Microstructure and coupling mechanisms in MnBi-FeSiB nanocomposites obtained by spark plasma sintering.

Fabrication and extensive characterization of hard-soft nanocomposites composed of hard magnetic low-temperature phase LTP-MnBi and amorphous Fe70Si10B20 soft magnetic phase for bulk magnets are reported. Samples with compositions Mn55Bi45 + x⋅(Fe70Si10B20) (x = 0, 3, 5, 10, 20 wt.%) were prepared by spark plasma sintering of powder mixtures. Characterization has been performed by X-ray diffraction, scanning and transmission electron microscopy, magnetometry and 57Fe MÓ§ssbauer spectroscopy. It was shown that samples contain crystallized and nanometric LTP-MnBi phases with various elemental compositions depending on the degree of Bi clustering. Complex correlations between starting compositions, processes during fabrication, and functional magnetic characteristics were observed. Unexpected special situations of the relation between microstructure and magnetic coupling mechanisms are discovered. Exchange spring effects of different strengths occur, being very sensitive to morpho-structural and compositional features, which in turn are controlled by processing conditions. An in-depth analysis of related microscopic characteristics is provided. Results of this work suggest that fabrication by powder metallurgy routes, such as spark plasma sintering of hard and soft magnetic powder mixtures, of MnBi-based composites with exchange spring phenomena have a high potential in designing and optimization of suitable materials with tunable magnetic properties towards rare-earth-free permanent magnet applications.

Influences of Dispersions' Shapes and Processing in Magnetic Field on Thermal Conductibility of PDMS-Fe3O4 Composites.

Composites of magnetite (Fe3O4) nanoparticles dispersed in a polydimethylsiloxane (PDMS) matrix were prepared by a molding process. Two types of samples were obtained by free polymerization with randomly dispersed particles and by polymerization in an applied magnetic field. The magnetite nanoparticles were obtained from magnetic micrograins of acicular goethite (α-FeOOH) and spherical hematite (α-Fe2O3), as demonstrated by XRD measurements. The evaluation of morphological and compositional properties of the PDMS:Fe3O4 composites, performed by SEM and EDX, showed that the magnetic particles were uniformly distributed in the polymer matrix. Addition of magnetic dispersions promotes an increase of thermal conductivity compared with pristine PDMS, while further orienting the powders in a magnetic field during the polymerization process induces a decrease of the thermal conductivity compared with the un-oriented samples. The shape of the magnetic dispersions is an important factor, acicular dispersions providing a higher value for thermal conductivity compared with classic commercial powders with almost spherical shapes.

Tuning structural and magnetic properties of Fe oxide nanoparticles by specific hydrogenation treatments.

Structural and magnetic properties of Fe oxide nanoparticles prepared by laser pyrolysis and annealed in high pressure hydrogen atmosphere were investigated. The annealing treatments were performed at 200 °C (sample A200C) and 300 °C (sample A300C). The as prepared sample, A, consists of nanoparticles with ~ 4 nm mean particle size and contains C (~ 11 at.%), Fe and O. The Fe/O ratio is between γ-Fe2O3 and Fe3O4 stoichiometric ratios. A change in the oxidation state, crystallinity and particle size is evidenced for the nanoparticles in sample A200C. The Fe oxide nanoparticles are completely reduced in sample A300C to α-Fe single phase. The blocking temperature increases from 106 K in A to 110 K in A200C and above room temperature in A300C, where strong inter-particle interactions are evidenced. Magnetic parameters, of interest for applications, have been considerably varied by the specific hydrogenation treatments, in direct connection to the induced specific changes of particle size, crystallinity and phase composition. For the A and A200C samples, a field cooling dependent unidirectional anisotropy was observed especially at low temperatures, supporting the presence of nanoparticles with core-shell-like structures. Surprisingly high MS values, almost 50% higher than for bulk metallic Fe, were evidenced in sample A300C.

Magnetic Phase Coexistence and Hard-Soft Exchange Coupling in FePt Nanocomposite Magnets.

