Steady-state Relationship Research Articles

Voltage-dependent potassium currents in feline sino-atrial node myocytes

We characterized the properties of the voltage-dependent K(+) currents I (to), I (Kr), and I (Ks) in isolated feline sino-atrial node (SAN) myocytes. I (to) activated rapidly and then inactivated with a single exponential and voltage-independent time course. Recovery from inactivation of I (to) followed a single exponential time course with τ = 21.1 ± 2.5 ms, at -80 mV. Steady-state inactivation relationship showed a V½ of inactivation at -47.9 ± 2.3 mV. These biophysical properties are similar to the fast I (to) phenotype of other mammals. I (Kr) exhibited typical negative slope conductance at test potentials > 0 mV and slow deactivation. I (Ks) activated very slowly. The functional contribution of I (to), I (Kr), and I (Ks) to the sustained pacemaking activity of feline SAN myocytes was analyzed. Similar to other mammals, I (to) underlies the initial repolarization phase of the SAN action potential, whereas I (Kr) and I (Ks) mediate repolarization back to the maximal diastolic potential. I (Kr) and I (Ks) also contribute to diastolic depolarization because of their slow deactivation kinetics. The I (Kr) specific blocker E-4031 and the I (Ks) blocker HMR 1556 significantly increased action potential duration, but had negligible effects on the maximum diastolic potential and only modest effects on the frequency of spontaneous activity, suggesting that each one of these two currents itself is capable of supporting action potential repolarization in the feline sinus node.

Who's afraid of a globalized world? Foreign Direct Investments, local knowledge and allocation of talents

We study the distributional effects of globalization within a model of heterogeneous agents where both managerial talent and knowledge of the local economic environment are required in order to become a successful entrepreneur. Agents willing to set up a firm abroad incur a learning cost that depends on how different the foreign and domestic entrepreneurial environments are. In this context, we show that globalization fosters FDI and raises wages, output and productivity. However, not everybody wins. The steady state relationship between globalization and income is U-shaped: high- and low-income agents are better off in a globalized world, while middle-income agents (domestic entrepreneurs) are worse off. Thus, consistently with recent empirical evidence, the model predicts globalization to increase inequality at the top of the income distribution while decreasing it at the bottom.

Quantitative Measurement of Ca2+-Dependent Calmodulin–Target Binding by Fura-2 and CFP and YFP FRET Imaging in Living Cells

Calcium dynamics and its linked molecular interactions cause a variety of biological responses; thus, exploiting techniques for detecting both concurrently is essential. Here we describe a method for measuring the cytosolic Ca(2+) concentration ([Ca(2+)](i)) and protein-protein interactions within the same cell, using Fura-2 and superenhanced cyan and yellow fluorescence protein (seCFP and seYFP, respectively) FRET imaging techniques. Concentration-independent corrections for bleed-through of Fura-2 into FRET cubes across different time points and [Ca(2+)](i) values allowed for an effective separation of Fura-2 cross-talk signals and seCFP and seYFP cross-talk signals, permitting calculation of [Ca(2+)](i) and FRET with high fidelity. This correction approach was particularly effective at lower [Ca(2+)](i) levels, eliminating bleed-through signals that resulted in an artificial enhancement of FRET. By adopting this correction approach combined with stepwise [Ca(2+)](i) increases produced in living cells, we successfully elucidated steady-state relationships between [Ca(2+)](i) and FRET derived from the interaction of seCFP-tagged calmodulin (CaM) and the seYFP-fused CaM binding domain of myosin light chain kinase. The [Ca(2+)](i) versus FRET relationship for voltage-gated sodium, calcium, and TRPC6 channel CaM binding domains (IQ domain or CBD) revealed distinct sensitivities for [Ca(2+)](i). Moreover, the CaM binding strength at basal or subbasal [Ca(2+)](i) levels provided evidence of CaM tethering or apoCaM binding in living cells. Of the ion channel studies, apoCaM binding was weakest for the TRPC6 channel, suggesting that more global Ca(2+) and CaM changes rather than the local CaM-channel interface domain may be involved in Ca(2+)CaM-mediated regulation of this channel. This simultaneous Fura-2 and CFP- and YFP-based FRET imaging system will thus serve as a simple but powerful means of quantitatively elucidating cellular events associated with Ca(2+)-dependent functions.

