Three-dimensional finite element analyses of four designs of a high-strength silicon nitride implant

The effects of implant shape and size on the stress distribution around high-strength silicon nitride implants under vertical and oblique forces were determined using a three-dimensional finite element analysis. Finite element models were designed using as a basis the serial sections of the mandible. Using Auto-CAD software, the model simulated the placement of implants in the molar region of the left mandible. Results of the analyses demonstrated that mainly the implant root shape and the directions of bite forces influence the stress distributions in the supporting bone around each implant. Implant size is a lesser factor. The serrated implants presented a larger surface area to the bone than either the cylindrical or tapered implants, which resulted in lower compressive stress around the serrated implants. With increasing implant diameter and length, compressive stress decreased. The mean compressive stress distribution on the serrated implants was more flat (platykurtic) than on either the cylindrical or tapered implants. Results of studies on two load directions (vertical and oblique) showed that, in either case, the compressive stress in the cortical bone around the neck of the implant was higher than in the cancellous bone along the length of the implant. The most extreme principal compressive stress was found with oblique force. This study provides the first information on the relationship between shape of the silicon nitride implant and stress on the supporting bone.

Roles of acyl-coenzyme A:cholesterol acyltransferase-1 and -2

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an intracellular enzyme that produces cholesteryl esters in various tissues. In mammals, two ACAT genes (ACAT1 and ACAT2) have been identified. Together, these two enzymes are involved in storing cholesteryl esters as lipid droplets, in macrophage foam-cell formation, in absorbing dietary cholesterol, and in supplying cholesteryl esters as part of the core lipid for lipoprotein synthesis and assembly. The key difference in tissue distribution of ACAT1 and ACAT2 between humans, mice and monkeys is that, in adult human liver (including hepatocytes and bile duct cells), the major enzyme is ACAT1, rather than ACAT2. There is compelling evidence implicating a role for ACAT1 in macrophage foam-cell formation, and for ACAT2 in intestinal cholesterol absorption. However, further studies at the biochemical and cell biological levels are needed in order to clarify the functional roles of ACAT1 and ACAT2 in the VLDL or chylomicron synthesis/assembly process.

A Chlamydia trachomatis UDP-N-acetylglucosamine acyltransferase selective for myristoyl-acyl carrier protein. Expression in Escherichia coli and formation of hybrid lipid A species

Chlamydia trachomatis lipid A is unusual in that it is acylated with myristoyl chains at the glucosamine 3 and 3' positions. We have cloned and expressed the gene encoding UDP-N-acetylglucosamine 3-O-acyltransferase of C. trachomatis (CtlpxA), the first enzyme of lipid A biosynthesis. C. trachomatis LpxA displays approximately 20-fold selectivity for myristoyl-ACP over R/S-3-hydroxymyristoyl-ACP under standard assay conditions, consistent with the proposed structure of C. trachomatis lipid A. CtLpxA is the first reported UDP-N-acetylglucosamine acyltransferase that prefers a non-hydroxylated acyl-ACP to a hydroxyacyl-ACP. When CtlpxA was expressed in RO138, a temperature-sensitive lpxA mutant of Escherichia coli, five new hybrid lipid A species were made in vivo after 2 h at 42 degrees C, in place of Escherichia coli lipid A. These compounds were purified and analyzed by matrix-assisted laser desorption ionization/time of flight mass spectrometry. In each case, a myristoyl chain replaced one or both of the ester linked 3-hydroxymyristoyl residues of E. coli lipid A. With prolonged growth at 42 degrees C, all the ester-linked 3-hydroxymyristoyl residues were replaced with myristate chains. Re-engineering the structure of E. coli lipid A should facilitate the microbiological production of novel agonists or antagonists of the innate immunity receptor TLR-4, with possible uses as adjuvants or anti-inflammatory agents.

[The prognostic research of expression and relationship between Helicobacter pylori of c-met oncogene correlation with gastric mucosal proliferation]

OBJECTIVE

To determine whether there is a relationship between Helicobacter pylori (Hp) and expression of c-met oncogene and proliferating cell nuclear antigen (PCNA) in gastric mucosal lesions, and prognostic significance of gastric carcinomas(GC).

