Effects of 2,4-dinitrophenol or low [ATP]i on cell excitability and action potential propagation in guinea pig ventricular myocytes

The role of sulfate in aerobic granular sludge process for emerging sulfate-laden wastewater treatment

Sulfate-rich wastewaters pose a major threat to mainstream wastewater treatment due to the unpreventable production of sulfide and associated shift in functional bacteria. Aerobic granular sludge could mitigate these challenges in view of its high tolerance and resilience against changes in various environmental conditions. This study aims to confirm the feasibility of aerobic granular sludge in the treatment of sulfate containing wastewater, investigate the impact of sulfate on nutrient removal and granulation, and reveal metabolic relationships in the above processes. Experiments were conducted using five sequencing batch reactors with different sulfate concentrations operated under alternating anoxic/aerobic condition. Results showed that effect of sulfate on chemical oxygen demand (COD) removal is negligible, while phosphate removal was enhanced from 12% to 87% with an increase in sulfate from 0 to 200 mg/L. However, a long acclimatization of the biomass (more than 70 days) is needed at a sulfate concentration of 500 mg/L and a total deterioration of phosphate removal at 1000 mg/L. Batch tests revealed that sulfide promoted volatile fatty acids (VFAs) uptake, producing more energy for phosphate uptake when sulfate concentrations were beneath 200 mg/L. However, sulfide detoxification became energy dominating, leaving insufficient energy for Polyhydroxyalkanoate (PHA) synthesis and phosphate uptake when sulfate content was further increased. Granulation accelerated with increasing sulfate levels by enhanced production of N-Acyl homoserine lactones (AHLs), a kind of quorum sensing (QS) auto-inducer, using S-Adenosyl Methionine (SAM) as primer. The current study demonstrates interactions among sulfate metabolism, nutrients removal and granulation, and confirms the feasibility of using the aerobic granular sludge process for sulfate-laden wastewaters treatment with low to medium sulfate content.

Does the combination of hyperkalemia and KATP activation determine excitation rate gradient and electrical failure in the globally ischemic fibrillating heart?

Ventricular fibrillation (VF) in the globally ischemic heart is characterized by a progressive electrical depression manifested as a decline in the VF excitation rate (VFR) and loss of excitability, which occur first in the subepicardium (Epi) and spread to the subendocardium (Endo). Early electrical failure is detrimental to successful defibrillation and resuscitation during cardiac arrest. Hyperkalemia and/or the activation of ATP-sensitive K(+) (KATP) channels have been implicated in electrical failure, but the role of these factors in ischemic VF is poorly understood. We determined the VFR-extracellular K(+) concentration ([K(+)]o) relationship in the Endo and Epi of the left ventricle during VF in globally ischemic hearts (Isch group) and normoxic hearts subjected to hyperkalemia (HighK group) or a combination of hyperkalemia and the KATP channel opener cromakalim (HighK-Crom group). In the Isch group, Endo and Epi values of [K(+)]o and VFR were compared in the early (0-6 min), middle (7-13 min), and late (14-20 min) phases of ischemic VF. A significant transmural gradient in VFR (Endo > Epi) was observed in all three phases, whereas a significant transmural gradient in [K(+)]o (Epi > Endo) occurred only in the late phase of ischemic VF. In the Isch group, the VFR decrease and inexcitability started to occur at much lower [K(+)]o than in the HighK group, especially in the Epi. Combining KATP activation with hyperkalemia only shifted the VFR-[K(+)]o curve upward (an effect opposite to real ischemia) without changing the [K(+)]o threshold for asystole. We conclude that hyperkalemia and/or KATP activation cannot adequately explain the heterogeneous electrical depression and electrical failure during ischemic VF.

Dystrophin is required for the normal function of the cardio-protective K(ATP) channel in cardiomyocytes

Duchenne and Becker muscular dystrophy patients often develop a cardiomyopathy for which the pathogenesis is still unknown. We have employed the murine animal model of Duchenne muscular dystrophy (mdx), which develops a cardiomyopathy that includes some characteristics of the human disease, to study the molecular basis of this pathology. Here we show that the mdx mouse heart has defects consistent with alteration in compounds that regulate energy homeostasis including a marked decrease in creatine-phosphate (PC). In addition, the mdx heart is more susceptible to anoxia than controls. Since the cardio-protective ATP sensitive potassium channel (K_ATP) complex and PC have been shown to interact we investigated whether deficits in PC levels correlate with other molecular events including K_ATP ion channel complex presence, its functionality and interaction with dystrophin. We found that this channel complex is present in the dystrophic cardiac cell membrane but its ability to sense a drop in the intracellular ATP concentration and consequently open is compromised by the absence of dystrophin. We further demonstrate that the creatine kinase muscle isoform (CKm) is displaced from the plasma membrane of the mdx cardiac cells. Considering that CKm is a determinant of K_ATP channel complex function we hypothesize that dystrophin acts as a scaffolding protein organizing the K_ATP channel complex and the enzymes necessary for its correct functioning. Therefore, the lack of proper functioning of the cardio-protective K_ATP system in the mdx cardiomyocytes may be part of the mechanism contributing to development of cardiac disease in dystrophic patients.