Piezoelectrically actuated hydraulic valve design for high bandwidth and flow performance

The performance of hydraulically actuated machine systems could be improved with the use of valves that have high bandwidth and high flowrates under low pressure drops. Although high flowrates can be achieved using very large spool strokes and/or diameters, the overall bandwidth of the valve will be reduced. Research has therefore been undertaken on a prototype valve design incorporating the Horbiger plate principle, which utilizes multiple metering edges to allow high flowrates to be obtained at low pressure drops and small poppet displacements. The valve is directly activated using a piezoelectric actuator to achieve a fast dynamic response. Valve performance is assessed using a mathematical model that includes the piezoelectric actuator and power amplifier, the supply flow, fluid squeeze forces, end stop response, and valve mechanical components. The steady state relationship between valve flow, force and pressure drop, and the fluid inertance, were determined using computational fluid dynamics software. The simulation model has been validated using test data obtained from experimental tests undertaken on a prototype valve. Good agreement is obtained between the predicted and measured results and it is shown that the valve is capable of opening or closing fully in less than 1.5 ms, and can pass a flow of 65l/min at a pressure drop of 20 bar.

Simulation of the diurnal variations of the oxygen isotope anomaly (Δ<sup>17</sup>O) of reactive atmospheric species

Abstract. The isotope anomaly (Δ17O) of secondary atmospheric species such as nitrate (NO3−) or hydrogen peroxide (H2O2) has potential to provide useful constrains on their formation pathways. Indeed, the Δ17O of their precursors (NOx, HOx etc.) differs and depends on their interactions with ozone, which is the main source of non-zero Δ17O in the atmosphere. Interpreting variations of Δ17O in secondary species requires an in-depth understanding of the Δ17O of their precursors taking into account non-linear chemical regimes operating under various environmental settings. This article reviews and illustrates a series of basic concepts relevant to the propagation of the Δ17O of ozone to other reactive or secondary atmospheric species within a photochemical box model. We present results from numerical simulations carried out using the atmospheric chemistry box model CAABA/MECCA to explicitly compute the diurnal variations of the isotope anomaly of short-lived species such as NOx and HOx. Using a simplified but realistic tropospheric gas-phase chemistry mechanism, Δ17O was propagated from ozone to other species (NO, NO2, OH, HO2, RO2, NO3, N2O5, HONO, HNO3, HNO4, H2O2) according to the mass-balance equations, through the implementation of various sets of hypotheses pertaining to the transfer of Δ17O during chemical reactions. The model results confirm that diurnal variations in Δ17O of NOx predicted by the photochemical steady-state relationship during the day match those from the explicit treatment, but not at night. Indeed, the Δ17O of NOx is "frozen" at night as soon as the photolytical lifetime of NOx drops below ca. 10 min. We introduce and quantify the diurnally-integrated isotopic signature (DIIS) of sources of atmospheric nitrate and H2O2, which is of particular relevance to larger-scale simulations of Δ17O where high computational costs cannot be afforded.

Centrifugal Compressors During Fast Transients

Transient studies for compressor systems allow the prediction of the compressor system behavior during fast transients such as they occur during emergency shutdowns. For the system simulations, the compressor behavior is assumed to be quasi-steady-state. This means in particular that the steady-state compressor flow-head-efficiency-speed map remains valid. During well instrumented emergency shutdown tests conducted on a centrifugal compressor system under realistic operating conditions, data showing the head-flow-speed relationship of the rapidly decelerating compressor were taken. These data are compared with steady-state head-flow relationships taken at a number of speeds. This allows the determination of the relative deviation between the transient and steady-state head-flow-relationships and thus answers the question of the validity of steady-state assumptions during rapid transients. The impact of the fast transients on efficiency and consumed power, which can be derived from the speed decay of the system, as well as the impact of nonstationary heat transfer are also evaluated and reported.

