TRP-PLIK, a bifunctional protein with kinase and ion channel activities

We cloned and characterized a protein kinase and ion channel, TRP-PLIK. As part of the long transient receptor potential channel subfamily implicated in control of cell division, it is a protein that is both an ion channel and a protein kinase. TRP-PLIK phosphorylated itself, displayed a wide tissue distribution, and, when expressed in CHO-K1 cells, constituted a nonselective, calcium-permeant, 105-picosiemen, steeply outwardly rectifying conductance. The zinc finger containing alpha-kinase domain was functional. Inactivation of the kinase activity by site-directed mutagenesis and the channel's dependence on intracellular adenosine triphosphate (ATP) demonstrated that the channel's kinase activity is essential for channel function.

Ionic remodeling underlying action potential changes in a canine model of atrial fibrillation

Rapid electrical activation, as occurs during atrial fibrillation (AF), is known to cause reductions in atrial refractoriness and in adaptation to heart rate of the atrial refractory period, which promote the maintenance of AF, but the underlying ionic mechanisms are unknown. In order to determine the cellular and ionic changes caused by chronic atrial tachycardia, we studied right atrial myocytes from dogs subjected to 1, 7, or 42 days of atrial pacing at 400/min and compared them with myocytes from sham-operated dogs (pacemaker inserted but not activated). Rapid pacing led to progressive increases in the duration of AF induced by bursts of 10-Hz stimuli (from 3 +/- 2 seconds in sham-operated dogs to 3060 +/- 707 seconds in dogs after 42 days of pacing, P < .001) and reduced atrial refractoriness and adaptation to rate of the atrial refractory period. Voltage-clamp studies showed that chronic rapid pacing did not alter inward rectifier K+ current, rapid or slow components of the delayed rectifier current, the ultrarapid delayed rectifier current, T-type Ca2+ current, or Ca(2+)-dependent Cl- current. In contrast, the densities of transient outward current (Ito) and L-type Ca2+ current (ICa) were progressively reduced as the duration of rapid pacing increased, without concomitant changes in kinetics or voltage dependence. In keeping with in vivo changes in refractoriness, action potential duration (APD) and APD adaptation to rate were decreased by rapid pacing. The response of the action potential and ionic currents flowing during the action potential (as exposed by action-potential voltage clamp) to nifedipine in normal canine cells and in cells from rapidly paced dogs suggested that the APD changes in paced dogs were largely due to reductions in ICa. We conclude that sustained atrial tachycardia reduces Ito and ICa, that the reduced ICa decreases APD and APD adaptation to rate, and that these cellular changes likely account for the alterations in atrial refractoriness associated with enhanced ability to maintain AF in the model.

A prokaryotic voltage-gated sodium channel

The pore-forming subunits of canonical voltage-gated sodium and calcium channels are encoded by four repeated domains of six-transmembrane (6TM) segments. We expressed and characterized a bacterial ion channel (NaChBac) from Bacillus halodurans that is encoded by one 6TM segment. The sequence, especially in the pore region, is similar to that of voltage-gated calcium channels. The expressed channel was activated by voltage and was blocked by calcium channel blockers. However, the channel was selective for sodium. The identification of NaChBac as a functionally expressed bacterial voltage-sensitive ion-selective channel provides insight into both voltage-dependent activation and divalent cation selectivity.

The Drosophila fusome, a germline-specific organelle, contains membrane skeletal proteins and functions in cyst formation

Oogenesis in Drosophila takes place within germline cysts that support polarized transport through ring canals interconnecting their 15 nurse cells and single oocyte. Developing cystocytes are spanned by a large cytoplasmic structure known as the fusome that has been postulated to help form ring canals and determine the pattern of nurse cell-oocyte interconnections. We identified the adducin-like hts product and alpha-spectrin as molecular components of fusomes, discovered a related structure in germline stem cells and documented regular associations between fusomes and cystocyte centrosomes. hts mutations completely eliminated fusomes, causing abnormal cysts containing a reduced number of cells to form. Our results imply that Drosophila fusomes are required for ovarian cyst formation and suggest that membrane skeletal proteins regulate cystocyte divisions.

