Distribution of a persistent sodium current across the ventricular wall in guinea pigs

A tetrodotoxin-sensitive persistent sodium current, I(pNa), was found in guinea pig ventricular myocytes by whole-cell patch clamping. This current was characterized in cells derived from the basal left ventricular subendocardium, midmyocardium, and subepicardium. Midmyocardial cells show a statistically significant (P<0.05) smaller I(pNa) than subendocardial and subepicardial myocytes. There was no significant difference in I(pNa) current density between subepicardial and subendocardial cells. Computer modeling studies support a role of this current in the dispersion of action potential duration across the ventricular wall.

The effect of compressed air foam on the detection of hydrocarbon fuels in fire debris samples

In 1998/99 the New Zealand Fire Service implemented compressed air foam delivery systems for the suppression of fires in rural areas. This study investigated whether the introduction of the foam to the seat of the fire created any problems in subsequent analyses of fire debris samples. No significant interferences from the foam were found when the samples were analysed by direct headspace using activated carbon strips. The only foam component detected was limonene.

Characterization of Na/Ca exchange in plasmalemmal vesicles from zona fasciculata cells of the bovine adrenal gland

The presence of an Na/Ca exchange system in fasciculata cells of the bovine adrenal gland was tested using isolated plasmalemmal vesicles. In the presence of an outwardly Na(+) gradient, Ca(2+) uptake was about 2-fold higher than in K(+) condition. Li(+) did not substitute for Na(+) and 5 mM Ni(2+) inhibited Ca(2+) uptake. Ca(2+) efflux from Ca(2+)-loaded vesicles was Na(+)-stimulated and Ni(2+)-inhibited. The saturable part of Na(+)-dependent Ca(2+) uptake displayed Michaelis-Menten kinetics. The relationship of Na(+)-dependent Ca(2+) uptake versus intravesicular Na(+) concentration was sigmoid (apparent K(0.5) approximately 24 mM; Hill number approximately 3) and Na(+) acted on V(max) without significant effect on K(m). Na(+)-stimulated Ca(2+) uptake was temperature-dependent (apparent Q(10) approximately 2.2). The inhibition properties of several divalent cations (Cd(2+), Sr(2+), Ni(2+), Ba(2+), Mn(2+), Mg(2+)) were tested and were similar to those observed in kidney basolateral membrane. The above results indicate the presence of an Na/Ca exchanger located on plasma membrane of zona fasciculata cells of bovine adrenal gland. This exchanger displays similarities with that of renal basolateral cell membrane.

Vagal control of heart rate is modulated by extracellular potassium

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).

Comparison of step and ramp voltage clamp on background currents in guinea-pig ventricular cells

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 (i K1). 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 i K1 (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 i K1 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.

Non-muscarinic and non-nicotinic inhibition by the acetylcholine analogue carbachol of the delayed rectifier potassium current, iK, in rabbit isolated sino-atrial node cells

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.

Characterization of metabolites in intact Streptomyces citricolor culture supernatants using high-resolution nuclear magnetic resonance and directly coupled high-pressure liquid chromatography-nuclear magnetic resonance spectroscopy

A novel NMR spectroscopic approach to the direct biochemical characterization of bacterial culture broths is presented. A variety of one- and two-dimensional 1H NMR spectroscopic methods were used to characterize low-molecular-weight organic components of broth supernatants from cultures of Streptomyces citricolor. By applying 1H NMR spectroscopy to analyze whole, untreated culture supernatants, it was possible to identify and monitor simultaneously a range of media substrates and excreted metabolites. Identified metabolites include 2-phenylethylamine, trehalose, succinate, acetate, uridine, and aristeromycin, a secondary metabolite with antibiotic properties. Directly coupled HPLC-NMR spectroscopy was also applied to the analysis of broth supernatants for the first time, to aid spectral assignments, especially where signals were extensively overlapped in the 1H NMR spectra of the whole broth mixtures. Two-dimensional NMR methods such as 1H-1H correlation spectroscopy, 1H-13C heteronuclear single quantum correlation, and 1H-13C heteronuclear multiple bond correlation aided the structure elucidation and peak assignments of individual components in the mixtures by providing information on 1H-1H coupling networks and 13C chemical shifts. This work shows that high-resolution NMR spectroscopic methods provide a rapid and efficient means of investigating microbial metabolism directly without invasive or destructive sample pretreatment.