Electrodialysis with spacers: Effects of variation and correlation of boundary layer thickness

The performance of electrodialysis is strongly dependent on the boundary layer thickness near an ion-exchange membrane. While a thinner boundary layer is known to enhance the ionic separation, the effects of statistical properties of the boundary layer thickness, such as the variation of the thickness and the correlation between the two boundary layers facing across the spacer, have not been elucidated. These effects were estimated by the Monte Carlo method incorporated into an analytical model. The analytical model simulates the binary ionic transport in four distinct regions of an electrodialysis cell pair: the bulk solution, boundary layer, ion-exchange membrane, and interface between the aqueous solution and ion-exchange membrane. The model current and potential relationships found that a greater variation or more positive correlation improves the ionic separation in the non-Ohmic regime. A bench-scale electrodialyzer was operated in a batch recycle system to develop steady-state current and potential relationships. Comparison between the model and experimental results found that the mean boundary layer thickness was tens of micrometers and the standard deviation of the thickness was similar to or greater than the mean thickness with the sheet-flow type mesh spacer.

Structural Basis for Calcium Modulation of the Cardiac Sodium Channel

Voltage gated sodium channels drive the upstroke of the cardiac action potential and then undergo a fast inactivation process which is essential for the termination of their contribution to excitability and for the maintenance of cardiac rhythms. The degree to which the channel opens and inactivates relies on many cellular factors. Increased cytoplasmic calcium, for instance, results in a rightward shifted steady-state inactivation relationship for the cardiac Nav1.5 isoform, resulting in more channels being available to contribute to the action potential. We have previously shown using isothermal titration calorimetry (ITC) that Ca2+ /calmodulin can bind the sodium channel inactivation gate comprised of the cytoplasmic linker between domains III and IV in a calcium dependent manner (Kd at 1mM free [Ca2+]=3uM). To further examine this interaction in greater detail, we have determined a high-resolution (1.35A) crystal structure of the rigid portion of the two proteins in complex. This new structure confirms our previous findings that tyrosine 1494 acts as a major interaction point with the calmodulin C-lobe. With this structural data in hand we have designed channel mutations that increase (Kd=600nM) or decrease (approximately Kd=30uM) the affinity of calmodulin to the inactivation gate. We then analyzed the effect of these novel mutations on channel gating via patch clamp electrophysiology to further investigate the mechanisms of calcium regulation of the cardiac sodium channel. We account for our observations obtained from ITC and patch clamp experiments with a kinetic model describing calcium modulation of cardiac sodium channels. Our results suggest that the inactivation gate is a key molecular determinant in the regulation of cardiac sodium channels by calcium.

N-Glycans Modulate hERG1A Window Current

Voltage-gated K+ channels are primarily responsible for the repolarization phases of neuronal, skeletal and cardiac muscle action potentials. Activity of the human ether-a-go-go-related gene 1 (hERG1) voltage-gated K+ channel produces a K+ current responsible for much of late phase II and phase III repolarization of the human cardiac action potential. HERG1A has two putative N-glycosylation sites located in the S5-S6 linker region, one of which is N-glycosylated. The aim of this study was to determine whether and how N-linked glycosylation modifies hERG1A channel function. Voltage-dependent gating of hERG1A were evaluated under conditions of full glycosylation, no sialylation, in the absence of complex N-glycans, and following the removal of the full N-glycosylation structure. The hERG1A steady-state activation relationships were shifted along the voltage axis by a significant, depolarizing ∼9 mV under each condition of reduced glycosylation. Steady state channel availability curves were shifted by a much greater depolarizing 20-30 mV under conditions of reduced glycosylation. The depolarizing, non-uniform shifts in voltage-dependent steady state activation and inactivation caused a large rightward shift and an increase in the hERG1A window current. This suggests that reduced glycosylation caused an increase in the persistent hERG current that occurs at more depolarized potentials as observed. This increase and shift in window current would lead to increased hERG1A activity during the action potential, effectively increasing the rate of repolarization, and reducing AP duration, as predicted through in silico modeling of the ventricular AP. Overall, these data indicate a functional role for N-glycosylation in the modulation of hERG1A channel activity, suggesting that even small changes in channel N-glycosylation modulate hERG1A activity, and thereby likely impact the rate of action potential repolarization.

