Organic cation transporters OCT1 and OCT2 determine the accumulation of lamivudine in CD4 cells of HIV-infected patients

PURPOSE

Identifying factors that determine concentrations of antiretroviral drugs in CD4 cells are important for improving therapeutic efficacy. Experimental models indicate that the nucleoside reverse transcriptase inhibitor lamivudine is transported by the organic cation transporters 1 and 2 (OCT1 and OCT2, respectively). Here, we tested whether OCT1 and OCT2 contribute to the uptake of lamivudine into native CD4 cells of human immunodeficiency virus (HIV)-infected individuals.

METHODS

CD4 cells obtained by non-activated cell sorting from 35 individuals with HIV-1 infection were incubated with lamivudine (10 μM, 30 min), and intracellular concentrations of lamivudine and its active metabolite lamivudine triphosphate were determined by liquid chromatography tandem mass spectrometry. The expression of OCT1 and OCT2 mRNA was measured by quantitative real-time polymerase chain reaction (PCR). A model of OCT2-transfected CD4 cells was established for mechanistic investigations.

RESULTS

Intracellular concentrations of lamivudine and its active metabolite lamivudine triphosphate showed strong linear correlations with each other and with the CD4 mRNA expression of OCT1 and OCT2 (r > 0.80). Coincubation with protease inhibitors (ritonavir, nelfinavir) that inhibit OCT1 and OCT2 yielded decreased intracellular concentrations of lamivudine and lamivudine triphosphate. Incubation of CD4 cells from healthy donors transfected with an OCT2 expression vector yielded increased concentrations of lamivudine and lamivudine triphosphate.

CONCLUSION

Our studies indicate a role of OCT1 and OCT2 for the cellular accumulation of lamivudine in HIV-infected individuals.

Paramount levels of ergothioneine transporter SLC22A4 mRNA in boar seminal vesicles and cross-species analysis of ergothioneine and glutathione in seminal plasma

Ergothioneine (ET) is a unique natural antioxidant which mammalia acquire exclusively from their food. Recently, we have discovered an ET transporter (ETT; gene symbol SLC22A4). The existence of a specific transporter suggests a beneficial role for ET; however, the precise physiological purpose of ET is still unclear. A conspicuous site of high extracellular ET accumulation is boar seminal plasma. Here, we have investigated whether ETT is responsible for specific accumulation of ET in the boar reproductive tract. The putative ETT from pig (ETTp) was cloned and validated by functional expression in 293 cells. The highest levels of ETTp mRNA were detected by real-time RT-PCR in seminal vesicles, eye, and kidney; much less was present in bulbourethral gland, testis, and prostate. By contrast, there was virtually no ETT mRNA in rat seminal vesicles. ET content in boar reproductive tissues, determined by LC-MS/MS, closely matched the ETT expression profile. Thus, strong and specific expression of ETTp in boar seminal vesicles explains high accumulation of ET in this gland and hence also in seminal plasma. Previous reports suggest that the glutathione (GSH) content of seminal plasma correlates directly with ET content; however, a comprehensive analysis across several species is not available. We have measured ET and GSH in seminal plasma from human, boar, bull, stallion, and rabbit by LC-MS/MS. GSH levels in seminal plasma do not correlate with ET levels. This suggests that the function of ET, at least in this extracellular context, does not depend on redox cycling with GSH.

Extraneuronal monoamine transporter and organic cation transporters 1 and 2: a review of transport efficiency

The extraneuronal monoamine transporter (EMT) corresponds to the classical steroid-sensitive monoamine transport mechanism that was first described as "uptake2" in rat heart with noradrenaline as substrate. The organic cation transporters OCT1 and OCT2 are related to EMT. The three carriers share basic structural and functional characteristics. Hence, EMT, OCT1 and OCT2 constitute a group referred to as non-neuronal monoamine transporters or organic cation transporters. After a brief general introduction, this review focuses on the critical analysis of substrate specificity. We calculate from the available literature and compare consensus transport efficiency (clearance) data for human and rat EMT, OCT1 and OCT2, expressed in transfected cell lines. From the plethora of inhibitors that have been tested, the casual observer likely gets the impression that these carriers indiscriminately transport very many compounds. However, our knowledge about actual substrates is rather limited. 1-Methyl-4-phenylpyridinium (MPP+) is an excellent substrate for all three carriers, with clearances typically in the range of 20-50 microl min(-1) mg protein(-1). The second-best general substrate is tyramine with a transport efficiency (TE) range relative to MPP+ of 20%-70%. The TEs of OCT1 and OCT2 for dopamine, noradrenaline, adrenaline and 5-HT in general are rather low, in the range relative to MPP+ of 5%-15%. This suggests that OCT1 and OCT2 are not primarily dedicated to transport these monoamine transmitters; only EMT may play a significant role in catecholamine inactivation. For many substrates, such as tetraethylammonium, histamine, agmatine, guanidine, cimetidine, creatinine, choline and acetylcholine, the transport efficiencies are markedly different among the carriers.

Multiple Organ Failure due to 5-Fluorouracil Chemotherapy in a Patient with a Rare Dihydropyrimidine Dehydrogenase Gene Variant

BACKGROUND

Dihydropyrimidine dehydrogenase (DPD) is the initial and rate-limiting enzyme in the metabolism of the chemotherapeutic drug 5-fluorouracil (5-FU). Application of 5-FU is restricted by a narrow therapeutic index because of severe toxicity of WHO grades III-IV. The exon 14-skipping mutation (c.1905+1G>A) accounts for approximately a quarter of all severely toxic cases. However, numerous other polymorphisms have been identified within the DPYD gene in affected patients, and the pathophysiological significance of most of them is unclear.

PATIENT AND METHODS

We report a patient with advanced caecum cancer who twice received 950 mg 5-FU and 45 mg folinic acid as adjuvant by bolus injection. 2 days after onset of chemotherapy, the patient developed a multiple organ dysfunction exhibiting a cardiogenic shock with severe left ventricular insufficiency, marked reduction of renal function, and beginning hepatic encephalopathy with somnolence, myoclonus, and a seizure. In order to investigate a possible defect within the DPYD gene direct sequencing of all 23 exons was carried out.

RESULTS

Genotyping revealed a rare c.1601G>A polymorphism which causes a change in the protein sequence (S534N). Data regarding the clinical relevance are ambiguous. The polymorphism has been detected together with an intronic mutation and both polymorphisms have consistently been reported with reduced enzyme activity.

CONCLUSION

The present case provides further evidence of an etiologic role of the c.1601G>A mutation for DPD deficiency and the occurrence of severe 5-FU-related toxicity and underlines the value of comprehensive pharmakogenetic diagnostics with respect to the dihydropyrimidine dehydrogenase.

Acute effects of glucose and insulin on vascular endothelium

AIMS/HYPOTHESIS

Chronic exposure to high concentrations of glucose has consistently been demonstrated to impair endothelium-dependent, nitric oxide (NO)-mediated vasodilation. In contrast, several clinical investigations have reported that acute exposure to high glucose, alone or in combination with insulin, triggers vasodilation. The aim of this study was to examine whether elevated glucose itself stimulates endothelial NO formation or enhances insulin-mediated endothelial NO release.

METHODS

We measured NO release and vessel tone ex vivo in porcine coronary conduit arteries (PCAs). Intracellular Ca(2+) was monitored in porcine aortic endothelial cells (PAECs) by fura-2 fluorescence. Expression of the Na(+)/glucose cotransporter-1 (SGLT-1) was assayed in PAECs and PCA endothelium by RT-PCR.

RESULTS

Stimulation of PCAs with D: -glucose, but not the osmotic control L: -glucose, induced a transient increase in NO release (EC(50) approximately 10 mmol/l), mediated by a rise in intracellular Ca(2+) levels due to an influx from the extracellular space. This effect was abolished by inhibitors of the plasmalemmal Na(+)/Ca(2+) exchanger (dichlorobenzamil) and the SGLT-1 (phlorizin), which was found to be expressed in aortic and coronary endothelium. Alone, D: -glucose did not relax PCA, but did augment the effect of insulin on NO release and vasodilation.