Stretch-induced changes in heart rate and rhythm: clinical observations, experiments and mathematical models

Clinical and research data indicate that active and passive changes in the mechanical environment of the heart are capable of influencing both the initiation and the spread of cardiac excitation via pathways that are intrinsic to the heart. This direction of the cross-talk between cardiac electrical and mechanical activity is referred to as mechano-electric feedback (MEF). MEF is thought to be involved in the adjustment of heart rate to changes in mechanical load and would help to explain the precise beat-to-beat regulation of cardiac performance as it occurs even in the recently transplanted (and, thus, denervated) heart. Furthermore, there is clinical evidence that MEF may be involved in mechanical initiation of arrhythmias and fibrillation, as well as in the re-setting of disturbed heart rhythm by 'mechanical' first aid procedures. This review will outline the clinical relevance of cardiac MEF, describe cellular correlates to the responses observed in situ, and discuss the role that quantitative mathematical models may play in identifying the involvement of cardiac MEF in the regulation of heart rate and rhythm.

Biological simulations in drug discovery

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

Effect of isoprenaline, carbachol, and Cs+ on Na+ activity and pacemaker potential in rabbit SA node cells

Effects of isoprenaline, carbachol, and Cs+ on intracellular Na+ activity (a(i)Na) and spontaneous action potentials were studied in multicellular and single cell preparations isolated from rabbit sinoatrial (SA) nodes. a(i)Na was measured with double-barreled Na+-selective microelectrodes and the fluorescent Na+-indicator sodium-binding benzofuran isophthalate (SBFI). In spontaneously beating cells, aiNa measured with Na+-selective microelectrodes and SBFI were 4.5 +/- 1.2 mM (means +/- SD, n = 21) in multicellular preparations and 4.0 +/- 1.1 mM (n = 16) in single cells, respectively. Measurements of a(i)Na with microelectrodes showed that isoprenaline increased a(i)Na from 4.7 +/- 1.2 to 5.5 +/- 1.6 mM (n = 16, P < 0.01) and shortened the action potential cycle length (ACL) from 338 +/- 46 to 269 +/- 35 ms (n = 16, P < 0.01). However, increasing the action potential rate by pacing produced a much smaller increase in a(i)Na. Changes in a(i)Na and ACL produced by isoprenaline were blocked by Cs+. The selective hyperpolarization-activated inward current (If) blocker ZD-7288 decreased a(i)Na from 5.2 +/- 1.0 to 4.6 +/- 1.3 mM (n = 4, P < 0.01) and prolonged ACL from 394 +/- 20 to 553 +/- 68 ms (n = 4, P < 0.01). The If blocker substantially inhibited the increase in a(i)Na produced by isoprenaline. Carbachol and Cs+ decreased aiNa from 4.6 +/- 1.4 to 3.9 +/- 1.2 mM (n = 15, P < 0.01) and from 4.9 +/- 1.0 to 3.9 +/- 1.3 mM (n = 18, P < 0.01), respectively. In addition, carbachol and Cs+ prolonged ACL from 345 +/- 44 to 587 +/- 100 ms (n = 15, P < 0.01) and from 353 +/- 30 to 464 +/- 87 ms (n = 18, P < 0.01), respectively. However, carbachol and Cs+ almost did not change a(i)Na when SA node cells became quiescent in a 25.4 mM extracellular K+ concentration. The results suggest that isoprenaline, ZD-7288, carbachol, or Cs+ might have changed a(i)Na and action potential rate by possibly stimulating or inhibiting If carried by Na+. Measurements of a(i)Na with SBFI showed that isoprenaline, carbachol, and Cs+ produced a(i)Na changes that were similar to those measured with the microelectrodes.

Isolation of translactone-containing triterpenes with thrombin inhibitory activities from the leaves of Lantana camara

Methanolic extracts prepared from the leaves of Lantana camara have been found to inhibit human thrombin. An assay, in which thrombin activity is measured as a function of clot formation from fibrinogen, was used to guide the fractionation and purification of five principal active constituents (1-5), which were all characterized as 5,5-trans-fused cyclic lactone-containing euphane triterpenes.

