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SUMMARY Isolated cardiac ventricular myocytes from guinea-pig were used to investigate the effect of voltage clamp protocols on background Na+ current (ib.Na) and inward rectifier current (iK1). Using long (4 s) clamp pulses and very long step clamps, the i-V relations showed that removal of Na+ reduces the amplitude and shifts the voltage dependence of iK1 (Spindler et al. 1998). Ramp clamps, however, gave more complicated results, with slower ramps more often giving the same results as steps and pulses. Both iK1 itself and, during faster ramps, other currents show hysteresis, so masking the steady-state changes. Using pulses, TTX had no effect on steady-state current. Small differences occurred in the ramps, but even at fast ramp speeds the effects are very much smaller than in Purkinje tissue. Only part of ib,Na is TTX sensitive and the effect does not occur in all cells.

Heart rate (HR) recovery from heavy exercise is associated with a shift in cardiac sympatho-vagal balance and a transient hypokalaemia. Since changes in extracellular potassium ([K+]0) affect membrane currents in the sino-atrial node, in particular the acetylcholine-activated potassium current (I(K,ACh)), the hyperpolarization-activated current (I(f)) and the L-type calcium current (I(Ca,L)), we investigated whether mimicking [K+]0 concentrations seen during and immediately after exercise could directly modulate the HR response to vagal nerve stimulation (VNS) in the isolated guinea-pig atria preparation pre-stimulated with noradrenaline (NA, 1 microM). Lowering [K+]0 from 4 to 3 mM significantly enhanced the HR response to VNS (5 Hz, 5 V, 30 s, deltaHR 84.5 +/- 14.1 bpm and 119.3 +/- 18.2 bpm, respectively). Increasing [K+]0 to 8 or 10 mM significantly decreased the drop in HR with VNS in comparison to the response to 3 mM K+ Tyrode (deltaHR 56.4 +/- 9.1 bpm and 52.1 +/- 8.7 bpm, respectively). These results could be simulated using the OXSOFT heart sino-atrial node computer model by activating I(K,ACh) during changes in [K+]0. However, changing [K+]0 in the model had no significant effect on the decrease in beating frequency brought about by decreasing I(f) or I(Ca,L). We conclude that the magnitude of the decrease in HR with VNS is enhanced in low [K +]0 and reduced in high [K+]0. The increased efficacy of cardiac vagal activation in low [K+]0 might therefore facilitate the drop in HR after heavy exercise where there is a transient hypokalaemia. Modelling suggests this result may be explained by the effects of changes in [K+]0 on the current-voltage relationship for I(K,ACh).

The effect of carbachol, an analogue of acetylcholine, on the delayed rectifier potassium current, iK, was investigated in rabbit isolated sino-atrial node cells using the whole cell patch clamp technique with amphotericin-permeabilized patches. In the presence of 500 nM atropine and 500 nM hexamethonium to block muscarinic and nicotinic receptors, respectively, 500 nM carbachol decreased the amplitude and rate of deactivation of iK without, however, affecting the slope of the iK activation curve. The same concentration of carbachol decreased the pacemaking rate of spontaneously active sino-atrial node cells by more than 13%. Thus, there is a non-muscarinic and non-nicotinic pathway for cholinergically induced reduction in the amplitude and rate of deactivation of iK that would appear to contribute to negative chronotropy in rabbit sinoatrial node pacemaker cells.

Simulation of biological processes, systems and organs is a necessary complement to genetic and molecular sequencing. Using the heart as an example, the authors describe the way in which such modelling can be used in drug discovery, development and assessment. The technology is readily applicable to other organs and systems as well; models of neurones and neuronal systems, the pancreas and the lungs have already been developed. Ultimately, a virtual corpus is feasible. Genes can only specify the properties of the proteins they code for, and any integrative properties of the system must be ‘computed’ by their interactions—Sydney Brenner

At the closing ceremony of the Congress in Kyoto, Japan, in 2009, the President of IUPS made a clear promise on behalf of the whole of Council: What on earth does IUPS exist for? We need to give back to you, to the young and upcoming physiologists the conviction that we are creating the environment

