MRI characterisation of a novel rat model of focal astrocyte loss

Blood-brain barrier (BBB) integrity is lost in several neurological conditions in which astrocytes are damaged. We studied 3-chloropropanediol-induced focal lesions, a toxicant that induces early astrocytic (but not neuronal) death followed by BBB leakage. T2-weighted images illustrate regional selectivity of the lesions, affected areas including the inferior colliculi and red nuclei. Gd-DTPA intensity quantified the degree of vascular leakage in the lesioned areas. MRI intensity in lesioned areas peaked at 2 days, correlating with BBB breakdown, and diminished thereafter, returning to pre-injection levels by 30 days in parallel with the return of astrocytes. T2 measurements were unchanged at 6 h, a time when astrocyte swelling is marked but the vasculature is intact, but increased at 2 days, consistent with cellular damage and BBB leakage. Gd-DTPA enhancement was also greatest at 2 days then decreased over the next 28 days, indicating a tracer-size-dependent rate of BBB repair. A simple model based on experimentally acquired data indicated that the vascular breakdown was the result of leakage of only a small percentage of blood vessels in the affected areas. Loss of astrocytes contributes to barrier loss, and restoration of astrocytes is needed for full barrier recovery.

High-energy particle acceleration in the shell of a supernova remnant

A significant fraction of the energy density of the interstellar medium is in the form of high-energy charged particles (cosmic rays). The origin of these particles remains uncertain. Although it is generally accepted that the only sources capable of supplying the energy required to accelerate the bulk of Galactic cosmic rays are supernova explosions, and even though the mechanism of particle acceleration in expanding supernova remnant (SNR) shocks is thought to be well understood theoretically, unequivocal evidence for the production of high-energy particles in supernova shells has proven remarkably hard to find. Here we report on observations of the SNR RX J1713.7 - 3946 (G347.3 - 0.5), which was discovered by ROSAT in the X-ray spectrum and later claimed as a source of high-energy gamma-rays of TeV energies (1 TeV = 10(12) eV). We present a TeV gamma-ray image of the SNR: the spatially resolved remnant has a shell morphology similar to that seen in X-rays, which demonstrates that very-high-energy particles are accelerated there. The energy spectrum indicates efficient acceleration of charged particles to energies beyond 100 TeV, consistent with current ideas of particle acceleration in young SNR shocks.

Effects of cyclic steroid hormone replacement on prolactin and luteinizing hormone surges in female rats

The ability of steroid hormones to produce an LH or prolactin (PRL) surge was determined in rats ovariectomized at 6, 9 or 13 weeks of age and subjected to one, three or six cycles of estrogen and progesterone replacement. Sensitivity to steroid replacement was dependent on the age of the animal at the time of ovariectomy. Repeated cyclic steroid hormone replacement significantly increased the magnitude of the PRL response, but not the LH response, in animals ovariectomized at 6 weeks. The LH response was significantly altered by cyclic steroid replacement only in animals ovariectomized at 13 weeks. These results indicate that the mechanisms involved in the regulation of PRL secretion are influenced by steroid hormone replacement and that cyclic steroid hormone exposure increases the magnitude of the PRL secretory response.

Drugs, hERG and sudden death

Early recognition of potential QT/TdP liability is now an essential component of the drug discovery/drug development program. The hERG assay is an indispensable step and a high-quality assay must accompany any investigational new drug (IND) application. While it is the gold standard at present, the hERG assay is too labor-intensive and too low throughput to be used as a screen early in the discovery/development process. A variety of indirect high throughput screens have been used.

Tradeoff between no-call reduction in genotyping error rate and loss of sample size for genetic case/control association studies

Single nucleotide polymorphisms (SNP) may be genotyped for use in case-control designs to test for association between a SNP marker and a disease using a 2 x 3 chi-squared test of independence. Genotyping is often based on underlying continuous measurements, which are classified into genotypes. A "no-call" procedure is sometimes used in which borderline observations are not classified. This procedure has the simultaneous effect of reducing the genotype error rate and the expected number of genotypes observed. Both quantities affect the power of the statistic. We develop methods for calculating the genotype error rate, the expected number of genotypes observed, and the expected power of the resulting test as a function of the no-call procedure. We examine the statistical properties of the chi-squared test using a no-call procedure when the underlying continuous measure of genotype classification is a three-component mixture of univariate normal distributions under a range of parameter specifications. The genotype error rate decreases as the no-call region is increased. The expected number of observations genotyped also decreases. Our key finding is that the expected power of the chi-squared test is not sensitive to the no-call procedure. That is, the benefits of reduced genotype error rate are almost exactly balanced by the losses due to reduced genotype observations. For an underlying univariate normal mixture of genotype classification to be analyzed with a 2 x 3 chi-squared test, there is little, if any, increase in power using a no-call procedure.