CONCLUSIONS/INTERPRETATION

An increased supply of extracellular D: -glucose appears to enhance the activity of the endothelial isoform of nitric oxide synthase by increasing intracellular Na(+) concentrations via SGLT-1, which in turn stimulates an extracellular Ca(2+) influx through the Na(+)/Ca(2+) exchanger. This mechanism may be responsible for glucose-enhanced, insulin-dependent increases in tissue perfusion (including coronary blood-flow), thus accelerating glucose extraction from the blood circulation to limit the adverse vascular effects of prolonged hyperglycaemia.

Aspirin induces nitric oxide release from vascular endothelium: a novel mechanism of action

1. The study was designed to test the hypothesis that aspirin may stimulate nitric oxide (NO) release from vascular endothelium, a pivotal factor for maintenance of vascular homeostasis. 2. Clinical evidence suggests that low-dose aspirin may improve vascular endothelial function. Since other cyclooxygenase (COX) inhibitors showed no beneficial vascular effects, aspirin may exhibit a vasculoprotective, COX-independent mechanism. 3. Luminal NO release was monitored in real time on dissected porcine coronary arteries (PCA) by an amperometric, NO-selective sensor. Additionally, endothelial NO synthase (eNOS) activity was measured in EA.hy 926 cell homogenates by an l-[(3)H]citrulline/l-[(3)H]arginine conversion assay. Superoxide scavenging capacity was assessed by lucigenin-enhanced luminescence. 4. Aspirin induced an immediate concentration-dependent NO release from PCA with an EC(50) of 50 nm and potentiated the NO stimulation by the receptor-dependent agonist substance P. These effects were independent of an increase in intracellular calcium and could be mimicked by stimulation with acetylating aspirin derivatives. The aspirin metabolite salicylic acid or the reversible cyclooxygenase inhibitor indomethacin failed to modulate NO release. Incubation of soluble eNOS for 15 min with 100 microm aspirin or acetylating aspirin analogues increased the l-[(3)H]citrulline yield by 40-80%, while salicylic acid had no effect. Aspirin and salicylic acid showed a similar, but only modest, magnitude and velocity of superoxide scavenging. 5. Our findings demonstrate that therapeutically relevant concentrations of aspirin elicit NO release from vascular endothelium. This effect appears to be due to a direct acetylation of the eNOS protein, but is independent of COX inhibition or inhibition of superoxide-mediated NO degradation.

Regulation of human extraneuronal monoamine transporter (hEMT) expressed in HEK293 cells by intracellular second messenger systems

Several transmembrane transporters of organic compounds are regulated by phosphorylation/dephosphorylation mechanisms. The aim of this study was to investigate the possible regulation of the human extraneuronal monoamine transporter, hEMT, by these mechanisms. The experiments were performed using HEK293 cells stably transfected with pcDNA3hEMT (293hEMT). The characteristics of hEMT-mediated uptake of [3H]1-methyl-4-phenylpyridinium ([3H]MPP+) were studied by incubating the cells at 37 degrees C for 1 min with 200 nM [3H]MPP+. Uptake of [3H]MPP+ by 293hEMT cells was not affected or only slightly reduced by modulators of protein kinase A, protein kinase C, or protein kinase G. It was not affected by an inhibitor of protein tyrosine kinase and was reduced by mitogen-activated protein kinase inhibitors. Uptake of [3H]MPP+ by 293hEMT cells was independent of extracellular Ca2+ and strongly reduced by Ca2+/calmodulin pathway inhibitors. Uptake of [3H]MPP+ by 293hEMT cells was strongly reduced in the presence of non-selective phosphodiesterase inhibitors (IBMX, caffeine, theophylline). The effect of IBMX was independent of extracellular Ca2+ its IC50 was found to be 82.0 microM (66.2-101.6 microM; n=4), and its inhibitory effect resulted from a significant decrease in the maximal velocity of [3H]MPP+ uptake, with no change in the Michaelis-Menten constant. [3H]MPP+ uptake was reduced by 8-methoxy-methyl-IBMX, a selective inhibitor of the Ca2+/calmodulin-dependent phosphodiesterase (PDE1), but not by zaprinast, a selective inhibitor of PDE5. Uptake of [3H]MPP+ by 293hEMT cells was strongly reduced by protein tyrosine phosphatase inhibitors, by an alkaline phosphatase inhibitor and, by contrast. showed an increase in the presence of exogenous alkaline phosphatase. In conclusion, these results suggest that hEMT is regulated by phosphorylation/dephosphorylation mechanisms, being active in the dephosphorylated state.

Apical uptake of organic cations by human intestinal Caco-2 cells: putative involvement of ASF transporters

The aim of this work was to characterise the intestinal absorption of organic cations, by testing the possibility of involvement of known members of the amphiphilic solute facilitator (ASF) family in this process. For that purpose, the characteristics of the uptake of 1-methyl-4-phenylpyridinium, a model organic cation, at the brush-border membrane of Caco-2 cells were compared with those of the extraneuronal monoamine transporter (EMT)-mediated transport. Uptake of [3H]MPP+ by Caco-2 and 293hEMT cells showed pH-dependence: it was significantly reduced (to 86% and 62% of control, respectively) when the pH of the extracellular medium was decreased to 6.2, and increased (to 116% and 136% of control, respectively) when the extracellular pH was increased to 8.2. Uptake of [3H]MPP+ by Caco-2 cells and 293hEMT cells showed potential-dependence: substitution of KCl for NaCl in the incubation medium resulted in a reduction in the inward transport of [3H]MPP+ (to 70% and 40% of control, respectively). Uptake of [3H]MPP+ by Caco-2 and 293hEMT cells showed only little dependence on Na+: substitution of NaCl of the incubation media with LiCl resulted in a small decrease (of 19% and 14%, respectively) in [3H]MPP+ uptake. However, when NaCl was substituted with choline chloride, a significant reduction in [3H]MPP+ uptake by Caco-2 and 2931hEMT cells (of 56% and 68%, respectively) was observed. The effect of various compounds on initial rates of [3H]MPP+ uptake into Caco-2 and 293hEMT cells was tested. All compounds tested interacted with the specific [3H]MPP+ uptake in both cell lines. There was no correlation between the IC50s in relation to inhibition of [3H]MPP+ uptake into Caco-2 cells and into 293hEMT cells. Reverse transcriptase-polymerase chain reaction indicates that mRNA of hEMT and of the human organic cation transporter 1 (hOCT1) are present in Caco-2 cells. In conclusion, our results suggest that uptake of organic cations at the brush-border membrane of Caco-2 cells may occur through two distinct Na+-independent transporters belonging to the ASF family: hEMT and hOCT1.

Gene structures of the human non-neuronal monoamine transporters EMT and OCT2

Non-neuronal monoamine transporters OCT1, OCT2, and EMT, which are all members of the amphiphilic solute facilitator family, control signal transmission by removing released transmitters, such as dopamine, noradrenaline, adrenaline, 5-hydroxytryptamine, and histamine, from the extracellular space. In the current study, we have isolated human EMT (gene symbol SLC22A3) and OCT2 (SLC22A2) genes and report the gene and promoter organization. Both genes consist of 11 coding exons, with consensus GT/AG splice sites and conserved intron locations. The EMT gene is 77 kb, and the OCT2 gene is 45 kb in size. For the EMT gene, two transcription start points were identified by inverse polymerase chain reaction based on mRNA from Caki-1 cells. The EMT promoter, located within a CpG island, lacks a consensus TATA box but contains a prototypical initiator element and a number of potential binding sites for ubiquitous transcription factors Sp1 and NF-1. In contrast, the OCT2 promoter is not associated with a CpG island, contains a putative TATA box, and potential binding sites for specific transcription factors, such as HFH-8 and IK2. Since EMT and OCT2 may play important roles in catecholamine homeostasis and, as such, are candidate genes in human disease, the present results provide a basis for the analysis of genetic variation and the regulation of transcription.