Substrate depletion and increased intracellular acidity are believed to underlie clinically important manifestations of myocardial ischaemia. Recent advances in measuring ion concentrations and metabolite changes have provided a wealth of detail on the processes involved. Coupled with the rapid increase in computing power, this has allowed the development of a mathematical model of cardiac metabolism in normal and ischaemic conditions. Pre-existing models of cardiac cells such as Oxsoft HEART contain highly developed dynamic descriptions of cardiac electrical activity. While biophysically detailed, these models do not yet incorporate biochemical changes. Modelling of bioenergetic changes was based and verified against whole heart NMR spectroscopy. In the model, ATP hydrolysis and generation are calculated simultaneously as a function of [Pi]i. Simulation of pH regulation was based on the pHi dependency of acid efflux, examined in time-course studies of pHi recovery (measured in myocytes with the fluorophore carboxy-SNARF-1) from imposed acid and alkali loads. The force-[Ca2+]i relationship of myofibrils was used as the basis of modelling H+ competition with Ca2+, and thus of pH effects on contraction. This complex description of biochemically important changes in myocardial ischaemia was integrated into the OXSOFT models. The model is sufficiently complete to simulate calcium-overload arrhythmias during ischaemia and reperfusion-induced arrhythmias. The timecourse of both metabolite and pH changes correlates well with clinical and experimental studies. The model possesses predictive power, as it aided the identification of electrophysiological effects of therapeutic interventions such as Na(+)-H+ block. It also suggests a strategy for the control of cardiac arrhythmias during calcium overload by regulating sodium-calcium exchange. In summary, we have developed a biochemically and biophysically detailed model that provides a novel approach to studying myocardial ischaemia and reperfusion.

Computational cardiac models provide important insights into the underlying mechanisms of heart function. Parameter estimation in these models is an ongoing challenge with many existing models being overparameterised. Sensitivity analysis presents a key tool for exploring the parameter identifiability. While existing methods provide insights into the significance of the parameters, they are unable to identify redundant parameters in an efficient manner. We present a new singular value decomposition based algorithm for determining parameter identifiability in cardiac models. Using this local sensitivity approach, we investigate the Ten Tusscher 2004 rapid inward rectifier potassium and the Mahajan 2008 rabbit L-type calcium currents in ventricular myocyte models. We identify non-significant and redundant parameters and improve the models by reducing them to minimum ones that are validated to have only identifiable parameters. The newly proposed approach provides a new method for model validation and evaluation of the predictive power of cardiac models. Highlights? New local sensitivity approach for determining parameter identifiability is presented. ? Non-significant and redundant parameters in ionic cardiac models of human and rabbit ventricular myocytes are identified. ? Application of the newly developed method to cardiac models' development and removal of the issue of overparameterisation is demonstrated.

Connective tissue is an essential component of cardiac histo-architecture. In particular the sinus node region of the healthy heart is rich in fibrous connective tissue [l]. The distinctive division of sino-atria1 node cells in multiple strands of different size reported in the original first description of the node by Keith and Flack [2] appears to be brought about by cardiac fibroblasts forming sheet-like extensions that enwrap groups of pacemaker cells [3]. According to different quantitative studies, connective tissue occupies between 45% [4] and 73% [5] of the volume of the sinus node in man. A number of pathological states is associated with excessive growth of fibrous tissue in other parts of the heart. These pathologies include focal scar development in regions of myocardial ischaemia and infarction, or more scattered fibrosis induced by inflammation during rheumatic heart disease and other processes. The predominant contribution of fibroblasts to post-traumatic atria1 or ventricular restoration is based on their sustained proliferative potential in the adult organism, while myocyte proliferation is confined mainly to fetal periods of development t6,71. Cardiac fibroblasts are known to be mechanosensitive. They respond to mechanical stimulation by changes in gene expression and collagen synthesis in vivo [8,9] and in vitro [lo]. This generative response to changes in the mechanical environment is assumed to find clinical expression in fibrosis induced by pressure overload [l l] and tissue dilatation [ 121. Recently, cardiac fibroblasts have been implicated as a possible alternative substrate for some forms of cardiac mechano-electric feedback [13] observed in the sino-atria1 node region and in cardiac scar tissue [14]. This short