Controlling Money Supply And Price Level With Unknown Regime Shifts: The Case Of Chile

<p class="MsoNormal" style="text-align: justify; margin: 0in 0.5in 0pt;"><span style="font-size: 10pt;" lang="EN-GB"><span style="font-family: Times New Roman;">This paper investigates the relationship between money supply and price level using new tests for cointegration with two unknown regime shifts and bootstrap causality tests. Quarterly Chilean data from 1973: I to 2006: III is used. We find empirical evidence that the variables establish a long-run steady state relationship in the presence of two regime shifts. The elasticity of price level with regard to money supply is close to unity during the first period (prior to 1978: II). The elasticity is reduced during the second period (1978: III-1986: I) and it is also reduced for the remaining period but the reduction is smaller. We also conducted bootstrap causality tests that reveal the following: in the first sub-period there is bidirectional causality between the underlying variables. In the last two sub-periods money supply causes the price level only. This implies that money supply is weakly exogenous concerning the price level and that the monetary authority had enough independence to execute an active monetary policy in Chile. <span style="mso-bidi-font-weight: bold;"></span></span></span></p>

Robust Control of Solid Oxide Fuel Cell Ultracapacitor Hybrid System

Mitigating fuel starvation and improving load-following capability of solid oxide fuel cells (SOFC) are conflicting control objectives. In this paper, we address this issue using a hybrid SOFC ultracapacitor configuration. Fuel starvation is prevented by regulating the fuel cell current using a steady-state invariant relationship involving fuel utilization, fuel flow, and current. Two comprehensive control strategies are developed. The first is a Lyapunov-based nonlinear control and the second is a standard H∞ robust control. Both strategies additionally control the state of charge of the ultracapacitor that provides transient power compensation. A hardware-in-the-loop test stand is developed where the proposed control strategies are verified.

Instability and freezing in a solidifying melt conduit

Previous works have shown that when liquid flows in a pipe whose boundary temperature is below freezing, a tubular drainage conduit forms surrounded by solidified material that freezes shut under the appropriate combination of forcing conditions. We conduct laboratory experiments with wax in which the tube freezes shut below a certain value of flux from a pump. As the flux is gradually decreased to this value, the total pressure drop across the length of the tube first decreases to a minimum value and then rises before freezing. Previous theoretical models of a tube driven by a constant pressure drop suggest that once the pressure minimum is reached, the states for a lower flux should be unstable and the tube should therefore freeze-up. In our experiments, flux and pressure drop were coupled, and this motivates us to extend the theory for low Reynolds number flow through a tube with solidification to incorporate a simple pressure-drop–flux relationship. Our model predicts a steady-state relationship between flux and pressure drop that has a minimum pressure as the flux is varied. The stability properties of these steady states depend on the boundary conditions: for a fixed flux, they are all stable, whereas for fixed pressure drop, only those with a flux larger than that at the pressure drop minimum are stable. For a mixed pressure–flux condition, the stability threshold of the steady states lies between these two end members. This provides a possible mechanism for the experimental observations.

Native and recombinant Slc26a3 (downregulated in adenoma, Dra) do not exhibit properties of 2Cl−/1HCO3− exchange

The recent proposal that Dra/Slc26a3 mediates electrogenic 2Cl(-)/1HCO(3)(-) exchange suggests a required revision of classical concepts of electroneutral Cl(-) transport across epithelia such as the intestine. We investigated 1) the effect of endogenous Dra Cl(-)/HCO(3)(-) activity on apical membrane potential (V(a)) of the cecal surface epithelium using wild-type (WT) and knockout (KO) mice; and 2) the electrical properties of Cl(-)/(OH(-))HCO(3)(-) exchange by mouse and human orthologs of Dra expressed in Xenopus oocytes. Ex vivo (36)Cl(-) fluxes and microfluorometry revealed that cecal Cl(-)/HCO(3)(-) exchange was abolished in the Dra KO without concordant changes in short-circuit current. In microelectrode studies, baseline V(a) of Dra KO surface epithelium was slightly hyperpolarized relative to WT but depolarized to the same extent as WT during luminal Cl(-) substitution. Subsequent studies indicated that Cl(-)-dependent V(a) depolarization requires the anion channel Cftr. Oocyte studies demonstrated that Dra-mediated exchange of intracellular Cl(-) for extracellular HCO(3)(-) is accompanied by slow hyperpolarization and a modest outward current, but that the steady-state current-voltage relationship is unaffected by Cl(-) removal or pharmacological blockade. Further, Dra-dependent (36)Cl(-) efflux was voltage-insensitive in oocytes coexpressing the cation channels ENaC or ROMK. We conclude that 1) endogenous Dra and recombinant human/mouse Dra orthologs do not exhibit electrogenic 2Cl(-)/1HCO(3)(-) exchange; and 2) acute induction of Dra Cl(-)/HCO(3)(-) exchange is associated with secondary membrane potential changes representing homeostatic responses. Thus, participation of Dra in coupled NaCl absorption and in uncoupled HCO(3)(-) secretion remains compatible with electroneutrality of these processes, and with the utility of electroneutral transport models for predicting epithelial responses in health and disease.