Human infection by genetically diverse SIVSM-related HIV-2 in west Africa

Our understanding of the biology and origins of human immunodeficiency virus type 2 (HIV-2) derives from studies of cultured isolates from urban populations experiencing epidemic infection and disease. To test the hypothesis that such isolates might represent only a subset of a larger, genetically more diverse group of viruses, we used nested polymerase chain reactions to characterize HIV-2 sequences in uncultured mononuclear blood cells of two healthy Liberian agricultural workers, from whom virus isolation was repeatedly unsuccessful, and from a culture-positive symptomatic urban dweller. Analysis of pol, env and long terminal repeat regions revealed the presence of three highly divergent HIV-2 strains, one of which (from one of the healthy subjects) was significantly more closely related to simian immunodeficiency viruses infecting sooty mangabeys and rhesus macaques (SIVSM/SIVMAC) than to any virus of human derivation. This subject also harboured multiply defective viral genotypes that resulted from hypermutation of G to A bases. Our results indicate that HIV-2, SIVSM and SIVMAC comprise a single, highly diverse group of lentiviruses which cannot be separated into distinct phylogenetic lineages according to species of origin.

Effect of genetic variation in the organic cation transporter 1, OCT1, on metformin pharmacokinetics

The goal of this study was to determine the effects of genetic variation in the organic cation transporter 1, OCT1, on the pharmacokinetics of the antidiabetic drug, metformin. Twenty healthy volunteers with known OCT1 genotype agreed to participate in the study. Each subject received two oral doses of metformin followed by collection of blood and urine samples. OCT1 genotypes had a significant (P<0.05) effect on metformin pharmacokinetics, with a higher area under the plasma concentration-time curve (AUC), higher maximal plasma concentration (Cmax), and lower oral volume of distribution (V/F) in the individuals carrying a reduced function OCT1 allele (R61C, G401S, 420del, or G465R). The effect of OCT1 on metformin pharmacokinetics in mice was less than in humans possibly reflecting species differences in hepatic expression level of the transporter. Our studies suggest that OCT1 genotype is a determinant of metformin pharmacokinetics.

Evidence for two components of delayed rectifier K+ current in human ventricular myocytes

Previous voltage-clamp studies have suggested that the delayed rectifier current (IK) is small or absent in the human ventricle and, when present, consists only of the rapid component (IKr); however, molecular studies suggest the presence of functionally important IK in the human heart, specific IKr blockers are known to delay ventricular repolarization and cause the long QT syndrome in humans, and we have shown that the expression of IK is strongly influenced by cell isolation techniques. The present experiments were designed to assess the expression of IK in myocytes obtained by arterial perfusion of right ventricular tissue from explanted human hearts. Of 35 cells from three hearts, 33 (94%) showed time-dependent currents typical of IK. The envelope-of-tails test was not satisfied under control conditions but became satisfied in the presence of the benzenesulfonamide E-4031 (5 micromol/L). E-4031 suppressed a portion of IK in 32 of 33 cells, with properties of the drug-sensitive and -resistant components consistent with previous descriptions of IKr and the slow component (IKs), respectively. Action potential duration to 95% repolarization at 1 Hz was prolonged by E-4031 from 336+/-16 (mean +/- SEM) to 421 +/- 19ms (n = 5, P < .01), indicating a functional role for IK. Indapamide, a diuretic agent previously shown to inhibit IKs selectively, suppressed E-4031-resistant current. The presence of a third type of delayed rectifier, the ultrarapid delayed rectifier current (IKur), was evaluated with the use of depolarizing prepulses and low concentrations (50 micromol/L) of 4-aminopyridine. Although these techniques revealed clear IKur in five of five human atrial cells, no corresponding component was observed in any of five human ventricular myocytes. We conclude that a functionally significant IK, with components corresponding to IKr and IKs, is present in human ventricular cells, whereas IKur appears to be absent. These findings are important for understanding the molecular, physiological, and pharmacological determinants of human ventricular repolarization and arrhythmias.

CaT1 manifests the pore properties of the calcium-release-activated calcium channel

The calcium-release-activated Ca2+channel, ICRAC, is a highly Ca2+-selective ion channel that is activated on depletion of either intracellular Ca2+ levels or intracellular Ca2+ stores. The unique gating of ICRAC has made it a favourite target of investigation for new signal transduction mechanisms; however, without molecular identification of the channel protein, such studies have been inconclusive. Here we show that the protein CaT1 (ref. 4), which has six membrane-spanning domains, exhibits the unique biophysical properties of ICRAC when expressed in mammalian cells. Like ICRAC, expressed CaT1 protein is Ca2+ selective, activated by a reduction in intracellular Ca2+ concentration, and inactivated by higher intracellular concentrations of Ca2+. The channel is indistinguishable from ICRAC in the following features: sequence of selectivity to divalent cations; an anomalous mole fraction effect; whole-cell current kinetics; block by lanthanum; loss of selectivity in the absence of divalent cations; and single-channel conductance to Na+ in divalent-ion-free conditions. CaT1 is activated by both passive and active depletion of calcium stores. We propose that CaT1 comprises all or part of the ICRAC pore.