Mathematical modelling of cardiac cell activity has developed in roughly ten-year cycles over the last 30 years, with each major phase of modelling being focused around particular experimental questions. The first phase (around 1962) was based on the dissection of the K-currents in heart cells into the inward rectifier and delayed current components. The second phase (1975) was based on identifying separate slow current mechanisms in the plateau and pacemaker ranges of potential and on the discovery of the calcium current in cardiac muscle. The most recent phase (starting with the 1985 DiFrancesco-Noble model) was based on the identification of the hyperpolarization-activated pacemaker current and on the electrogenicity of sodium-calcium exchange. Although each of these developments has depended on advances in experiment method, it is also true that each has also needed to theorize ahead of the experiment work. There is, therefore, a bi-directional interaction between theory and experiment. Sometimes experimental work leads, sometimes the theoretical work does so. A major use of such models in the case of cardiac cells is their incorporation into integrative studies of how large networks of cardiac cells interact to produce normal and abnormal rhythms. This work has received a major boost from the introduction of massively parallel computers that provide the required speed and capacity. Already, models of networks of sinus node and atrial cells have been constructed.

Drug-induced pro-arrhythmic risk has been one of the main causes for drug candidate failure and has even led to compound withdrawn from the market. A new cardiac safety paradigm that combines in-vitro screening of multiple ion channels and in-silico tools, particularly biophysically-detailed cardiac electrophysiology models, has been encouraged by the FDA, HESI, Cardiac Safety Research Consortium & Safety Pharmacology Society.CaV1.2 channel (conducting the L-type Calcium current (ICaL)) plays an important role in cardiac electrophysiology. Blockade of ICaL can mitigate the pro-arrhythmic risk for multi-channel blockers (e.g. verapamil). For drug development, L-type calcium current data can be obtained from different experimental sources. Cell lines over-expressing the human CaV1.2 channel are normally used for HT compound screening. And recently iPS-derived cardiomyocytes have become an increasingly widespread model in pharmaceutical industry.In this study, the kinetics (steady state activation, inactivation and recovery from inactivation) of CaV1.2 CaL from over-expressed CHO cells and iPS-derived cardiomyocytes were measured using Ca2+ containing or Ba2+ containing solutions (to distinguish between Calcium-dependent and Voltage-dependent inactivations). Several in-silico models were used to emulate the experimental protocols, the simulated ICaL was extracted and compared with experimental data.Comparing simulated traces using data obtained from these two experiment sources, we can learn a lot about the biophysical and biological models and their translational value.

IUPS speaks for the international community of physiological scientists, with adhering member societies, associations, and academies from over 50 countries worldwide. Its council has members drawn from all continents. Through its nominating committee, IUPS tries to ensure that it is carefully

Global sensitivity analysis is critical in understanding the role of ionic currents variability in modulating cellular electrophysiological properties of ventricular myocytes as well as in assessing the cardiac single cell arrhythmic risk biomarkers. This work involves a systematic investigation into the sensitivity of various preclinical cellular biomarkers of arrhythmic risk to changes in ionic current conductances and kinetics. We compare local and global sensitivity analysis approaches. The nonlinearity of the system is confirmed to play an important role when considering the effect of parameters on markers that involve information from different frequencies. In the case of such biomarkers as action potential duration (APD), maxima Cai and Nai concentrations, we find no significant changes in the overall order of the parameters' significance between local and global sensitivity studies. However, even for these single frequency markers we are able to give a more precise relative contribution of each parameter. Importantly, in the case of maximum S1-S2 slope marker, which integrates the information across a spectrum of frequencies of electrical stimuli, we find significant changes in the order of parameters as compared to the previously used methods for studying the sensitivity of cellular biomarkers. Moreover, the models consistently identify the L-type Calcium current (ICaL) as one of the dominant currents affecting the plateau of the action potential. Furthermore, in consistence with experimental data, the simulation results show that variation in conductances and kinetics of other channels does not affect the significance of ICaL's contribution. We demonstrate the critical role of global (vs local) sensitivity analysis in providing insights into the sensitivity of preclinical biomarkers, and hence mechanisms of repolarization and their rate dependence.

