Protective role of NHE-3 inhibition in rat renal transplantation undergoing acute rejection

Acute rejection in renal transplantation disturbs solute and volume maintenance in humans accompanied by delayed graft function and poor prognosis. We recently reported that decreased expression and function of Na+/H+ exchanger type 3 (NHE-3) in proximal tubules and epithelial Na+ channels and aquaporin 2 in collecting ducts are major mechanisms involved in Na+ and water imbalances shortly after transplantation in rat undergoing acute rejection. We performed kidney transplantations in rats with bilaterally nephrectomized recipients with acute rejection and, in addition, systemically administered a specific inhibitor of NHE-3 (NHE-I). NHE inhibition in acute renal failure was shown to improve tubular function and recovery. The aim of this therapy was to reduce energy consumption of the graft and preserve NHE-3 function. Imbalances in electrolyte excretion declined in NHE-I-treated animals and NHE-3 activity was preserved. Observed NHE-I-dependent changes in electrolyte excretion, polyuria, and reduced protein reabsorption in the acute postoperative phase are predictors of favorable graft outcome in humans.

Activation of counter-regulatory mechanisms in a rat renal acute rejection model

Background

Microarray analysis provides a powerful approach to identify gene expression alterations following transplantation. In patients the heterogeneity of graft specimens, co-morbidity, co-medications and the challenges in sample collection and preparation complicate conclusions regarding the underlying mechanisms of graft injury, rejection and immune regulation.

Results

We used a rat kidney transplantation model with strict transplant and sample preparation procedures to analyze genome wide changes in gene expression four days after syngeneic and allogeneic transplantation. Both interventions were associated with substantial changes in gene expression. After allogeneic transplantation, genes and pathways related to transport and metabolism were predominantly down-regulated consistent with rejection-mediated graft injury and dysfunction. Up-regulated genes were primarily related to the acute immune response including antigen presentation, T-cell receptor signaling, apoptosis, interferon signaling and complement cascades. We observed a cytokine and chemokine expression profile consistent with activation of a Th1-cell response. A novel finding was up-regulation of several regulatory and protective genes after allogeneic transplantation, specifically IL10, Bcl2a1, C4bpa, Ctla4, HO-1 and the SOCS family.

Conclusion

Our data indicate that in parallel with the predicted activation of immune response and tissue injury pathways, there is simultaneous activation of pathways for counter regulatory and protective mechanisms that would balance and limit the ongoing inflammatory/immune responses. The pathophysiological mechanisms behind and the clinical consequences of alterations in expression of these gene classes in acute rejection, injury and dysfunction vs. protection and immunoregulation, prompt further analyses and open new aspects for therapeutic approaches.

Acute rejection modulates gene expression in the collecting duct

Kidney transplantation, especially when associated with acute rejection, leads to changes in the expression of many genes, including those encoding solute transporters and water channels. In a rat model of acute rejection after allogeneic renal transplantation, impaired renal function, increased urine volume, and increased fractional excretion of sodium were observed. Gene array analysis revealed that these findings were associated with significant downregulation of water channels (aquaporin-1, -2, -3, and -4) and transporters of sodium, glucose, urea, and other solutes. In addition, changes in expression of various receptors, kinases, and phosphatases that modulate the expression or activity of renal transport systems were observed. Syngeneic transplantation or treatment with cyclosporine A following allogeneic transplantation did not impair graft function but did lead to the downregulation of aquaporin-1, -3, and -4 and several solute transporters. However, expression of aquaporin-2 and the epithelial sodium channel did not change, suggesting that the downregulation of these transporters following allogeneic transplantation is rejection-dependent. In conclusion, changes in gene expression may explain the impaired handling of solute and water after allogeneic transplantation, especially during acute rejection. Treatment with cyclosporine A improves the regulation of solute and water by preventing the downregulation of aquaporin-2 and epithelial sodium channel, even though many other transporter genes remain downregulated.

Ligands and signaling of the G-protein-coupled receptor GPR14, expressed in human kidney cells

Activation of the urotensin II (U-II) receptor, GPR14, leads to an increase in Ca(2+), activation of phospholipase A(2) (PLA(2)) and an increase in arachidonic acid. The signaling pathway for guanylin peptides in the kidney involves an unknown G-protein coupled receptor which activates PLA(2) and increases arachidonic acid as well. To test if guanylin peptides could be, as U-II, agonists for the GPR14 receptor in the kidney, we used HEK293 and CHO cells transfected with hGPR14 (HEK293+hGPR14, CHO+hGPR14, respectively). Effects of guanylin peptides and U-II were studied by slow-whole-cell patch-clamp analysis and microfluorimetric measurements of intracellular Ca(2+). Guanylin peptides and U-II depolarized HEK293+hGPR14 significantly more than wild type cells. These effects were inhibited in the presence of Ba(2+) or PLA(2) inhibition (AACOCF(3)), suggesting that guanylin peptides and U-II increase arachidonic acid and inhibit ROMK channels in these cells. However, only U-II was capable to increase the cellular Ca(2+), suggesting different mechanism of GPR14 activation by guanylin peptides and U-II. This signaling pathway of U-II involves PKC, because U-II effects in HEK293+hGPR14 cells were inhibited by calphostin C. Guanylin peptides activate PLA(2) and inhibit ROMK channels in HEK293 cells transfected with the human GPR14 receptor. Since GPR14 is present in mouse and human CCD it is a candidate for the guanylate cyclase independent receptor for guanylin peptides.

Renal electrolyte effects of guanylin and uroguanylin

PURPOSE OF REVIEW

Guanylin peptides are secreted from the intestine and influence electrolyte and water transport in intestine and kidney, suggesting that these peptides act as intestinal natriuretic peptides. This review presents recent research on renal guanylin and uroguanylin effects.

RECENT FINDINGS

After salty meals guanylin peptides are produced in the intestine activating anion secretion and inhibiting sodium absorption. In the kidney guanylin peptides induce saluresis and diuresis. The signaling of guanylin peptides in the intestine is well known, involving guanylate cyclase C and increases in cellular cGMP concentrations. As in the intestine in proximal tubule cells a cGMP and guanylate cyclase C-dependent signaling pathway exists. In guanylate cyclase C-deficient mice, renal effects are unaltered, which could be by explained by recently described new cGMP-independent signaling pathways. In proximal tubules, Uroguanylin activates a pertussis toxin-sensitive receptor. Another cGMP-independent signaling pathway of guanylin peptides involving phospholipase A2 and arachidonic acid is shown for principal cells of human and mouse cortical collecting ducts.

SUMMARY

Mechanisms and sites of renal actions of guanylin peptides are still not completely understood. Renal receptors for guanylin peptides are probably G-protein-coupled. The influences of guanylin peptides on natriuresis, kaliuresis, and diuresis are complex and only further detailed studies will allow a complete understanding of the function of these peptides.

Characterization of regulatory mechanisms and states of human organic cation transporter 2

Polyspecific organic cation transporters (OCTs) have a large substrate binding pocket with different interaction domains. To determine whether OCT regulation is substrate specific, suitable fluorescent organic cations were selected by comparing their uptake in wild-type (WT) human embryonic kidney (HEK)-293 cells and in HEK-293 cells stably transfected with hOCT2. N-amidino-3,5-diamino-6-chloropyrazine-carboxamide (amiloride) and 4-[4-(dimethylamino)-styryl]- N-methylpyridinium (ASP) showed concentration-dependent uptake in hOCT2 at 37°C. After subtraction of unspecific uptake determined in WT at 37°C or in hOCT2 at 8°C saturable specific uptake of both substrates was measured. Kmvalues of hOCT2-mediated uptake of 95 μM amiloride and 24 μM ASP were calculated. Inhibition of amiloride and ASP uptake by several organic cations was also measured [IC50(in μM) for amiloride and ASP, respectively, tetraethylammonium (TEA) 98 and 30, cimetidine 14 and 26, and tetrapentylammonium (TPA) 7 and 2]. Amiloride and ASP uptake were significantly reduced by inhibition of Ca2+/CaM complex (−55 ± 5%, n = 10 and −63 ± 2%, n = 15, for amiloride and ASP, respectively) and stimulation of PKC (−54 ± 5%, n = 14, and −31 ± 6%, n = 26) and PKA (−16 ± 5%, n = 16, and −18 ± 4%, n = 40), and they were increased by inhibition of phosphatidylinositol 3-kinase (+28 ± 6%, n = 8, and +55 ± 17%, n = 16). Inhibition of Ca2+/CaM complex resulted in a significant decrease of Vmax(160–99 photons/s) that can be explained in part by a reduction of the membrane-associated hOCT2 (−22 ± 6%, n = 9) as determined using FACScan flow cytometry. The data indicate that saturable transport by hOCT2 can be measured by the fluorescent substrates amiloride and ASP and that transport activity for both substrates is regulated similarly. Inhibition of the Ca2+/CaM complex causes changes in transport capacity via hOCT2 trafficking.