Selective substrates for non-neuronal monoamine transporters

The recently identified transport proteins organic cation transporter 1 (OCT1), OCT2, and extraneuronal monoamine transporter (EMT) accept dopamine, noradrenaline, adrenaline, and 5-hydroxytryptamine as substrates and hence qualify as non-neuronal monoamine transporters. In the present study, selective transport substrates were identified that allow, by analogy to receptor agonists, functional discrimination of these transporters. To contrast efficiency of solute transport, stably transfected 293 cell lines, each expressing a single transporter, were examined side by side in uptake experiments with radiolabeled substrates. Normalized uptake rates indicate that tetraethylammonium, with a rate of about 0.5 relative to 1-methyl-4-phenylpyridinium (MPP+), is a good substrate for OCT1 and OCT2. It was not, however, accepted as substrate by EMT. Choline was transported exclusively by OCT1, with a rate of about 0.5 relative to MPP+. Histamine was a good substrate with a rate of about 0.6 relative to MPP+ for OCT2 and EMT, but was not transported by OCT1. Guanidine was an excellent substrate for OCT2, with a rate as high as that of MPP+. Transport of guanidine by OCT1 was low, and transport by EMT was negligible. With the guanidine derivatives cimetidine and creatinine, a pattern strikingly similar to guanidine was observed. Collectively, these substrates reveal key differences in solute recognition and turnover and thus challenge the concept of "polyspecific" organic cation transporters. In addition, our data, when compared with previous studies, suggest that OCT2 corresponds to the organic cation/H+ antiport mechanism in renal brush-border membrane vesicles, and that EMT corresponds to the guanidine/H+ antiport mechanism in membrane vesicles from placenta and intestine.

Differential regulation of distinct phenotypic features of serotonergic neurons by bone morphogenetic proteins

Bone morphogenetic proteins (BMPs), growth and differentiation factor 5 (GDF5) and glial cell line-derived neurotrophic factor (GDNF) are members of the transforming growth factor-beta superfamily that have been implicated in tissue growth and differentiation. Several BMPs are expressed in embryonic and adult brain. We show now that BMP-2, -6 and -7 and GDF5 are expressed in the embryonic rat hindbrain raphe. To start to define roles for BMPs in the regulation of serotonergic (5-HT) neuron development, we have generated serum-free cultures of 5-HT neurons isolated from the embryonic (E14) rat raphe. Addition of saturating concentrations (10 ng/mL) of BMP-6 and GDF5 augmented numbers of tryptophan hydroxylase (TpOH) -immunoreactive neurons and cells specifically taking up 5, 7-dihydroxytryptamine (5,7-DHT) by about two-fold. Alterations in 5-HT neuron numbers were due to the induction of serotonergic markers rather than increased survival, as shown by the efficacy of short-term treatments. Importantly, BMP-7 selectively induced 5, 7-DHT uptake without affecting TpOH immunoreactivity. BMP-6 and -7 also promoted DNA synthesis and increased numbers of cells immunoreactive for vimentin and glial fibrillary acidic protein (GFAP). Pharmacological suppression of cell proliferation or glial development abolished the induction of serotonergic markers by BMP-6 and -7, suggesting that BMPs act indirectly by stimulating synthesis or release of glial-derived serotonergic differentiation factors. Receptor bodies for the neurotrophin receptor trkB, but not trkC, abolished the BMP-mediated effects on serotonergic development, suggesting that the glia-derived factor is probably brain-derived neurotrophic factor (BDNF) or neurotrophin-4. In support of this notion, we detected increased levels of BDNF mRNA in BMP-treated cultures. Together, these data suggest both distinct and overlapping roles of several BMPs in regulating 5-HT neuron development.

Transport of monoamine transmitters by the organic cation transporter type 2, OCT2

The recently cloned apical renal transport system for organic cations (OCT2) exists in dopamine-rich tissues such as kidney and some brain areas (Gründemann, D., Babin-Ebell, J., Martel, F., Ording, N., Schmidt, A., and Schömig, E. (1997) J. Biol. Chem. 272, 10408-10413). The study at hand was performed to answer the question of whether OCT2 accepts dopamine and other monoamine transmitters as substrate. 293 cells were stably transfected with the OCT2r cDNA resulting in the 293OCT2r cell line. Expression of OCT2r in 293 cells induces specific transport of tritiated dopamine, noradrenaline, adrenaline, and 5-hydroxytryptamine (5-HT). Initial rates of specific 3H-dopamine, 3H-noradrenaline, 3H-adrenaline, and 3H-5-HT transport were saturable, the Km values being 2.1, 4.4, 1.9, and 3.6 mmol/liter. The corresponding Vmax values were 3.9, 1.0, 0. 59, and 2.5 nmol min-1.mg of protein-1, respectively. 1, 1'-diisopropyl-2,4'-cyanine (disprocynium24), a known inhibitor of OCT2 with a potent eukaliuric diuretic activity, inhibited 3H-dopamine uptake into 293OCT2r cells with an Ki of 5.1 (2.6, 9.9) nmol/liter. In situ hybridization reveals that, within the kidney, the OCT2r mRNA is restricted to the outer medulla and deep portions of the medullary rays indicating selective expression in the S3 segment of the proximal tubule. These findings open the possibility that OCT2r plays a role in renal dopamine handling.

Catecholamine transport by the organic cation transporter type 1 (OCT1)

1. Liver and kidney extract adrenaline and noradrenaline from the circulation by a mechanism which does not seem to be one of the classical catecholamine transporters. The hypothesis that OCT1 is involved the organic cation transporter type 1 which exists in rat kidney and liver-was tested. 2. Based on human embryonic kidney cells (293), we constructed a cell line which stably expresses OCT1r (293OCT1r cells). Transfection with OCT1 resulted in a transport activity not only for prototypical known substrates of OCT1 such as 3H-1-methyl-4-phenylpyridinium and 14C-tetraethylammonium but also for the catecholamines 3H-adrenaline, 3H-noradrenaline (3H-NA) and 3H-dopamine (3H-DA), the indoleamine 3H-5-hydroxytryptamine (3H-5HT) as well as the indirect sympathomimetic 14C-tyramine. 3. For 3H-DA, 3H-5HT and 3H-NA, at non-saturating concentrations, the rate constants for inwardly directed substrate flux (kin) were 6.9+/-0.8, 3.1+/-0.2, and 1.2+/-0.1 microl min(-1) mg protein(-1). In wild type cells (293WT) the corresponding kin's were considerably lower, being 0.94+/-0.40, 0.47+/-0.08 and 0.23+/-0.05 microl min(-1) mg protein ' (n=12). The indirectly determined half-saturating concentrations of DA, 5HT, and NA were 1.1 (95% c.i.: 0.8, 1.4), 0.65 (0.49, 0.86), and 2.8 (2.1, 3.7) mmol l(-1) (n=3). 4. Specific 3H-DA uptake in 293OCT1r cells was resistant to cocaine (1 micromoll(-1)), 3H-5HT uptake was resistant to citalopram (300 nmol l(-1)) and 3H-NA uptake was resistant to desipramine (100 nmoll(-1)), corticosterone (1 micromol l(-1)), and reserpine (10 nmoll(-1)) which rules out the involvement of classical transporters for biogenic amines. 5. The findings demonstrate that OCTI efficiently transports catecholamines and other biogenic amines and support the hypothesis that OCT1 is responsible for hepatic and renal inactivation of circulating catecholamines.