Microstructure and creep behavior of the rare-earth doped Mg–6Zn–3Cu cast alloy

The effect of 1, 2 and 3wt.% rare earth (RE) additions on the microstructure and creep resistance of the Mg–6Zn–3Cu alloy (ZC63) was investigated by impression creep tests in the temperature range 423–498K and under punching stresses in the range 150–700MPa for dwell times up to 3600s. The results showed that the creep strength of the base alloy was remarkably improved by RE addition. This was attributed to the grain refinement effect of RE elements, and formation of thermally stable Mg12RE, MgRE, and Cu2RE compounds which strengthen both matrix and grain boundaries during creep deformation. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.017<σimp/G<0.037. Based on the steady-state power-law creep relationship, the stress exponents of about 4.4–6 and 7–12.7 were obtained at low and high stresses, respectively. The creep activation energies were found to be in the range 73–78 and 122–130kJ/mol, at low- and high-stress regimes, respectively. It is suggested that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide in the low-stress regime, and dislocation climb with some sort of back stress, similar to those noted in dispersion strengthening alloys, in the high-stress regime.

Re-examination of the long-run purchasing power parity: further evidence from Turkey

In this article, we re-examine the empirical validity of the Purchasing Power Parity (PPP) theory for the Turkish economy. For this purpose, an empirical model is constructed using some contemporaneous estimation techniques such as multivariate co-integration and vector error correction methodology. Our estimation results reveal that the PPP can strongly be supported as a long-run stationary steady-state relationship for the Turkish economy.

Dynamic Response of Rigid Foundations of Arbitrary Shape Using Half-Space Green’s Function

This paper deals with the dynamic steady-state force-displacement relationships (impedance) for rigid foundations of arbitrary shape resting on a semi-infinite medium consisting of homogeneous, isotropic, linear elastic materials using the boundary element method (BEM). An elaborate numerical technique is used to calculate Green’s function, yielding a highly accurate numeric approximation in an explicit form. By using a BEM formulation for triangular elements (linear and infinite shape function) compliance of rigid massless surface foundations was obtained. The use of triangular elements causes stress continuity between elements to be satisfied. Meanwhile, the tendency for foundation edge stresses toward infinity can be modeled by using these triangular infinite stress elements. As a result of this, computations are more accurate but less time-consuming. This procedure is used to evaluate the vertical rocking compliance functions for rigid rectangular, circular, and square foundations with internal holes....

Cellular modelling: experiments and simulation to develop a physiological model of G-protein control of muscarinic K + channels in mammalian atrial cells

The first model of G-protein-K(ACh) channel interaction was developed 14 years ago and then expanded to include both the receptor-G-protein cycle and G-protein-K(ACh) channel interaction. The G-protein-K(ACh) channel interaction used the Monod-Wyman-Changeux allosteric model with the idea that one K(ACh) channel is composed of four subunits, each of which binds one active G-protein subunit (G(betagamma)). The receptor-G-protein cycle used a previous model to account for the steady-state relationship between K(ACh) current and intracellular guanosine-5-triphosphate at various extracellular concentrations of acetylcholine (ACh). However, simulations of the activation and deactivation of K(ACh) current upon ACh application or removal were much slower than experimental results. This clearly indicates some essential elements were absent from the model. We recently found that regulators of G-protein signalling are involved in the control of K(ACh) channel activity. They are responsible for the voltage-dependent relaxation behaviour of K(ACh) channels. Based on this finding, we have improved the receptor-G-protein cycle model to reproduce the relaxation behaviour. With this modification, the activation and deactivation of K(ACh) current in the model are much faster and now fall within physiological ranges.