Cisplatin nephrotoxicity is critically mediated via the human organic cation transporter 2

Cis-platin is an effective anti-neoplastic agent, but it is also highly nephrotoxic. Here, we clearly identify the human organic cation transporter 2 (hOCT2) as the critical transporter for cis-platin nephrotoxicity in isolated human proximal tubules and offer a potential mechanism for reducing nephrotoxicity in clinical practice. Interaction of cis-platin with hOCT2 in kidney or hOCT1 in liver was investigated with the fluorescent cation 4-[4-(dimethyl-amino)styril]-methylpyridinium in stably transfected HEK293 cells and for the first time in tissues physiologically expressing these transporters, human proximal tubules, and human hepatocyte couplets. Cis-platin (100 micromol/L) inhibited transport via hOCT2-HEK293 but not hOCT1-HEK293. In human proximal tubules cis-platin competed with basolateral organic cation transport, whereas it had no effect in tubules from a diabetic kidney or in hepatocytes. In hOCT2-HEK293 cells treated for 15 hours, incubation with cis-platin induced apoptosis, which was completely suppressed by contemporaneous incubation with the hOCT2 substrate cimetidine (100 micromol/L). These findings demonstrate that uptake of cis-platin is mediated by hOCT2 in renal proximal tubules, explaining its organ-specific toxicity. A combination of cis-platin with other substrates that compete for hOCT2 offers an effective option to decrease nephrotoxicity in the clinical setting.

Uroguanylin and guanylin regulate transport of mouse cortical collecting duct independent of guanylate cyclase C

BACKGROUND

Electrolyte and water homeostasis mostly depend on differentially regulated intestinal and renal transport. Guanylin and uroguanylin were proposed as first hormones linking intestinal with renal electrolyte and water transport, which is disturbed in pathophysiology. Guanylate cyclase C is the intestinal receptor for these peptides, but in guanylate cyclase C-deficient mice renal effects are retained. Unlike for the intestine the sites of renal actions and cellular mechanisms of guanylin peptides are still unclear.

METHODS

After first data on proximal tubular effects in this study their effects are examined in detail in mouse cortical collecting duct (CCD). Effects of guanylin peptides on principal cells of isolated mouse CCD were studied by slow whole-cell patch-clamp analysis, reverse transcription-polymerase chain reaction (RT-PCR), and microfluorimetric measurements of intracellular Ca2+.

RESULTS

Guanylin peptides depolarized or hyperpolarized principal cells. Whereas 8-Br-cyclic guanosine monophosphate (8-Br-cGMP) hyperpolarized, 8-Br-cyclic adenosine monophosphate (8-Br-cAMP) depolarized principal cells. All effects of guanylin peptides were inhibited by Ba2+. Hyperpolarizations were blocked by clotrimazole or protein kinase G (PKG) inhibition, suggesting an involvement of basolateral Ca2+- and cGMP-dependent K+ channels. Effects remained in CCD isolated from guanylate cyclase C-deficient mice. Depolarizations were inhibited by arachidonic acid or inhibition of phospholipase A2 (PLA2), but not by protein kinase A (PKA) inhibition. Conclusion. These results suggest the existence of two signaling pathways for guanylin peptides in principal cells of mouse CCD. One pathway is cGMP- and PKG-dependent but not mediated by guanylate cyclase C, the second involves PLA2 and arachidonic acid. The first pathway most likely leads to an activation of the basolateral K+-conductance while the latter probably results in decreased activity of ROMK channels in the luminal membrane.

Cellular effects of guanylin and uroguanylin

Ingestion of a salty meal induces secretion of guanylin (GN) and uroguanylin (UGN) into the intestinal lumen, where they inhibit Na+ absorption and induce Cl-, HCO3-, and water secretion. Simultaneously, these hormones stimulate renal electrolyte excretion by inducing natriuresis, kaliuresis, and diuresis. GN and UGN therefore participate in the prevention of hypernatremia and hypervolemia after salty meals. The signaling pathway of GN and UGN in the intestine is well known. They activate enterocytes via guanylate cyclase C (GC-C), which leads to cGMP-dependent inhibition of Na+/H+ exchange and activation of the cystic fibrosis transmembrane regulator. In GC-C-deficient mice, GN and UGN still produce renal natriuresis, kaliuresis, and diuresis, suggesting different signaling pathways in the kidney compared with the intestine. Signaling pathways for GN and UGN in the kidney differ along the various nephron segments. In proximal tubule cells, a cGMP- and GC-C-dependent signaling was demonstrated for both peptides. In addition, UGN activates a pertussis toxin-sensitive G-protein-coupled receptor. A similar dual signaling pathway is also known for atrial natriuretic peptide. Recently, a cGMP-independent signaling pathway for GN and UGN was also shown in principal cells of the human and mouse cortical collecting duct. Because GN and UGN activate different signaling pathways in specific organs and even within the kidney, this review focuses on more recent findings on cellular effects and signaling mechanisms of these peptides and their pathophysiologic implications in the intestine and the kidney.

Enhanced Activity of the Myocardial Na + /H + Exchanger NHE-1 Contributes to Cardiac Remodeling in Atrial Natriuretic Peptide Receptor–Deficient Mice

Background— Atrial natriuretic peptide (ANP), through its guanylyl cyclase-A (GC-A) receptor, not only is critically involved in the endocrine regulation of arterial blood pressure but also locally moderates cardiomyocyte growth. The mechanisms underlying the antihypertrophic effects of ANP remain largely uncharacterized. We examined the contribution of the Na + /H + exchanger NHE-1 to cardiac remodeling in GC-A–deficient (GC-A −/− ) mice.

Methods and Results— Fluorometric measurements in isolated adult cardiomyocytes demonstrated that cardiac hypertrophy in GC-A −/− mice was associated with enhanced NHE-1 activity, alkalinization of intracellular pH, and increased Ca 2+ levels. Chronic treatment of GC-A −/− mice with the NHE-1 inhibitor cariporide normalized cardiomyocyte pH and Ca 2+ levels and regressed cardiac hypertrophy and fibrosis, despite persistent arterial hypertension. To characterize the molecular pathways driving cardiac hypertrophy in GC-A −/− mice, we evaluated the activity of 4 prohypertrophic signaling pathways: the mitogen-activated protein kinases (MAPK), the serine-threonine kinase Akt, calcineurin, and Ca 2+ /calmodulin-dependent kinase II (CaMKII). The results demonstrate that all 4 pathways were activated in GC-A −/− mice, but only CaMKII and Akt activity regressed during reversal of the hypertrophic phenotype by cariporide treatment. In contrast, the MAPK and calcineurin/NFAT signaling pathways remained activated during regression of hypertrophy.

Conclusions— On the basis of these results, we conclude that the ANP/GC-A system moderates the cardiac growth response to pressure overload by preventing excessive activation of NHE-1 and subsequent increases in cardiomyocyte intracellular pH, Ca 2+ , and CaMKII as well as Akt activity.

Guanylin and uroguanylin regulate electrolyte transport in isolated human cortical collecting ducts

BACKGROUND

Guanylin and uroguanylin link intestinal and renal electrolyte and water transport. Their function in intestine is well studied, but renal actions are less understood. Uroguanylin concentrations are increased in patients with chronic renal failure, nephrotic syndrome, or those on dialysis. Guanylate cyclase C (GC-C) is the receptor first described for these peptides. In guanylate cyclase C-deficient mice guanylin- and uroguanylin-induced renal natriuresis, kaliuresis, and diuresis are retained.

METHODS

Effects of guanylin and uroguanylin on principal cells of human cortical collecting ducts (CCD) isolated from kidneys after tumor nephrectomy were investigated. Reverse transcription-polymerase chain reaction (RT-PCR), slow whole-cell patch-clamp, and microfluorimetric analysis of intracellular Ca(2+) were used. Here we present first functional measurements of isolated human CCD.

RESULTS

Principal cells of CCD were identified by the amiloride-induced hyperpolarization of principal cells (-3.8 +/- 0.3 mV) (N= 52). Cells depolarized upon guanylin or uroguanylin (each 10 nmol/L) by 3.3 +/- 0.8 mV (N= 12) and 3.4 +/- 0.5 mV (N= 18), respectively, but were hyperpolarized by 8Br-cyclic guanosine monophosphate (cGMP) (100 micromol/L) (-3.0 +/- 0.2 mV) (N= 4). mRNA for GC-C was not detected in CCD. Effects of both peptides were inhibited by Ba(2+) (1 mmol/L) or phospholipase A(2) (PLA(2)) inhibition (AACOCF(3)) (5 micromol/L).