Eukaliuric natriuresis and diuresis in response to disprocynium24: studies on the tubular site of action

We previously described that 1,1'-diisopropyl-2,4'-cyanine (disprocynium24, DP24) exerts an eukaliuric diuresis and natriuresis in the anesthetized rat. The purpose of the present study was to localize the tubular site of action of DP24. Employing micropuncture experiments in anesthetized rats, we first tested the effect of systemic application of DP24 (300 microg/kg + 300 microg/kg h, i.v.) on whole kidney excretion rates as well as on fluid, sodium and potassium ion delivery to the early distal tubule (V(ED), Na+(ED), K+(ED)). It was found that the eukaliuric diuresis and natriuresis in response to DP24 was accompanied by a substantial increase in V(ED) and Na+(ED), suggesting a predominant tubular site of action upstream to the early distal tubule, most likely in the proximal tubule. DP24 caused a comparable fractional, although minor absolute increase in K+(ED) as compared to Na+(ED). Second, application of DP24 into the first surface loop of the proximal tubule significantly increased V(ED) and Na+(ED) at a concentration of about 10(-7) M, indicating that DP24 may act from the intratubular site. Third, microperfusion of tubular segments revealed that effects of DP24 on the proximal convoluted tubule and the loop of Henle accounted for about 70 and 30%, respectively, of its diuretic and natriuretic action upstream to the early distal tubule. With regard to the loop of Henle, the quantitative effect of DP24 on fluid and Na+ reabsorption proposed a predominant effect on the straight part of the proximal tubule rather than the thick ascending limb. Intratubular DP24 did not affect reabsorption in the distal tubule. In summary, the present findings indicate that: (1) the diuretic and natriuretic effect of DP24 resides predominantly in the proximal tubule, and (2) DP24 may act from the intratubular site. Since DP24 increased V(ED) and Na+(ED) without apparently affecting sodium or potassium ion transport in the distal tubule, the mechanism of the eukaliuric response remains unclear.

Molecular cloning and characterization of two novel transport proteins from rat kidney

The recent cloning of renal transport systems for organic anions and cations (OAT1, OCT1, and OCT2) opened the possibility to search, via polymerase chain reaction (PCR) homology screening, for novel transport proteins. Two integral membrane proteins, UST1 and UST2, were cloned from rat kidney. RT-PCR revealed that UST1 is confined to the kidney whereas UST2 mRNA was detected in all tested tissues. Sequence analyses suggest that UST1 and UST2, together with four related transporters, comprise, within the major facilitator superfamily, a so far unrecognized transporter family, termed amphiphilic solute facilitator (ASF) family. Characteristic signatures for the ASF family were identified.

Disprocynium24 induces a dopamine-independent, eukaliuric diuresis and natriuresis in the anaesthetized rat

In the anaesthetized rat, intravenous administration of the isocyanine 1,1'-diisopropyl-2,4'-cyanine (disprocynium24) at doses up to 600 microg/kg resulted in marked diuresis and natriuresis without affecting urinary potassium excretion. Fractional sodium excretion was increased over 10-fold indicating a high ceiling-diuretic action. The effects of disprocynium24 on renal function were accompanied by a dose-dependent reduction in heart rate (HR) and mean arterial blood pressure (MAP). Acute administration of 600 microg/kg disprocynium24 decreased MAP by 25% and, in addition, caused a fall in glomerular filtration rate (GFR). Since i) disprocynium24 has been shown to interfere with urinary dopamine excretion (UDAV) and ii) dopamine has been implicated with the regulation of renal sodium excretion, we hypothesized that the effects of disprocynium24 might be mediated by its effects on renal dopamine handling. The following findings, however, argue against this hypothesis. First, administration of disprocynium24 in single doses up to 600 microg/kg caused a diuresis and natriuresis, but did not significantly affect U(DA)V. Second, neither the systemic nor the renal response to disprocynium24 were markedly altered by pretreatment with the dopamine D1- or D2-receptor blockers SCH23390 (10 microg x kg(-1) x min[-1]) or S(-)sulpiride (15 microg x kg(-1) x min[-1]), respectively.

1,1'-Diisopropyl-2,4'-cyanine (disprocynium24), a potent uptake2 blocker, inhibits the renal excretion of catecholamines

1,1'-Diisopropyl-2,4'-cyanine (disprocynium24), a potent inhibitor of the extraneuronal monoamine transport system (uptake2), was previously shown to reduce the clearance of catecholamines from plasma not only by blocking uptake2 but presumably also by blocking organic cation transport. To provide more direct evidence for the latter conclusion, the present study was carried out in anaesthetized rabbits. It aimed at determining the effect of disprocynium24 on the renal excretion of catecholamines which is known to be, at least in part, a consequence of organic cation transport in the kidney. To this end, the plasma clearance due to renal excretion (Cl(u)) of endogenous as well as infused 3H-labelled adrenaline, noradrenaline and dopamine was determined for 60-min periods of urine collection in rabbits treated either with disprocynium24 (270 nmol kg(-1) i.v. followed by i.v. infusion of 80 nmol kg(-1) min(-1)) or vehicle. Two groups of animals were studied: group I (monoamine oxidase and catechol-O-methyltransferase intact) and group II (monoamine oxidase and catechol-O-methyltransferase inhibited). A third group of animals with intact monoamine oxidase and catechol-O-methyltransferase was used to study the effect of disprocynium24 on the glomerular filtration rate (as determined by measuring the plasma clearance of inulin). In vehicle controls, Cl(u) of endogenous adrenaline, noradrenaline and dopamine was 7.2, 5.2 and 153.6 ml kg(-1) min(-1), respectively, in group I and 10.4, 7.0 and 134.3 ml kg(-1) min(-1), respectively, in group II. Similar control values of Cl(u) were obtained for infused 3H-adrenaline and 3H-noradrenaline, but not for infused 3H-dopamine; Cl(u) of 3H-dopamine (4.9 ml kg(-1) min(-1) in group I and 15.4 ml kg(-1) min(-1) in group II) was considerably smaller than Cl(u) of endogenous dopamine, indicating that most of the dopamine in urine (i.e., 98% in group I and 92% in group II) was derived from the kidneys rather than from the circulation. By contrast, only about one quarter of the noradrenaline in urine (32% in group I and 24% in group II) and none of the urinary adrenaline were of renal origin. In both groups, disprocynium24 markedly reduced the Cl(u) of endogenous catecholamines (by 72-90%) and of infused 3H-catecholamines (by 49-69%). Moreover, it preferentially inhibited the renal excretion of those components of urinary dopamine and noradrenaline which were derived from the kidney. Therefore, disprocynium24 inhibits the tubular secretion of catecholamines and, hence, organic cation transport in the kidney. This conclusion was substantiated by the observation that disprocynium24 did not alter the glomerular filtration rate.

Primary structure and functional expression of the apical organic cation transporter from kidney epithelial LLC-PK1 cells

Renal secretion of organic cations involves at least two distinct transporters, located in the basolateral and apical membranes of proximal tubule cells. Whereas the basolateral transporter has recently been cloned, sequence information about the apical type was not yet available. An organic cation transporter, OCT2p, was cloned from LLC-PK1 cells, a porcine cell line with properties of proximal tubular epithelial cells. OCT2p was heterologously expressed and characterized in human embryonic kidney 293 cells. OCT2p-mediated uptake of the prototypical organic cation [14C]tetraethylammonium ([14C]TEA) into 293 cells was saturable. There was a highly significant correlation between the Ki values for the inhibition of apical [14C]TEA uptake into LLC-PK1 cells and 293 cells transfected with OCT2p (r = 0.995; p < 0.001; n = 6). Although OCT2p is structurally related to OCT1r, the basolateral organic cation transporter from rat kidney, the transporters could be clearly discriminated pharmacologically with corticosterone, decynium22, and O-methylisoprenaline. The findings at hand suggest that OCT2 corresponds to the apical type of organic cation transporter. Reverse transcriptase-polymerase chain reaction indicates that mRNA of OCT1r is limited to non-neuronal tissue, whereas OCT2r, the OCT2p homologue from rat, was found in both the kidney and central nervous regions known to be rich in the monoamine transmitter dopamine.