METHODS

Expression of c-met and PCNA were investigated in 145 gastric mucosal lesions by using immunohistochemistry. Hp was examined by Warthin-Starry method.

RESULTS

For chronic superficial gastritis (CSG), chronic atrophic gastritis and intestinal metaplasia (CAS + IM), dysplasia(DYS), early GC and advanced GC, the positive rates of c-met and PCNA were higher in IM, DYS and GC than in CSG (P < 0.05). Expressions of c-met and PCNA increased with the increasing proliferation of gastric mucosa, in rank correlation test, they had a close relationship (P < 0.01). Expressions of c-met and PCNA were correlated significantly with histologic type, serosal invasion and lymph node metastasis. In particular, the expression of c-met and PCNA were significantly higher in Borrmann type 4 GC, type 3 GC and type 1, 2 than in early GC(P < 0.05). By rank correlation test, c-met and PCNA have a close relationship (P < 0.001). Expressions of c-met of Hp positive were higher than Hp negative in intestinal GC (P < 0.05). Expressions of c-met were correlated significantly with Hp positive in CAG + IM, DYS and GC. The survival rate of patients with Hp infection was significantly poorer than that of patients with Hp non-infection.

CONCLUSIONS

The expression of c-met may be is associated with proliferation and malignant transformation of gastric mucosa. These results suggest that expression of c-met may be a new prognostic factor in gastric carcinoma. Hp infection and expression of c-met may be associated with in proliferation and malignant transformation of gastric mucosa. Hp infection is associated with prognosis of gastric carcinoma.

[Study of susceptibility loci located within Xp11 in attention deficit hyperactivity disorder]

OBJECTIVE

To detect the genetic relationship between monoamine oxidase(MAO) A type gene and attention deficit hyperactivity disorder(ADHD) in Chinese.

METHODS

The haplotype-based haplotype relative risk(HHRR) and the transmission disequilibrium test(TDT) methods were used to analyze the genetic association and linkage in 60 ADHD children and their parents.

RESULTS

In this sample were found significant association (chi(2)=4.90, P<0.05) and linkage (chi(2)=4.84, P<0.05) between the MAOCA 114bp allele and DSM-III-R-diagnosed ADHD in trios composed of father, mother and affected offspring.

CONCLUSION

The above results suggested that ADHD was associated and in linkage with MAO A gene, and the susceptibility loci might reside in chromosome Xp11 for ADHD.

Sequencing cyclic peptide inhibitors of mammalian ribonucleotide reductase by electrospray ionization mass spectrometry

Mammalian ribonucleotide reductase (mRR) is a potential target for cancer intervention. A series of lactam-bridged cyclic peptide inhibitors (1-9) of mRR have been synthesized and tested in previous work. These inhibitors consist of cyclic and linear regions, causing their mass spectral characterization to be a challenge. We determined the fragmentation mechanism of cyclic peptides 1-9 using an ion-trap mass spectrometer equipped with an ESI source. Low-energy collision-induced dissociation of sodiated cyclic peptides containing linear branches follows a general pathway. Fragmentation of the linear peptide region produced mainly a and b ions. The ring peptide region was more stable and ring opening required higher collision energy, mainly occurring at the amide bond adjacent to the lactam bridge. The sodium ion, which bound to the carbonyl oxygen of the lactam bridge, acted as a fixed charge site and directed a charge-remote, sequence-specific fragmentation of the ring-opened peptide. Amino acid residues were cleaved sequentially from the C-terminus to the N-terminus. Our findings have established a new way to sequence cyclic peptides containing a lactam bridge based on charge-remote fragmentation. This methodology will permit unambiguous identification of high-affinity ligands within cyclic peptide libraries.