With the aim of demonstrating phase coexistence of two magnetic phases in an intermediate annealing regime and obtaining highly coercive FePt nanocomposite magnets, two alloys of slightly off-equiatomic composition of a binary Fe-Pt system were prepared by dynamic rotation switching and ball milling. The alloys, with a composition Fe53Pt47 and Fe55Pt45, were subsequently annealed at 400 °C and 550 °C and structurally and magnetically characterized by means of X-ray diffraction, 57Fe Mössbauer spectrometry and Superconducting Quantum Interference Device (SQUID) magnetometry measurements. Gradual disorder-order phase transformation and temperature-dependent evolution of the phase structure were monitored using X-ray diffraction of synchrotron radiation. It was shown that for annealing temperatures as low as 400 °C, a predominant, highly ordered L10 phase is formed in both alloys, coexisting with a cubic L12 soft magnetic FePt phase. The coexistence of the two phases is evidenced through all the investigating techniques that we employed. SQUID magnetometry hysteresis loops of samples annealed at 400 °C exhibit inflection points that witness the coexistence of the soft and hard magnetic phases and high values of coercivity and remanence are obtained. For the samples annealed at 500 °C, the hysteresis loops are continuous, without inflection points, witnessing complete exchange coupling of the hard and soft magnetic phases and further enhancement of the coercive field. Maximum energy products comparable with values of current permanent magnets are found for both samples for annealing temperatures as low as 500 °C. These findings demonstrate an interesting method to obtain rare earth-free permanent nanocomposite magnets with hard-soft exchange-coupled magnetic phases.

Stepped heating procedure for experimental SAR evaluation of ferrofluids.

The aim of this paper is to present a reliable procedure for the experimental determination of the specific absorption rate (SAR) in case of superparamagnetic Fe oxide nanoparticles dispersed in liquid environments. It is based on the acquisition of consecutive steps of time-temperature dependences along of both heating and cooling processes. Linear fitting of these recorded steps provides the heating and cooling speeds at different temperatures, which finally allow the determination of the heating profile in adiabatic-like conditions over a broad temperature range. The presented methodology represents on one hand, a useful alternative tool for the experimental evaluation of the heating capability of nanoparticulate systems for magnetic hyperthermia applications and on the other hand, gives support for a more accurate modeling of bio-heat transfer phenomena.

The nature of the hydrogen bond in the LaNiSnH2 and NdNiSnH hydrides.

The electronic structure of LaNiSn and NdNiSn compounds and their hydrides has been studied by first principles calculations and variable temperature 119Sn Mossbauer spectroscopy and the nature of the hydrogen-metal bond is discussed. The analysis of the electronic density of states (DOS) in both compounds before and after hydrogenation indicates an hybridization of the Sn, Ni, and H orbitals. The partial Sn-p DOS of LaNiSnH2 gives evidence for a lower symmetry of electron density around tin atoms compared to LaNiSn, according to the larger quadrupole splitting in the corresponding Mossbauer spectrum. Theoretical and experimental Mossbauer parameters agree very well for all samples.

FPL-64176 modifies pore properties of L-type Ca(2+) channels.

In addition to its known effects on Ca(2+) and Ba(2+) currents, the L-type Ca(2+) channel agonist FPL-64176 was found to affect channel function in isolated rat ventricular myocytes in the absence of Ca(2+), with other ions as current carriers through the channel. FPL-64176 induced Cd(2+) current through the L-type Ca(2+) channel, suggesting that certain selectivity properties had changed, perhaps indicative of a small change in pore structure. FPL-64176 slightly but significantly decreased the effectiveness of Co(2+) as a blocker of the channel. FPL-64176 also increased conductance through single L-type Ca(2+) channels recorded in the cell-attached configuration, from 71.9 +/- 11.6 to 94.1 +/- 8.3 pS, with Na(+) carrying the current at pH 9.0. At present it is uncertain whether FPL-64176 produces small alterations of a sole open state of the channel or whether it increases the prevalence of a second, higher conductance open state. These changes, particularly the conversion of Cd(2+) from a pure blocker to a permeant ion, may be of eventual help in discriminating among different models for Ca(2+) channel selectivity.

Ras reduces L-type calcium channel current in cardiac myocytes. Corrective effects of L-channels and SERCA2 on [Ca(2+)](i) regulation and cell morphology.