Protection of DNA during preparative agarose gel electrophoresis against damage induced by ultraviolet light

Preparative gel electrophoresis of double-stranded DNA usually includes staining the gel with ethidium bromide followed by illumination with ultraviolet (UV-B) light. In this report, DNA isolated from agarose gels was found to be a poor substrate for in vitro transcription, transformation of E. coli and PCR. Inhibition was not caused by enzyme-inhibiting impurities in the agarose gel, but was induced by a standard transilluminator fitted with 312-nm tubes. Interestingly, it was possible to protect the DNA against UV damage by the addition of cytidine or guanosine to the electrophoresis buffer.

Pharmacokinetic and alpha 1-adrenoceptor antagonistic properties of two cyanine-type inhibitors of extraneuronal monoamine transport

1,1'-Diethyl-2,2'-cyanine (decynium22) and 1,1'-diisopropyl-2,4'-cyanine (disprocynium24) are highly potent inhibitors of the extraneuronal monamine transporter. When given as i.v. bolus injections (4 mumol kg-1) to anaesthetized rabbits, both drugs elicited a transient fall in blood pressure without altering heart rate. The observed maximum fall in diastolic blood pressure was 59% after decynium22 and 43% after disprocynium24 administration. The pharmacokinetics of decynium22 and disprocynium24 were similar; they were characterized by short half-lives for elimination (8.2 and 4.5 min, respectively) and very high plasma clearances (173 and 180 ml kg-1 min-1, respectively). The mechanism underlying the blood pressure-lowering effect of decynium22 was explored in the isolated incubated rabbit aorta. Decynium22 antagonized the noradrenaline-induced contraction; the pA2 for this interaction was 7.6, and the slope of the corresponding Schild plot was unity. In a membrane preparation from rat myocardium, decynium22 as well as disprocynium24 inhibited the specific binding of [125I]-2-[beta-(4-hydroxy-3-iodophenyl)-ethylaminomethyl]- tetralone (125I-HEAT), a selective ligand to alpha 1-adrenoceptors. The Ki's were 5.3 and 240 mumol l-1 for decynium22 and disprocynium24, respectively. The type of binding inhibition by decynium22 was competitive. It is concluded that the two inhibitors of extraneuronal monoamine transport decynium22 and disprocynium24 lower blood pressure by blocking alpha 1-adrenoceptors. A comparison of their potencies in blocking extraneuronal monoamine transport and alpha 1-adrenoceptors clearly indicates that disprocynium24 is more suitable for studies designed to determine the role of extraneuronal monoamine transport in vivo. Considering its very fast elimination kinetics, disprocynium24 must be administered by constant rate-infusions in order to avoid large fluctuations of plasma levels.

Disprocynium24, a novel inhibitor of the extraneuronal monoamine transporter, has potent effects on the inactivation of circulating noradrenaline and adrenaline in conscious rat

The role of extraneuronal uptake in terminating the actions of catecholamines has been difficult to evaluate in vivo, largely because of lack of suitable inhibitors. The compound, 1,1'-diisopropyl-2,4'-cyanine iodide or disprocynium24 (D24), is a novel inhibitor of extraneuronal uptake with a high degree of potency in vitro. This study examined the actions of D24 on the inactivation and metabolism of circulating noradrenaline and adrenaline in conscious rats. Animals received i.v. infusions of 3H-labelled noradrenaline and adrenaline, and their extraneuronal O-methylated metabolites, normetanephrine and metanephrine. Plasma concentrations of endogeneous and 3H-labelled catecholamines and metanephrines were measured before and after D24. D24 caused large increases in plasma concentrations of noradrenaline and adrenaline, effects due to both decreases in their plasma clearances and increases in their rates of release into plasma. Plasma concentrations of normetanephrine and metanephrine also increased due to their decreased clearance from plasma. Increased release of normetanephrine into plasma did not contribute to increased plasma concentrations of normetanephrine. In fact, the contribution of extraneuronal O-methylation to noradrenaline clearance decreased substantially after D24. The data indicate that D24 is a potent inhibitor of the extraneuronal catecholamine transporter in vivo and that this process contributes importantly to the removal of circulating catecholamines and their O-methylated amine metabolites. Increased release of noradrenaline into plasma may reflect an increase in the proportion of transmitter that escapes from sites of release into the circulation. However, increased adrenaline release indicates that the drug also causes sympathoadrenal activation.

Transport of small organic cations in the rat liver. The role of the organic cation transporter OCT1

The kidneys and the liver are the principal organs for the inactivation of circulating organic cations. Recently, an organic cation transporter (OCT1) has been cloned from rat kidney. In order to answer the question whether OCT1 is involved also in hepatic uptake of organic cations, the pharmacological characteristics of organic cation transport in hepatocytes were compared to the characteristics of transiently expressed OCT1. Primary cultures of rat hepatocytes avidly accumulated the small organic cation 3H-1-methyl-4-phenylpyridinium (3H-MPP+). At equilibrium, the hepatocytes accumulated 3H-MPP+ 56-fold. Initial rates of specific 3H-MPP+ transport in hepatocytes were saturable. The half-saturating concentration was 13 mumol/l. 3H-MPP+ transport was sensitive to quinine (Ki = 0.79 mumol/l) and cyanine863 (Ki = 0.097 mumol/l). Quinine and cyanine863 are known inhibitors of type I hepatic transport of cationic drugs and of renal excretion of organic cations, respectively. To compare the functional characteristics of 3H-MPP+ transport in hepatocytes with those of OCT1, OCT1 has been heterologously expressed and characterized in a mammalian cell line (293 cells). Initial rates of 3H-MPP+ transport were saturable, the Km being 13 mumol/l. The rank order of inhibitory potencies of various inhibitors was almost identical in hepatocytes and 293 cells transiently transfected with OCT1. There was a positive correlation between the Ki's for the inhibition of 3H-MPP+ transport in isolated hepatocytes and transfected 293 cells (r = 0.85; P < 0.01; n = 8). The results indicate that OCT1 is functionally expressed not only in the kidney but also in hepatocytes where it is responsible for the transport of small organic cations which, in the past, have been classified as type I substrates.

The role of extraneuronal amine transport systems for the removal of extracellular catecholamines in the rabbit

As selective inhibitors of the extraneuronal monoamine uptake system (uptake2) suitable for in-vivo studies were not available, the question of whether uptake2 plays a definite role in vivo is largely unresolved. We attempted to resolve the question by using 1,1'-diisopropyl-2,4'-cyanine iodide (disprocynium24), a novel agent that blocks uptake2 in vitro with high potency. Anaesthetized rabbits were infused with 3H-labelled noradrenaline, adrenaline and dopamine, and catecholamine plasma clearances as well as rates of spillover of endogenous catecholamines into plasma were measured before and during treatment with either disprocynium24 or vehicle. Four groups of animals were studied: group I, no further treatment: group II, monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT) inhibited; group III, neuronal uptake (uptake1) inhibited; group IV, uptake1 as well as MAO and COMT inhibited. Disprocynium24 (270 nmol kg-1 i.v. followed by an i.v. infusion of 80 nmol kg-1 min-1) did not alter heart rate and mean arterial blood pressure, but increased cardiac output by 22% and decreased the total peripheral vascular resistance by 16% with no difference between groups. When compared with vehicle controls, catecholamine clearances (normalized for the cardiac output of plasma) were decreased and spillover rates increased in response to disprocynium24. Although there were statistically significant between-group differences in baseline clearances (which decreased in the order: group I > group II > group III > group IV), the drug-induced clearance reductions relative to vehicle controls were similar in groups I to IV and amounted to 29-38% for noradrenaline, 22-31% for adrenaline and 16-22% for dopamine. Hence, there was still a significant % reduction in catecholamine clearances even after the combined inhibition of MAO and COMT, and there was no increase in the % reduction of clearances after inhibition of uptake1. Noradrenaline spillover increased in response to disprocynium24 in all four groups by 1.6- to 1.9-fold, whereas a 1.5- to 2.0-fold increase in adrenaline and dopamine spillover was observed in groups II and IV only. The results indicate that disprocynium24 interferes with the removal of circulating catecholamines not only by inhibiting uptake2, but also by inhibiting related organic cation transporters. As disprocynium24 increased the spillover of endogenous catecholamines into plasma even after inhibition of MAO and COMT, organic cation transporters may also be involved in the removal of endogenous catecholamines before they enter the circulation.