Inhibition of the Escherichia coli pyruvate dehydrogenase complex E1 subunit and its tyrosine 177 variants by thiamin 2-thiazolone and thiamin 2-thiothiazolone diphosphates. Evidence for reversible tight-binding inhibition

Variants of the pyruvate dehydrogenase subunit (E1; EC ) of the Escherichia coli pyruvate dehydrogenase multienzyme complex with Y177A and Y177F substitutions were created. Both variants displayed pyruvate dehydrogenase multienzyme complex activity at levels of 11% (Y177A E1) and 7% (Y177F E1) of the parental enzyme. The K(m) values for thiamin diphosphate (ThDP) were 1.58 microm (parental E1) and 6.65 microm (Y177A E1), whereas the Y177F E1 variant was not saturated at 200 microm. According to fluorescence studies, binding of ThDP was unaffected by the Tyr(177) substitutions. The ThDP analogs thiamin 2-thiazolone diphosphate (ThTDP) and thiamin 2-thiothiazolone diphosphate (ThTTDP) behaved as tight-binding inhibitors of parental E1 (K(i) = 0.003 microm for ThTDP and K(i) = 0.064 microm for ThTTDP) and the Y177A and Y177F variants. This analysis revealed that ThTDP and ThTTDP bound to parental E1 via a two-step mechanism, but that ThTDP bound to the Y177A variant via a one-step mechanism. Binding of ThTDP was affected and that of ThTTDP was unaffected by substitutions at Tyr(177). Addition of ThDP or ThTDP to parental E1 resulted in similar CD spectral changes in the near-UV region. In contrast, binding of ThTTDP to either parental E1 or the Y177A and Y177F variants was accompanied by the appearance of a positive band at 330 nm, indicating that ThTTDP was bound in a chiral environment. In combination with x-ray structural evidence on the location of Tyr(177), the kinetic and spectroscopic data suggest that Tyr(177) has a role in stabilization of some transition state(s) in the reaction pathway, starting with the free enzyme and culminating with the first irreversible step (decarboxylation), as well as in reductive acetylation of the dihydrolipoamide acetyltransferase component.

Metabolic substrates other than glucose support axon function in central white matter

We tested the hypothesis that non-glucose energy sources can support axon function in the rat optic nerve. Axon function was assessed by monitoring the stimulus-evoked compound action potential (CAP). CAP was maintained at full amplitude for 2 hr in 10 mM glucose. 20 mM lactate, 20 mM pyruvate, 10 mM fructose, or 10 mM mannose supported axon function as effectively as did glucose, and 10 mM glutamine provided partial support, but beta-hydroxybutyrate, octanoate, sorbitol, alanine, aspartate, and glutamate failed to support axon function. Our results indicated that a variety of compounds can sustain function in CNS myelinated axons. Axons probably use lactate, pyruvate, and glutamine directly as energy substrates, whereas mannose and fructose could be shuttled through astrocytes to lactate, which is then exported to axons.

Two nonredundant SecA homologues function in mycobacteria

The proper extracytoplasmic localization of proteins is an important aspect of mycobacterial physiology and the pathogenesis of Mycobacterium tuberculosis. The protein export systems of mycobacteria have remained unexplored. The Sec-dependent protein export pathway has been well characterized in Escherichia coli and is responsible for transport across the cytoplasmic membrane of proteins containing signal sequences at their amino termini. SecA is a central component of this pathway, and it is highly conserved throughout bacteria. Here we report on an unusual property of mycobacterial protein export--the presence of two homologues of SecA (SecA1 and SecA2). Using an allelic-exchange strategy in Mycobacterium smegmatis, we demonstrate that secA1 is an essential gene. In contrast, secA2 can be deleted and is the first example of a nonessential secA homologue. The essential nature of secA1, which is consistent with the conserved Sec pathway, leads us to believe that secA1 represents the equivalent of E. coli secA. The results of a phenotypic analysis of a Delta secA2 mutant of M. smegmatis are presented here and also indicate a role for SecA2 in protein export. Based on our study, it appears that SecA2 can assist SecA1 in the export of some proteins via the Sec pathway. However, SecA2 is not the functional equivalent of SecA1. This finding, in combination with the fact that SecA2 is highly conserved throughout mycobacteria, suggests a second role for SecA2. The possibility exists that another role for SecA2 is to export a specific subset of proteins.