Genetic diversity of human immunodeficiency virus type 2: evidence for distinct sequence subtypes with differences in virus biology

The virulence properties of human immunodeficiency virus type 2 (HIV-2) are known to vary significantly and to range from relative attenuation in certain individuals to high-level pathogenicity in others. These differences in clinical manifestations may, at least in part, be determined by genetic differences among infecting virus strains. Evaluation of the full spectrum of HIV-2 genetic diversity is thus a necessary first step towards understanding its molecular epidemiology, natural history of infection, and biological diversity. In this study, we have used nested PCR techniques to amplify viral sequences from the DNA of uncultured peripheral blood mononuclear cells from 12 patients with HIV-2 seroreactivity. Sequence analysis of four nonoverlapping genomic regions allowed a comprehensive analysis of HIV-2 phylogeny. The results revealed (i) the existence of five distinct and roughly equidistant evolutionary lineages of HIV-2 which, by analogy with HIV-1, have been termed sequence subtypes A to E; (ii) evidence for a mosaic HIV-2 genome, indicating that coinfection with genetically divergent strains and recombination can occur in HIV-2-infected individuals; and (iii) evidence supporting the conclusion that some of the HIV-2 subtypes may have arisen from independent introductions of genetically diverse sooty mangabey viruses into the human population. Importantly, only a subset of HIV-2 strains replicated in culture: all subtype A viruses grew to high titers, but attempts to isolate representatives of subtypes C, D, and E, as well as the majority of subtype B viruses, remained unsuccessful. Infection with all five viral subtypes was detectable by commercially available serological (Western immunoblot) assays, despite intersubtype sequence differences of up to 25% in the gag, pol, and env regions. These results indicate that the genetic and biological diversity of HIV-2 is far greater than previously appreciated and suggest that there may be subtype-specific differences in virus biology. Systematic natural history studies are needed to determine whether this heterogeneity has clinical relevance and whether the various HIV-2 subtypes differ in their in vivo pathogenicity.

hu-li tai shao, a gene required for ring canal formation during Drosophila oogenesis, encodes a homolog of adducin

Drosophila females bearing mutations in a previously undescribed gene, hu-li tai shao [(hts) too little nursing], produced egg chambers that contained fewer than the normal 15 nurse cells and that usually lacked an oocyte. The cytoplasmic bridges (ring canals) interconnecting nurse cells and the oocyte appeared abnormal, and lacked associated actin rings. The hts locus was found to encode a homolog of the mammalian membrane skeletal protein adducin. During oogenesis, hts mRNA became localized at the anterior of the oocyte and was subsequently expressed in a variety of embryonic tissues. These studies suggested that Drosophila adducin is needed to assemble actin at specialized regions of cell-cell contact in developing egg chambers and may also function at other times during the Drosophila life cycle.

Molecular mechanisms underlying ionic remodeling in a dog model of atrial fibrillation

The rapid atrial rate during atrial fibrillation (AF) decreases the ionic current density of transient outward K+ current, L-type Ca2+ current, and Na+ current, thereby altering cardiac electrophysiology and promoting arrhythmia maintenance. To assess possible underlying changes in cardiac gene expression, we applied competitive reverse transcriptase-polymerase chain reaction to quantify mRNA concentrations in dogs subjected to 7 (group P7 dogs) or 42 (group P42 dogs) days of atrial pacing at 400 bpm and in sham controls. Rapid pacing reduced mRNA concentrations of Kv4.3 (putative gene encoding transient outward K+ current; by 60% in P7 and 74% in P42 dogs; P<0.01 and P<0.001, respectively, versus shams), the alpha1c subunit of L-type Ca2+ channels (by 57% in P7 and 72% in P42 dogs; P<0.01 versus shams for each) and the alpha subunit of cardiac Na+ channels (by 18% in P7 and 42% in P42; P=NS and P<0.01, respectively, versus shams) genes. The observed changes in ion channel mRNA concentrations paralleled previously measured changes in corresponding atrial ionic current densities. Atrial tachycardia did not affect mRNA concentrations of genes encoding delayed or Kir2.1 inward rectifier K+ currents (of which the densities are unchanged by atrial tachycardia) or of the Na+,Ca2+ exchanger. Western blot techniques were used to quantify protein expression for Kv4.3 and Na+ channel alpha subunits, which were decreased by 72% and 47%, respectively, in P42 dogs (P<0.001 versus control for each), in a manner quantitatively similar to measured changes in mRNA and currents, whereas Na+,Ca2+ exchanger protein concentration was unchanged. We conclude that chronic atrial tachycardia alters atrial ion channel gene expression, thereby altering ionic currents in a fashion that promotes the occurrence of AF. These observations provide a potential molecular basis for the self-perpetuating nature of AF.