CONCLUSION

These findings suggest a new cGMP- and GC-C-independent but PLA(2)-dependent signaling pathway for these peptides in the kidney. Most likely guanylin and uroguanylin inhibit luminal K(+) channels of principal cells of human CCD via this pathway. This depolarization of principal cells consequently reduces the driving force of Na(+) and water reabsorption, explaining natriuresis and diuresis caused by these peptides.

Prior vasorelaxation enhances diadenosine polyphosphate-induced contractility of rat mesenteric resistance arteries

Low-threshold concentrations of diadenosine polyphosphates (ApnA: Ap3A, Ap4A, Ap5A, Ap6A) or ATP, which at basal vessel tone induce just measurable vasoconstrictions, induce up to ten times enhanced vasoconstrictions of previously relaxed (by acetylcholine or sodium nitroprusside or 8Br2 cGMP or isoproterenol or levcromakalim) pre-contracted rat mesenteric resistance arteries (MrA) in a microvessel-myograph. These enhanced vasoconstrictions were of similar magnitude for threshold concentrations of all ApnA.Possibly, the low concentrations of ApnA reverse the prior vasorelaxation by inhibiting a common vasorelaxation pathway, but obviously this is not due to inhibition of guanylate cyclase, which has been previously described to be inhibited by ApnA, because the enhanced vasoconstrictions can be observed with guanylate cyclase-independent vasorelaxants (8Br2 cGMP, isoproterenol or levcromakalim), too. The enhanced vasoconstrictions are endothelium-independent because after mechanical vascular de-endothelialization the results were identical. De-endothelialized vessels, which fail to relax by acetylcholine, showed no enhanced ApnA-induced vasoconstrictions, demonstrating that the mere prior vasorelaxation of the vessel is required to provide the enhanced vasoconstriction by ApnA. Furthermore, the enhanced contractility is not based on a potentiation of the phenylephrine contraction because it equally occurs with other agents used for arterial pre-contraction. Systemically applied ApnA considerably decrease arteriovascular resistance, resulting in hypotension. But here it is demonstrated that a preceding vasorelaxation enables the resistance arteries to generate a strong and persistent ApnA-induced vasoconstriction. Thus, in vivo at very low concentrations ApnA may serve to counteract severe conditions of hypotension (e.g., shock syndrome or anaphylaxis) by the constriction of resistance arteries.

Mechanisms of actions of guanylin peptides in the kidney

After a salty meal, stimulation of salt excretion via the kidney is a possible mechanism to prevent hypernatremia and hypervolemia. Besides the well known hormonal regulators of salt and water excretion in the distal nephron, arginine vasopressin and aldosterone, guanylin (GN) peptides produced in the intestine were proposed to be intestinal natriuretic peptides. These peptides inhibit Na+ absorption in the intestine and induce natriuresis, kaliuresis and diuresis in the kidney. The signaling pathway of GN peptides in the intestine is well known. They activate enterocytes via guanylate cyclase C (GC-C) and increase the cellular concentration of cGMP which leads to secretion of Cl-, HCO3- and water into the intestinal lumen and to inhibition of Na+ absorption. Guanylin peptides are filtered in the glomerulus, and additionally synthesized and excreted by tubular cells. They activate receptors located in the luminal membrane of the tubular cells along the nephron. In GC-C deficient mice renal effects of GN peptides are retained. In human, rat, and opossum proximal tubule cells, a cGMP-dependent signaling was demonstrated, but in addition GN peptides apparently also activate a PT-sensitive G-protein coupled receptor. A similar dual signaling pathway is also known for other natriuretic peptides like atrial natriuretic peptide. A cGMP-independent signaling pathway of GN peptides is also shown for principal cells of the human cortical collecting duct where the final hormonal regulation of electrolyte homeostasis takes place. This review will focus on the current knowledge on renal actions of GN peptides and specifically address novel GC-C- and cGMP-independent signaling mechanisms.

Acute rejection after rat renal transplantation leads to downregulation of NA+ and water channels in the collecting duct

Renal transplantation is associated with alterations of tubular functions and of the renin-angiotensin-aldosterone system. The underlying cellular and molecular mechanisms are unclear. We used an allogeneic rat renal transplantation model of acute rejection with and without immunosuppression by cyclosporine A (CsA) and a syngeneic model as control. Uninephrectomized Lewis or Lewis-Brown-Norway (LBN) rats received a kidney from LBN-rats. Renal transporters and receptors were analyzed by immunohistochemistry, semiquantitative RT-PCR and Western-blot analysis. Intracellular Na(+) was analyzed microfluorimetrically in isolated cortical collecting ducts. mRNA expression and function of the epithelial Na(+)-channel (ENaC) and mRNA and protein expression of the water-channel AQP2 were downregulated in transplanted kidneys undergoing rejection. Expression of the serum- and glucocorticoid-kinase (Sgk1) was decreased and that of the ubiquitin-protein ligase Nedd4-2 was increased. These changes were absent under CsA-therapy and in syngeneic model. Expression and function of the Na(+)-K(+)-ATPase, expression of the secretory K(+)-channel and of the mineralocorticoid receptor remained unchanged. Reduced ENaC function is likely due to decreased Sgk1- and increased Nedd4-2 mRNA expression leading to reduced ENaC expression in the membrane. These acute downregulations of ENaC and AQP2 may be triggered to reduce energy consumption in the distal nephron to protect the kidney immediately after transplantation.

Individual PKC-Phosphorylation Sites in Organic Cation Transporter 1 Determine Substrate Selectivity and Transport Regulation

To elucidate the molecular mechanisms underlying stimulation of rat organic cation transporter type 1 (rOCT1) by protein kinase C (PKC) activation, functional properties and regulation of rOCT1 stably expressed in HEK293 cells after site-directed mutagenesis of putative PKC phosphorylation-sites were compared with wild-type (WT) rOCT1 using microfluorometric measurements with the fluorescence organic cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP(+)). Either substitutions of single (S286A, S292A, T296A, S328A, and T550A) or of all five PKC-sites (5x-PKC) with alanine suppressed PKC-induced stimulation of ASP(+) uptake, whereas regulation by p56(lck) tyrosine kinase was conserved in all mutants. Remarkably, the apparent affinities for TEA(+), TPA(+), and quinine were changed differently in each mutant (EC(50) in WT, S286A, S292A, T296A, S328A, T550A, and 5x-PKC in mumol: TEA(+): 105, 153, 56, 1135, 484, 498, 518; TPA(+): 0.1, 2.1, 0.3, 1.0, 43, 0.3, 2.2; quinine: 1.5, 3.0, 2.5, 4.8, 81, 7.6, 8.9, respectively). After mutations, no effects of PKC activation on apparent affinity of rOCT1 for these substrates could be detected, in contrast to what was observed in WT. PKC activation had no significant effect on rOCT1 trafficking from intracellular pools to the cell membrane. Substitution of all PKC sites suppressed PKC-induced phosphorylation of rOCT1. In conclusion, it was found that the presence of all five potential PKC phosphorylation sites is necessary for the PKC-induced stimulation of rOCT1. The different effects on the EC(50) values by the different mutations suggest that the large intracellular loop participates in building the substrate binding pocket of rOCT1 or specifically modulates its structure.

Regulation of the human organic cation transporter hOCT1

The human organic cation transporter type 1 (hOCT1) is an important transport system for small organic cations in the liver. Organic cation transporters are regulated by different signaling pathways, but the regulation of hOCT1 has not yet been studied. In this work, we have for the first time investigated the regulation of hOCT1. hOCT1 was expressed in Chinese hamster ovary cells (CHO-hOCT1) and in human embryonic kidney cells (HEK293-hOCT1). Its activity was monitored using microfluorimetry with the fluorescent organic cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP(+)) as substrate. hOCT1 expressed in CHO-cells was inhibited by protein kinase A (PKA) activation (1 microM forskolin, -58 +/- 6%, n = 12), calmodulin inhibition (0.1 microM calmidazolium, -68 +/- 3%, n = 6; 10 microM ophiobolin A, -48 +/- 10%, n = 7), calmodulin-dependent kinase II inhibition (1 microM KN62, -78 +/- 4%, n = 12), and inhibition of p56(lck) tyrosine kinase (10 microM aminogenistein, -35 +/- 7%, n = 12). The apparent affinities for TEA(+) were lower in CHO-hOCT1 than in HEK293-hOCT1, while those for TPA(+) and quinine were almost identical; the rank order of EC(50) values (TPA(+) > quinine > TEA(+)) was independent of the expression system. EC(50) values for TEA(+) in CHO-hOCT1 or HEK293-hOCT1 were increased under calmidazolium incubation (6.3 and 1.4 mM, respectively). hOCT1 was inhibited by PKA and endogenously activated by calmodulin, calmodulin-dependent kinase II, and p56(lck) tyrosine kinase. Regulation pathways were the same in the two expression systems. Since apparent substrate affinities depend on activity of regulatory pathways, the expression system plays a role in determining the substrate affinities.