Ion Channel Density Regulates Switches between Regular and Fast Spiking in Soma but Not in Axons

The threshold firing frequency of a neuron is a characterizing feature of its dynamical behaviour, in turn determining its role in the oscillatory activity of the brain. Two main types of dynamics have been identified in brain neurons. Type 1 dynamics (regular spiking) shows a continuous relationship between frequency and stimulation current (f-Istim) and, thus, an arbitrarily low frequency at threshold current; Type 2 (fast spiking) shows a discontinuous f-Istim relationship and a minimum threshold frequency. In a previous study of a hippocampal neuron model, we demonstrated that its dynamics could be of both Type 1 and Type 2, depending on ion channel density. In the present study we analyse the effect of varying channel density on threshold firing frequency on two well-studied axon membranes, namely the frog myelinated axon and the squid giant axon. Moreover, we analyse the hippocampal neuron model in more detail. The models are all based on voltage-clamp studies, thus comprising experimentally measurable parameters. The choice of analysing effects of channel density modifications is due to their physiological and pharmacological relevance. We show, using bifurcation analysis, that both axon models display exclusively Type 2 dynamics, independently of ion channel density. Nevertheless, both models have a region in the channel-density plane characterized by an N-shaped steady-state current-voltage relationship (a prerequisite for Type 1 dynamics and associated with this type of dynamics in the hippocampal model). In summary, our results suggest that the hippocampal soma and the two axon membranes represent two distinct kinds of membranes; membranes with a channel-density dependent switching between Type 1 and 2 dynamics, and membranes with a channel-density independent dynamics. The difference between the two membrane types suggests functional differences, compatible with a more flexible role of the soma membrane than that of the axon membrane.

Enhanced creep properties of a cast Mg–5Sn alloy subjected to aging-treatment

The microstructure and impression creep behavior of the Mg–5Sn alloy are studied in the as-cast and aged conditions. Impression creep tests were performed in the temperature range 423–498K and under punching stresses in the range 125–400MPa. Analysis of the data showed that for all loads and temperatures, the creep resistance of the aged alloys was much better than that of as-cast materials, due to the higher volume fraction of Mg2Sn precipitates distributed in the microstructure. This phase strengthened both matrix and grain boundaries during creep deformation in the investigated system. Based on the steady-state power-law creep relationship, the stress exponents of about 5.2–6.9 were obtained at different temperatures. When the experimental creep rates were normalized to the pipe-diffusion coefficient, stress exponents of 5 and 6.8 were, respectively obtained for the cast and aged conditions, which are in complete agreement with stress exponents determined by the conventional creep testing of the same material reported in the literature. The activation energies in the range 90–98kJ/mole, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents of about 5 and 7 suggest that the operative creep mechanism is dislocation viscous glide in the as-cast condition, and dislocation climb controlled by dislocation pipe diffusion in the aged material.

Potassium Channel Silencing by Constitutive Endocytosis and Intracellular Sequestration

Tandem of P domains in a weak inwardly rectifying K(+) channel 1 (TWIK1) is a K(+) channel that produces unusually low levels of current. Replacement of lysine 274 by a glutamic acid (K274E) is associated with stronger currents. This mutation would prevent conjugation of a small ubiquitin modifier peptide to Lys-274, a mechanism proposed to be responsible for channel silencing. However, we found no biochemical evidence of TWIK1 sumoylation, and we showed that the conservative change K274R did not increase current, suggesting that K274E modifies TWIK1 gating through a charge effect. Now we rule out an eventual effect of K274E on TWIK1 trafficking, and we provide convincing evidence that TWIK1 silencing results from its rapid retrieval from the cell surface. TWIK1 is internalized via a dynamin-dependent mechanism and addressed to the recycling endosomal compartment. Mutation of a diisoleucine repeat located in its cytoplasmic C terminus (I293A,I294A) stabilizes TWIK1 at the plasma membrane, resulting in robust currents. The effects of I293A,I294A on channel trafficking and of K274E on channel activity are cumulative, promoting even more currents. Activation of serotoninergic receptor 5-HT(1)R or adrenoreceptor alpha2A-AR stimulates TWIK1 but has no effect on TWIK1I293A,I294A, suggesting that G(i) protein activation is a physiological signal for increasing the number of active channels at the plasma membrane.