Cloning, expression, and characterization of a human inosine triphosphate pyrophosphatase encoded by the itpa gene

ITP and dITP exist in all cells. dITP is potentially mutagenic, and the levels of these nucleotides are controlled by inosine triphosphate pyrophosphatase (EC ). Here we report the cloning, expression, and characterization of a 21.5-kDa human inosine triphosphate pyrophosphatase (hITPase), an enzyme whose activity has been reported in many animal tissues and studied in populations but whose protein sequence has not been determined before. At the optimal pH of 10.0, recombinant hITPase hydrolyzed ITP, dITP, and xanthosine 5'-triphosphate to their respective monophosphates whereas activity with other nucleoside triphosphates was low. K(m) values for ITP, dITP, and xanthosine 5'-triphosphate were 0.51, 0.31, and 0.57 mm, respectively, and k(cat) values were 580, 360, and 640 s(-1), respectively. A divalent cation was absolutely required for activity. The gene encoding the hITPase cDNA sequence was localized by radiation hybrid mapping to chromosome 20p in the interval D20S113-D20S97, the same interval in which the ITPA inosine triphosphatase gene was previously localized. A BLAST search revealed the existence of many similar sequences in organisms ranging from bacteria to mammals. The function of this ubiquitous protein family is proposed to be the elimination of minor potentially mutagenic or clastogenic purine nucleoside triphosphates from the cell.

[Observation of preventing of bone loss during early postmenopause by percutaneous estradiol in Chinese postmenopausal women]

OBJECTIVE

To investigate the optimal regimen for application of percutaneous estradiol gel in preventing bone loss in Chinese postmenopausal women.

METHODS

A 3-year open randomized clinical study was designed. The percutaneous estradiol gel was used in a cyclic regimen combined with micronized progesterone (MP) or medroxyprogesterone acetate (MPA). Sixty healthy women (naturally menopause for 1 to 5 years) were recruited and divided into four groups according to estrogen dosage and two kinds of progestin. All were given for 25 d/month. The cortical bone mineral density (BMD) of right radius was measured by single photon absorptiometry. The trabecular BMD in lumbar vertebrae was measured by quantitative CT. The spine and hip BMD were also measured by dual energy X-ray absorptiometry at baseline, 6, 12, 18, 24 and 36 months, respectively. The bone metabolic markers, scores of menopausal symptoms were also evaluated.

RESULTS

Fifty-nine patients (98%) completed 1 year and 56 patients (93%) 2 years, 51 (85%) 3 years of study. The symptoms were alleviated by 80% after 6 months treatment on average. By the end of 24 month, the mean increases of BMD ranged from 4.3% to 7.5% in trabecular bone, and by the end of 36 month 4.2% to 6.2% in the lumbar (L) 2-4, 1.6% to 3.8% in the femur neck, with significant differences (P < 0.05). Comparing the 4 groups with each other, no significant differences (P > 0.05) were found in improvement of symptoms, bone markers and BMD.

CONCLUSIONS

Both daily estrogen containing 0.75 mg and 1.5 mg E2 are effective to prevent the early postmenopausal bone loss and improve the menopausal symptoms. During 3 years treatment, the BMD of lumbar increased continuously and the BMD of hip increased in the first 2 years and then plateau.

[A clinical evaluation on reconstruction in extended partial laryngectomy for transglottic cancer]

OBJECTIVE

Transglottic cancer has been a definite indication for total laryngectomy in decades, compromising function of the larynx. The purpose of this study is to present a new radical surgery of extended partial laryngectomy and reconstruction to restore the essential function of larynx.

METHODS

Forty-one cases of transglottic cancer were treated by extended vertical laryngectomy (26/41), extended frontolateral laryngectomy (5/41) and subtotal laryngectomy (10/41); the defects of larynx were reconstructed by rotary door myocutaneous flap in twenty-seven cases, by sternohyoideus flap in seven cases, by osteomuscular flap in five cases and by sternocleidomastoid flap in two cases.

RESULTS

The 3 and 5 year survival rates were 85.7% (30/35) and 74.1% (20/27) respectively, those for stage III were 84.6% (22/26) and 76.2% (16/21), for stage IV were 3/4 and 1/2 respectively. All cases resumed acceptable voice, 92.7% (38/41) and enjoyed satisfactory phonation. The over-all decannulation rate was 87.8% (36/41), those with rotary door myocutaneous flap was 96.3% (26/27), with sternohyoideus flap was 5/7, with osteomuscular flap was 4/5 and with sternocleidomastoid flap was 1/2. All patients had normal swallow function.