Cardiac simulation is used to integrate information on drug action to predict side effects on the whole heart. Could simulation begin to replace animal models?

The positive chronotropic response of the heart to stretch of the right atrium is one of the major mechanisms adjusting the heart rate to variations in venous return on a beat-by-beat basis. The precise pathway of this mechano-electric feedback and its cellular basis are uncertain. In this study, a possible contribution of mechanosensitive fibroblasts, abundant in the sino-atrial node region, was investigated using a mathematical model of the electrical interaction of a mechanosensitive fibroblast and a sino-atrial pacemaker cell. Electrophysiological evidence for a bio-electrical interaction of mechanosensitive fibroblasts with surrounding cardiomyocytes has been studied in (i) the isolated spontaneously beating atrium of rat hearts, and (ii) cell cultures of the neonatal rat heart. These investigations were performed using (i) double-barrelled floating microelectrodes for intracellular potential registrations, and (ii) the double whole cell patch-clamp technique. It was shown that cardiac fibroblasts and surrounding cardiomyocytes can be either electrically well isolated from each other, or coupled both capacitively and electrotonically. The electrophysiological data obtained were incorporated into the OXSOFT HEART program. Assuming that equivalent coupling may occur between mechanosensitive fibroblasts and sino-atrial pacemaker cells, a heterologous cell pair consisting of one fibroblast and one sino-atrial node myocyte connected by ten to thirty single gap junctional channels with a conductance of 30 pS was modelled. The model of the electrotonic interaction of these cells showed that stretch of the fibroblast during atrial diastole, simulating increased atrial wall tension during atrial filling, can raise the spontaneous depolarization rate of the pacemaker cell in a stretch-dependent manner by up to 24%. These results show that cardiac mechanosensitive fibroblasts could form a cellular basis for the positive chronotropic response of the heart to stretch of the right atrium.

Mathematical models of the electrical activity of the heart using equations for protein ion channels and other transporters began with the Noble 1962 model. These models then developed over a period of about 50 years. Cell types in all regions have been modeled and now are available for download from the CellML website (www.cellml.org). Simulation is a necessary tool of analysis in attempting to understand biological complexity. We often learn as much from the failures as from the successes of mathematical models. It is the iterative interaction between experiment and simulation that is important.

With scale relativity theory, Laurent Nottale has provided a powerful conceptual and mathematical framework with numerous validated predictions that has fundamental implications and applications for all sciences. We discuss how this extended framework reviewed in Nottale (Found Sci 152 (3):101-152, 2010a) may help facilitating integration across multiple size and time frames in systems biology, and the development of a scale relative biology with increased explanatory power. © 2010 Springer Science+Business Media B.V.

Based on the similarity between a reentrant wave in cardiac tissue and a vortex in fluid dynamics, the authors hypothesised that a new non-dimensional index, like the Reynolds number in fluid dynamics, may play a critical role in categorising reentrant wave dynamics. Therefore the goal of the present study is to devise a new index to characterise electric wave conduction in cardiac tissue and examined whether this index can be used as a biomarker for categorising the reentrant wave pattern in cardiac tissue. Similar to the procedure used to derive the Reynolds number in fluid dynamics, the authors used a non-dimensionalisation technique to obtain the new index. Its usefulness was verified using a two-dimensional simulation model of electrical wave propagation in cardiac tissue. The simulation results showed that electrical waves in cardiac tissue move into an unstable region when the index exceeds a threshold value.