Cdc37 is a molecular chaperone with specific functions in signal transduction

Cdc37 is required for cyclin-dependent kinase activation and is genetically linked with the activity of several other kinases, including oncogenic v-Src, casein kinase II, MPS-1 kinase, and sevenless. Strikingly, many pathways involving Cdc37 also involve the protein chaperone Hsp90. The identification of Cdc37 as the 50-kD protein in several Hsp90-kinase complexes, together with other data, led to the recent suggestion that Cdc37 is a kinase-targeting "subunit" of Hsp90. We directly examined the effect of Cdc37 on Hsp90 functions. Rather than simply acting as an accessory factor for Hsp90, Cdc37 is itself a protein chaperone with properties remarkably similar to those of Hsp90. In vitro, Cdc37 maintains denatured beta-galactosidase in an activation-competent state without reactivating it and stabilizes mature, but unstable, casein kinase II. In vivo, Cdc37 overexpression can compensate for decreased Hsp90 function, but the proteins are not interchangeable. Cdc37 can compensate for Hsp90 in maintaining the activity of v-Src kinase but does not maintain the activity of the glucocorticoid receptor. Thus, the very similar chaperone activities of the two proteins, uncovered through in vitro analysis, diverge in vivo in specific signal transduction pathways.

Differential distribution of inward rectifier potassium channel transcripts in human atrium versus ventricle

BACKGROUND

The inward rectifier K+ current (IK1) plays an important role in governing cardiac electrical activity and is well known to have different properties in the atrium compared with the ventricle. Several inward rectifier K+ channel (IRK) subunits (hIRK, HH-IRK1, HIR, and TWIK-1) with different properties have been cloned from human tissues, but their relative expression in cardiac tissues has not been quantified. The present study was designed to define the relative levels of mRNA for various IRKs in human atrium and in failing and nonfailing ventricle.

METHODS AND RESULTS

Competitive reverse transcription-polymerase chain reaction was used to quantify in human atrium and ventricle the mRNA levels of hIRK, HH-IRK1, HIR, and TWIK-1. The absence of important noncardiac contamination was confirmed by demonstrating a lack of detectable mRNA markers for neuronal (acetylcholine receptor) and vascular (maxi-K channel) tissue. mRNA of HIR was more abundant in normal atrium (7.1+/-1.3 amol/ microg total RNA) than ventricle (0.6+/-0.1 amol/ microg, P<0. 05), whereas TWIK-1 mRNA was more concentrated in ventricle (18. 1+/-4.3 amol/ microg) than atrium (1.4+/-0.3 amol/ microg, P<0.05). Concentrations of hIRK (42.7+/-6.7 amol/ microg in atrium vs 57. 1+/-9.2 amol/ microg in ventricle) and HH-IRK1 (2.0+/-0.5 amol/ microg in atrium vs 1.5+/-0.5 amol/ microg in ventricle) were comparable. No significant differences in IRK subunit transcript concentrations were found between normal and failing ventricles.

CONCLUSIONS

mRNAs for all 4 IRKs are detected in human atrium and ventricle, but the mRNA copy number of a low-conductance subunit (HIR) is larger in atrium and the copy number of a weakly rectifying subunit (TWIK-1) is larger in ventricle. These differences in relative message levels may provide a potential molecular basis for different properties of IK1 in human atrium compared with ventricle.

Transmural heterogeneity of action potentials and Ito1 in myocytes isolated from the human right ventricle