Modelling myocardial ischaemia and reperfusion

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.

Reduction and integration in understanding the heart

The heart provides an excellent example of the limits of the reductive approach. Cardiac cells function through the interaction of a very large number of ion transporters, and the processes that link these to metabolic states and to contraction. Yet, the great majority of the advances made recently have been at the cellular and molecular levels. The pressing problem now is to begin to understand the highly complex interactions that create physiological function at a cellular level and, in turn, to understand the way in which large numbers of cells interact to produce the activity of the whole heart. Many kinds of arrhythmia, for example, can only be understood at the whole organ level. Successful interventions using drugs designed to treat cardiac disease depend on an integrative understanding, which at present we do not have. This is one of the reasons why clinical trials of drugs treating cardiac arrhythmias have been spectacularly disappointing. This paper illustrates some of these problems by analysing normal and abnormal heart rhythms, and by focusing on one particular transporter, the sodium-calcium exchanger, that is deeply involved both in normal calcium balance in the heart and in the generation of pathological states, including life-threatening arrhythmias. It will be shown that some surprising counterintuitive results appear when computations are done at an integrative level.

Anaesthetic management of the morbidly obese patient

Morbidly obese patients are prone to many clinical conditions that can effect anaesthesia. Of major concern to the anaesthetist are difficulties with airway management and abnormalities of cardiorespiratory function. Safe anaesthesia requires an appreciation of potential problems and a thorough understanding of the pathophysiological changes that accompany morbid obesity.

The effects of sodium substitution on currents determining the resting potential in guinea-pig ventricular cells

It has recently been shown that a sodium background current, ib,Na, exists in cardiac muscle cells whose effect is to depolarize the membrane so that the resting potential, Vm, is positive to the potassium equilibrium potential, EK. In ventricular cells, where ib,Na is smallest, Vm is about 10 mV positive to EK (EK = -87 mV at 37 degrees C). Yet, replacement of Na+ ions by large impermeant cations does not cause the expected hyperpolarization. We have studied this problem in guinea-pig myocytes using a single microelectrode recording technique in combination with a rapid external solution switch. Cells depolarized < or = 0.5 mV from potentials between -80 and -73 mV and hyperpolarized up to 5 mV from potentials between -73 and -64 mV when 70 mM choline chloride or N-methyl-D-glucamine chloride were used to replace 70 mM Na+ in the bathing solution. Replacement by 70 mM lithium chloride, however, only caused hyperpolarization in very depolarized cells when the voltage change was much smaller. The changes were complete almost as soon as the solution change, i.e. within 250 ms, indicating that the actions are attributable to the external solution change rather than to secondary changes in intracellular concentrations. Patch clamp recording was used to investigate the mechanism involved. These experiments showed that the presence or absence of the inward rectifier current iK1 determines in which direction Na+ removal acts. In the absence of iK1 the changes are attributable to removal of ib,Na, whereas in the presence of iK1 the changes resemble the i(V) relation for iK1, implying that Na+ regulates iK1 in a way that can mask the changes in ib,Na. These results explain why removal of Na+ does not lead to hyperpolarization in ventricular cells as would be expected if changes in ib,Na were solely responsible. Computer reconstruction shows that the effects may be attributed to actions of sodium removal on the conductance and gating of iK1.

Cellular mechanisms of cardiac mechano-electric feedback in a mathematical model

BACKGROUND

Cardiac mechanical and electrical activity are closely interrelated. While excitation-contraction coupling is rather well characterized, less is known about cellular mechanisms that promote mechanically induced changes in cardiac electrical activity--mechano-electric feedback.

OBJECTIVE

To integrate experimental findings on stretch activation of ion channels and length-dependent changes in intracellular calcium handling into a mathematical description of cardiac cellular activity.

METHODS

Simulations are based on the cellular OXSOFT HEART v4.8 models of electrical activity of single cardiac cells of different populations and species. Sarcolemmal stretch-activated channels, mechanically induced changes in the affinity of troponin C to calcium, and length-dependent modulation of calcium handling by the sarcoplasmic reticulum were introduced into the models and linked to a description of sarcomere length or isometric tension.