Heart failure is associated with dysregulation of intracellular calcium ([Ca(2+)](i)), reduction in myofibrils, and increased activation of Ras, a regulator of signal-transduction pathways. To evaluate the potential effects of Ras on [Ca(2+)](i), we expressed constitutively active Ras (Ha-Ras(V12)) in cardiac myocytes and monitored [Ca(2+)](i) via fluorescence and electrophysiological techniques. Ha-Ras(V12) reduced the magnitude of the contractile calcium transients. Unexpectedly, however, calcium loading of the sarcoplasmic reticulum was increased, suggesting that Ha-Ras(V12) introduces a defect in excitation-calcium release coupling. Consistent with this idea, L-channel calcium currents were reduced by Ha-Ras(V12), which also downregulated the activity of the L-channel gene promoter. Coexpression of L-channels and SERCA2 largely corrected Ha-Ras(V12)-induced dysregulation of [Ca(2+)](i). Furthermore, whereas Ha-Ras(V12) downregulated myofibrils, this effect was blocked by coexpression of L-channels. These results suggest that Ras downregulates L-channel expression, which may play a pathophysiological role in cardiac disease.

Sarcoplasmic reticulum calcium release is stimulated and inhibited by daunorubicin and daunorubicinol.

Cardiac effects of anthracyclines or their metabolites may include both the stimulation and inhibition of Ca(2+) release from sarcoplasmic reticulum. In this study, the ability of daunorubicin and its primary metabolite, daunorubicinol, to stimulate and inhibit Ca(2+) release from canine sarcoplasmic reticulum (SR) vesicles was investigated. It was observed that both daunorubicin and daunorubicinol were several fold more potent at inhibiting than they were at stimulating SR Ca(2+) release. Respective IC50 inhibition of daunorubicin and daunorubicinol for caffeine-induced calcium release was 1.2 and 0.6 microM, and for spontaneous Ca(2+) release was 3 and 1 microM. EC50's for daunorubicin- and daunorubicinol-induced calcium release were 30 and 15 microM, respectively. Inhibition of either spontaneous or caffeine-induced SR Ca(2+) release was inversely related to the amount of Ca(2+) loaded into the SR before exposure to daunorubicin or daunorubicinol. The free-radical scavenger dithiothreitol did not attenuate the ability of anthracyclines to inhibit SR Ca(2+) release. A nonquinone daunorubicin derivative, 5-iminodaunorubicin, was less potent than daunorubicin at inhibiting caffeine-induced Ca(2+) release. These data suggest anthracyclines and their metabolites may produce cardiotoxicity through free-radical independent, concentration-dependent effects on SR Ca(2+) release. These effects involve either inhibition or stimulation of SR Ca(2+) release and are partly dependent upon the presence of the quinone moiety.

A model of the L-type Ca2+ channel in rat ventricular myocytes: ion selectivity and inactivation mechanisms.

1. We have developed a mathematical model of the L-type Ca2+ current, which is based on data from whole-cell voltage clamp experiments on rat ventricular myocytes. Ion substitution methods were employed to investigate the ionic selectivity of the channel. Experiments were configured with Na+, Ca2+ or Ba2+ as the majority current carrier. 2. The amplitude of current through the channel is attenuated in the presence of extracellular Ca2+ or Ba2+. Our model accounts for channel selectivity by using a modified Goldman-Hodgkin-Katz (GHK) configuration that employs voltage-dependent channel binding functions for external divalent ions. Stronger binding functions were used for Ca2+ than for Ba2+. 3. Decay of the ionic current during maintained depolarization was characterized by means of voltage- and Ca2+-dependent inactivation pathways embedded in a five-state dynamic channel model. Particularly, Ca2+ first binds to calmodulin and the Ca2+-calmodulin complex is the mediator of Ca2+ inactivation. Ba2+-dependent inactivation was characterized using the ttau same scheme, but with a decreased binding to calmodulin. 4. A reduced amount of steady-state inactivation, as evidenced by a U-shaped curve at higher depolarization levels (>40 mV) in the presence of [Ca2+]o, was observed in double-pulse protocols used to study channel inactivation. To characterize this phenomenon, a mechanism was incorporated into the model whereby Ca2+ or Ba2+ also inhibits the voltage-dependent inactivation pathway. 5. The five-state dynamic channel model was also used to simulate single channel activity. Calculations of the open probability of the channel model are generally consistent with experimental data. A sixth state can be used to simulate modal activity by way of introducing long silent intervals. 6. Our model has been tested extensively using experimental data from a wide variety of voltage clamp protocols and bathing solution manipulations. It provides: (a) biophysically based explanations of putative mechanisms underlying Ca2+- and voltage-dependent channel inactivation, and (b) close fits to voltage clamp data. We conclude that the model can serve as a predictive tool in generating testable hypotheses for further investigation of this complex ion channel.