Dual effect of nicotine on cardiac noradrenaline release during metabolic blockade

Nicotine-induced noradrenaline was investigated in perfused guinea pig hearts subjected to metabolic blockade that was caused either by anoxia or by cyanide intoxication. Noradrenaline, neuropeptide Y, and dihydroxyphenylethyleneglycol (DOPEG) were determined in the coronary venous overflow Neuropeptide Y is a sympathetic cotransmitter of noradrenaline, and concomitant release of both transmitters indicates an exocytotic, calcium-dependent release mechanism, whereas neuropeptide Y overflow does not occur during nonexocytotic noradrenaline release. Nonexocytotic, calcium-independent noradrenaline release, however, is associated with an increase of DOPEG overflow, which is the main intraneuronal metabolite of noradrenaline formed by monoamine oxidase if oxygen is present. Anoxia per se caused a nonexocytotic release of noradrenaline starting after 10 min of anoxia and reaching peak levels at 30 min. During anoxia, nicotine (3 and 10 mumol/l) accelerated and enhanced noradrenaline overflow, i.e., the period between the onset of anoxia and the begin of noradrenaline release was shortened and peak levels were increased. Nicotine-induced noradrenaline release was accompanied by neuropeptide Y overflow. The action of nicotine was further evaluated during energy depletion caused by cyanide. As anoxia did, cyanide administration alone resulted in noradrenaline release. In accordance with a nonexocytotic mechanism and due to the presence of oxygen, this release of noradrenaline was accompanied by an increase of DOPEG. When added 10 min after the onset of energy depletion, nicotine (10 mumol/l) caused a brief but marked enhancement of exocytotic noradrenaline release, since this release was calcium-dependent and was accompanied by a significant rise of neuropeptide Y overflow. In absence of extracellular calcium to avoid exocytosis, concomitant administration of nicotine (3-100 mumol/l) and cyanide caused a concentration-dependent acceleration of both the overflow of noradrenaline and DOPEG, whereas overflow of neuropeptide Y was not increased, thus indicating a nonexocytotic release mechanism. In conclusion, the application of nicotine during myocardial energy depletion increases overflow of noradrenaline by both calcium-dependent exocytotic release and calcium-independent nonexocytotic release mechanisms.

The neuronal transport mechanism for noradrenaline is not influenced by the ATP analogue ATP gamma S

Noradrenaline is taken up in noradrenergic neurones via a desipramine-sensitive transport system (uptake1). There are reports which describe the activity of uptake1 being increased by extracellular ATP and ATP gamma S. This was attributed to a protein kinase-mediated mechanism. We reevaluated this phenomenon since a modulation of uptake1 by extracellular ATP would be of great physiological relevance. In clonal rat pheochromocytoma cells (PC12) we determined the kinetic parameters, Km and Vmax, of noradrenaline transport in the absence and presence of ATP gamma S. The addition of 0.1 mumol/L ATP gamma S had no effect on initial rates of specific noradrenaline transport. Only a slightly increased non-specific noradrenaline flux in the presence of ATP gamma S could be observed. On the basis of our results, there is no evidence for the modulation of the uptake1 carrier by ATP gamma S.

Isocyanines and pseudoisocyanines as a novel class of potent noradrenaline transport inhibitors: synthesis, detection, and biological activity

Noradrenaline, the neurotransmitter of the sympathetic nervous system, is removed from the extracellular space by both neuronal and extraneuronal transport mechanisms. In the past, further investigation of the extraneuronal type of noradrenaline transporter was severely hampered by the lack of potent inhibitors. Here, we describe the synthesis of various novel noradrenaline transport inhibitors which belong to the chemical class of isocyanine and pseudoisocyanine dyes. The biological activity of these compounds was investigated in a tissue culture system (Caki-1 cells). 1,1'-Diisopropyl-2,4'-cyanine, 1,1'-diethyl-2,2'-cyanine, and 1-ethyl-1'-isopropyl-2,2'-cyanine turned out as the most potent inhibitors of the extraneuronal noradrenaline transport known so far. At 100 nmol/L, these compounds diminished extraneuronal noradrenaline transport by about 95%. Their IC50's were below 20 nmol/L. In addition, a rapid and sensitive method (based on HPLC with fluorometric detection) to measure these compounds in body fluids is reported.

Cyanine-related compounds: a novel class of potent inhibitors of extraneuronal noradrenaline transport

The neurotransmitter noradrenaline is removed from the extracellular space by neuronal and extraneuronal transport mechanisms. In the past, further functional and biochemical characterisation of the corticosterone-sensitive extraneuronal transporter was hampered by the lack of highly potent inhibitors. Here we describe a new class of selective and highly potent inhibitors of the extraneuronal noradrenaline transporter. Clonal Caki-1 cells possess the human type of extraneuronal noradrenaline carrier. The effect of various steroids and steroid-like compounds on initial rates of specific 3H-noradrenaline transport in Caki-1 cells was investigated. None of these steroids had an inhibitory potency higher than that of corticosterone which hitherto was generally accepted as the most potent inhibitor of the extraneuronal noradrenaline transport. On the other hand, a variety of quinoline and isoquinoline derivatives interacted with the extraneuronal noradrenaline transporter. Several cationic quinolines that belong to the chemical class of the cyanine dyes turned out to be very potent inhibitors of 3H-noradrenaline transport in Caki-1 cells. The isocyanines, 1,1'-diisopropyl-2,4'-cyanine (disprocynium24) and 1-methyl-1'-isopropyl-2,4'-cyanine as well as the pseudoisocyanines 1,1'-diethyl-2,2'-cyanine (decynium22) and 1-isopropyl-1'-ethyl-2,2'-cyanine (iprecynium22) were most potent with IC50's of 14, 62, 16, and 18 nmol/l, respectively. The inhibitory potency on extraneuronal noradrenaline transport of 1-methyl-1'-isopropyl-2,4'-cyanine was determined also in isolated organs, namely the isolated incubated rabbit aorta and the isolated perfused rat heart.(ABSTRACT TRUNCATED AT 250 WORDS)

1,1'-diethyl-2,2'-cyanine (decynium22) potently inhibits the renal transport of organic cations

The excretion of cationic compounds by renal proximal tubule cells involves at least two distinct transporters: the basolateral type which transports organic cations from the plasma into the proximal tubule cell, and the apical type which secretes the organic cations into the lumen of the tubule. However, potent inhibitors were known for neither type of transporter. Here we introduce a compound, decynium22, that potently, competitively, and selectively inhibits the apical type of the renal organic cation transporter. The transport of the prototypical organic cation 14C-tetraethylammonium through the apical plasma membrane of clonal proximal tubule cells (LLC-PK1) was used as experimental system. Initial rates of 14C-tetraethylammonium transport into LLC-PK1 cells were saturable, the Km and Vmax being 27 mumol/l and 200 pmol/(mg protein.min), respectively. Decynium22 competitively and potently inhibited 14C-tetraethylammonium transport (Ki = 5.6 nmol/l). Moreover, the effect of decynium22 on basolateral to apical directed transepithelial transport of 14C-tetraethylammonium through a confluent monolayer of LLC-PK1 cells was determined. Decynium22 (30 nmol/l) applied to the apical medium, reduced transepithelial transport by 76% and increased intracellular accumulation of 14C-tetraethylammonium 1.5-fold. In contrast, application of 30 nmol/l decynium22 to the basolateral medium failed to affect transepithelial transport and intracellular accumulation of 14C-tetraethylammonium. Decynium22 is the most potent inhibitor of the renal transport of organic cations known so far. With decynium22 it is now possible to distinguish precisely between a decynium22-sensitive apical type and a decynium22-resistant basolateral type of renal organic cation transporter in renal proximal tubule cells.