Molecular determinants of inactivation and dofetilide block in ether a-go-go (EAG) channels and EAG-related K(+) channels

The major subunit of the cardiac delayed rectifier current I(Kr) is encoded by the human ether a-go-go related gene (HERG). HERG/I(Kr) channels are blocked selectively by class III antiarrhythmic methanesulfonanilide drugs such as dofetilide. The binding site for methanesulfonanilides is believed to be similar for nonantiarrhythmic drugs such as antihistamines, antibiotics, and antipsychotics. To gain further insight into the binding site, we examined the minimal structural changes necessary to transform low-affinity binding of dofetilide by the related bovine ether a-go-go channel bEAG to high-affinity binding of HERG. Previously, it was shown that high-affinity binding in HERG required intact C-type inactivation; the bovine ether a-go-go K(+) channel (bEAG), unlike HERG, is noninactivating. Therefore, we introduced C-type inactivation into noninactivating bEAG using site-directed mutagenesis. Two point mutations in the pore region, T432S and A443S, were sufficient to produce C-type inactivation. Low concentrations of dofetilide produced block of bEAG T432S/A443S; unlike HERG, block was almost irreversible. Substitution of an additional amino acid in transmembrane domain S6 made the block reversible. Dofetilide blocked the triply mutated bEAG T432S/A443S/A453S with an IC(50) value of 1.1 microM. The blocking potency was 30-fold greater than bEAG WT and about one third that of HERG WT. We conclude that high affinity methanesulfonanilide binding to HERG channels is strongly dependent on C-type inactivation.

Jejunal diverticulosis and gastrointestinal bleeding

Occult gastrointestinal bleeding frequently frustrates clinicians' attempts to locate the source. Foci of hemorrhage within the small bowel are often found only at laparotomy and can be attributed to Meckel's diverticula, carcinomas, or less frequently, pulsion-type diverticula. We report our experience with two patients whose jejunal diverticula resulted in recurrent episodes of massive gastrointestinal hemorrhage.

Increased cholesterol efflux in apolipoprotein AI (ApoAI)-producing macrophages as a mechanism for reduced atherosclerosis in ApoAI((-/-)) mice

The concentration of apolipoprotein (apo) AI in the artery wall is thought to enhance cellular cholesterol efflux and protect against atherosclerosis. It has been shown that although macrophages do not make apoAI, they respond to it by increased cholesterol efflux. We hypothesized that macrophage production of apoAI would increase cholesterol efflux and reduce atherogenesis. In this study, we produced mice expressing human apoAI under the control of the macrophage-specific scavenger receptor-A promoter (mphi-AI). Human apoAI was detectable in the serum HDL fraction of mphi-AI transgenic mice at concentrations too low to affect serum cholesterol or HDL levels. Immunoblotting showed the presence of human apoAI in transgenic macrophage culture supernatants, mostly as lipoprotein-free protein, with a small component associated with HDL-like particles. Atherosclerosis studies using apoAI((-/-)) mice transplanted with mphi-AI bone marrow showed that in the absence of macrophage-derived apoE, local expression of apoAI reduced diet-induced lesions in the proximal aorta. Additionally, mphi-AI macrophages showed a 40% increase in cholesterol efflux compared with control macrophages. These data support the hypothesis that apoAI production by macrophages in the artery wall is protective against atherosclerosis. This protection is likely mediated by increased cholesterol efflux and decreased foam cell formation in vivo.

The distinct spectra of tumor-associated Apc mutations in mismatch repair-deficient Apc1638N mice define the roles of MSH3 and MSH6 in DNA repair and intestinal tumorigenesis