Expression and characterization of Edg-1 receptors in rat cardiomyocytes: calcium deregulation in response to sphingosine 1-phosphate.

Recent evidence indicates that sphingolipids are produced by the heart during hypoxic stress and by blood platelets during thrombus formation. It is therefore possible that sphingolipids may influence heart cell function by interacting with G-protein-coupled receptors of the Edg family. In the present study, it was found that sphingosine 1-phosphate (Sph1P), the prototypical ligand for Edg receptors, produced calcium overload in rat cardiomyocytes. The cDNA for Edg-1 was cloned from rat cardiomyocytes and, when transfected in an antisense orientation, effectively blocked Edg-1 protein expression and reduced the Sph1P-mediated calcium deregulation. Taken together, these results demonstrate that cardiomyocytes express an extracellular lipid-sensitive receptorsystem that can respond to sphingolipid mediators. Because the major source of Sph1P is from blood platelets, we speculate that Edg-mediated Sph1P negative inotropic and cardiotoxic effects may play important roles in acute myocardial ischemia where Sph1P levels are probably elevated in response to thrombus.

Kinetic effects of FPL 64176 on L-type Ca2+ channels in cardiac myocytes.

To characterize the effects of the Ca2+ channel agonist FPL 64176 on L-type Ca2+ current in isolated rat ventricular myocytes, certain of its effects were compared with those of a better known agonist, S (-) Bay K 8644. Both drugs enhance currents elicited by depolarizing pulses and enhance and slow the decay of tail currents elicited by subsequent repolarization. Both drugs shift the voltage dependence of activation and of inactivation approximately 10 mV in the negative direction, but FPL 64176 slows the rate of both activation and the decline of Ca2+ current during a depolarization, whereas Bay K 8644 accelerates the rate of current decay under the same conditions. In single channel studies in on-cell recording mode, FPL 64176 produced a great lengthening of the channel open time, produced very long openings when the channels were repolarized after a depolarizing stimulus, and had only modest effects on mean closed times and on first latency distributions. FPL 64176 and Bay K 8644 also had minimal effects on L-type channel "on" gating currents, while the "off" gating currents were slowed, particularly at positive potentials. However, the effects on gating currents were too small to account for the prolonged tails observed in FPL 64176. Once the channel is open, FPL 64176 slows transitions to closed or inactivated channel states.

One calcium ion may suffice to open the tetrameric cardiac ryanodine receptor in rat ventricular myocytes.

1. The release of Ca2+ from sarcoplasmic reticulum in response to Ca2+ entering through L-type Ca2+ channels was studied in isolated voltage clamped rat ventricular myocytes at room temperature using the fluorescent Ca2+ indicators fluo-3 and Oregon Green 488 Bapta 5N. 2. Depolarizations to positive potentials elicited fluo-3 Ca2+ transients with rates of rise that were linearly related to the magnitude of the peak measured Ca2+ current in the presence of Cs+-containing pipette solutions. 3. Further experiments utilizing prepulses to preactivate a constant number of channels also revealed a linear relationship between the Ca2+ transient rate of rise and the magnitude of entering Ca2+ current at positive potentials. Under these conditions as well, the maximal rates of rise of global myoplasmic Ca2+ transients were due primarily to Ca2+ release from the sarcoplasmic reticulum as revealed by effects of ryanodine and caffeine on the Ca2+ transients. Using such prepulses, linearity between the Ca2+ transient rate of rise and the magnitude of the peak Ca2+ current was found under a variety of pulse protocols. 4. Using one such pulse protocol, linearity between the Ca2+ transient rate of rise and the magnitude of the peak Ca2+ current was also found when Ca2+ currents assessed at one potential were reduced in magnitude during the onset of block by application of Co2+. Using the same pulse protocol, linearity between the Ca2+ transient rate of rise and the magnitude of the peak Ca2+ current was also found when use of Cs+ was avoided by blocking K+ currents with extracellular TEA and 4-aminopyridine. Linearity in the relationship between the Ca2+ transient rate of rise and the magnitude of the peak Ca2+ current was also found when Ca2+ transients were measured using the low affinity Ca2+ indicator Oregon Green 488 Bapta 5N in place of fluo-3. 5. These results appear to indicate that the cardiac ryanodine receptor is capable of being activated by only one calcium ion. Alternative interpretations of the data are discussed.