RESULTS

Transient or sustained stretch of cardiomyocytes was simulated during electrical systole and diastole. The electrophysiological response observed in the model depended on timing and severity of mechanical stimulation and on the main subcellular target of the intervention. Responses ranged from triggering of premature action potentials, over changes in action potential shape and duration, to length-dependent variations in contractile behaviour. Modelling findings could be related to experimental observations and may help to explain some of the contradictory data in the literature. The model is sufficiently complete to reproduce experimental findings and to help identify causally linked events.

Improved guinea-pig ventricular cell model incorporating a diadic space, IKr and IKs, and length- and tension-dependent processes

The guinea-pig ventricular cell model, originally developed by Noble et al in 1991, has been greatly extended to include accumulation and depletion of calcium in a diadic space between the sarcolemma and the sarcoplasmic reticulum where, according to contempory understanding, the majority of calcium-induced calcium release is triggered. The calcium in this space is also assumed to play the major role in calcium-induced inactivation of the calcium current. Delayed potassium current equations have been developed to include the rapid (IKr) and slow (IKs) components of the delayed rectifier current based on the data of of Heath and Terrar, along with data from Sanguinetti and Jurkiewicz. Length- and tension-dependent changes in mechanical and electrophysiological processes have been incorporated as described recently by Kohl et al. Drug receptor interactions have started to be developed, using the sodium channel as the first target. The new model has been tested against experimental data on action potential clamp, and on force-interval and duration-interval relations; it has been found to reliably reproduce experimental observations.

Modelling of sodium-overload arrhythmias and their suppression

Models of the electrophysiological properties of cardiac cells are now capable of accounting not only for normal activity, but also for some of the mechanisms of arrhythmia. A good example is the reconstruction of the inotropic and arrhythmogenic effects of sodium pump inhibition leading to the generation of ectopic beats. The models can also be used to investigate the possible mechanisms of suppression of arrhythmias. The authors show how the antiarrhythmic actions of lidocaine may be reproduced. The results account for the large difference between therapeutic levels (approximately 30 microM) in whole tissue and the concentration (approximately 500 microM) required to suppress excitation in isolated cells.

Modeling of internal pH, ion concentration, and bioenergetic changes during myocardial ischemia

Arrhythmias are caused by the interdependent processes of change in energy metabolism and alterations in sarcolemmal ion gradients that occur during ischemia. Depletion of energy metabolites and increased proton concentrations in ischemic heart may underlie the observed phenomena of reduced contractile force and also of malignant ventricular arrhythmias that can lead to tachycardia and ventricular fibrillation. Recent advances in measuring changes in ion concentrations and metabolites during cardiac ischemia have provided a wealth of detail on the processes involved. Some of the experimental data have been used to construct a computer model that integrates cardiac energetics with electrophysiological changes. This is a novel approach to studying myocardial ischemia, and the resulting model would aid in the prediction of the effects of therapeutic interventions.

An efficient process for production of N-acetylneuraminic acid using N-acetylneuraminic acid aldolase

N-acetyl-D-neuraminic acid (Neu5Ac) aldolase (EC 4.1.3.3) has bee reported for synthesis of Neu5Ac,1-5 but there are no reports of processes which do not have significant drawbacks for large-scale operation. Here, Neu5Ac aldolase from an overexpressing recombinant strain of Escherichia coli has been used to develop an immobilized enzyme process for production of Neu5Ac. The enzyme was immobilized onto Eupergit-C and could be reused many times in the reaction. Base-catalyzed epimerization of N-acetyl-D-glucosamine (GlcNAc) yielded GlcNAc/N-acetyl-D-mannosamine (ManNAc) mixtures (c 4:1) which could be used directly in the aldolase reaction; however, inhibition of the enzyme by GlcNAc limited the concentration of ManNAc which could be used in the reaction by this approach. This necessitated the addition of a large molar excess of pyruvate (five- to seven-fold) to drive the equilibrium over to Neu5Ac; nevertheless, a method has been developed to remove the excess pyruvate effectively by complexation with bisulfite, thus allowing Neu5Ac to be recovered by absorption onto an anion-exchange resin. In a second approach, a method has been developed to enrich GlcNAc/ManNAc mixtures for ManNAc. ManNAc can be used at high concentrations in the reaction, thus obviating the need to use a large molar excess of pyruvate. Neu5Ac can be isolated from such reaction mixtures by a simple crystallization. This work shows the importance of integrated process solutions for the effective scale-up of biotransformation reactions.