CONCLUSION

Extended partial laryngectomy is a kind of radical operation in selected cases of transglottic cancer. The method of reconstruction with rotary door myocutaneous flap can improve curative effect and the essential functions of the larynx.

Bronchial hyperresponsiveness and exhaled nitric oxide in patients with cardiac disease

BACKGROUND

Increased concentrations of exhaled nitric oxide (NO) correlate with increased airway inflammation and measurement of exhaled NO is a noninvasive method for the management of bronchial asthma. In various cardiac diseases, bronchial hyperresponsiveness is observed, as is bronchial asthma. However, there have been few studies on the relationship between exhaled NO and bronchial responsiveness in cardiac diseases.

OBJECTIVE

The aim of this study was to clarify the association between exhaled NO and bronchial hyperresponsiveness in patients with cardiac disease.

METHODS

We measured expired NO and bronchial responsiveness to inhaled methacholine in 19 patients with cardiac diseases and 17 with bronchial asthma. We divided the cardiac disease patients into two groups according to their bronchial responsiveness to inhaled methacholine: BHR(+) group consisted of 12 patients with bronchial hyperresponsiveness and BHR(-) group consisted of 7 patients without bronchial hyperresponsiveness.

RESULTS

The concentration of exhaled NO in the asthmatic patients was significantly higher than that in the BHR(+) and BHR(-) groups (142.0 +/- 17.0, 33.6 +/- 6.4 and 42.3 +/- 10.3 ppb, respectively, p < 0.01). There was no significant difference in exhaled NO between BHR(+) and BHR(-) groups. There were also no significant differences in the parameters of bronchial hyperresponsiveness between the cardiac BHR(+) and bronchial asthma groups. These results indicate that bronchial hyperresponsiveness in patients with cardiac diseases is not a consequence of eosinophilic inflammation or of exhaled NO.

CONCLUSION

We conclude that bronchial hyperresponsiveness in patients with cardiac diseases can occur independently of NO production.

Analysis of pancreatic development in living transgenic zebrafish embryos

Using DNA constructs containing regulatory sequences of the zebrafish Pdx-1 and insulin genes, germline transgenic zebrafish expressing the green fluorescent protein (GFP) reporter gene in the pancreas were generated. For both constructs, the GFP expression patterns in transgenic embryos were consistent with the mRNA expression patterns detected by RNA in situ hybridization. A deletion promoter analysis revealed that positive and negative cis-acting elements were involved in regulation of insulin gene expression. Three-dimensional reconstructions imaged from living embryos using two-photon laser-scanning microscopy (TPLSM) demonstrated that the zebrafish pancreas is formed from a single dorsal pancreatic cell mass. This is in contrast to mammals where the pancreas derives from both dorsal and ventral anlage. These transgenic fish should be useful for in vivo studies of factors involved in specifying and regulating pancreatic development and function.

Collectrin, a collecting duct-specific transmembrane glycoprotein, is a novel homolog of ACE2 and is developmentally regulated in embryonic kidneys

Collectrin, a novel homolog of angiotensin-converting enzyme-related carboxypeptidase (ACE2), was identified during polymerase chain reaction-based cDNA subtraction and up-regulated in 5/6 ablated kidneys at hypertrophic phase. Collectrin, with 222 amino acids, has an apparent signal peptide and a transmembrane domain; the sequence is conserved in mouse, rat, and human and shares 81.9% identity. Human collectrin has 47.8% identity with non-catalytic extracellular, transmembrane, and cytosolic domains of ACE2; however, unlike ACE and ACE2, collectrin lacks active dipeptidyl carboxypeptidase catalytic domains. The collectrin mRNA transcripts are expressed exclusively in the kidney. In situ hybridization reveals its mRNA expression in renal collecting ducts, and immunohistochemistry shows that it is localized to the luminal surface and cytoplasm of collecting ducts. Immunoprecipitation studies, using [35S]methionine-labeled renal cortical and inner medullar collecting duct cells, i.e. M-1 and mIMCD-3, indicate that the protein size is approximately 32 kDa. During the development of mouse kidney, mRNA signal is detectable at day 13 of gestation, and the protein product is observed in the ureteric bud branches. Its expression is progressively increased during later stages of the gestation extending into the neonatal periods and then is decreased in adult life. Up-regulated expression of collectrin in the hypertrophic kidneys after renal ablation and restricted spatio-temporal expression during development indicates a possible role(s)in the process of progressive renal failure and renal organogenesis.