The extraneuronal transport mechanism for noradrenaline (uptake2) avidly transports 1-methyl-4-phenylpyridinium (MPP+)

The corticosterone-sensitive extraneuronal transport mechanism for noradrenaline (uptake2) removes the neurotransmitter from the extracellular space. Recently, an experimental model for uptake2 has been introduced which is based on tissue culture techniques (human Caki-1 cells). The present study describes some properties of uptake2 in Caki-1 cells and introduces a new substrate, i.e., 1-methyl-4-phenylpyridinium (MPP+). Experiments on Caki-1 cells disclosed disadvantages of tritiated noradrenaline as substrate for the investigation of uptake2. The initial rate of 3H-noradrenaline transport [kin = 0.58 microliter/(mg protein.min)] was low compared with other cellular transport systems and intracellular noradrenaline was subject to rapid metabolism (kO-methylation = 0.54 min-1). The neurotoxin MPP+ was found to be a good substrate of uptake2. Initial rates of specific 3H-MPP+ transport into Caki-1 cells were saturable, the Km being 24 micromol/l and the Vmax being 420 pmol/(mg protein.min). The rate constant of specific inward transport was 34 times higher [19.6 microliters/(mg protein.min)] than that of 3H-noradrenaline. The ratio specific over non-specific transport was considerably higher for 3H-MPP+ (12.6) than for 3H-noradrenaline (3.0). 3H-MPP+ transport into Caki-1 cells was inhibited by various inhibitors of uptake2. The highly significant positive correlation (p less than 0.001, r = 0.986, n = 7) between the IC50's for the inhibition of the transport of 3H-noradrenaline and 3H-MPP+, respectively, proves the hypothesis that MPP+ enters Caki-1 cells via uptake2. 3H-MPP+ is taken up via uptake2 not only by Caki-1 cells but also by the isolated perfused rat heart which is another established model of uptake2.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of the density of adrenergic innervation on the extracellular steady-state concentration gradient for 3H-noradrenaline

After inhibition of extraneuronal uptake by corticosterone, isolated right atria and lengthwise halved vasa deferentia of the rat were incubated with 0.2 mumol/l 3H-noradrenaline for 60 min, washed out for 100 min and then prepared for autoradiography. The autoradiographic images were digitized, and silver grain density was determined as a function of the distance from the surface. Silver grain density declined towards the centre of the tissue; the decline was monophasic exponential and significantly steeper in the vas deferens (0.016 microns-1) than in the less densely innervated right atrium (0.011 microns-1). Silver grain density at the surface of the tissue was higher in vas deferens than in right atrium. The results show that the extracellular steady-state concentration gradient for 3H-noradrenaline (generated by uptake1 during the incubation with this amine) largely depends on the density of the adrenergic innervation.

The force driving the extraneuronal transport mechanism for catecholamines (uptake2)

Recently, uptake2 was shown to exist in the clonal Caki-1 cell line. The aim of this study was two-fold: a) to determine, in Caki-1 cells, the intracellular fate of 3H-noradrenaline after its translocation by uptake2 and b) to analyse the force driving uptake2. Caki-1 cells have the characteristics of a "metabolizing system" in which the activity of catechol-O-methyl transferase (COMT) greatly exceeds that of monoamine oxidase (MAO). In all subsequent experiments these enzymes were inhibited. The determination of initial rates of uptake2 into Caki-1 cells at an extracellular pH between 6.9 and 7.9 indicated that the protonated species of 3H-noradrenaline is transported. Depolarization of Caki-1 cells (by three different procedures) inhibited the inward transport. Determination of the time course of the specific accumulation of 3H-noradrenaline in Caki-1 cells and of 3H-isoprenaline in the perfused rat heart (both mediated by uptake2) revealed that depolarization (by high K+) reduced the rate constant for inward transport (kIN) and increased that for outward movement (kOUT). Consequently, depolarization reduced the steady-state factor of accumulation. It is proposed that, as the protonated species of the substrates of uptake2 is transported, the membrane potential is likely to provide the driving force for uptake2. The fact that depolarization decreased kIN and increased kOUT agrees with this proposal, as do the magnitudes of the steady-state accumulation factors determined in Caki-1 cells and perfused rat heart.

Nicotine-induced release of noradrenaline and neuropeptide Y in guinea-pig heart: role of calcium channels and protein kinase C

The role of calcium, calcium influx through calcium channels, and activation of protein kinase C for the nicotine-induced release of noradrenaline and of the sympathetic co-transmitter neuropeptide Y (NPY) was investigated in the guinea-pig isolated perfused heart. In the coronary venous overflow noradrenaline and NPY were determined by high-pressure liquid chromatography and radioimmunoassay, respectively. In the presence of extracellular calcium (1.85 mmol/l) nicotine (1-100 mumol/l) evoked a concentration-dependent overflow of both transmitters with a molar ratio of approximately 1500 (noradrenaline):1 (NPY). The nicotine-induced (100 mumol/l) overflow of noradrenaline and NPY was in a linear manner related (r = 0.79 and 0.90, respectively; p less than 0.05) to the extracellular calcium concentration (0-1.85 mmol/l), and it was prevented by calcium-free perfusion. The L-type calcium channel blocker felodipine (100 nmol/l) did not affect the nicotine-induced (100 mumol/l) transmitter overflow. On the other hand, the neuronal (N-type) calcium channel blockers omega-conotoxin (100 nmol/l) and cadmium chloride (50 mumol/l) reduced the nicotine-induced (100 mumol/l) transmitter overflow to 20% of the control value, suggesting a role of N-type calcium channels in mediating the calcium influx for the nicotine-induced transmitter release. The nicotine-induced (30 mumol/l) overflow of both transmitters was two- to three-fold increased by activation of protein kinase C (phorbol 12-myristate 13-acetate; 100 nmol/l). The transmitter overflow was unaffected by 4 alpha-phorbol 12,13-didecanoate (100 nmol/l), a phorbol ester which does not stimulate protein kinase C.(ABSTRACT TRUNCATED AT 250 WORDS)

The energy requirements for the basal efflux of 3H-noradrenaline from sympathetically innervated organs

In the rat vas deferens (preloaded with 3H-noradrenaline, catechol-O-methyl transferase inhibited, calcium-free solution) ouabain, glucose deprivation or the combination of hypoxia plus presence of lactate were found to induce a carrier-mediated (desipramine-sensitive) outward transport of the 3H-amine. Glucose deprivation additionally increased the efflux of deaminated 3H-metabolites, as a consequence of an increased net leakage of vesicular 3H-noradrenaline; moreover, 3H-dihydroxymandelic acid then became the predominant neuronal metabolite. The simultaneous lack of oxygen and glucose resulted in a very pronounced release of the 3H-amine. Moreover, during spontaneous efflux more outward transport of 3H-noradrenaline was observed in the absence than in the presence of extracellular calcium. In rat atria (under the same experimental conditions) the contribution by carrier-mediated outward transport to the spontaneous efflux of tritium exceeded that in vasa deferentia. Moreover, the efflux of lactate (as an index of hypoxia of the tissue) exceeded that observed in vasa deferentia, under aerobic and anaerobic conditions. It is proposed that the greater contribution by outward transport of 3H-noradrenaline to spontaneous efflux in atria than in vasa deferentia does not reflect any basic difference between the varicosities in two different organs. It is likely that the less heterogeneous distribution of the 3H-amine in atria than in vasa deferentia is responsible for storage of the exogenous amine in atrial varicosities that are subject to some hypoxia, to an increased extracellular lactate level and to perhaps a minor degree of glucose deficiency; these factors may well be responsible for the difference with regard to outward transport of 3H-noradrenaline during spontaneous efflux. Thus, in addition to the heterogeneity of the distribution of 3H-noradrenaline, an additional heterogeneity with regard to the energy supply must be expected for incubated organs.