Regulation of organic cation transport

Transport of organic cations (OC) is important for the recycling of endogenous OC and also a necessary step for detoxification of exogenous OC in the body. Even though the identification and characterisation of numerous OC transporters in recent years has allowed the elucidation of molecular mechanisms underlying OC transport, elucidation of the regulation of this transport is just beginning. This review summarises the general properties of OC transport and then analyses the literature on the regulation of these processes. Studies on short- and long-term regulation of OC transport are considered separately. Important aspects of short-term regulation have been clarified and the regulatory pathways of several OC transporters have been characterised. Short-term regulation appears to be transporter subtype-, tissue- and species-dependent and to involve transporter phosphorylation. Transporter phosphorylation may alter the affinity for substrates or/and expression on the plasma membrane. Even though several studies have shown long-term regulation of OC transport, the pathophysiological meaning of these changes are not well understood. In this case, regulation seems to be subtype-, tissue- and gender-specific. Further research is necessary to clarify this important issue of regulation of OC transport.

Vasoactivity of diadenosine polyphosphates in human small renal resistance arteries

BACKGROUND

We examined for the first time the vascular effects of purinergic agents that contribute to the regulation of peripheral vascular resistance in human small renal resistance arteries (hRRAs).

METHODS AND RESULTS

Diadenosine polyphosphates (ApnAs, n = 3-6) and ATP, mounted in a microvessel myograph, caused vasoconstriction in hRRAs (rank order of potency: Ap5A > Ap6A = Ap4A > Ap3A = ATP). ADP, AMP and adenosine had less contractile potency than ApnA, suggesting that the observed effects were not induced by ApnA degradation products. The ApnA agent, Ap5A, but not Ap4A, induced vasoconstrictions that were inhibited by pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; a P2X purinoceptor antagonist), but not by ADP3'5' (a P2Y purinoceptor antagonist). In pre-contracted hRRAs, all of the ApnA agents caused vasorelaxation, and the potencies did not differ from each other. The ApnA degradation products had less vasorelaxing potencies than ApnA, suggesting that the vasorelaxation was caused by the ApnA agents themselves. Ap4A-induced vasorelaxation was inhibited by ADP3'5' and PPADS. In contrast, Ap5A-induced vasorelaxation was not antagonized by ADP3'5', but was antagonized more strongly by PPADS than was Ap4A.

CONCLUSIONS

We found that the tone of resistance arteries in human kidneys can be considerably influenced by these purinergic agonists, and most potently by ApnAs. Ap5A-induced vasoconstriction appeared to be mediated by P2X purinoceptors, whereas constriction due to Ap4A was caused by a different purinoceptor. Vasorelaxation due to Ap4A, but not Ap5A, appeared to be mediated by P2Y purinoceptors.

Genistein potentiates the ANP effect on a K(+)-conductance in HEK-293 cells

HEK-293 cells are known to reflect many features of the late distal tubule. Furthermore, they have the ability to release urodilatin, the structural analog to ANP. RT-PCR was performed to test for the expression of natriuretic peptide receptors. While the mRNA for the human ANP receptor (NPR-A, GC-A) could be amplified, the CNP-specific receptor NPR-B (GC-B) and the receptor specific for guanylins, GC-C, could not be detected. In patch clamp experiments the effects of ANP (10 nM) on membrane voltage (V(m)) were monitored and HEK-293 cells depolarized by 2.3 +/- 0.5 mV (n=14). In the presence of the EGF receptor blocker genistein (10 microM) the effect of ANP was increased by 65% to 3.9 +/- 0.8 mV (n=14). After removal of genistein the ANP-mediated depolarization further increased by 147% to 5.7 +/- 1.0 mV (n=14). ANP given repetitively without genistein had no increasing depolarizing effect in HEK-293 cells with time. The ANP effect could be fully blocked by 1 mM Ba(2+) and by 1 microM of the specific PKG inhibitor KT5823 indicating that ANP inhibits a K(+)-conductance via a cGMP-dependent protein kinase. Genistein itself hyperpolarized the membrane voltage of HEK-293 cells by -3.9 +/- 0.6 mV (n=11) and this effect could also be fully blocked by Ba(2+) (-0.3 +/- 0.1 mV, n=5), indicating that genistein activates a K(+)-conductance which contributes significantly to the membrane potential of HEK-293 cells.

Renal Transplantation Modulates Expression and Function of Receptors and Transporters of Rat Proximal Tubules

Kidney transplantation often leads to disturbances of solute and volume maintenance in humans. To investigate underlying mechanisms, expression and function of renal transporters and receptors of the proximal tubule (PT) were analyzed in an acute rejection model of rat kidney transplantation. Semiquantitative RT-PCR and Western blot, histology, immunohistochemistry, and microfluorometry were performed on whole kidneys and isolated PT. With acute rejection, Na+/H+-exchanger type-3 (NHE-3) was markedly downregulated. Na+-HCO(3)(-)-cotransporter (NBC-1) and Na+-glucose transporter type-2 (SGLT2) were upregulated after transplantation. Expressions of Na+/H+-exchanger type-1 (NHE-1), Na+/K+-ATPase (NKA), angiotensin II (AngII) receptor (AT-1), or natriuretic peptide receptor (GC-A) were unaltered. Microfluorometric analyses of intracellular pH, Na+, and Ca2+ demonstrated a decrease in NHE-3 function and AngII-mediated stimulation of NHE-3. AngII-mediated inhibition of NHE-1 and function of all other transporters tested remained unaltered. Function of AT-1 and GC-A were unaffected. Reduced expression of NHE-3 was also confirmed by semiquantitative immunohistochemistry. These findings suggest that expression and function of transmembrane proteins involved in Na+-transport after transplantation and rejection is specifically modulated. The local renin-angiotensin-system is apparently not altered. Downregulation of NHE-3 may be a protective mechanism occurring in the graft.

Signaling and distribution of NPR-Bi, the human splice form of the natriuretic peptide receptor type B

Recently, we described a splice variant of the human natriuretic peptide receptor type B (NPR-Bi) in human proximal tubule cells [immortalized human kidney epithelial cells (IHKE-1) that lacks a functional guanylate cyclase domain (Hirsch JR, Meyer M, Mägert HJ, Forssmann WG, Mollerup S, Herter P, Weber G, Cermak R, Ankorina-Stark I, Schlatter E, and Kruhøffer M. J Am Soc Nephrol 10: 472-480, 1999). Its signaling pathway does not include cGMP, cAMP, or Ca2+ but leads to inhibition of K+ channels. In patch-clamp experiments, effects of tyrosine kinase receptor blockers on C-type natriuretic peptide (CNP)-mediated depolarizations of membrane voltages (Vm) of IHKE-1 cells were tested. The epidermal growth factor (EGF) receptor blocker genistein (10 microM) abolished the effect of CNP (0.2 +/- 0.4 mV, n = 7), and comparable results were obtained with 10 microM daidzein (n = 8). Aminogenistein (10 microM, n = 5) and tyrphostin AG1295 (10 microM, n = 5) had no significant effects. EGF (1 nM) hyperpolarized cells by -5.3 +/- 0.8 mV (n = 5). This effect was completely blocked by genistein or daidzein. The Cl- channel blocker NPPB (10 microM, n = 5) inhibited the EGF-mediated hyperpolarization. mRNA expression of NPR-B and NPR-Bi shows reversed patterns along the human nephron. NPR-B is highly expressed in glomeruli and proximal tubules, whereas NPR-Bi shows strong signals in the distal nephron. Expression of NPR-Bi in the cortical collecting duct was also confirmed with immunohistochemistry. In other human tissues, NPR-Bi shows strongest expression in pancreas and lung, whereas in the heart and liver NPR-B is the dominating receptor. In conclusion, CNP inhibits an apical K+ channel in IHKE-1 cells independently of cGMP and so far this effect can only be blocked by genistein and daidzein. Tyrosine phosphorylation might be the missing link in the signaling pathway of CNP/NPR-Bi.