In mammalian cells, mismatch recognition has been attributed to two partially redundant heterodimeric protein complexes of MutS homologues, MSH2-MSH3 and MSH2-MSH6. We have conducted a comparative analysis of Msh3 and Msh6 deficiency in mouse intestinal tumorigenesis by generating Apc1638N mice deficient in Msh3, Msh6 or both. We have found that Apc1638N mice defective in Msh6 show reduced survival and a 6-7-fold increase in intestinal tumor multiplicity. In contrast, Msh3-deficient Apc1638N mice showed no difference in survival and intestinal tumor multiplicity as compared with Apc1638N mice. However, when Msh3 deficiency is combined with Msh6 deficiency (Msh3(-/-)Msh6(-/-)Apc1638N), the survival rate of the mice was further reduced compared to Msh6(-/-)Apc(1638N) mice because of a high multiplicity of intestinal tumors at a younger age. Almost 90% of the intestinal tumors from both Msh6(-/-)Apc1638N and Msh3(-/-)Msh6(-/-)Apc1638N mice contained truncation mutations in the wild-type Apc allele. Apc mutations in Msh6(-/-)Apc1638N mice consisted predominantly of base substitutions (93%) creating stop codons, consistent with a major role for Msh6 in the repair of base-base mismatches. However, in Msh3(-/-)Msh6(-/-)Apc1638N tumors, we observed a mixture of base substitutions (46%) and frameshifts (54%), indicating that in Msh6(-/-)Apc1638N mice frameshift mutations in the Apc gene were suppressed by Msh3. Interestingly, all except one of the Apc mutations detected in mismatch repair-deficient intestinal tumors were located upstream of the third 20-amino acid beta-catenin binding repeat and before all of the Ser-Ala-Met-Pro repeats, suggesting that there is selection for loss of multiple domains involved in beta-catenin regulation. Our analysis therefore has revealed distinct mutational spectra and clarified the roles of Msh3 and Msh6 in DNA repair and intestinal tumorigenesis.

The gene structure and minimal promoter of the human agouti related protein

The murine agouti related protein (mAGRP) is upregulated in obese and diabetic mice and stimulates hyperphagia when administered intracerebroventricularly (i.c.v.) or when overexpressed in transgenic mice. The human ortholog, hAGRP, has been isolated and has similar molecular and physiological properties. Here, we report the complete gene structure of the human AGRP gene and upstream regions with differential promoter activity. A polymorphism, A67T, in the third exon was identified but was not associated with obesity- or type 2 diabetes-related phenotypes. Putative binding sites for transcription factors were identified in the promoter of the gene including recognition sites for the signal transducers and activators of transcription (STATs) that may potentially mediate leptin's action in the hypothalamus. The upstream non-coding exon had significant promoter activity in a periphery- but not so in a hypothalamus-derived cell line, suggesting that it might contain the minimal promoter required for expression of the short transcript of hAGRP in the periphery.

Insulin inhibits the maturation phase of VLDL assembly via a phosphoinositide 3-kinase-mediated event

LY 294002 (80 micromol/L), an inhibitor of phosphoinositide 3-kinase, was used to investigate the involvement of this enzyme in the insulin-mediated regulation of very low density lipoprotein (VLDL) apolipoprotein B (apoB) output from cultured rat hepatocytes. Newly synthesized apoB was pulse-labeled with [(35)S]methionine and was then allowed to assemble, via an intermediate precursor stage, into mature VLDL during subsequent chase periods. Brefeldin A (BFA, 0.2 microgram/mL) was used to discriminate between the role of insulin in the regulation of the early, compared with the later, events of VLDL assembly, including apoB degradation. Insulin (78 nmol/L), when present during the pulse-labeling and subsequent chase periods, inhibited the secretion of apoB-100 and apoB-48 as VLDL by 53% and 56%, respectively. Degradation of both was concomitantly increased. Secretion of high density lipoprotein apoB, derived from VLDL precursors, was relatively unaffected under these conditions, as was the net synthesis of apoB-100 and apoB-48. The presence of BFA during the pulse-labeling period and subsequent chase period prevented the maturation of VLDL in the insulin-treated and the non-insulin-treated cells. BFA was then removed, allowing the maturation of VLDL to proceed. Removal of insulin at this stage reversed the overall inhibitory effect of insulin. Furthermore, when insulin remained present during this period, the simultaneous presence of LY 294002 also reversed the inhibitory effect of insulin on VLDL apoB output and abolished the increase in apoB degradation. The results suggest that insulin signaling via phosphoinositide 3-kinase inhibited the maturation phase of VLDL assembly by preventing bulk lipid transfer to a VLDL precursor, thus enhancing the degradation of apoB. There was no inhibition of the conversion of newly synthesized apoB into the VLDL precursor form.

A role for the Agouti-Related Protein promoter in obesity and type 2 diabetes

The murine Agouti-Related Protein (mAGRP) is upregulated in obese and diabetic mice and can stimulate hyperphagia when overexpressed in transgenic models. Here we report upstream nucleotide sequences of the human hAGRP gene with putative recognition sites for transcription factors including a site for the STAT transactivators. A polymorphism (-38C-->T) was identified in the promoter region and the C/C genotype had significantly higher promoter activity and affinity for transcription factors as tested in periphery- and hypothalamus-derived cell lines. The polymorphic site could affect the expression levels of hAGRP and the high expressing C/C genotype was significantly associated with high BMI and type 2 diabetes in Africans.

