Surface oxide and the role of magnesium during the sintering of aluminum

The effect of trace additions of magnesium on the sintering of aluminum and its alloys is examined. Magnesium, especially at low concentrations, has a disproportionate effect on sintering because it disrupts the passivating Al2O3 layer through the formation of a spinel phase. Magnesium penetrates the sintering compact by solid-state diffusion, and the oxide is reduced at the metal-oxide interface. This facilitates solid-state sintering, as well as wetting of the underlying metal by sintering liquids, when these are present. The optimum magnesium concentration is approximately 0.1 to 1.0 wt pet, but this is dependent on the volume of oxide and, hence, the particle size, as well as the sintering conditions. Small particle-size fractions require proportionally more magnesium than large-size fractions do.

The nurse lecturer role in clinical practice conceptualized: helping clinical teachers provide optimal student learning

Despite the fairly wide reporting in the literature of the ma ny roles of clinical supervision by the nursing teacher, little attention has been given to conceptualizing the relative priorities these roles take during the process of supervising nursing students in clinical practice. The purpose of this paper is to consider the manifestations and implications of conflicting roles when nurse lecturers undertake clinical supervision. Previously published research will provide working examples of issues in a conceptual framework for clinical teaching.

Role of the 6-20 disulfide bridge in the structure and activity of epidermal growth factor

Two synthetic analogues of murine epidermal. growth factor, [Abu6, 20] mEGF4-48 (where Abu denotes amino-butyric acid) and [G1, M3, K21, H40] mEGF1-48, have been investigated by NMR spectroscopy. [Abu6, 20] mEGF4-48 was designed to determine the contribution of the 6-20 disulfide bridge to the structure and function of mEGF The overall structure of this analogue was similar to that of native mEGF, indicating that the loss of the 6-20 disulfide bridge did not affect the global fold of the molecule. Significant structural differences were observed near the N-terminus, however, with the direction of the polypeptide chain between residues four and nine being altered such that these residues were now located on the opposite face of the main beta-sheet from their position in native mEGF Thermal denaturation experiments also showed that the structure of [Abu6, 20] mEGF4-48 was less stable than that of mEGF. Removal of this disulfide bridge resulted in a significant loss of both mitogenic activity in Balb/c 3T3 cells and receptor binding on A431 cells compared with native mEGF and mEGF4-48, implying that the structural changes in [Abu6, 20] mEGF4-48, although limited to the N-terminus, were sufficient to interfere with receptor binding. The loss of binding affinity probably arose mainly from steric interactions of the dislocated N-terminal region with part of the receptor binding surface of EGF [G1, M3, K21, H40] mEGF1-48 was also synthesized in order to compare the synthetic polypeptide with the corresponding product of recombinant expression. Its mitogenic activity in Balb/c 3T3 cells was similar to that of native mEGF and analysis of its H-1 chemical shifts suggested that its structure was also very similar to native.

Strategies of accommodation: Development of a coding system for conversational interaction

This study describes a coding system developed to operationalize the sociolinguistic strategies proposed by communication accommodation theory (CAT) in an academic context. Fifty interactions between two students (of Australian or Chinese ethnic background) or a student and faculty member were videotaped. A turn- and episode-based coding system was developed, focusing on verbal and nonverbal behavior. The development of this system is described in detail, before results are presented. Results indicated that status was the main influence on choice of strategies, particularly the extent and type of discourse management and interpersonal control. Participants' sew and ethnicity also played a role: Male participants made more use of interpretability (largely questions), whereas female participants used discourse management to develop a shared perspective. The results make clear that there is no automatic correspondence between behaviors and the strategies they constitute, and they point to the appropriateness of conceptualizing behavior and strategies separately in CAT.

