the bath. The total ammonia production rate by unperfused proximal segments was 6.0 +/- 0.2 (+/- SE) pmol/mm per minute, which was significantly lower than segments perfused at a flow rate of 22.7 +/- 3.4 nl/min with KRB buffer (21.5 +/- 1.4 pmol/mm per minute; P less than 0.001) or with KRB buffer containing 0.5 mM L-glutamine (31.9 +/- 2.5; P less than 0.001). The rate of NH3 production was higher in segments perfused with glutamine than in segments perfused without glutamine (P less than 0.01). The perfusion-associated stimulation of NH3 production was characterized further. Analysis of collected luminal fluid samples revealed that the luminal fluid total NH3 leaving the distal end of the perfused proximal segment accounted for 91% of the increment in NH3 production observed with perfusion. Increasing the perfusion flow rate from 3.7 +/- 0.1 to 22.7 +/- 3.4 nl/min by raising the perfusion pressure resulted in an increased rate of total NH3 production in the presence or absence of perfusate glutamine (mean rise in rate of total NH3 production was 14.9 +/- 3.7 pmol/mm per minute in segments perfused with glutamine and 7.8 +/- 0.9 in those perfused without glutamine). In addition, increasing the perfusion flow rate at a constant perfusion pressure increased the rate of luminal output of NH3. Total NH3 production was not affected by reducing perfusate sodium concentration to 25 mM and adding 1.0 mM amiloride to the perfusate, a condition that was shown to inhibit proximal tubule fluid reabsorption. These observations demonstrate that the rate of total NH3 production by the mouse proximal tubule is accelerated by perfusion of the lumen of the segment, by the presence of glutamine in the perfusate, and by increased perfusion flow rates. The increased rate of NH3 production with perfusion seems not to depend upon normal rates of sodium reabsorption. The mechanism underlying the stimulation of NH3 production by luminal flow is unknown and requires further study.

Dissociation of gluconeogenesis from fluid and phosphate reabsorption in isolated rabbit proximal tubules

Gluconeogenesis in the kidney is a metabolic function characteristic of proximal tubules. Recent studies suggest that renal gluconeogenesis (GNG) may in some way be coupled to fluid and phosphate reabsorption in the proximal tubule. Therefore, the present studies examined more directly the relationship between GNG and transport of fluid and phosphate using isolated proximal straight tubules of rabbit kidney. Glucose production rates were determined in isolated tubules with a glucose microassay while both phosphate (Jp) and net fluid fluxes (Jv) were measured by the in vitro isolated tubule perfusion technique. Glucose production rates from individual substrates, including pyruvate, lactate, glutamate and alpha-ketoglutarate, differed significantly, but neither Jv nor Jp was altered when different substrates in the bath medium were used. Inhibition of GNG by 3-mercaptopicolinate did not alter the Jv or Jp. Acid pH (7.0) stimulated GNG and suppressed both Jv and Jp. The addition of 3-mercaptopicolinate at acid pH abolished the stimulatory effect of acid pH on GNG but had no effect on Jv or Jp. These data thus indicate that, in the isolated rabbit renal proximal tubule, GNG rates can be dissociated from both the Jv and Jp and suggest that renal GNG may not directly regulate the fluid and phosphate transport rates in the proximal straight tubule.

Inhibition of synaptosomal accumulation of l-norepinephrine. I: N-arylalkyl and N-aryloxyalkyl dl-amphetamines and related compounds

The ability of a group of systematically modified amphetamines to inhibit the accumulation of l-norepinephrine by nonstriatal synaptosomes was investigated. N-Substitution by the proper bulky hydrophobic groups can be well tolerated. Structure-activity relationships generate a qualitative picture of the inhibitor-carrier interaction site.