We propose an innovative, integrated, cost-effective health system to combat major non-communicable diseases (NCDs), including cardiovascular, chronic respiratory, metabolic, rheumatologic and neurologic disorders and cancers, which together are the predominant health problem of the 21st century. This proposed holistic strategy involves comprehensive patient-centered integrated care and multi-scale, multi-modal and multi-level systems approaches to tackle NCDs as a common group of diseases. Rather than studying each disease individually, it will take into account their intertwined gene-environment, socio-economic interactions and co-morbidities that lead to individual-specific complex phenotypes. It will implement a road map for predictive, preventive, personalized and participatory (P4) medicine based on a robust and extensive knowledge management infrastructure that contains individual patient information. It will be supported by strategic partnerships involving all stakeholders, including general practitioners associated with patient-centered care. This systems medicine strategy, which will take a holistic approach to disease, is designed to allow the results to be used globally, taking into account the needs and specificities of local economies and health systems. Part of the conceptual work presented has received support from the European commission FP7 projects AIRProm (Grant Agreement FP7 270194), BioSHaRE-EU (Grant Agreement FP7 261433), MeDALL (Grant Agreement FP7 264357), SYNERGY-COPD (Grant Agreement) and U-BIOPRED (Grant Agreement IMI 115010). JB, JMA, AC-T, FK, MLK, SP, CP and CA were supported by MeDALL; PJS, IMA and KFC were supported by U-BIOPRED; JR and AA were supported by SYNERGY-COPD; CB was supported by AIRProm; AC-T was supported by BioSHaRE-EU

Ca2+-induced delayed afterdepolarizations (DADs) are depolarizations that occur after full repolarization. They have been observed across multiple species and cell types. Experimental results have indicated that the main cause of DADs is Ca2+ overload. The main hypothesis as to their initiation has been Ca2+ overflow from the overloaded sarcoplasmic reticulum (SR). Our results using 37 previously published mathematical models provide evidence that Ca2+-induced DADs are initiated by the same mechanism as Ca2+-induced Ca2+ release, i.e., the modulation of the opening of ryanodine receptors (RyR) by Ca2+ in the dyadic subspace; an SR overflow mechanism was not necessary for the induction of DADs in any of the models. The SR Ca2+ level is better viewed as a modulator of the appearance of DADs and the magnitude of Ca2+ release. The threshold for the total Ca2+ level within the cell (not only the SR) at which Ca2+ oscillations arise in the models is close to their baseline level (∼1- to 3-fold). It is most sensitive to changes in the maximum sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pump rate (directly proportional), the opening probability of RyRs, and the Ca2+ diffusion rate from the dyadic subspace into the cytosol (both indirectly proportional), indicating that the appearance of DADs is multifactorial. This shift in emphasis away from SR overload as the trigger for DADs toward a multifactorial analysis could explain why SERCA overexpression has been shown to suppress DADs (while increasing contractility) and why DADs appear during heart failure (at low SR Ca2+ levels).

The ability to produce rhythmic, spontaneous contractions is a primary function of the heart, and it has intrigued generations of cardiac physiologists in the search for the underlying processes. Spontaneous electrical activity was recorded intracellularly from cardiac tissue by Silvio Weidmann since the mid-1950s. 1 In the early 1960s, the first numerical reconstruction of cardiac activity showed that the cardiac action potential could be interpreted on the basis of specific kinetic properties of identified ionic currents and this opened the way to investigation of the contribution of cellular mechanisms to action potential generation in cardiac cells by numerical modeling. 2 The idea that pacemaking originates from a membrane ionic mechanism was validated by the finding of ion currents specifically committed to this function. A “funny” (If) current with properties especially designed for the generation of a depolarizing process was identified in the rabbit sino-atrial node (SAN) in the late 1970s 3 ; the current, inward and activating upon hyperpolarization in the diastolic range of voltages, had features apt to generate a diastolic depolarization and control its steepness, and thus represented a mechanism able to not only induce rhythmic behavior but also regulate cardiac rate. Further indication of the involvement of If in pacemaker regulation came from evidence that the current is modulated by autonomic transmitters and contributes to the acceleratory action of sympathetic stimuli and the slowing action of parasympathetic stimuli. 3‐7