Perforated patch recording with beta-escin.

Perforated patch recording with nystatin, amphotericin B and gramicidin can be more difficult in the hands of some investigators than others. In addition, it is difficult to introduce low molecular weight substances such as dyes into the cytoplasm in such experiments. We have determined that beta-escin represents a convenient, easy-to-use alternative to less water-soluble ionophores.

Dihydropyridine receptor isoform expression in adult rat skeletal muscle.

The expression of isoform-specific dihydropyridine receptor Ca2+ channel (DHPR) alpha1-subunit genes in rat diaphragm, soleus and extensor digitorum longus muscles was investigated using RNase protection assays. As expected, mRNA expression levels for the DHPR skeletal muscle isoform were highest in extensor digitorum longus. Unexpectedly, both diaphragm and soleus expressed mRNA for the cardiac isoform at a significant level. Moreover, immunohistochemical experiments provided evidence of the cardiac DHPR isoform at the protein level in muscle fibres. The presence of the cardiac DHPR in the soleus and diaphragm is consistent with a degree of reported cardiac-like excitation-contraction coupling in these muscles, and may be an explanation for some of the therapeutic effects of theophylline in asthmatics, but is likely to serve some other role(s) as well.

Effects of three sarcoplasmic/endoplasmic reticulum Ca++ pump inhibitors on release channels of intracellular stores.

The three principal sarcoplasmic/endoplasmic reticulum Ca++ pump inhibitors have been compared for their effects on Ca++ fluxes across intracellular stores present in isolated skeletal muscle and brain membrane preparations. At moderate concentrations that only partially inhibited Ca++ pumping, all three inhibitors induced transient release of Ca++ from isolated sarcoplasmic reticulum membranes, and release was ruthenium red-sensitive, much faster and sustained at higher pump inhibitor concentrations. In contrast, in unidirectional 45Ca efflux assays, cyclopiazonic acid appeared to have little effect, thapsigargin decreased efflux and 2,5-di(tert-butyl)-1,4-benzohydroquinone increased efflux only slightly. These observations taken together suggest that transient releases were manifest primarily by vesicles with a high ratio of ryanodine receptors to pumps (and thus more susceptible to becoming leaky with only some pumps inhibited), and that Ca(++)-induced Ca++ release amplified releases when all pumps were blocked. These mostly indirect side effects were specific for ryanodine receptors. In similar experiments with brain cerebellar membranes, none of the three inhibitors appeared to directly reduce release induced by inositol 1,4,5-trisphosphate. These findings may affect interpretation of results of experiments involving application of these compounds to isolated membranes, cells or tissue preparations.

Ca2+ feedback on "quantal" Ca2+ release involving ryanodine receptors.