Impact of renal transplantation on small vessel reactivity

BACKGROUND

The function of large arteries is altered after renal transplantation. Whether transplantation also induces agonist-dependent functional changes in small arterial renal and extrarenal vessels has not yet been studied.

METHODS

Chronic rejection was induced by grafting Lewis rats with kidneys from Fischer rats (FL). Rats that underwent transplantation were bilaterally nephrectomized. Rats that underwent syngeneic transplantation, uninephrectomized rats, uninephrectomized rats with denervated kidneys or with kidneys made ischemic, and native rats served as controls. All animals were treated with cyclosporine for 10 days. Eighteen weeks after surgery, the reactivity of small arteries (220-270 microm) was tested by myography.

RESULTS

Weight gain, glomerular filtration rate, and arterial pressure were similar in all groups, whereas proteinuria was elevated in FL. Only kidneys from FL showed glomerular lesions, tubular atrophy, and vasculopathy. Responsiveness of coronary, mesenteric, and femoral resistance vessels to both constrictor and dilator agonists was similar in transplanted and nontransplanted animals. Resistance vessels obtained from both allogeneically and syngeneically transplanted kidneys were more sensitive to norepinephrine, phenylephrine, angiotensin II, and vasopressin than renal vessels from weight-matched controls. Vasodilation in response to acetylcholine and sodium nitroprusside was mitigated in transplanted versus nontransplanted kidneys.

CONCLUSIONS

In rat renal transplantation, renal resistance vessel responsiveness to constrictor or dilator stimuli is altered. Extrarenal small vessel function is not affected. The changes in function of renal resistance vessels are not explained by reduction of nephron mass, denervation, ischemia, or chronic rejection.

Regulation of human organic cation transporter hOCT2 by PKA, PI3K, and calmodulin-dependent kinases

Properties and regulation of the human organic cation (OC) transporter type 2 (hOCT2) expressed in HEK-293 cells were extensively characterized using the fluorescent OC 4-[4-(dimethylamino)styryl]-N-methylpyridinium (ASP(+)). ASP(+) uptake was electrogenic and inhibited by TPA(+) (EC(50) = 2.7 microM), tetraethylammonium (EC(50) = 35 microM), cimetidine (EC(50) = 36 microM), or quinine (EC(50) = 6.7 microM). Stimulation with carbachol or ATP decreased initial uptake by 44 +/- 3 (n = 14) and 34 +/- 4% (n = 21), respectively, independently of PKC but dependent on phosphatidylinositol 3-kinase (PI3K). PKA stimulation decreased uptake by 18 +/- 4% (n = 40). Inhibition of calmodulin (CaM), Ca(2+)/CaM-dependent kinase II, or myosin light chain kinase decreased uptake by 63 +/- 2 (n = 15), 40 +/- 4 (n = 30), and 31 +/- 4% (n = 16), respectively. Inhibition of CaM resulted in a significant change in the EC(50) value for the inhibition of ASP(+) uptake by tetraethylammonium. In conclusion, we demonstrate that the hOCT2 is inhibited by PI3K and PKA and activated by a CaM-dependent signaling pathway, probably via a change in substrate affinity.

Mesenteric and renal vascular effects of diadenosine polyphosphates (APnA)

Diadenosine polyphosphates (APnA) are endogenous dinucleoside molecules consisting of two adenosine moieties linked via their 5'-ribose positions by a variable number of phosphate groups. APnA have been shown to be present in different cell types and to be released from platelets as well as co-released with catecholamines and ATP from bovine adrenal medulla. Candidate metabolites of APnA are ATP, ADP, AMP and adenosine. Vascular effects induced by APnA and their metabolites in several models have been reported to be mediated by A1- and A2-adenosine receptors as well as P2-purinoceptors. APnA have been demonstrated to differentially affect regional perfusion, to influence cardiac output and blood pressure as well as the reactivity of isolated blood vessels and vascular beds. Vascular effects of APnA vary with the number of phosphate groups linking the adenosine molecules. This review outlines the effects of APnA on mesenteric and renal circulation. The effects of the antagonists varying with the type of vascular bed and the heterogeneous and dynamic vascular effects of diadenosine polyphosphates indicate a regionally different distribution of P2X and of P2Y purinoceptors in resistance arteries from different vascular beds. Although APnA have vasoconstrictor effects on the local level, it was repeatedly confirmed that systemically applied APnA induce hypotensive effects. The vasoconstrictor effects of APnA in isolated vessels are most prominent under resting tone conditions. In vivo, the vasculature exhibits a vasotone which makes dilatory effects more likely. Information on effects of APnA in vivo is still limited despite the fact that these compounds already have been used in man.

The organic cation transporters rOCT1 and hOCT2 are inhibited by cGMP

The electrogenic cation transporters OCT1 and OCT2 in the basolateral membrane of renal proximal tubules mediate the first step during secretion of organic cations. Previously we demonstrated stimulation and change of selectivity for rat OCT1 (rOCT1) by protein kinase C. Here we investigated the effect of cGMP on cation transport by rOCT1 or human OCT2 (hOCT2) after expression in human embryonic kidney cells (HEK293) or oocytes of Xenopus laevis. In HEK293 cells, uptake was measured by microfluorimetry using the fluorescent cation 4-(4-(dimethyl-amino)styryl)-N-methylpyridinium iodide (ASP + ) as substrate, whereas uptake into Xenopus laevis oocytes was measured with radioactively labelled cations. In addition, ASP +-induced depolarizations of membrane voltages (Vm) were measured in HEK293 cells using the slow whole-cell patch-clamp method. Incubation of rOCT1-expressing HEK293 cells for 10 min with 100 mM 8-Br-cGMP reduced initial ASP + uptake by maximally 78% with an IC50 value of 24 +/- 16 mM. This effect was not abolished by the specific PKG inhibitor KT5823, indicating that a cGMP-dependent kinase is not involved. An inhibition of ASP + uptake by rOCT1 in HEK293 cells was also obtained when the cells were incubated for 10 min with 100 mM cGMP, whereas no effect was obtained when cGMP was given together with ASP +. ASP + (100 mM)-induced depolarizations of Vm were reduced in the presence of 8-Br-cGMP (100 mM) by 44 +/- 11% (n = 6). Since it could be demonstrated that [3H]cGMP is taken up by an endogeneous cyanine863-inhibitable transporter, the effect of cGMP is probably mediated from inside the cell. Uptake measurements with [14C]tetraethylammonium and [3H]2-methyl-4-phenylpyridinium in Xenopus laevis oocytes expressing rOCT1 performed in the absence and presence of 8-Br-cGMP showed that cGMP does not interact directly with the transporter. The data suggest that the inhibition mediated by cGMP observed in HEK293 cells occurs most likely via a mammalian cGMP-binding protein that interacts with OCT1-2 transporters.

Vasoactivity of diadenosine polyphosphates in human small mesenteric resistance arteries

Diadenosine polyphosphates (ApnA) (n = 3-6) induced vasoconstrictions in isolated human mesenteric resistance arteries (hMRAs) mounted in a microvessel myograph (rank order of potency: Ap5A > Ap6A > Ap4A > Ap3A). The contractile effects of ApnA in hMRA were similar to their effects in rat MRA investigated previously. ATP, ADP, AMP, and adenosine had less contractile potency than ApnA, suggesting that the observed effects were not induced by the degradation products of ApnA. Ap4A- and Ap5A-induced vasoconstriction was inhibited by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (P2X purinoceptor antagonist) but not by ADP3'5' (P2Y purinoceptor antagonist). Thus, this purinergic vasoconstriction of hMRA seems to be P2X but not P2Y purinoceptor-mediated. In precontracted hMRA all ApnA caused vasorelaxations but (in contrast to rat MRA) the potencies of the ApnA did not differ significantly from each other. The ApnA degradation products had less vasorelaxing potency than ApnA, demonstrating that the vasorelaxations can be ascribed to the ApnA themselves. Ap5A-induced vasorelaxation of hMRA could neither be inhibited with ADP3'5' nor with PPADS, which reveals a decisive difference to the rat MRA where the inhibitory profile demonstrated the importance of the P2Y purinoceptor for Ap5A-induced vasorelaxation. However, Ap4A-induced vasorelaxation in hMRA could be inhibited by ADP3'5'. These findings show that Ap4A-induced vasorelaxation in hMRA is due to P2Y purinoceptor activation, that Ap5A evokes vasorelaxation in hMRA via another mechanism than Ap4A, and that data derived from the animal model cannot be simply transferred to human conditions.