The use of mathematical models to study cardiac electrophysiology has a long history, and numerous cellular scale models are now available, covering a range of species and cell types. Their use to study emergent properties in tissue is also widespread, typically using the monodomain or bidomain equations coupled to one or more cell models. Despite the relative maturity of this field, little has been written looking in detail at the interface between the cellular and tissue-level models. Mathematically this is relatively straightforward and well-defined. There are however many details and potential inconsistencies that need to be addressed, in order to ensure correct operation of a cellular model within a tissue simulation. This paper will describe these issues and how to address them. Simply having models available in a common format such as CellML is still of limited utility, with significant manual effort being required to integrate these models within a tissue simulation. We will thus also discuss the facilities available for automating this in a consistent fashion within Chaste, our robust and high-performance cardiac electrophysiology simulator. It will be seen that a common theme arising is the need to go beyond a representation of the model mathematics in a standard language, to include additional semantic information required in determining the model’s interface, and hence to enhance interoperability. Such information can be added as metadata, but agreement is needed on the terms to use, including development of appropriate ontologies, if reliable automated use of CellML models is to become common.

We have investigated the effect of change in external Na+ concentration on the hyperpolarization-activated inward current (I(f)) in the presence of different internal cations. Rabbit single isolated sinoatrial node cells were studied using the whole-cell patch-clamp technique. With 140 mM K+ pipettes, lowering [Na+]o causes the fully activated I/V curve for I(f) to shift in a negative direction without a significant decrease of the slope conductance. The PNa/PK ratio, as defined by the Goldman-Hodgkin-Katz equation, is concentration-dependent: the lower the [Na+]o, the higher PNa/PK. The conductance/concentration relationship for I(f) shows saturation at low [Na+]o or [K+]o, indicating that the channel has a strong affinity for external cations. With 140 mM Cs+ pipettes, the I/V curve shows strong inward rectification and inward I(f) current decreases almost proportionally to the decrease in [Na+]o; the conductance/concentration relationship for I(f) shifts to the right suggesting that the binding affinity of the external binding site is reduced. These results suggest that the I(f) channel is a multi-ion channel with a high-affinity external binding site, the affinity of which is modulated by internal cations.

Atrial and ventricular myocytes were exposed to Ca$^{2+}$- and K$^{+}$-free solutions containing blockers of gated channel and exchange currents. Replacement of external sodium by large organic cations revealed a background sodium current i$\_{\text{b,Na}}$. In atrial cells, the average conductance was 5.0 pS pF$^{-1}$. In ventricular cells the conductance was 2.3 pS pF$^{-1}$. Together with previous results, these figures reveal a strong gradient of background current density: sinus > atrium > ventricle. Replacement of sodium with inorganic cations showed that the channel selectivity behaves like an Eisenman group III/IV sequence, in agreement with previous results. The permeability of the channel to TMA was found to be pH dependent, suggesting that protonation of the channel is a factor determining permeation in addition to ionic size. The values of g$\_{\text{b,Na}}$ obtained from these experiments are very similar to those assumed in computer modelling of cardiac cell electrical activity.

Computational methods for simulating biophysically detailed, large-scale models of mammalian cardiac sinus and atrial networks on the massively parallel Connection Machine CM-2, and techniques for visualization of simulation data, are presented. Individual cells are modeled using the formulations of Noble et al. Models incorporate properties of voltage-dependent membrane currents, ion pumps and exchangers, and internal calcium sequestering and release mechanisms. Network models are used to investigate factors determining the site of generation and direction of propagation of the pacemaker potential. Models of the isolated sinus node are used to show that very few gap junction channels are required to support frequency entrainment. When cell membrane properties in the isolated sinus node models are modified to reproduce regional differences in oscillation properties, as described by the data of Kodama and Boyett, an excitatory wave is generated in the node periphery which propagates towards the node center. This agrees with activation patterns measured in the isolated sinus node by Kirchoff. When the model sinus node is surrounded by a region of atrial cells, the site of pacemaker potential generation is shifted away from the periphery towards the node center. This is in agreement with activation patterns measured by Kirchoff in the intact sinus node of the rabbit heart, and demonstrates the importance of sinus node boundary conditions on shaping the site of generation and direction of propagation of the pacemaker potential.