Voltage- and time-dependent block of delayed rectifier K+ current in rabbit sino-atrial node cells by external Ca2+ and Mg2+

1. The properties of the delayed rectifier K+ current (IK) of rabbit isolated sino-atrial node cells were investigated in high (140 mM) [K+]o using the whole-cell-clamp technique. 2. Hyperpolarizing clamp pulses from 0 mV induced an instantaneous current jump (I-V relation linear) followed by a time-dependent increase in inward current to a peak, whereas depolarizing clamp pulses induced little outward current. The peak I-V relation showed a strong inward rectification. The inwardly rectifying current was blocked by E-4031. 3. The inward K+ current induced by hyperpolarizing clamp pulses from 0 mV relaxed after reaching its peak. The rate of the relaxation increased as the membrane potential became more negative and concentrations of external Ca2+ or Mg2+ were increased. The steady-state current was smaller as the relaxation of the current accelerated on increasing [Ca2+]o or [Mg2+]o. 4. Depolarizing clamp pulses from -80 mV induced an increase in inward current, reaching a steady state. The amplitude of the steady-state current became smaller and the rate of current increase became slower as [Ca2+]o or [Mg2+]o was increased. 5. The effects of Ca2+ and Mg2+ are well explained by a time- and voltage-dependent blockade of the K+ channel by these ions. The fractional electrical distance of the binding site calculated from the voltage dependence of the blocking rate constant is 0.69 for Ca2+ and 0.88 for Mg2+. The blocking rate constant at 0 mV for Ca2+ is about 15 times faster than that for Mg2+, indicating stronger effects of Ca2+. 6. A re-interpretation of IK in sino-atrial node cells is proposed: there are two independent gates (an activation gate which opens on hyperpolarization and an inactivation gate which closes on hyperpolarization) and a binding site for Ca2+ and Mg2+ inside the channel. Binding of these ions, which is facilitated by hyperpolarization, causes channel blockade, resulting in the observed voltage dependence of IK in physiological concentrations of Ca2+ and Mg2+.

Induction of phenazine biosynthesis in cultures of Pseudomonas aeruginosa by L-N-(3-oxohexanoyl)homoserine lactone

A range of Pseudomonas spp. and other Gram-negative bacteria were screened for induction of antimicrobial activity in response to the autoregulatory factor L-N-(3-oxohexanoyl)homoserine lactone. In one of these, P. aeruginosa ATCC 10145, the production of phenazine metabolites was shown to be inducible in a dose-dependent manner. The production of phenazine-1-carboxamide increased over 50-fold compared to control cultures when supplemented with 200 micrograms/ml of the autoregulator. In addition, the production of an unidentified polar antibacterial substance by this strain increased with autoregulator concentration.

Novel squalestatins produced by biotransformation

Microorganisms were screened for the ability to modify the squalene synthase inhibitor squalestatin 1. Biotransformation of 1 by two actinomycetes, S15106 and S15138, yielded three products hydroxylated on the 4,6-dimethyl-oct-2-enoyl side chain either at the 6 position (5) or 7 position (4 two diastereoisomers), and lacking the acetyl ester from the C-1 side chain. Many strains were found to hydrolyse the 4,6-dimethyl-oct-2-enoyl or acetyl esters to yield squalestatins 2 or 3. The 3-methyl ester (6) of 1 was obtained using Fusarium sp. F13945. This fungus also produced a farnesoic acid derivative, possibly in response to inhibition of its squalene synthase by 1. The biotransformation products of 1 all retained potent squalene synthase inhibitory activity.

Mechanosensitive fibroblasts in the sino-atrial node region of rat heart: interaction with cardiomyocytes and possible role

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.