Differentiation of the mononuclear phagocyte system during mouse embryogenesis: The role of transcription factor PU.1

During mouse embryogenesis, macrophage-like cells arise first in the yolk sac and are produced subsequently in the liver. The onset of liver hematopoiesis is associated with the transition from primitive to definitive erythrocyte production. This report addresses the hypothesis that a similar transition in phenotype occurs in myelopoiesis. We have used whole mount in situ hybridization to detect macrophage-specific genes expressed during mouse development. The mouse c-fms mRNA, encoding the receptor for macrophage colony-stimulating factor (CSF-1), was expressed on phagocytic cells in the yolk sac and throughout the embryo before the onset of liver hematopoiesis, Similar cells were detected using the mannose receptor, the complement receptor (CR3), or the Microphthalmia transcription factor (MITF) as mRNA markers. By contrast, other markers including the F4/80 antigen, the macrophage scavenger receptor, the S-100 proteins, S100A8 and S100A9, and the secretory product lysozyme appeared later in development and appeared restricted to only a subset of c-fms-positive cells. Two-color immunolabeling on disaggregated cells confirmed that CR3 and c-fms proteins are expressed on the same cells. Among the genes appearing later in development was the macrophage-restricted transcription factor, PU.1, which has been shown to be required for normal adult myelopoiesis. Mice with null mutations in PU.1 had normal numbers of c-fms-positive phagocytes at 11.5dpc. PU.1(-/-) embryonic stem cells were able to give rise to macrophagelike cells after cultivation in vitro. The results support previous evidence that yolk sac-derived fetal phagocytes are functionally distinct from those arising in the liver and develop via a different pathway. (C) 1999 by The American Society of Hematology.

A null mutation in the inflammation-associated S100 protein S100A8 causes early resorption of the mouse embryo

S100A8 (also known as CP10 or MRP8) was the first member of the S100 family of calcium-binding proteins shown to be chemotactic for myeloid cells. The gene is expressed together with its dimerization partner S100A9 during myelopoiesis in the fetal liver and in adult bone marrow as well as in mature granulocytes. In this paper we show that S100A8 mRNA is expressed without S100A9 mRNA between 6.5 and 8.5 days postcoitum within fetal cells infiltrating the deciduum in the vicinity of the ectoplacental cone. Targeted disruption of the S100A8 gene caused rapid and synchronous embryo resorption by day 9.5 of development in 100% of homozygous null embryos. Until this point there was no evidence of developmental delay in S100A8(-/-) embryos and decidualization was normal. The results of PCR genotyping around 7.5-8.5 days postcoitum suggest that the null embryos are infiltrated with maternal cells before overt signs of resorption. This work is the first evidence for nonredundant function of a member of the S100 gene family and implies a role in prevention of maternal rejection of the implanting embryo. The S100A8 null provides a new model for studying fetal-maternal interactions during implantation.

The plasma membrane calcium pump, its role and regulation: New complexities and possibilities

Significant progress has been achieved in elucidating the role of the plasma membrane Ca2+-ATPase in cellular Ca2+ homeostasis and physiology since the enzyme was first purified and physiology since the enzyme was first purified and cloned a number of years ago. The simple notion that the PM Ca2+-ATPase controls resting levels of [Ca2+](CYT) has been challenged by the complexity arising from the finding of four major isoforms and splice variants of the Ca2+ pump, and the finding that these are differentially localized in various organs and subcellular regions. Furthermore, the isoforms exhibit differential sensitivities to Ca2+, calmodulin, ATP, and kinase-mediated phosphorylation. The latter pathways of regulation can give rise to activation or inhibition of the Ca2+ pump activity, depending on the kinase and the particular Ca2+ pump isoform. Significant progress is being made in elucidating subtle and more profound roles of the PM Ca2+-ATPase in the control of cellular function. Further understanding of these roles awaits new studies in both transfected cells and intact organelles, a process that will be greatly aided by the development of new and selective Ca2+ pump inhibitors. (C) 1999 Elsevier Science Inc.

Rapid evolution towards equal sex ratios in a system with heterogamety

The equal sex ratios found in many species with heterogametic sex determination may be a consequence of selection for equality or the result of the Mendelian segregation of the two sex chromosomes. A lack of genetic variation in sex ratio in species with heterogamety has been the major obstacle in distinguishing between these two hypotheses. We overcome this obstacle by generating hybrids between two species of Drosophila. The resulting hybrid lines had biased sex ratios, allowing us to observe the evolution of sex ratio in replicate populations. Sex ratio converged towards 1:1 after 16 generations of natural selection. These changes in sex ratio were not due to differences in viability between the sexes and the loci underlying the variation in sex ratio were not sex-linked. Equal sex ratios may therefore be the result of natural selection as Fisher predicted.