Because of the reported presence of a Na(+)-Mg2+ exchanger in guinea-pig but not in ferret myocardium, the Mg2+ extrusion mechanism in guinea-pig myocardium has been reinvestigated using Mg(2+)- and Na(+)- selective microelectrodes and the fluorochromes mag-fura-2 and -5. The mean [Mg2+]i measured with microelectrodes in trabeculae or papillary muscles was 0.72 mmol/l (n = 22, thirteen experiments; range 0.42-1.23 mmol/l). Increasing [Mg2+]o from 0.5 mmol/l to either 10.5 or 20 mmol/l caused small increases in [Mg2+]i. Decreasing [Na+]o by 50% had no effect on the [Mg2+]i and there was no change in [Na+]i on increasing [Mg2+]o from 0.5 to 10.5 mmol/l. Varying pHo or changing pHi with NH4Cl did not influence the [Mg2+]i. In vitro calibration of mag-fura-2 and -5 using the ratio method gave values for K'd (experimentally determined dissociation constant) of 22.2 +/- 2.7 (mean +/- S.D., n = 7) and 25.7 +/- 1.3 (n = 4) mmol/l respectively. Mag-fura-2 reacted to physiological concentrations of Ca2+ and mag-fura-5 to changes in pH. In isolated myocytes, Na+ removal gave an apparent increase of [Mg2+]i with mag-fura-2 but not with mag-fura-5. However, when the pHi was altered with NH4Cl mag-fura-5 showed an apparent decrease in [Mg2+]i on application and an apparent increase on removal, with a time course similar to the pHi changes. It is concluded that Mg2+ extrusion in guinea-pig myocardium is not via a Na(+)-Mg2+ exchanger. The use of mag-fura-2 and -5 are limited in their application because of Ca2+ and H+ sensitivity respectively.

The world of physiology is on a roll. And it's rolling big. The messages from IUPS 2013 are clear. First, the physiological sciences are now where much of the important action is in biological and medical science. The bloggers and tweeters also all said the same thing during and after the event.

This article argues that the gene-centric interpretations of evolution, and more particularly the selfish gene expression of those interpretations, form barriers to the integration of physiological science with evolutionary theory. A gene-centred approach analyses the relationships between genotypes and phenotypes in terms of differences (change the genotype and observe changes in phenotype). We now know that, most frequently, this does not correctly reveal the relationships because of extensive buffering by robust networks of interactions. By contrast, understanding biological function through physiological analysis requires an integrative approach in which the activity of the proteins and RNAs formed from each DNA template is analysed in networks of interactions. These networks also include components that are not specified by nuclear DNA. Inheritance is not through DNA sequences alone. The selfish gene idea is not useful in the physiological sciences, since selfishness cannot be defined as an intrinsic property of nucleotide sequences independently of gene frequency, i.e. the ‘success’ in the gene pool that is supposed to be attributable to the ‘selfish’ property. It is not a physiologically testable hypothesis.

In just over a decade, Systems Biology has moved from being an idea, or rather a disparate set of ideas, to a mainstream feature of research and funding priorities. Institutes, departments, and centers of various flavors of Systems Biology have sprung up all over the world. An Internet search now produces more than 2 million hits. Of the 2,800 entries in PubMed with "Systems Biology" in either the title or the abstract, only two papers were published before 2000, and >90% were published in the past five years. In this article, we interpret Systems Biology as an approach rather than as a field or a destination of research. We illustrate that this approach is productive for the exploration of systems behavior, or "phenotypes," at all levels of structural and functional complexity, explicitly including the supracellular domain, and suggest how this may be related conceptually to genomes and biochemical networks. We discuss the role of models in Systems Biology and conclude with a consideration of their utility in biomedical research and development.

Abstract : Interaction between a membrane oscillator generated by voltage-dependent ion channels and an intracellular calcium signal oscillator was present in the earliest models (1984 to 1985) using representations of the sarcoplasmic reticulum. Oscillatory release of calcium is inherent in the calcium-induced calcium release process. Those historical results fully support the synthesis proposed in the articles in this review series. The oscillator mechanisms do not primarily compete with each; they entrain each other. However, there is some asymmetry: the membrane oscillator can continue indefinitely in the absence of the calcium oscillator. The reverse seems to be true only in pathological conditions. Studies from tissue-level work and on the development of the heart also provide valuable insights into the integrative action of the cardiac pacemaker.