Pharmacological assessment of the role of the glycine transporter GlyT-1 in mediating high-affinity glycine uptake by rat cerebral cortex and cerebellum synaptosomes

Two distinct types of glycine transporter, GlyT-1 and GlyT-2, have been characterised. GlyT-1 and GlyT-2 are known to be differentially expressed amongst CNS areas, but direct functional evidence for their relative contributions to high-affinity glycine uptake by brain tissues is lacking. In the present study, we have used the selective GlyT-1 inhibitor N[3-(4"-fluorophenyl)-3-(4"-phenylphenoxy)propyl]sarcosine (NFPS) to investigate the role of GlyT-1 in mediating glycine uptake. HEK293 cells expressing human GlyT-1c or GlyT-2 showed high levels of Na(+)-dependent glycine uptake, with K(m) values of 117+/-13 and 200+/-22 microM, respectively. NFPS potently inhibited uptake in GlyT-1c cells (IC(50) value 0.22+/-0.03 microM), being around 500-fold more potent than glycine or sarcosine, but had no effect on uptake in GlyT-2 cells (IC(50) >10 microM). Efflux of pre-loaded [3H]-glycine from GlyT-1c cells was increased by glycine or sarcosine, whereas NFPS had no effect on its own but blocked the effects of glycine or sarcosine. These results confirm that NFPS is a potent, selective and non-transportable GlyT-1 inhibitor. Rat cortex and cerebellum synaptosomes also showed a high-affinity Na(+)-dependent component of glycine uptake, with affinities similar to those observed for uptake in GlyT-1c or GlyT-2 cells. In cortex synaptosomes, NFPS and sarcosine produced the same maximal inhibition of uptake as glycine itself. However, in cerebellum synaptosomes, the maximal inhibition produced by NFPS and sarcosine was only half that produced by glycine. In both tissues NFPS was around 1000-fold more potent than glycine or sarcosine. Overall, our findings indicate that high-affinity glycine uptake in cerebral cortex occurs predominantly via GlyT-1. However, in cerebellum, only a part of the high-affinity uptake is mediated by GlyT-1, with the remaining NFPS-insensitive component most likely mediated by GlyT-2.

KChAP/Kvbeta1.2 interactions and their effects on cardiac Kv channel expression

KChAP and voltage-dependent K+ (Kv) beta-subunits are two different types of cytoplasmic proteins that interact with Kv channels. KChAP acts as a chaperone for Kv2.1 and Kv4.3 channels. It also binds to Kv1.x channels but, with the exception of Kv1.3, does not increase Kv1.x currents. Kvbeta-subunits are assembled with Kv1.x channels; they exhibit "chaperone-like" behavior and change gating properties. In addition, KChAP and Kvbeta-subunits interact with each other. Here we examine the consequences of this interaction on Kv currents in Xenopus oocytes injected with different combinations of cRNAs, including Kvbeta1.2, KChAP, and either Kv1.4, Kv1.5, Kv2.1, or Kv4.3. We found that KChAP attenuated the depression of Kv1.5 currents produced by Kvbeta1.2, and Kvbeta1.2 eliminated the increase of Kv2.1 and Kv4.3 currents produced by KChAP. Both KChAP and Kvbeta1.2 are expressed in cardiomyocytes, where Kv1.5 and Kv2.1 produce sustained outward currents and Kv4.3 and Kv1.4 generate transient outward currents. Because they interact, either KChAP or Kvbeta1.2 may alter both sustained and transient cardiac Kv currents. The interaction of these two different classes of modulatory proteins may constitute a novel mechanism for regulating cardiac K+ currents.