Effects of rapid changes of external Na+ concentration at different moments during the action potential in guinea-pig myocytes

1. A rapid solution-changing system using a solenoid was set up. The half-time for changing the external solution surrounding a ventricular cardiac cell was 7.2 +/- 1.4 ms, whereas the time needed to change 90% of this solution was 48.5 +/- 7.9 ms. This rapid switching system was used to reduce the external sodium concentration at different moments during the action potential (recorded using the whole-cell method) to 50% of its original value. This was performed in order to investigate the effect on the shape and duration of the action potential of modifying the activity of the sodium-calcium exchanger. 2. A diminution of the action potential duration was seen irrespective of the substitute used for reducing the NaCl concentration from 140 to 70 mM. The magnitude of this diminution depended on the presence or absence of EGTA (5 mM) in the pipette solution and also on the moment during the action potential at which the NaCl substitution occurred. 3. Some differences were observed depending on whether the NaCl substitute used was lithium chloride or choline chloride. When choline chloride or N-methyl-D-glucamine was used as the NaCl substitute, the amplitude of the action potential was slightly reduced (by 2-5 mV) when the solution was changed 40 ms before the action potential was triggered. This reduction was never observed when LiCl was used as the NaCl substitute. 4. The effects on the shape of the action potential of changing from a solution containing 140 mM NaCl to one containing 70 mM NaCl and 70 mM LiCl were much more rapid when these changes occurred at a later stage during the action potential. The rate of repolarization was more than doubled when the change occurred at a late stage of the action potential but was hardly changed at the beginning of the plateau. 5. These experiments confirm the role of the sodium-calcium exchange current in determining the duration of the mammalian ventricular action potential. However, it is also possible that the sodium background current plays a significant role in determining the shape of the action potential.

The squalestatins, novel inhibitors of squalene synthase produced by a species of Phoma. V. Minor metabolites

The isolation and structure determination of 3H and 13C NMR and MS of 24 novel squalestatins from cultures of Phoma sp. C2932 is described.

The influence of dietary creatine supplementation on performance during repeated bouts of maximal isokinetic cycling in man

The effect of dietary creatine (Cr) supplementation on performance during 3, 30 s bouts maximal isokinetic cycling and on plasma ammonia and blood lactate accumulation during exercise was investigated. Placebo (P) ingestion had no effect on peak power output (PPO), mean power output (MPO) and total work output during each bout of exercise. Cr ingestion (4 x 5 g.day-1 for 5 days) significantly increased PPO in exercise bout 1 (p < 0.05) and MPO and total work output in exercise bouts 1 (p < 0.05, p < 0.05, respectively) and 2 (p < 0.05, p < 0.05, respectively). Cr ingestion had no effect on any of the measures of performance during exercise bout 3. No difference was observed in peak plasma ammonia accumulation before (146 + 30 mumol.l-1) and after (122 +/- 17 mumol.l-1) P ingestion, however the corresponding concentration was lower following Cr ingestion (129 +/- 22 mumol.l-1) compared with before Cr ingestion (160 +/- 18 mumol.l-1, p < 0.05), despite subjects performing more work. No difference in peak blood lactate accumulation was observed before and after P or Cr ingestion. The results demonstrate that Cr ingestion can increase whole body exercise performance during the initial two, but not a third, successive bout of maximal exercise lasting 30 s. The lower accumulation of plasma ammonia under these conditions suggests this response is achieved by an effect on muscle ATP turnover.

Effects of gap junction conductance on dynamics of sinoatrial node cells: two-cell and large-scale network models

A computational model of single rabbit sinoatrial (SA) node cells has been revised to fit data on regional variation of rabbit SA node cell oscillation properties. The revised model simulates differences in oscillation frequency, maximum diastolic potential, overshoot potential, and peak upstroke velocity observed in cells from different regions of the node. Dynamic properties of electrically coupled cells, each with different intrinsic oscillation frequency, are studied as a function of coupling conductance. Simulation results demonstrate at least four distinct regimes of behavior as coupling conductance is varied: a) independent oscillation (Gc < 1 pS); b) complex oscillation (1 < or = Gc < 220 pS); c) frequency, but not waveform entrainment (Gc > or = 220 pS); and d) frequency and waveform entrainment (Gc > or = 50 nS). The conductance of single cardiac myocyte gap junction channels is about 50 pS. These simulations therefore show that very few gap junction channels between each cell are required for frequency entrainment. Analyses of large-scale SA node network models implemented on the Connection Machine CM-200 supercomputer indicate that frequency entrainment of large networks is also supported by a small number of gap junction channels between neighboring cells.