Cardiovascular disease, and the cardiovascular side effects of drugs, are essentially multifactorial problems involving interactions between many proteins, dependent on highly organized cell, tissue and organ structures. This is one reason why the side effects of drugs are often unanticipated. It is impossible to unravel such problems without using a systems approach, i.e. focussing on processes, not just molecular components. This inevitably involves modelling as the interactions require quantitative analysis. Modelling is a tool of analysis aimed at understanding, first, and predicting, eventually. We illustrate these principles using modelling of the heart. Models of the cardiac myocyte have benefited from several decades of interaction between experimentation and simulation. They are now sufficiently detailed to have been of use in the development of new drug compounds like ranolazine and ivabradine. With the help of cardiac modelling, we have also been able to unravel the mechanisms underlying the beneficial effect of sodium calcium exchange block for long QT syndrome (LQTS) 2 and LQTS3 patients. Detailed models of the interaction between ion channels and blocking agents provide the basis for modelling drug action from basic principles and predict changes in the inhomogeneous tissue of the heart. We demonstrate that mathematical models are beneficial for unravelling the complex interactions of pharmacodynamics in the heart. Embedding these detailed biophysical cellular scale models into anatomically correct models of the ventricle geometry will enable reconstructions of Torsades de Pointes arrhythmias and of fibrillation, providing a mechanism for linking detailed cellular scale experimental data to clinical applications.

Biophysics at the systems level, as distinct from molecular biophysics, acquired its most famous paradigm in the work of Hodgkin and Huxley, who integrated their equations for the nerve impulse in 1952. Their approach has since been extended to other organs of the body, notably including the heart. The modern field of computational biology has expanded rapidly during the first decade of the twenty-first century and, through its contribution to what is now called systems biology, it is set to revise many of the fundamental principles of biology, including the relations between genotypes and phenotypes. Evolutionary theory, in particular, will require re-assessment. To succeed in this, computational and systems biology will need to develop the theoretical framework required to deal with multilevel interactions. While computational power is necessary, and is forthcoming, it is not sufficient. We will also require mathematical insight, perhaps of a nature we have not yet identified. This article is therefore also a challenge to mathematicians to develop such insights.

Mol Syst Biol. 5: 292 The virtual physiological human (VPH) initiative is intended to support the development of patient‐specific computer models and their application in personalised and predictive healthcare. The VPH, a core target of the European Commission's 7th Framework Programme, will serve as a ‘ methodological and technological framework that, once established, will enable collaborative investigation of the human body as a single complex system ’ (http://www.europhysiome.org/roadmap/). As such, the VPH initiative constitutes an integral part of the international Physiome Project (http://www.physiome.org.nz/), a worldwide public domain effort to develop a computational framework for the quantitative description of biological processes in living systems across all relevant levels of structural and functional integration, from molecule to organism, including the human (Kohl et al , 2000; Bassingthwaighte et al , 2009). So, what is the connection between this grand challenge and systems biology? To explore this, we must first agree on what we take systems biology to mean. ### Description versus definition Descriptions of systems biology range from the view that it is merely ‘new wording, more fashionable, for physiology’ (http://is.gd/tQJL), to the all‐inclusive ‘systems biology involves the application of experimental, theoretical, and computational techniques to the study of biological organisms at all levels, from the molecular, through the cellular, to the organ, organism, and populations. Its aim is to understand biological processes as integrated systems instead of as isolated parts’ (http://is.gd/tQK0). At the same time, attempts to concisely define systems biology have not yielded definitive form of words that is acceptable to the majority of researchers engaged in what they consider to be systems biology. One of the reasons for this situation may be that many different scientific streams have come together in the systems biology pool (see also Bassingthwaighte et al , 2009), each with its own conceptual and terminological …