A confidence-set approach for finding tightly linked genomic regions

As more studies adopt the approach of whole-genome screening, geneticists are faced with the challenge of having to interpret results from traditional approaches that were not designed for genome-scan data. Frequently, two-point analysis by the LOD method is performed to search for signals of linkage throughout the genome, for each of hundreds or even thousands of markers. This practice has raised the question of how to adjust the significance level for the fact that multiple tests are being performed. Various recommendations have been made, but no consensus has emerged. In this article, we propose a new method, the confidence-set approach, that circumvents the need to correct for the level of significance according to the number of markers tested. In the search for the gene location of a monogenic disorder, multiplicity adjustment is not needed in order to maintain the desired level of confidence. For complex diseases involving multiple genes, one needs only to adjust the level of significance according to the number of disease genes--a much smaller number than the number of markers in a genome screen-to ensure a predetermined genomewide confidence level. Furthermore, our formulation of the tests enables us to localize disease genes to small genomic regions, an extremely desirable feature that the traditional LOD method lacks. Our simulation study shows that, for sib-pair data, even when the coverage probability of the confidence set is chosen to be as high as 99%, our approach is able to implicate only the markers that are closely linked to the disease genes.

Relevance of aldo-keto reductase family members to the pathobiology of diabetic nephropathy and renal development

Aldo-keto reductases (AKRs) are a family of monomeric oxido-reductases with molecular weight ranging from 35-40 kDa and currently includes upwards of 60 members. They are expressed in a wide variety of tissues, where they catalyze the NADPH-dependent reduction of various aliphatic and aromatic aldehydes and ketones. The functions of most of the family members are not well defined. But two members, aldehyde reductase (AKRIA) and aldose reductase (AKRIB), have been extensively studied. The latter has received the most attention since being relevant to the complications of diabetes mellitus. It is up-regulated during hyperglycemia, and at the same time there is an increased activity of the sorbitol pathway and non-enzymatic glycation of proteins with ensuing damage in various tissues. It is developmentally regulated in the ocular lens, and is believed to modulate lens fiber morphogenesis during fetal life. Unlike the other AKR family members that are ubiquitously expressed, recently a renal-specific oxio-reductase has been described that is expressed exclusively in the proximal tubules. Although, it has no homology with other AKR members, it binds to NADPH with high affinity and is up-regulated in streptozotocin-induced diabetes in mice. It is also developmentally regulated and seems to selectively modulate renal tubulogenesis during embryonic life.

Purification and Characterization of a Chymotrypsin Inhibitor from the Venom of Ophiophagus hannah (King Cobra)

A chymotrypsin inhibitor from the venom of Ophiophagus hannah was isolated by a combination of ion-exchange chromatography and reverse phase HPLC. Amino acid sequence analysis revealed that this protein consists of 58 amino acids, six of these being cysteine residues and is highly homologous to Kunitz-type protease inhibitors. ESI-mass spectrum showed that the protein had a mass of 6493, which is in agreement with that predicted from its primary structure. In contrast to P1 Leu, Met, Phe, Trp, and Tyr appearing in other chymotrypsin inhibitors, a P1 Asn in the novel inhibitor may cause a weak binding (Ki = 3.52 microM) with chymotrypsin. Phylogenetic analysis suggests that the functional variations of the chymotrypsin inhibitor and other Kunitz-type inhibitors probably distinguish from dendrotoxins by accelerated evolution.