Limited information is available about transmural heterogeneity in cardiac electrophysiology in man. The present study was designed to evaluate heterogeneity of cardiac action potential (AP), transient outward K+ current (Ito1) and inwardly rectifying K+ current (IK1) in human right ventricle. AP and membrane currents were recorded using whole cell current- and voltage-clamp techniques in myocytes isolated from subepicardial, midmyocardial, and subendocardial layers of the right ventricle of explanted failing human hearts. AP morphology differed among the regional cell types. AP duration (APD) at 0.5-2 Hz was longer in midmyocardial cells (M cells) than in subepicardial and subendocardial cells. At room temperature, observed Ito1, on step to +60 mV, was significantly greater in subepicardial (6.9 +/- 0.8 pA/pF) and M cells (6.0 +/- 1.1 pA/pF) than in subendocardial cells (2.2 +/- 0.7 pA/pF, P < 0.01). Slower recovery of Ito1 was observed in subendocardial cells. The half-inactivation voltage of Ito1 was more negative in subendocardial cells than in M and subepicardial cells. At 36 degrees C, the density of Ito1 increased, the time-dependent inactivation and reactivation accelerated, and the frequency-dependent reduction attenuated in all regional cell types. No significant difference was observed in IK1 density among the regional cell types. The results indicate that M cells in humans, as in canines, show the greatest APD and that a gradient of Ito1 density is present in the transmural ventricular wall. Therefore, the human right ventricle shows significant transmural heterogeneity in AP morphology and Ito1 properties.

Basic mechanisms of atrial fibrillation--very new insights into very old ideas

Atrial fibrillation (AF) was recognized and studied extensively in the early twentieth century, but many fundamental aspects of the arrhythmia were poorly understood until quite recently. It is now recognized that AF can be initiated by a variety of mechanisms that share the ability to cause extremely rapid, irregular atrial electrical activity. Once initiated, AF causes alterations in atrial electrical properties (electrical remodeling), including both rapid functional changes and slower alterations in ion channel gene expression, which promote the maintenance of AF and facilitate reinitiation of the arrhythmia should it terminate. Electrical remodeling decreases the atrial refractory period in a heterogeneous way, thus decreasing the size and stability of potential functional atrial reentry waves and promoting multiple-circuit reentry. Whatever the initial cause of AF, electrical remodeling is likely to be a final common pathway that ultimately supervenes. Recent advances in understanding ion channel function, regulation, and remodeling at the molecular level have allowed for a much more detailed appreciation of the basic determinants of AF. Improvements in the clinical management of AF will inevitably follow the recent advances in our understanding of its detailed pathophysiology.

Ionic mechanisms of regional action potential heterogeneity in the canine right atrium

Atrial action potential heterogeneity is a major determinant of atrial reentrant arrhythmias, but the underlying ionic mechanisms are poorly understood. To evaluate the basis of spatial heterogeneity in canine right atrial repolarization, we isolated cells from 4 regions: the crista terminalis (CT), appendage (APG), atrioventricular ring (AVR) area, and pectinate muscles. Systematic action potential (AP) differences were noted: CT cells had a "spike-and-dome" morphology and the longest AP duration (APD; value to 95% repolarization at 1 Hz, 270+/-10 ms [mean+/-SEM]); APG and pectinate muscle cells had intermediate APDs (180+/-3 and 190+/-3 ms, respectively; P<0.001 versus CT for each), with APG cells having a small phase 1; and AVR cells had the shortest APD (160+/-4 ms, P<0.001 versus other regions). The inward rectifier and the slow and ultrarapid delayed rectifier currents were similar in all regions. The transient outward K+ current was significantly smaller in APG cells, explaining their small phase 1 and high plateau. L-type Ca2+ current was greatest in CT cells and least in AVR cells, contributing to their longer and shorter APD, respectively. The E-4031-sensitive rapid delayed rectifier K+ current was larger in AVR cells compared with other regions. Voltage- and time-dependent current properties were constant across regions. We conclude that myocytes from different right atrial regions of the dog show systematic variations in AP properties and ionic currents and that the spatial variation in ionic current density may explain AP differences. Regional variation in atrial ionic currents may play an important role in atrial arrhythmia generation and may present opportunities for improving antiarrhythmic drug therapy.

Genetic variation of HIV type 1 in four World Health Organization-sponsored vaccine evaluation sites: generation of functional envelope (glycoprotein 160) clones representative of sequence subtypes A, B, C, and E. WHO Network for HIV Isolation and Characterization

As part of the WHO Network for HIV Isolation and Characterization, we PCR amplified, cloned, and sequenced gp120 and gp160 genes from 12 HIV-1 isolates collected in four WHO-sponsored vaccine evaluation sites (Brazil, Rwanda, Thailand, Uganda). Envelope clones were derived from PBMC-grown isolates obtained from asymptomatic individuals within 2 years of seroconversion. Analysis of their deduced amino acid sequences identified all but one to contain an uninterrupted open reading frame. Transient expression and biological characterization of selected gp160 constructs identified six clones to encode full length and functional envelope glycoproteins. Phylogenetic analysis of their nucleotide sequences revealed that they represent HIV-1 subtypes A, B, C, and E. Since current knowledge of HIV-1 envelope immunobiology is almost exclusively derived from subtype B viruses, these reagents should facilitate future envelope structure, function and antigenicity studies on a broader spectrum of viruses. This should assist in the design and evaluation of effective vaccines against HIV-1.