Inhibition of Na(+)-dependent transporters in cystine-loaded human renal cells: electrophysiological studies on the Fanconi syndrome of cystinosis

Cystinosis is the most common cause of the renal Fanconi syndrome in children, leading to severe electrolyte disturbances and growth failure. A defective lysosomal transporter, cystinosin, results in intralysosomal accumulation of cystine. Loading cells with cystine dimethyl ester (CDME) is the only available model for this disease. This model was used to present electrophysiologic studies on immortalized human kidney epithelial (IHKE-1) cells that had been derived from the proximal tubule with the slow whole-cell patch clamp technique. Basal membrane voltages (V(m)) of IHKE-1 cells were -30.7 +/- 0.4 mV (n = 151). CDME concentration-dependently altered V(m) with an initial depolarization (2.7 +/- 0.2 mV;n = 76; 1 mM CDME) followed by a more pronounced hyperpolarization (-9.9 +/- 1.0 mV;n = 49). Three Na(+)-dependent transporters were examined. Alanine (1 mM) depolarized IHKE-1 cells by 17.6 +/- 0.7 mV (n = 59), and phosphate (1.8 mM) depolarized by 9.7 +/- 1.1 mV (n = 18). Acidification of IHKE-1 cells with propionate (20 mM) resulted in a depolarization of V(m) by 7.1 +/- 0.3 mV (n = 21) followed by a repolarization by 2.9 +/- 0.3 mV/min (n = 17), reflecting Na(+)/H(+)-exchanger activity. Acute addition of 1 mM CDME did not alter the alanine- and propionate-induced changes in V(m), but it reduced the phosphate-induced depolarization by 37 +/- 9% (n = 10). Incubation with 1 mM CDME reduced the activity of all three transporters. Depolarizations by alanine and phosphate and the repolarization after propionate were inhibited by 57 +/- 4% (n =30), 45 +/- 9% (n = 9), and 78 +/- 15% (n = 8), respectively. In conclusion, this study demonstrates that CDME acutely alters V(m) of IHKE-1 cells and that at least three Na(+)-dependent transporters are inhibited, the Na(+)-phosphate cotransporter most sensitively. This might suggest that phosphate depletion and dissipation of the Na(+)-gradient are involved in the development of the Fanconi syndrome of cystinosis.

Guanylin, uroguanylin, and heat-stable euterotoxin activate guanylate cyclase C and/or a pertussis toxin-sensitive G protein in human proximal tubule cells

Membrane guanylate cyclase C (GC-C) is the receptor for guanylin, uroguanylin, and heat-stable enterotoxin (STa) in the intestine. GC-C-deficient mice show resistance to STa in intestine but saluretic and diuretic effects of uroguanylin and STa are not disturbed. Here we describe the cellular effects of these peptides using immortalized human kidney epithelial (IHKE-1) cells with properties of the proximal tubule, analyzed with the slow-whole-cell patch clamp technique. Uroguanylin (10 or 100 nm) either hyperpolarized or depolarized membrane voltages (V(m)). Guanylin and STa (both 10 or 100 nm), as well as 8-Br-cGMP (100 microm), depolarized V(m). All peptide effects were absent in the presence of 1 mm Ba(2+). Uroguanylin and guanylin changed V(m) pH dependently. Pertussis toxin (1 microg/ml, 24 h) inhibited hyperpolarizations caused by uroguanylin. Depolarizations caused by guanylin and uroguanylin were blocked by the tyrosine kinase inhibitor, genistein (10 microm). All three peptides increased cellular cGMP. mRNA for GC-C was detected in IHKE-1 cells and in isolated human proximal tubules. In IHKE-1 cells GC-C was also detected by immunostaining. These findings suggest that GC-C is probably the receptor for guanylin and STa. For uroguanylin two distinct signaling pathways exist in IHKE-1 cells, one involves GC-C and cGMP as second messenger, the other is cGMP-independent and connected to a pertussis toxin-sensitive G protein.

Properties and regulation of organic cation transport in freshly isolated human proximal tubules

The kidney, and more specifically the proximal tubule, is the main site of elimination of cationic endogenous metabolites and xenobiotics. Although numerous studies exist on renal organic cation transport of rat and rabbit, no information is available from humans. Therefore, we examined organic cation transport and its regulation across the basolateral membrane of isolated human proximal tubules. mRNA for the cation transporters hOCT1 and hOCT2 as well as hOCTN1 and hOCTN2 was detected in these tubules. Organic cation transport across the basolateral membrane of isolated collapsed proximal tubules was recorded with the fluorescent dye 4-(4-dimethylamino)styryl-N-methylpyridinium (ASP(+)). Depolarization of the cells by rising extracellular K(+) concentration to 145 mm reduced ASP(+) uptake by 20 +/- 5% (n = 15), indicating its electrogeneity. The substrates of organic cation transport tetraethylammonium (K(i) = 63 microm) and cimetidine (K(i) = 11 microm) as well as the inhibitor quinine (K(i) = 2.9 microm) reduced ASP(+) uptake concentration dependently. Maximal inhibition reached with these substances was approximately 60%. Stimulation of protein kinase C with 1,2-dioctanoyl-sn-glycerol (DOG, 1 microm) or ATP (100 microm) inhibited ASP(+) uptake by 30 +/- 3 (n = 16) and 38 +/- 13% (n = 6), respectively. The effect of DOG could be reduced with calphostin C (0.1 microm, n = 7). Activation of adenylate cyclase by forskolin (1 microm) decreased ASP(+) uptake by 29 +/- 3% (n = 10). hANP (10 nm) or 8-bromo-cGMP (100 microm) also decreased ASP(+) uptake by 17 +/- 3 (n = 9) or 32 +/- 5% (n = 10), respectively. We show for the first time that organic cation transport across the basolateral membrane of isolated human proximal tubules, most likely mediated via hOCT2, is electrogenic and regulated by protein kinase C, the cAMP- and the cGMP-dependent protein kinases.

Cellular localization, membrane distribution, and possible function of guanylyl cyclases A and 1 in collecting ducts of rat

BACKGROUND

Natriuretic peptides regulate Na+ and H(2)O transport in the cortical collecting duct (CCD). We have shown that natriuretic peptides have no effect on ion conductances or water transport of principal cells (PC) even though a cGMP-regulated K+ channel is located in the basolateral membrane of these cells.

METHODS

RT-PCR was used to screen for different guanylyl cyclases (GC) in CCD and to look for the expression of GC-1 and GC-A mRNA in CCD of male and female Wistar and Sprague-Dawley rats. Polyclonal antibodies were raised against the detected GC. BCECF was used to investigate the effects of ANP on intracellular pH in intercalated cells (IC).

RESULTS

GC-A and GC-1 were detected. GC-A was immunolocalized in the luminal membrane of IC while GC-1 was mainly found in the luminal membrane of PC. GC-1 is expressed in Sprague-Dawley and Wistar rats except for male Sprague-Dawley rats, while GC-A is expressed in all strains. ANP (160 nM, n=11), urodilatin (140 nM, n=6), which had no effect in PC, significantly decreased pH(i) by 0.02+/-0.01 and 0.03 +/- 0.01 Units in IC, respectively. ANP as well as urodilatin and 8-Br-cGMP decreased the pH(i) recovery after acidification by 30 +/- 6% (n=12), 37 +/- 7% (n=8), and 19 +/- 3% (n=8), respectively.

CONCLUSION

GC-A is located in the luminal membrane of IC of rat CCD and ANP acts through this receptor when regulating pH(i) via an inhibition of the Na+/H+-exchanger. PC do not possess GC-A. GC-1 seems to be the only GC in these cells of most rat strains tested and therefore, it could be responsible for the regulation of K+ channels in the basolateral membrane via cGMP-dependent protein kinase.

cGMP serves as an extracellular regulator of a Ca(2+)-dependent K(+) channel in immortalized human proximal tubule cells

Recently we showed that a K(+) channel in immortalized human kidney epithelial (IHKE-1) cells derived from the proximal tubule is regulated by natriuretic peptides in cell-attached patches and directly regulated by cGMP in excised inside-out oriented membrane patches [1]. The patch clamp technique was used to investigate the regulatory effect of extracellular, non-membrane permeable cGMP on membrane voltage and the regulation of this K(+) channel in outside-out oriented membrane patches. In 7 out of 7 experiments the membrane voltage of IHKE-1 cells depolarized by 3.9 +/- 0.1 mV when the non-membrane permeable cGMP was added to the bath solution. In outside-out oriented membrane patches cGMP inhibited P(o) already at 1 microM (-12 +/- 4%, n=7), at 0.1 mM inhibition of P(o) reached -39 +/- 6% (n=14). cAMP (0.1 mM) only had a weak inhibitory effect (n=7). GTP and ATP (n=7 each) had no significant effect on P(o) from the outside. When cGMP was added to the pipette solution in experiments with outside-out oriented membranes cGMP still inhibited this K(+) channel from the outside by 36 +/- 6% (n=6). In 4 paired experiments 8-Br-cGMP (0.1 mM) showed a significantly higher inhibitory effect on P(o) compared to cGMP (0.1 mM). cGMP inhibits a K(+) channel in human proximal tubule cells from the outside and may serve as an autocrine and paracrine regulatory factor in the kidney.