PEA3 is up-regulated in response to Wnt1 and activates the expression of cyclooxygenase-2

The inducible prostaglandin synthase cyclooxygenase-2 (COX-2) is aberrantly expressed in intestinal tumors resulting from APC mutation, and is also transcriptionally up-regulated in mouse mammary epithelial cells in response to Wnt1 expression. beta-Catenin stabilization is a consequence of both APC mutation and Wnt signaling. We have previously observed coordinate regulation of the matrilysin promoter by beta-catenin and Ets family transcription factors of the PEA3 subfamily. Here we show that while beta-catenin only weakly activates the COX-2 promoter, PEA3 family transcription factors are potent activators of COX-2 transcription. Consistent with this, PEA3 is up-regulated in Wnt1-expressing mouse mammary epithelial cells, and PEA3 factors are highly expressed in tumors from Wnt1 transgenic mice, in which Cox-2 is also up-regulated. Promoter mapping experiments suggest that the NF-IL6 site in the COX-2 promoter is important for mediating PEA3 responsiveness. The NF-IL6 site is also important for COX-2 transcription in some colorectal cancer lines (Shao, J., Sheng, H., Inoue, H., Morrow, J. D., and DuBois, R. N. (2000) J. Biol. Chem. 275, 33951-33956), and PEA3 factors are highly expressed in colorectal cancer cell lines. Therefore, we speculate that PEA3 factors may contribute to the up-regulation of COX-2 expression resulting from both APC mutation and Wnt1 expression.

Cyclooxygenase-2: a target for the prevention and treatment of breast cancer

Cyclooxygenase-2 (COX-2), an inducible prostaglandin synthase, is normally expressed in parts of the kidney and brain. Aberrant COX-2 expression was first reported in colorectal carcinomas and adenomas, and has now been detected in various human cancers, including those of the breast. Strikingly, COX-2 overexpression in murine mammary gland is sufficient to cause tumour formation. To date, the role of COX-2 in tumorigenesis has been most intensively studied in the colon. Thus, the relationship between COX-2 and neoplasia can best be illustrated with reference to intestinal tumorigenesis. Here we consider the potential utility of selective COX-2 inhibitors for the prevention and treatment of breast cancer. Data for cancers of the colon and breast are compared where possible. In addition, the mechanisms by which COX-2 is upregulated in cancers and contributes to tumorigenesis are discussed. Importantly, several recent studies of mammary tumorigenesis in animal models have found selective COX-2 inhibitors to be effective in the prevention and treatment of breast cancer. Clinical trials will be needed to determine whether COX-2 inhibition represents a useful approach to preventing or treating human breast cancer.

A step-by-step guide to non-linear regression analysis of experimental data using a Microsoft Excel spreadsheet

The objective of this present study was to introduce a simple, easily understood method for carrying out non-linear regression analysis based on user input functions. While it is relatively straightforward to fit data with simple functions such as linear or logarithmic functions, fitting data with more complicated non-linear functions is more difficult. Commercial specialist programmes are available that will carry out this analysis, but these programmes are expensive and are not intuitive to learn. An alternative method described here is to use the SOLVER function of the ubiquitous spreadsheet programme Microsoft Excel, which employs an iterative least squares fitting routine to produce the optimal goodness of fit between data and function. The intent of this paper is to lead the reader through an easily understood step-by-step guide to implementing this method, which can be applied to any function in the form y=f(x), and is well suited to fast, reliable analysis of data in all fields of biology.

High-affinity blockade of human ether-a-go-go-related gene human cardiac potassium channels by the novel antiarrhythmic drug BRL-32872

Human ether-a-go-go-related gene (HERG) potassium channels are one primary target for the pharmacological treatment of cardiac arrhythmias by class III antiarrhythmic drugs. These drugs are characterized by high antiarrhythmic efficacy, but they can also initiate life-threatening "torsade de pointes" tachyarrhythmias. Recently, it has been suggested that combining potassium and calcium channel blocking mechanisms reduces the proarrhythmic potential of selective class III antiarrhythmic agents. BRL-32872 is a novel antiarrhythmic drug that inhibits potassium and calcium currents in isolated cardiomyocytes. In our study, we investigated the effects of BRL-32872 on cloned HERG channels heterologously expressed in Xenopus oocytes. Using the two-microelectrode voltage clamp technique, we found that BRL-32872 caused a high-affinity, state-dependent block of open HERG channels (IC(50) = 241 nM) in a frequency-dependent manner with slow unbinding kinetics. Inactivated channels mainly had to open to be blocked by BRL-32872. The HERG S620T mutant channel, which has a strongly reduced degree of inactivation, was 51-fold less sensitive to BRL-32872 block, indicating that BRL-32872 binding was enhanced by the inactivation process. In an additional approach, we studied HERG channels expressed in a human cell line (HEK 293) using the whole-cell patch-clamp technique. BRL-32872 inhibited HERG currents in HEK 293 cells in a dose-dependent manner, with an IC(50) value of 19.8 nM. We conclude that BRL-32872 is a potent blocker of HERG potassium channels, which accounts for the class III antiarrhythmic action of BRL-32872.