Cardiac ultrarapid delayed rectifiers: a novel potassium current family o f functional similarity and molecular diversity

Classical cardiac delayed rectifier currents activate at least two orders of magnitude slower than delayed rectifier currents in nerve and skeletal muscle tissue. It has recently become evident that many cardiac tissues express delayed rectifier currents with kinetics similar to those of nerve and muscle. These currents have been designated I(Kur) (for 'ultrarapid' delayed rectifier), in contrast to the classical cardiac rapid (I(Kr)) and slow (I(Ks)) delayed rectifier components. Although the kinetics of I(Kur) in different species and tissues are similar, their pharmacological properties vary greatly. It now appears that the differences among various I(Kur)s are due to differences in the molecular basis. A variety of Shaker-related clones (Kv1.2, 1.5, 2.1 and 3.1) that form I(Kur) channels upon heterologous expression have been identified with specific I(Kur)s (e.g. Kv1.2, rat atrium; Kv1.5, mouse ventricle and human atrium; Kv3.1, dog atrium). The present article reviews the distribution, biophysical and pharmacological properties, molecular basis and functional role of I(Kur), as well as the potential value of I(Kur) as a target for new antiarrhythmic drug development.

Genetic analysis of viable Hsp90 alleles reveals a critical role in Drosophila spermatogenesis

The Hsp90 chaperone protein maintains the activities of a remarkable variety of signal transducers, but its most critical functions in the context of the whole organism are unknown. Point mutations of Hsp83 (the Drosophila Hsp90 gene) obtained in two different screens are lethal as homozygotes. We report that eight transheterozygous mutant combinations produce viable adults. All exhibit the same developmental defects: sterile males and sterile or weakly fertile females. We also report that scratch, a previously identified male-sterile mutation, is an allele of Hsp82 with a P-element insertion in the intron that reduces expression. Thus, it is a simple reduction in Hsp90 function, rather than possible altered functions in the point mutants, that leads to male sterility. As shown by light and electron microscopy, all stages of spermatogenesis involving microtubule function are affected, from early mitotic divisions to later stages of sperm maturation, individualization, and motility. Aberrant microtubules are prominent in yeast cells carrying mutations in HSP82 (the yeast Hsp90 gene), confirming that Hsp90 function is connected to microtubule dynamics and that this connection is highly conserved. A small fraction of Hsp90 copurifies with taxol-stabilized microtubule proteins in Drosophila embryo extracts, but Hsp90 does not remain associated with microtubules through repeated temperature-induced assembly and disassembly reactions. If the spermatogenesis phenotypes are due to defects in microtubule dynamics, we suggest these are indirect, reflecting a role for Hsp90 in maintaining critical signal transduction pathways and microtubule effectors, rather than a direct role in the assembly and disassembly of microtubules themselves.

Characterization of an ultrarapid delayed rectifier potassium channel involved in canine atrial repolarization

1. Depolarizing pulses positive to 0 mV elicit a transient outward current (Ito) and a sustained 'pedestal' current in canine atrial myocytes. The pedestal current was highly sensitive to 4-aminopyridine (4-AP) and TEA, with 50% inhibitory concentrations (EC50) of 5.3 +/- 0.7 and 307 +/- 25 microM, respectively. When the pedestal current was separated from Ito with prepulses or by studying current sensitive to 10 mM TEA, it showed very rapid activation and deactivation. We therefore designated the current IKur,d, for 'ultrarapid delayed rectifier, dog'. IKur,d inactivation was bi-exponential, with mean time constants of 609 +/- 91 and 5563 +/- 676 ms during a 20 s pulse to +40 mV. 2. The reversal potential of IKur,d tail currents are dependent on extracellular potassium concentration ([K+]o; slope, 54.7 mV decade-1). The envelope of tails test was satisfied and the current inwardly rectified at > or = +40 mV. The current was insensitive to E-4031, dendrotoxin and chloride substitution, but was inhibited by barium, with an EC50 of 1.65 mM. Lanthanum ions caused a positive shift in voltage dependence without producing direct inhibition. 3. Single-channel activity was observed in cell-attached, inside-out and outside-out patches. Upon depolarization from -50 to +30 mV, single channels had similar time constants and [K+]o dependence to whole-cell current. Channel open probability (Po) increased with depolarization in a saturable fashion and the Po-voltage relation had a half-activation voltage and slope factor similar to whole-cell IKur,d. 4. Unitary channel current was linearly related to depolarization potential to +40 mV; at more positive potentials, inward rectification occurred. The unitary conductance was 20.3 and 35.5 pS for an [K+]o of 5.4 and 130 mM, respectively. Single-channel activity was strongly inhibited by 50 microM 4-AP or 10 mM TEA. Both 4-AP and TEA decreased open time, suggesting open-channel block. 5. Selective inhibition of IKur,d with 50 microM 4-AP or 0.3-5 mM TEA prolonged canine atrial action potentials, indicating that IKur,d contributes to canine atrial repolarization. The single-channel and macroscopic properties of IKur,d have many similarities to those of currents carried by Kv3.1 cloned channels and our findings thus suggest a possible role for Kv3.1 channels in cardiac repolarization.