High salt intake increases uroguanylin expression in mouse kidney

The intestinal peptides, guanylin and uroguanylin, may have an important role in the endocrine control of renal function. Both peptides and their receptor, guanylyl cyclase C (GC-C), are also expressed within the kidney, suggesting that they may act locally in an autocrine/paracrine fashion. However, their physiological regulation within the kidney has not been studied. To begin to address this issue, we evaluated the distribution of uroguanylin and guanylin messenger RNA (mRNA) in the mouse nephron and the regulation of renal expression by changes in dietary salt/water intake. Expression was determined in 1) wild-type mice, 2) two strains of receptor-guanylyl cyclase-deficient mice (ANP-receptor-deficient, GC-A-/-, and GC-C-deficient mice); and 3) cultured renal epithelial (M-1) cells, by RT-PCR, Northern blotting and immunocytochemistry. Renal uroguanylin messenger RNA expression was higher than guanylin and had a different distribution pattern, with highest levels in the proximal tubules, whereas guanylin was mainly expressed in the collecting ducts. Uroguanylin expression was significantly lower in GC-C-/- mice than in GC-A-/- and wild-types, suggesting that absence of a receptor was able to down-regulate ligand expression. Salt-loading (1% NaCl in drinking water) increased uroguanylin-mRNA expression by >1.8-fold but had no effect on guanylin expression. Uroguanylin but not guanylin transcripts were detected in M-1 cells and increased in response to hypertonic media (+NaCl or mannitol). Our results indicate that high-salt intake increases uroguanylin but not guanylin expression in the mouse kidney. The synthesis of these peptides by tubular epithelium may contribute to the local control of renal function and its adaptation to dietary salt.

Cellular localization of the potassium channel Kir7.1 in guinea pig and human kidney

BACKGROUND

K(+) channels have important functions in the kidney, such as maintenance of the membrane potential, volume regulation, recirculation, and secretion of potassium ions. The aim of this study was to obtain more information on the localization and possible functional role of the inwardly rectifying K(+) channel, Kir7.1.

METHODS

Kir7.1 cDNA (1114 bp) was isolated from guinea pig kidney (gpKir7.1), and its tissue distribution was analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, a genomic DNA fragment (6153 bp) was isolated from a genomic library. cRNA was expressed in Xenopus laevis oocytes for functional studies. Immunohistochemistry and RT-PCR were used to localize Kir7.1 in guinea pig and human kidney.

RESULTS

The expression of gpKir7.1 in Xenopus laevis oocytes revealed inwardly rectifying K(+) currents. The reversal potential was strongly dependent on the extracellular K(+) concentration, shifting from -14 mV at 96 mmol/L K(+) to -90 mV at 1 mmol/L K(+). gpKir7.1 showed a low affinity for Ba(2+). Significant expression of gpKir7.1 was found in brain, kidney, and lung, but not in heart, skeletal muscle, liver, or spleen. Immunocytochemical detection in guinea pig identified the gpKir7.1 protein in the basolateral membrane of epithelial cells of the proximal tubule. RT-PCR analysis identified strong gpKir7.1 expression in the proximal tubule and weak expression in glomeruli and thick ascending limb. In isolated human tubule fragments, RT-PCR showed expression in proximal tubule and thick ascending limb.

CONCLUSION

Our results suggest that Kir7.1 may contribute to basolateral K(+) recycling in the proximal tubule and in the thick ascending limb.

Amino acid transport in the renal proximal tubule

In the kidney the proximal tubule is responsible for the uptake of amino acids. This occurs via a variety of functionally and structurally different amino acid transporters located in the luminal and basolateral membrane. Some of these transporters show an ion-dependence (e.g. Na+, Cl- and K+) or use an H+-gradient to drive transport. Only a few amino acid transporters have been cloned or functionally characterized in detail so far and their structure is known, while little is known about a majority of amino acid transporters. Only few attempts have been untertaken looking at the regulation of amino acid transport. We summarized more recent information on amino acid transport in the renal proximal tubule emphasizing functional and regulatory aspects.

A novel assay for determination of diadenosine polyphosphates in human platelets: studies in normotensive subjects and in patients with essential hypertension

OBJECTIVE

Diadenosine polyphosphates (APnAs, n = 3-6) are a family of endogenous vasoactive purine dinucleotides which have been isolated from thrombocytes. Diadenosine pentaphosphate (AP5A) and diadenosine hexaphosphate (AP6A) are more potent than diadenosine tetraphosphate (AP4A) and diadenosine triphosphate (AP3A) and cause skeletal muscle vasoconstriction in rats. Little is known about their physiological and pathophysiological significance in humans. The aims of the present study were to compare thrombocyte APnA concentrations in patients with essential hypertension (HYP) and in healthy normotensive humans (CON) using a novel quantitative assay and to assess a possible relationship between thrombocyte APnA concentrations and skeletal muscle vascular resistance.

DESIGN AND METHODS

We describe a novel assay for quantification of APnAs in human platelets, involving platelet isolation from human blood, a solid-phase extracting procedure with a derivatized resin, desalting and quantitative determination of the substances with an ion-pair reversed-phase high-performance liquid chromatography (HPLC) system. The structural integrity of the isolated APnAs was confirmed by mixed assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF) measurements and co-elution with added standards. The detection threshold for all four APnAs was 1 pmol/l and the inter-assay coefficients of variation were < 11% (n = 12). After venous blood sampling, mean arterial blood pressure (MAP) and forearm blood flow (FBF, using venous occlusion plethysmography) were measured in HYP and CON. Forearm vascular resistance (FVR) was calculated as MAP/FBF. significantly differ in platelet AP3A and AP4A content, but HYP had significantly higher thrombocyte concentrations of AP5A (56 +/- 7 versus 32 +/- 3 ng/microg beta-thromboglobulin, P = 0.003) and AP6A (10 +/- 1 versus 6 +/- 1 ng/microg beta-thromboglobulin, P = 0.015) than CON. HYP had significantly elevated FVR (50 +/- 6 versus 33 +/- 2 arbitrary units, P = 0.01) compared to CON. Significant correlations were found between AP5A and FVR (p = 0.38, P = 0.04) as well as between AP6A and FVR (p = 0.42, P = 0.02). In contrast, there were no significant correlations between APnAs and MAP.

CONCLUSIONS

The study shows that thrombocyte concentrations of AP5A and AP6A are elevated in patients with essential hypertension. Vasoconstriction caused by release of AP5A and AP6A from thrombocytes may contribute to the increase of vascular resistance in hypertensive patients.

Regulation of mesangial cell apoptosis and proliferation by intracellular Ca(2+) signals

BACKGROUND

In inflammatory glomerular diseases, proliferation, as well as apoptosis of mesangial cells (MCs), has been shown histomorphologically. Both processes may regulate the cellular content of the mesangium by closely influencing each other. In the present study, we examined whether the cytoplasmic free Ca(2+) concentration [Ca(2+)](i) is involved as a key second messenger in the regulation of proliferative and apoptotic events.

METHODS

Thapsigargin, an inhibitor of the endoplasmic Ca(2+)-Mg(2+)-ATPase, was used as a test substance to investigate the role of [Ca(2+)](i) in signaling MC apoptosis and growth in vitro. Apoptosis was determined by nuclear chromatin staining with Hoechst 33258, by a [3H]-thymidine-based DNA fragmentation assay or by flow cytometry detecting binding of FITC-conjugated annexin V. Proliferation was measured by [3H]-thymidine incorporation into acid-precipitable material and corroborated by cell counting.