The moderating role of task characteristics in determining responses to a stressful work simulation

Two experimental studies were conducted to examine whether the stress-buffering effects of behavioral control on work task responses varied as a function of procedural information. Study 1 manipulated low and high levels of task demands, behavioral control, and procedural information for 128 introductory psychology students completing an in-basket activity. ANOVA procedures revealed a significant three-way interaction among these variables in the prediction of subjective task performance and task satisfaction. It was found that procedural information buffered the negative effects of task demands on ratings of performance and satisfaction only under conditions of low behavioral control. This pattern of results suggests that procedural information may have a compensatory effect when the work environment is characterized by a combination of high task demands and low behavioral control. Study 2 (N = 256) utilized simple and complex versions of the in-basket activity to examine the extent to which the interactive relationship among task demands, behavioral control, and procedural information varied as a function of task complexity. There was further support for the stress-buffering role of procedural information on work task responses under conditions of low behavioral control. This effect was, however, only present when the in-basket activity was characterized by high task complexity, suggesting that the interactive relationship among these variables may depend on the type of tasks performed at work. Copyright (C) 1999 John Wiley & Sons, Ltd.

A role for protein kinase B beta/Akt2 in insulin-stimulated GLUT4 translocation in adipocytes

Insulin stimulates glucose uptake into muscle and fat cells by promoting the translocation of glucose transporter 4 (GLUT4) to the cell surface. Phosphatidylinositide 3-kinase (PI3K) has been implicated in this process. However, the involvement of protein kinase B (PKB)/Akt, a downstream target of PI3K in regulation of GLUT4 translocation, has been controversial. Here we report that microinjection of a PKB substrate peptide or an antibody to PKB inhibited insulin-stimulated GLUT4 translocation to the plasma membrane by 66 or 56%, respectively. We further examined the activation of PKB isoforms following treatment of cells with insulin or platelet-derived growth factor (PDGF) and found that PKB beta is preferentially expressed in both rat and 3T3-L1 adipocytes, whereas PKB alpha expression is down-regulated in 3T3-L1 adipocytes. A switch in growth factor response was also observed when 3T3-L1 fibroblasts were differentiated into adipocytes. While PDGF was more efficacious than insulin in stimulating PKB phosphorylation in fibroblasts, PDGF did not stimulate PKB beta phosphorylation to any significant extent in adipocytes, as assessed by several methods. Moreover, insulin, but not PDGF, stimulated the translocation of PKB beta to the plasma membrane and high-density microsome fractions of 3T3-L1 adipocytes. These results support a role for PKB beta in insulin-stimulated glucose transport in adipocytes.

Monooxygenases play only a minor role in resistance to synthetic pyrethroids in the cattle tick, Boophilus microplus

We investigated the role of monooxygenases in resistance to synthetic pyrethroids (SPs) in the cattle tick, Boophilus microplus. We found that monooxygenases play only a minor role in resistance to SPs in both resistant and susceptible strains of B. microplus. We blocked the monooxygenases with piperonyl butoxide (PBO) and simultaneously applied the SPs, flumethrin and cypermethrin to larval B. microplus. PBO increased the effect of flumethrin (synergism ratios 2.7-8.9) more than it increased the effect of cypermethrin (synergism ratios 1.9-3.1). Of the four strains tested, Parkhurst, which is resistant to SPs, was the least affected by the addition of PBO (synergism ratios after cypermethrin was applied 1.9; after flumethrin 2.7) whereas N.R.F.S., the strain susceptible to SPs, was the most affected by synergism between PBO and SPs (synergism ratio after cypermethrin was applied 3.1; after flumethrin 8.9). We hypothesize that B. microplus lacks monooxygenases capable of conferring resistance to SPs because it and its recent ancestors were blood-feeders rather than herbivores.