Improved postprandial glycemic control during treatment with Humalog Mix25, a novel protamine-based insulin lispro formulation. Humalog Mix25 Study Group

OBJECTIVE

Humalog Mix25 is a manufactured premixed insulin formulation containing insulin lispro and a novel insulin lispro-protamine formulation (NPL) in a ratio of 25:75%. The objective of this study was to compare Humalog Mix25 to human insulin 30/70 (30% regular insulin/70% NPH) with respect to glycemic control.

RESEARCH DESIGN AND METHODS

Humalog Mix25 was compared with human insulin 30/70 in 89 individuals with type 2 diabetes during a 6-month randomized open-label two-period crossover study. Each insulin was administered twice daily, before the morning and evening meals. Information regarding self-monitored blood glucose (BG), hypoglycemic episodes (hypoglycemic signs or symptoms or BG < or = 3.0 mmol/l), insulin dose, and HbA1c was collected.

RESULTS

Treatment with Humalog Mix25 resulted in better postprandial glycemic control after the morning and evening meals compared with treatment with human insulin 30/70. Overall glycemic control and the incidence of hypoglycemia were comparable between the treatments.

CONCLUSIONS

In comparison to treatment with human insulin 30/70, twice daily administration of Humalog Mix25 resulted in improved postprandial glycemic control, similar overall glycemic control, and the convenience of dosing immediately before meals.

Transient outward and delayed rectifier currents in canine atrium: properties and role of isolation methods

Although the dog is the principal species used for in vivo studies of atrial arrhythmias, little is known about currents governing canine atrial repolarization. Cells were isolated from dog atria by exposure to collagenase of tissue in vitro ("chunk cells") and by arterial perfusion ("perfusion cells"). Whole cell voltage clamp revealed transient outward K+ current (Ito1), Ca(2+)-dependent Cl- current (Ito2), and delayed rectifier K+ current (IK). Ito1 recovered rapidly and showed little frequency dependence. Two components of IK were present as follows: a rapidly activating E-4031-sensitive current with marked inward recitification and a slower-activating E-4031-insensitive component. Ito1 and IK resembled corresponding currents previously described in human atrium. Transient outward currents were similar in chunk and perfusion cells, but IK was seen in 4% of chunk cells vs. 99% of perfusion cells (P < 0.001). Suppression of each identified current retarded canine action potential repolarization. We conclude that Ito1, Ito2, and both components of IK are present in dog atrium, IK is much more sensitive to the isolation method than Ito1 or Ito2, and the properties of two important repolarizing currents (Ito1 and IK) previously described in human atrium are similar to those in dog atrium.

Atorvastatin overcomes gefitinib resistance in KRAS mutant human non-small cell lung carcinoma cells

The exact influence of statins on gefitinib resistance in human non-small cell lung cancer (NSCLC) cells with KRAS mutation alone or KRAS/PIK3CA and KRAS/PTEN comutations remains unclear. This work found that transfection of mutant KRAS plasmids significantly suppressed the gefitinib cytotoxicity in Calu3 cells (wild-type KRAS). Gefitinib disrupted the Kras/PI3K and Kras/Raf complexes in Calu3 cells, whereas not in Calu3 KRAS mutant cells. These trends were corresponding to the expression of pAKT and pERK in gefitinib treatment. Atorvastatin (1 μM) plus gefitinib treatment inhibited proliferation, promoted cell apoptosis, and reduced the AKT activity in KRAS mutant NSCLC cells compared with gefitinib alone. Atorvastatin (5 μM) further enhanced the gefitinib cytotoxicity through concomitant inhibition of AKT and ERK activity. Atorvastatin could interrupt Kras/PI3K and Kras/Raf complexes, leading to suppression of AKT and ERK activity. Similar results were also obtained in comutant KRAS/PTEN or KRAS/PIK3CA NSCLC cells. Furthermore, mevalonate administration reversed the effects of atorvastatin on the Kras/Raf and Kras/PI3K complexes, as well as AKT and ERK activity in both A549 and Calu1 cells. The in vivo results were similar to those obtained in vitro. Therefore, mutant KRAS-mediated gefitinib insensitivity is mainly derived from failure to disrupt the Kras/Raf and Kras/PI3K complexes in KRAS mutant NSCLC cells. Atorvastatin overcomes gefitinib resistance in KRAS mutant NSCLC cells irrespective of PIK3CA and PTEN statuses through inhibition of HMG-CoA reductase-dependent disruption of the Kras/Raf and Kras/PI3K complexes.