RESULTS

Thapsigargin significantly induced apoptosis and inhibited proliferation dose dependently in nanomolar concentrations without evoking necrotic damage when administered not longer than 12 hours. Significant apoptosis was measurable after a six-hour treatment of MCs with thapsigargin. Determination of [Ca(2+)](i) by fura-2-dependent spectrofluorometry showed that thapsigargin was able to induce prolonged [Ca(2+)](i) rises that could be prevented by preincubation with the intracellular Ca(2+) chelator 1, 2-bis(2-aminophenoxy)-ethane-N,N,N', N'-tetra-acetic acid (BAPTA) acetomethyl ester (AM). BAPTA had no influence on MC viability but reversed thapsigargin-induced apoptosis to control levels. After thapsigargin treatment (100 nmol/L, 12 hours), apoptotic MCs had a significantly higher [Ca(2+)](i) of 251 +/- 25 nmol/L (N = 41) as compared with MCs that were not or not yet apoptotic ([Ca(2+)](i) of 116 +/- 20 nmol/L, N = 26, P < 0,05). Platelet-derived growth factor (PDGF), a well-characterized growth factor for MCs, reversed the effects of thapsigargin on proliferation and apoptosis in a similar fashion as BAPTA. PDGF acutely stimulated increases of [Ca2+]i but abolished thapsigargin-dependent, but not angiotensin II- or ATP-induced Ca(2+) rises when administered during a 12-hour preincubation.

CONCLUSIONS

Our data suggest that a sustained increase of [Ca(2+)](i) may serve as a signal to trigger MC apoptosis. Growth factors such as PDGF can abolish apoptosis induced by elevations of [Ca(2+)](i) by altering intracellular Ca(2+) signaling.

Diadenosine polyphosphates activate a Ca(2+)-dependent K(+)-conductance in porcine aortic smooth muscle cells via P2-purinoceptors

Effects of the diadenosine polyphosphates P(1),P(3)-diadenosine triphosphate (Ap3A), P(1),P(4)-diadenosine tetraphosphate (Ap4A), P(1),P(5)-diadenosine pentaphosphate (Ap5A) and P(1), P(6)-diadenosine hexaphosphate (Ap6A) and of adenosine, ATP, ADP, AMP, UTP on smooth muscle cells from porcine aorta were examined. Membrane voltages and cellular conductances were measured in the slow whole cell configuration of the patch clamp technique. All four diadenosine polyphosphates, adenosine, AMP and ADP predominantly hyperpolarized membrane voltages with only occasional transient initial depolarizations whereas ATP and UTP led to sustained depolarizations. All four diadenosine polyphosphates increased cellular conductances. The effects of Ap5A on membrane voltages were almost completely inhibited by the putative P2-purinoceptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 micromol/l) and only partially reduced by the putative A(2)-purinoceptor antagonist 3,7-dimethyl-1-propragyl-xanthine (DMPX, 10 micromol/l) or the Ap4A-receptor antagonist diinosine pentaphosphate (Ip5I, 10 micromol/l). The adenosine-induced hyperpolarization was partially reduced by the putative A(1)-purinoceptor antagonist 8-cyclopentyl-1,3-dipropargylxanthine (DPCPX, 0.1 micromol/l) or by DMPX while PPADS or Ip5I were without effects. Ap5A-induced hyperpolarizations were inhibited by Ba(2+) and clotrimazole but not by glibenclamide. We conclude that diadenosine polyphosphates activate predominantly a Ca(2+)-dependent K(+)-conductance in smooth muscle cells obtained from porcine aorta most likely mediated via P2Y-purinoceptors and possibly partially also by Ap4A receptors.

Effects of diadenosine polyphosphates on the intracellular Ca2+ concentration in endothelial cells

Diadenosine polyphosphates have differential hemodynamic effects. The role of the endothelium in the vascular effects of these agonists is still unclear. Primary cultures of rat aortal endothelial cells and Ea.hy 926 cells (a continuous endothelial cell line) were used to investigate the effects of Ap3A-Ap6A, adenosine triphosphate (ATP), and for comparison, arginine vasopressin (AVP) and angiotensin II (A II) on the intracellular Ca2+ concentration, [Ca2+]i. Fura-2 was used as Ca2+ indicator. In rat aortal endothelial cells, ATP and Ap4A concentration dependently increased [Ca2+]i with an initial peak followed by an elevated plateau. The half-maximal effects were reached at approximately 7 micromol/l for ATP and at approximately 10 micromol/l for Ap4A. The maximal peak effects at 100 micromol/l were 1,035 +/- 413 nmol/l (n = 3) and 437 +/- 271 nmol/l (n = 8) for ATP and Ap4A, respectively. At 100 micromol/l Ap3A and Ap6A slightly increased [Ca2+]i, while Ap5A had no significant effect. The known endothelial agonists AVP (100 nmol/l) and A II (10 nmol/l) increased [Ca2+]i initially by 1,549 +/- 913 nmol/l (n = 7) and 209 +/- 45 nmol/l (n = 9), respectively. In Ea.hy 926 cells an increase in [Ca2+]i was obtained only with ATP (10 micromol/l) and with Ap4A (100 micromol/l). Ap3A, Ap5A, and Ap6A (each 100 micromol/l) and also AVP (100 nmol/l) and A II (10 nmol/l) had no significant effects in these cells. These results show that a considerable increase in [Ca2+]i in endothelial cells can only be induced by Ap4A among the diadenosine polyphosphates, indicating that the vasoactive effects of only this polyphosphate could at least partly be mediated via Ca2+-dependent mechanisms in endothelial cells, comparable to the known effects of AVP, A II, and ATP. The fact that A II and AVP did not influence [Ca2+]i in Ea.hy 926 cells is probably due to the loss of the respective receptors in this cell line.

Na(+)-dependent and -independent amino acid transport systems in immortalized human kidney epithelial cells derived from the proximal tubule

In the proximal tubule Na(+)-dependent (SDAT) and Na(+)-independent (SIAT) amino acid (AA) transporters are present. The effects of neutral, basic, and acidic AA on membrane voltage (Vm) of immortalized human kidney epithelial (IHKE-1) cells derived from the proximal tubule were examined using the slow whole-cell patch-clamp technique. In the presence of Na+ AA depolarized Vm in a concentration-dependent manner (0.05-5 mM) with Asp = Arg = Glu = 2Cys < Pro = Leu < Phe = AIB = Ala = Pro = Asn < Gly. In the absence of extracellular Na+ a decreased depolarization was seen with most neutral AA (Ala, Pro, Asn, Gly, Phe, and Leu), and the depolarization was increased with Asp, Glu, Arg, and 2Cys (1 mM each). In the absence of Na+ and a reduction in Cl- (5 mM) the depolarization by Arg was reduced. Unlike that predicted for transport by system b0,+ which exchanges neutral against dibasic amino acids, Leu does not hyperpolarize but depolarize Vm of IHKE-1 cells in the absence of extracellular Na+. After removal of Na+ (0 mM) and a reduction in Cl- (5 mM) in the extracellular solution, Leu or Glu hyperpolarized Vm, indicating that IHKE-1 cells possess two different SIAT systems, one Cl(-)-dependent and similar to system b0,+ and one novel Cl(-)-dependent system, which might be a Cl-/AA exchanger and can be blocked by the Cl(-)-channel blockers 5-nitro-2-(3-phenylpropylamino)-benzoate (10 microM) and 4,4'-diisothiocyanostibene-2,2'-disulfonic acid (50 microM). B system-related AA transporters might be responsible for the C(-)-independent SIAT, since we were able to detect its signal by Northern blot analysis.

Effects of diadenosine polyphosphates on systemic and regional hemodynamics in anesthetized rats

Diadenosine polyphosphates (Ap4A, Ap5A, Ap6A) induce vasodilatation or vasoconstriction in various isolated vessels and influence central and peripheral hemodynamics. The influence of diadenosine polyphosphates on hemodynamics was studied in anesthetized rats in vivo. Mean arterial blood pressure (MABP) and heart rate (HR) measured in the carotid artery decreased with Ap4A, Ap5A, and Ap6A. Renal blood flow (RBF), femoral blood flow (FBF) and cardiac output (CO) were evaluated by an ultrasonic transit-time method. Renal superficial blood flow (RSBF) was measured by laser Doppler flowmetry. CO, RBF and RSBF were decreased initially by all three diadenosine polyphosphates. FBF was also slightly decreased. Total peripheral (TPR), renal (RVR) and femoral (FVR) vascular resistances were calculated. TPR was transiently increased by the dinucleotides following by a decrease. RVR and, to a lesser extent, FVR were also increased. These data show that diadenosine polyphosphates have effects on both the heart and the peripheral blood vessels. The effects on the heart and MABP were dominated by bradycardia and hypotension. In the kidney, diadenosine polyphosphates induced a predominant vascoconstriction. The effects on skeletal muscle blood flow were much smaller. Thus, the three diadenosine polyphosphates studied differ in the effects on heart and peripheral vessels.