The influence of luminal and cytoplasmic Ca2+ on the ability of ryanodine-sensitive stores to undergo multiple partial ("quantal") releases has been assessed. Increased luminal Ca2+ levels do indeed modulate sarcoplasmic reticulum Ca2+ release by lowering the threshold agonist concentration required to elicit release, but the decrease in luminal Ca2+ that accompanies a partial release is not sufficient by itself to terminate release. Similarly, an increase in cytoplasmic Ca2+ lowers the threshold agonist concentration required to elicit release; thus, the bulk cytoplasmic Ca2+ levels attained during a release would only stimulate further release, not terminate it before it reached completion. Very high cytoplasmic Ca2+ levels (1-3 mM) also triggered release but were unable to terminate release before reaching completion. Thus, even the high local cytoplasmic Ca2+ concentration that might accompany release would also not terminate release. It is concluded that Ca2+ feedback can modulate release through ryanodine receptors but that it does not account for the properties of quantal release. The low affinity inhibitor tetracaine induces a decrease in the extent of release that cannot be explained solely by heterogeneous caffeine sensitivity of the stores. The results are interpreted in terms of a scheme that includes (i) heterogeneous sensitivity of stores, conferred in part by differences in luminal Ca2+ content and (ii) adaptive behavior on the part of individual ryanodine receptors.

Dihydropyridine receptor and ryanodine receptor gene expression in long-term denervated rat muscles.

Following disruption of the nerve supply, extensor digitorum longus (EDL) and soleus (SOL) muscles in rats are known to exhibit alterations in excitation-contraction coupling. After total RNA isolation from the denervated and the contralateral control muscles performed at 25 and 50 days following denervation, RNase protection assays were carried out with four cDNA probes specific for the skeletal and cardiac isoforms of both the DHPR alpha 1-subunit and the RyR. Longterm denervation increased the expression of the mRNA for skeletal DHPR and skeletal RyR in SOL muscle, but it also significantly increased the expression of the mRNA for the cardiac isoform of the DHPR alpha 1 subunit in EDL muscle.

Chronic stimulation differentially modulates expression of mRNA for dihydropyridine receptor isoforms in rat fast twitch skeletal muscle.

This study examined the effects of low frequency chronic stimulation on expression of the mRNA encoding the two isoforms of the alpha1 subunit of the dihydropyridine receptor (DHPR) calcium channel, a critical component of skeletal muscle excitation-contraction coupling. RNase protection assay was used to determine alteration in isoform expression in 5-day, 9-day and 13-day chronically stimulated rat tibialis anterior muscle, and to compare it with soleus and extensor digitorum longus muscles. Low frequency chronic stimulation was associated not only with a significant decrease in the mRNA level of the skeletal isoform of the DHPR, but also with a significant increase in the mRNA level of the cardiac isoform of the DHPR, the overwhelming majority of which was the adult splice variant. Significant levels of cardiac DHPR mRNA expression were also found in normal adult slow twitch soleus muscle. These findings raise the question of a potential role for the cardiac DHPR in certain adult skeletal muscles.

Sphingosylphosphocholine modulates the ryanodine receptor/calcium-release channel of cardiac sarcoplasmic reticulum membranes.

Sphingosylphosphocholine (SPC) modulates Ca2+ release from isolated cardiac sarcoplasmic reticulum membranes; 50 microM SPC induces the release of 70 80% of the accumulated calcium. SPC release calcium from cardiac sarcoplasmic reticulum through the ryanodine receptor, since the release is inhibited by the ryanodine receptor channel antagonists ryanodine. Ruthenium Red and sphingosine. In intact cardiac myocytes, even in the absence of extracellular calcium. SPC causes a rise in diastolic Ca2+, which is greatly reduced when the sarcoplasmic reticulum is depleted of Ca2+ by prior thapsigargin treatment. SPC action on the ryanodine receptor is Ca(2+)-dependent. SPC shifts to the left the Ca(2+)-dependence of [3H]ryanodine binding, but only at high pCa values, suggesting that SPC might increase the sensitivity to calcium of the Ca(2+)-induced Ca(2+)-release mechanism. At high calcium concentrations (pCa 4.0 or lower), where [3H]ryanodine binding is maximally stimulated, no effect of SPC is observed. We conclude that SPC releases calcium from cardiac sarcoplasmic reticulum membranes by activating the ryanodine receptor and possibly another intracellular Ca(2+)-release channel, the sphingolipid Ca(2+)-release-mediating protein of endoplasmic reticulum (SCaMPER) [Mao, Kim, Almenoff, Rudner, Kearney and Kindman (1996) Proc.Natl.Acad.Sci. U.S.A 93, 1993-1996], which we have identified for the first time in cardiac tissue.