Plaque pH and associated parameters in relation to caries

Intensified plaque acidogenicity in caries-prone subjects was reported many years ago, but emerging evidence has suggested that the relationship may not be as strong as once thought. We have now determined a range of acidogenicity variables in subjects having both caries prevalence and incidence data, and have included plaque mineral data in the analysis. pH measurements were made in 20 randomly selected subjects from a high-caries group (mean DMFS = 8. 95) and 20 from a caries-free group of Beijing children aged 12 years participating in a caries prediction study. Subgroups with a 12-month DMFS increment >/=2 or = 0 were also formed from the two groups, respectively. Measurements were made with an iridium oxide electrode inserted between teeth 13/14, 23/24, 34/35 and 44/45, before and every 5 min for 30 min after rinsing with 10% sucrose, and the 4 resulting 'Stephan curves' averaged using a plaque pH analysis program. Supragingival plaque was collected from buccal and lingual smooth surfaces of posterior and upper anterior teeth and its acid extract analysed for Ca, P and F. Caries-free subjects (based on past experience) had a significantly higher maximum plaque pH and pH value after 30 min (reflecting a faster return to resting pH), a lower minimum enamel dissolution capacity of plaque and recorded less time below pH 7.0 than did high-caries subjects. No other differences were significant, including those of the principal acidogenic parameters 'minimum pH attained after a sugar rinse', 'curve area below the critical pH of 5.5' and 'time below the critical pH'. Selection of the caries groups on the basis of both experience and incidence did not reveal significant differences in more parameters. Upper arch plaque was significantly more acidogenic than lower arch plaque, and there was a consistently strong association between upper and lower arch values in individuals. Ca, P and F in the subjects' plaque had little or no influence on the principal acidogenic parameters. Our failure to find a relationship between caries prevalence or activity and these principal acidogenicity parameters may be related to differences between fissure and smooth surface plaque, temporal variations in acidogenicity and/or to use of F toothpaste during the 1-year observation period. These results support the view that factors such as the frequency of acidogenic episodes may be more important in caries progression than the degree of acidogenicity during any one episode.

Protein kinase C-regulated dynamitin-macrophage-enriched myristoylated alanine-rice C kinase substrate interaction is involved in macrophage cell spreading

Macrophage spreading requires the microtubule cytoskeleton and protein kinase C (PKC). The mechanism of involvement of the microtubules and PKC in this event is not fully understood. Dynamitin is a subunit of dynactin, which is important for linking the microtubule-dependent motor protein dynein to vesicle membranes. We report that dynamitin is a Ca(2+)/calmodulin-binding protein and that dynamitin binds directly to macrophage-enriched myristoylated alanine-rice C kinase substrate (MacMARCKS), a membrane-associated PKC substrate involved in macrophage spreading and integrin activation. Dynamitin was found to copurify with MacMARCKS both during MacMARCKS purification with conventional chromatography and during the immunoabsorption of MacMARCKS using anti-MacMARCKS antibody. Vice versa, MacMARCKS was also found to cosediment with the 20 S dynactin complex. We determined that the effector domain of MacMARCKS is required to interact with the N-terminal domain of dynamitin. MacMARCKS and dynamitin also partially colocalized at peripheral regions of macrophages and in the cell-cell border of 293 epithelial cells. Treatment with phorbol esters abolished this colocalization. Disrupting the interaction with a short peptide derived from the MacMARCKS-binding domain of dynamitin caused macrophages to spread and flatten. These data suggest that the dynamitin-MacMARCKS interaction is involved in cell spreading. Furthermore, the regulation of this interaction by PKC and Ca(2+)/calmodulin provides a possible regulatory mechanism for cell adhesion and spreading.