Discrimination of transiently applied mechanical loads: breathing vs. pulling

Two groups of 24 subjects each attempted to discriminate between large elastic and resistive loads during 50 randomized presentations of each load. Breathers inspired from the loads through a J valve, whereas pullers reciprocally stroked the plunger of a 2-liter syringe connected to the J valve. A range of load durations was obtained in each subject by prematurely unloading approximately one-half of the trials at graded times from their onset. Breathers produced random discrimination scores [50.8 +/- 2.5% (SE) correct] when loaded inspirations were shorter than unloaded inspirations [trials in which both loads induced equal airway pressure (and probably respiratory muscle tension) waveforms] and nonrandom discrimination scores (65.7 +/- 2.8% correct) when loaded inspirations were longer than unloaded inspirations (trials in which both loads induced different waveforms). In contrast, pullers produced nonrandom discrimination scores (62.2 +/- 2.9% correct) when loaded airflow durations were shorter than unloaded inspirations [trials in which both loads induced equal line pressure (and therefore limb muscle tension) waveforms]. Supplemental audio feedback related to instantaneous airflow (an expression of movement) improved load perception in breathers (to 64.2 +/- 3.0% correct; P < 0.01), indicating that airflow feedback introduced load-specific information that was lacking during breathing but redundant during pulling. In support of this hypothesis, airflow feedback by itself enabled a third group of listeners to identify load type with equal accuracy as pullers but with greater accuracy than breathers. These findings suggest that 1) uniformed subjects rely heavily on feedback from airway pressure and/or muscle tension receptors to perceive added loads to breathing and 2) limb mechanoreceptors provide a more sensitive appreciation of movement than do respiratory mechanoreceptors.

High selectivity of the i(f) channel to Na+ and K+ in rabbit isolated sinoatrial node cells

The ionic selectivity of the hyperpolarization-activated inward current (i(f)) channel to monovalent cations was investigated in single isolated sinoatrial node cells of the rabbit using the whole-cell patch-clamp technique. With a 140 mM K+ pipette, replacement of 90% external Na+ by Li+ caused a -24.5 mV shift of the fully activated current/voltage I/V curve without a significant decrease of the slope conductance. With a 140 mM Cs+ pipette, the i(f) current decreased almost proportionally to the decrease in external [Na+]o as Li+ was substituted. These responses are practically the same as those observed with N-methyl glucamine (NMG+) substitution, suggesting that the relative permeability of Li+ compared with Na+ for the i(f) channel is as low as that of NMG+. When Cs+ or Rb+ was substituted for internal K+, the fully activated I/V relationship for i(f) showed strong inward rectification with a positive reversal potential, indicating low permeability of the i(f) channel for Cs+ and Rb+. These results show that the i(f) channel is highly selective for Na+ and K+ and will not pass the similar ions Li+ and Rb+. Such a high degree of selectivity is unique and may imply that the structure of the i(f) channel differs greatly from that of other Na+ and K+ conducting channels.

Generation and propagation of ectopic beats induced by spatially localized Na-K pump inhibition in atrial network models

A biophysically detailed two-dimensional network model of the cardiac atrium has been implemented on the Thinking Machines massively parallel CM-5 supercomputer. The model is used to study the effects of spatially localized inhibition of the Na-K pump. Na overloading produced by pump inhibition can induce spontaneous, propagating ectopic beats within the network. At a cell-to-cell coupling value yielding a realistic plane wave conduction velocity of 0.6 m s-1, pump inhibition in roughly 1000 cells can induce propagating ectopic beats in a 512 x 512 lattice of cells.

The squalestatins, novel inhibitors of squalene synthase produced by a species of Phoma. IV. Preparation of fluorinated squalestatins by directed biosynthesis

Feeding of fluorinated benzoic acids to fermentations of a Phoma sp. resulted in the biosynthesis of a series of novel fluorinated squalestatins. The feeding studies, isolation, structural elucidation and biological activities of these compounds are reported.

Internal K ions modulate the action of external cations on hyperpolarization-activated inward current in rabbit isolated sinoatrial node cells

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.