The steady-state concentration gradient for 3H-noradrenaline generated by uptake1 in the extracellular space of the rat vas deferens incubated with this amine

The rat vas deferens was incubated with 0.2 mumol/l 3H-noradrenaline for 60 min, washed out with amine-free solution for 100 min and then prepared for autoradiography (same tissues as presented by Azevedo et al. (1990) Naunyn-Schmiedeberg's Arch Pharmacol 342:245-248). The autoradiographic images were then digitized, and grain density was determined as a function of the distance from the surface of the tissue. When neither monoamine oxidase nor vesicular uptake was impaired, i.e. under control conditions, grain density declined monophasically exponentially towards the centre of the tissue. Tis decline amounted to 0.017 micron-1 or 0.124 varicosity-1, since the average distance between varicosities was calculated to be 7.4 microns. After inhibition of monoamine oxidase and vesicular uptake the rate constant was significantly reduced, and the grain density in close proximity of the surface of the tissue was also reduced. It is proposed that the distribution of grain density observed in controls reflects the steady-state concentration gradient that is generated by uptake1 during the incubation with 3H-noradrenaline. During spontaneous efflux of 3H-noradrenaline one has to distinguish between "re-uptake of the 3H-amine into the leaking varicosity" and "uptake en passant" (during diffusion through the extracellular space). On the basis of the present results, the extent of "uptake en passant" was calculated (with a computer-assisted model) for the spontaneous efflux of heterogeneously distributed 3H-noradrenaline (after wash-out). "Uptake en passant" into varicosities located between the source of efflux and the medium amounted to about 55% of the net leakage of 3H-noradrenaline from all varicosities.(ABSTRACT TRUNCATED AT 250 WORDS)

Energy requirements for the basal efflux of noradrenaline and its metabolites from adrenergic varicosities

The combination of hypoxia plus glucose deprivation or of hypoxia plus lactate induces carrier-mediated outward transport of 3H-noradrenaline in the rat vas deferens. Lactate efflux is higher from atria than from vas deferens. Hence, the much lower contribution by outward transport to the spontaneous efflux of 3H-noradrenaline in vas deferens than atria is likely to be due to a better supply of oxygen (and perhaps also glucose) to the 3H-noradrenaline-storing varicosities in vas deferens than in atria.

The heterogeneity of the neuronal distribution of exogenous noradrenaline in the rat vas deferens

After loading of the incubated rat vas deferens with 0.2 mumol/l 3H-noradrenaline (followed by 100 min of wash-out with amine-free solution), the efflux of endogenous and exogenous compounds was determined by HPLC with electrochemical detection and by column chromatography with scintillation counting. Two different types of heterogeneity of labelling were found. The first one is due to the preferential labelling of varicosities close to the surface of the tissue, the second one to the preferential labelling of vesicles close to the surface of loaded varicosities. As diffusion distances within the tissue and within varicosities are then longer for endogenous than for exogenous amine and metabolites, the composition of spontaneous efflux of exogenous compounds differed from that for endogenous compounds. Because of preferential neuronal and vesicular re-uptake of endogenous noradrenaline, the percentage contribution by noradrenaline to overall efflux was: endogenous less than exogenous. While 3H-DOPEG was the predominant exogenous metabolite, DOPEG and MOPEG equally contributed to the "endogenous" efflux. Desipramine abolished the consequences of the first heterogeneity of labelling, i.e., it increased the efflux more for endogenous than for exogenous noradrenaline; moreover it decreased the efflux of 3H-DOPEG, but increased that of 3H-MOPEG. The reserpine-like compound Ro 4-1284, on the other hand, abolished the consequences of the second type of heterogeneity; it reduced the specific activity of "total efflux" (i.e., of the sum of noradrenaline + DOPEG + MOPEG) to the specific activity of the tissue noradrenaline. The degree of heterogeneity of labelling was reduced after inhibition of monoamine oxidase and also when the tissues were loaded with 2 or 20 mumol/l 3H-noradrenaline. It is proposed that the various "compartments" and "pools" of noradrenaline described in the literature reflect the two heterogeneities described here.

[Methoxyflurane and ethanol do not inhibit the neuronal uptake of noradrenaline (uptake 1) at the desipramine binding site]

We recently demonstrated that the net accumulation of 3H-norepinephrine in the rat pheochromocytoma cell line PC12 was reduced by anesthetic concentrations of n-alkanols and the volatile anesthetics halothane, enflurane, isoflurane, and methoxyflurane. In PC12 cells, as in adrenergic neurons, norepinephrine is transported across the plasma membrane by a saturable, high-affinity, carrier-mediated mechanism (uptake1), which follows Michaelis-Menten kinetics, is energy- and sodium-dependent, and is inhibited by low concentrations of cocaine and the tricyclic antidepressant desipramine. Although uptake1 is the most important process for the removal of norepinephrine from the synaptic cleft, the net accumulation of norepinephrine within the neuron also depends on other factors including its vesicular uptake and storage within the granules, its metabolism by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT), and the efflux of its more lipophilic metabolites. In our previous report we could not exclude the contribution of any of these factors to the observed inhibitory effects of volatile substances. Therefore, the aim of the present study with ethanol and methoxyflurane was: (1) to elucidate further the exact mechanism responsible for the reduction of the norepinephrine accumulation; and (2) to investigate the anesthetics' interaction with the substrate recognition site, which is identical with the desipramine binding site on the norepinephrine carrier. METHODS. For 3H-norepinephrine uptake experiments, PC12 cells were cultured on dishes (60 mm, Nunc) coated with polyornithine. Reserpine (10 microM) was added to the culture 24 h before the experiment to deplete endogenous norepinephrine. The initial carrier-mediated transport rate (60 s) was measured as previously described. 3H-desipramine equilibrium binding was determined with isolated plasma membranes prepared from PC12 cells grown in suspension culture. The carrier-mediated uptake of 3H-norepinephrine and the specific 3H-desipramine binding were defined as those inhibited by 1 microM nisoxetine. All buffers contained 10 microM pargyline and 10 microM U-0521 to inhibit MAO and COMT. Incubations were done in the presence and absence of methoxyflurane (1% and 2% vol/vol in synthetic air containing 5% CO2) or ethanol (5% vol/vol). Media had been equilibrated with methoxyflurane by bubbling (30 min) and were routinely checked by gas chromatography. RESULTS AND DISCUSSION. Methoxyflurane and ethanol inhibited uptake1. However, reduction of uptake1 was far less pronounced than that previously found for the net accumulation of norepinephrine. Even at a vaporous concentration of 2% (corresponding with an over 15-fold half-maximal inhibitory concentration for norepinephrine accumulation), methoxyflurane produced only 58% inhibition of the high-affinity uptake...

Extraneuronal noradrenaline transport (uptake2) in a human cell line (Caki-1 cells)

This study describes for the first time an experimental system for the extraneuronal transport mechanism of noradrenaline (uptake 2) which is based on a clonal cell line (Caki-1). Caki-1 cells were originally derived from a human renal cell carcinoma. The conclusion that these cells express uptake 2 is supported by several experimental findings. (1) The initial rate of 3H-noradrenaline uptake in Caki-1 cells is saturable, the Km being 450 mumol/l. (2) Inhibitors of uptake 2 such as corticosterone (1 mumol/l) and O-methyl-isoprenaline (100 mumol/l) largely inhibit 3H-noradrenaline uptake in Caki-1 cells. Whereas inhibitors of the neuronal transport mechanism for noradrenaline (uptake 1) such as desipramine (1 mumol/l) and cocaine (10 mumol/l) do not reduce it. (3) Depolarization of Caki-1 cells by the elevation of extracellular potassium inhibits 3H-noradrenaline uptake. (4) There is a highly significant correlation between the Ic50's of various compounds for the inhibition of 3H-noradrenaline uptake in Caki-1 cells and rabbit aorta known to possess uptake2. Interestingly enough, uptake 2 in Caki-1 cells and rabbit aorta is inhibited by cimetidine, quinidine and procainamide which are substrates of the renal transport mechanism for organic cations. Moreover, 3H-cimetidine is shown to be a substrate of uptake 2 in the isolated perfused rat heart. These results indicate a striking similarity between uptake 2 and the renal transport mechanism for organic cations.