A single-cell model of phase-driven control of ventricular fibrillation frequency

The mechanisms controlling the rotation frequency of functional reentry in ventricular fibrillation (VF) are poorly understood. It has been previously shown that Ba2+ at concentrations up to 50 mumol/L slows the rotation frequency in the intact guinea pig (GP) heart, suggesting a role of the inward rectifier current (I(K1)) in the mechanism governing the VF response to Ba2+. Given that other biological (e.g., sinoatrial node) and artificial systems display phase-locking behavior, we hypothesized that the mechanism for controlling the rotation frequency of a rotor by I(K1) blockade is phase-driven, i.e., the phase shift between transmembrane current and voltage remains constant at varying levels of I(K1) blockade. We measured whole-cell admittance in isolated GP myocytes and in transfected human embryonic kidney (HEK) cells stably expressing Kir 2.1 and 2.3 channels. The admittance phase, i.e., the phase difference between current and voltage, was plotted versus the frequency in control conditions and at 10 or 50 micromol/L Ba2+ (in GP heart cells) or 1 mM Ba2+ (in HEK cells). The horizontal distance between plots was called the "frequency shift in a single cell" and analyzed. The frequency shift in a single cell was -14.14 +/- 5.71 Hz (n = 14) at 10 microM Ba2+ and -18.51 +/- 4.00 Hz (n = 10) at 50 microM Ba2+, p < 0.05. The values perfectly matched the Ba2+-induced reduction of VF frequency observed previously in GP heart. A similar relationship was found in the computer simulations. The phase of Ba2+-sensitive admittance in GP cells was -2.65 +/- 0.32 rad at 10 Hz and -2.79 +/- 0.26 rad at 30 Hz. In HEK cells, the phase of Ba2+-sensitive admittance was 3.09 +/- 0.03 rad at 10 Hz and 3.00 +/- 0.17 rad at 30 Hz. We have developed a biological single-cell model of rotation-frequency control. The results show that although rotation frequency changes as a result of I(K1) blockade, the phase difference between transmembrane current and transmembrane voltage remains constant, enabling us to quantitatively predict the change of VF frequency resulting from I(K1) blockade, based on single-cell measurement.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Kinetics of ATP-sensitive K+ channels in isolated rat hearts assessed by 87Rb NMR spectroscopy

An experimental model was developed to evaluate the effects of activators and inhibitors of K(ATP) channels on unidirectional K+ fluxes in the whole heart. Isolated rat hearts perfused in the Langendorff mode were equilibrated with Pi-free Krebs-Henseleit buffer (KH buffer) containing 0.94-2.14 mM RbCl and 3.76 mM KCl (20-36% of K+ substituted by Rb+). Rb+ efflux was initiated by removing Rb+ from the perfusate and 87Rb spectra were acquired continuously with a 1-2 min time resolution. In hearts with normal energetics, the efflux of Rb+ fit a monoexponential function, and the rate constant did not depend on intracellular [Rb+]. Agents depressing excitability and heart rate (HR), such as 0.6 mM lidocaine (Lido), 10 microM carbachol (carb) and 20 mM MgSO4, inhibited Rb+ efflux such that the rate constant, k (10(3)/min), decreased from 50+/-1.2 in the beating heart to 26+/-1, 40+/-1.1 and 19+/-1.2, respectively. In contrast, high [K+] (21 mM) did not affect the k value (50+/-4.5), independently of the presence or absence of bumetanide (Bum, 30 microM) and glibenclamide (Glib, 5 microM). Dinitrophenol (DNP, 0.2 mM) added in the presence of high [K+] + Bum increased k three-fold, to 160+/-5. This effect was associated with a significant decrease in phosphocreatine (PCr, <10% of initial) and ATP ( 15%) levels, and a 7-fold increase in the Pi level, assessed by 31P-NMR spectroscopy. Glib completely reversed the effect of DNP. Pinacidil (Pin, 20-80 microM) did not affect the k value either in beating control hearts or in the presence of Carb or KCl + Bum. Moreover, under conditions of moderate metabolic stress induced by 0.05 mM DNP (PCr, 35%; ATP, 65%), where half-maximal activation of K(ATP) channels occurred, Pin did not further activate Rb+ efflux. We conclude that:(1) heart rate-independent Rb+ efflux accounts for 40-80% of the total Rb+ efflux in beating (300 bpm) rat hearts;(2) DNP-activated Rb+ efflux is a good model for testing inhibitors of KATP channels in whole hearts; and (3) Pin is not an effective K(ATP) channel opener in the rat heart model.

Antiarrhythmic and electrophysiological effects of the novel KATP channel opener, rilmakalim, in rabbit cardiac cells

1. The effects of rilmakalim, a potassium channel opener, were studied on rabbit cardiac Purkinje, ventricular muscle and atrial fibers, with the use of conventional microelectrode techniques. 2. Rilmakalim (0.24-7.2 microM) shortened, in a concentration-dependent manner, the action potential duration (APD) in Purkinje and ventricular muscle without affecting other parameters of the action potential. Pinacidil (30-300 microM) also decreased the APD of Purkinje fibers. 3. Rilmakalim (2.4 microM) and cromakalim (100 microM) hyperpolarized and abolished abnormal automaticity of cardiac Purkinje fibers pretreated with barium (0.2-0.3 mM). Glibenclamide (5 microM) blocked the hyperpolarizing effect. 4. Stable early afterdepolarizations induced in Purkinje fibers by berberine (100 microM) were reversibly blocked by rilmakalim (2.4 microM), which also suppressed late afterdepolarizations induced in Purkinje fibers treated with ouabain (0.3-0.5 microM). 5. The rate of spontaneous discharges of the rabbit sinoatrial node was not affected by rilmakalim (7.2 microM) or by pinacidil (100 microM). Both agents were also unable to affect the APD of atrial muscle fibers. 6. In cardiac Purkinje fibers, tetraethylammonium (TEA; 20 mM) significantly reduced the effects of rilmakalim (2. 4 microM) on the APD. However, neither TEA nor glibenclamide (100 microM) reduced the shortening of the APD induced by dinitrophenol (30 microM) or by salicylate (1 mM).