Status of glucose transporters in the mammalian kidney and renal development

Glucose is the main source or energy for the mammalian cells and its entry is mediated via various transporters. About 7 facilitative (GULT-1 to -7) and 2 concentrative glucose transporters (SGLT-1 and -2) have been identified. The facilitative glucose transporters allow the glucose entry into the cell interior due to the concentration gradient and the latter via the Na+-dependent electrochemical gradient. They have similar structural motifs with 12-14 putative transmembrane domains with a predicted protein size varying from 50 to 76kDa. Some of the facilitative glucose transporters (GLUT-1, -2, -4 and -5) and both the sodium glucose co-transporters (SGLT-1 and -2) are expressed in the kidney. The transporters that are involved in the major transport of glucose in the kidney include GLUT-2 and SGLT-2. They are of high capacity and low affinity type and are expressed in the S1 segment of the proximal tubule. All the transporters expressed in the kidney are developmentally regulated. The mRNA expression of renal GLUTs is variable during the fetal and postnatal periods. On the other hand the mRNA of SGLTs increases steadily from the fetal period to maturity along with the increase in their functional activity, i.e., glucose uptake. Recent studies indicate that the SGLTs are believed to selectively regulate tubulogenesis since they are expressed in the metanephric tubules very early in the embryonic life in mammals.

[Culture and identification of fetal human retinal photoreceptors]

OBJECTIVE

To investigate the methods of isolation and culture of fetal human retinal photoreceptors.

METHODS

Fetal human retinal photoreceptors were obtained by enzyme gradation digestion, cultured in media conditioned by retinal pigment epithelial cells (RPE-CM), identified by morphologic observation and immunohistochemically stained with rod specific opsin 4D2 (Rho 4D2) antibody. The digested retinal tissues were tested by hematoxylin-eosin (HE) staining.

RESULTS

Enzyme gradation digestion could obtain relatively pure retinal photoreceptors. These cells were stained positively for Rho 4D2 antibody, and survived in RPE-CM for a long time.

CONCLUSION

Fetal human retinal photoreceptors obtained by enzyme gradation digestion can survive in RPE-CM for a long time and express rod specific opsin. This culture system will provide a source for human retinal photoreceptor transplantation and further studies of photoreceptors in vitro.

[Expression and significance of interleukin-6,8 in human nasal polyps]

OBJECTIVE

To explore the role of interleukin-6,8 in nasal polyp formation and to search into the effect of allergy in the pathogenesis of nasal polyps (NP).

METHODS

The expression and significance of interleukin-6,8 were studied in 36 nasal polyps and 36 serum samples of NP patients by radioimmunoassay.

RESULTS

The mean value of IL-6 and IL-8 was (2.7658 +/- 0.3797) ng/L and (4.1877 +/- 0.1758) ng/L in all nasal polyp tissue homogenates. As compared with serum of NP patients, IL-6 and IL-8 were over expressed in nasal polyp tissue homogenates. No relation was found between the expression of IL-6/IL-8 and patients' gender, age and clinical stage. The expression of IL-6 and IL-8 in patients' serum, cord blood and normal serum showed no significant difference.

CONCLUSION

IL-6 and IL-8 are strongly correlated with the formation of nasal polyp. Neither allergy nor infection play a role in the pathogenesis of nasal polyps.

[The combination of cyclosporin A and androgen in the treatment of chronic aplastic anemia]

OBJECTIVE

To explore the therapeutic effectiveness of combination of cyclosporin A (CsA) and androgen in the treatment of chronic aplastic anemia (CAA).

METHOD

Androgen alone or combined with CsA for the treatment of CAA was compared by a randomized controlled clinical trial.

RESULT

The efficacy of androgen combined with CsA (87.9%) was higher than that of androgen alone (57.1%). Therapeutic effectiveness of the combination treatment between the patients with positive and negative peripheral blood mononuclear cells (PBMNCs) inhibiting normal colony formation unit-granulocyte and macrophage (CFU-GM) test showed a significant difference (P < 0.005).

CONCLUSION

Androgen combined with CsA had a much better efficacy than that of androgen alone in the treatment of CAA. The patients' PBMNCs inhibiting normal CFU-GM growth test can be used as an index of the treatment outcome. The side effects of the combination therapy are low and tolerable.

Concurrent expression of recombination activating genes 1 and 2 in zebrafish olfactory sensory neurons

Each olfactory sensory neuron (OSN) expresses a single odorant receptor (OR) from a large repertoire of clustered OR genes. It has been hypothesized that OR gene regulation may involve stochastic DNA rearrangement, which in lymphocytes requires the recombination activating genes, rag1 and rag2. We have recently demonstrated that rag1 is expressed in zebrafish OSNs. Here we report that rag2, the obligate partner for rag1 function, is also expressed in OSNs and that its expression pattern mimics that of rag1. The onset of rag1 and rag2 expression preceded that of known zebrafish ORs and the number of rag1-positive OSNs corresponded with the number expressing the olfactory cyclic nucleotide-gated cation channel, an OSN marker. Zebrafish OSNs are the first example of concurrent rag expression in a nonlymphoid tissue. The expression of rag1 and rag2 in OSNs adds to the list of similarities between the olfactory and immune systems that includes monoallelic and mutually exclusive gene expression.

Arsenicals inhibit thioredoxin reductase in cultured rat hepatocytes

Thioredoxin reductase (TR), an NADPH-dependent flavoenzyme that catalyzes the reduction of many disulfide-containing substrates, plays an important role in the cellular response to oxidative stress. Trivalent arsenicals, especially methyl As that contains trivalent arsenic (MAs(III)), are potent noncompetitive inhibitors of TR purified from mouse liver. Because MAs(III) is produced in the biomethylation of As, it was postulated that the extent of inhibition of TR in cultured rat hepatocytes would correlate with the intracellular concentration of methyl As. Exposure of cultured hepatocytes to inorganic As(III) (iAs(III)), MAs(III), or aurothioglucose (ATG, a competitive inhibitor of TR activity) for 30 min caused a concentration-dependent reduction in TR activity. The estimated IC(50) was >>100 microM for iAs(III), approximately 10 microM for ATG, and approximately 3 microM for MAs(III). In hepatocytes exposed to 1 microM MAs(III) for up to 24 h, the inhibition of TR activity was maximal ( approximately 40%) after exposure for 15 min. After exposure for 3 h [when most MAs(III) has been converted to dimethyl As (DMAs)], TR activity in these cells had returned to control levels. Notably, exposure of the cell to 50 microM DMAs(III) did not affect TR activity. In hepatocytes exposed to 10 microM iAs(III) for up to 24 h, the inhibition of TR activity was progressive; at 24 h, activity was reduced approximately 35%. Following exposure to iAs(III) or MAs(III), the extent of inhibition of TR activity correlated strongly with the intracellular concentration of MAs. Taken together, these results suggest that arsenicals formed in the course of cellular metabolism of As are potent inhibitors of TR activity. In particular, MAs(III), an intermediate in the metabolic pathway, is an especially potent inhibitor of TR. Hence, the capacity of cells to produce or consume the intermediates in the pathway for As methylation may be an important determinant of susceptibility to the toxic effects of As.

Influence of permeating ions on Kv1.5 channel block by nifedipine

Nifedipine can block K(+) currents through Kv1.5 channels in an open-channel manner (32). Replacement of internal and external K(+) with equimolar Rb(+) or Cs(+) reduced the potency of nifedipine block of Kv1.5 from an IC(50) of 7.3 microM (K(+)) to 16.0 microM (Rb(+)) and 26.9 microM (Cs(+)). The voltage dependence of block was unaffected, and a single binding site block model was used to describe block for all three ions. By varying ion species at the intra- and extracellular mouth of the channel and by using a nonconducting W472F-Kv1.5 mutant, we demonstrated that block was conditioned by the ion permeating the pore and, to a lesser extent, by the extracellular ion species alone. In Kv1.5, the outer pore mutations R487V and R487Y reduced nifedipine potency close to that of Kv4.2 and other Kv channels with an equivalent valine. Although changing this residue can affect C-type inactivation of Kv channels, the normalized reduction and time course of currents blocked by nifedipine in 5, 135, and 300 mM extracellular K(+) concentration was the same. Similarly, a mean recovery time constant from nifedipine block of 316 ms was unchanged (332 ms) after 5-s prepulses to allow C-type inactivation. This is consistent with the conclusion that nifedipine block and C-type inactivation in the Kv1.5 channel can coexist but are mediated by distinct mechanisms coordinated by outer pore conformation.