Inhibition of cloned human L-type cardiac calcium channels by 2,3-butanedione monoxime does not require PKA-dependent phosphorylation sites

Cardiac metabolic effects of KNa1.2 channel deletion and evidence for its mitochondrial localization

Controversy surrounds the molecular identity of mitochondrial K+ channels that are important for protection against cardiac ischemia-reperfusion injury. Although KNa1.2 (sodium-activated potassium channel encoded by Kcn2) is necessary for cardioprotection by volatile anesthetics, electrophysiological evidence for a channel of this type in mitochondria is lacking. The endogenous physiological role of a potential mito-KNa1.2 channel is also unclear. In this study, single channel patch-clamp of 27 independent cardiac mitochondrial inner membrane (mitoplast) preparations from wild-type (WT) mice yielded 6 channels matching the known ion sensitivity, ion selectivity, pharmacology, and conductance properties of KNa1.2 (slope conductance, 138 ± 1 pS). However, similar experiments on 40 preparations from Kcnt2-/- mice yielded no such channels. The KNa opener bithionol uncoupled respiration in WT but not Kcnt2-/- cardiomyocytes. Furthermore, when oxidizing only fat as substrate, Kcnt2-/- cardiomyocytes and hearts were less responsive to increases in energetic demand. Kcnt2-/- mice also had elevated body fat, but no baseline differences in the cardiac metabolome. These data support the existence of a cardiac mitochondrial KNa1.2 channel, and a role for cardiac KNa1.2 in regulating metabolism under conditions of high energetic demand.-Smith, C. O., Wang, Y. T., Nadtochiy, S. M., Miller, J. H., Jonas, E. A., Dirksen, R. T., Nehrke, K., Brookes, P. S. Cardiac metabolic effects of KNa1.2 channel deletion and evidence for its mitochondrial localization.

Inhibitors of myosin, but not actin, alter transport through Tradescantia plasmodesmata

Actin and myosin are components of plasmodesmata, the cytoplasmic channels between plant cells, but their role in regulating these channels is unclear. Here, we investigated the role of myosin in regulating plasmodesmata in a well-studied, simple system comprising single filaments of cells which form stamen hairs in Tradescantia virginiana flowers. Effects of myosin inhibitors were assessed by analysing cell-to-cell movement of fluorescent tracers microinjected into treated cells. Incubation in the myosin inhibitor, 2,3-butanedione monoxime (BDM) or injection of anti-myosin antibodies increased cell-cell transport of fluorescent dextrans, while treatment with the myosin inhibitor N-ethylmaleimide (NEM) decreased cell-cell transport. Pretreatment with the callose synthesis inhibitor, deoxy-D: -glucose (DDG), enhanced transport induced by BDM treatment or injection of myosin antibodies but did not relieve NEM-induced reduction in transport. In contrast to the myosin inhibitors, cell-to-cell transport was unaffected by treatment with the actin polymerisation inhibitor, latrunculin B, after controlling for callose synthesis with DDG. Transport was increased following azide treatment, and reduced after injection of ATP, as in previous studies. We propose that myosin detachment from actin, induced by BDM, opens T. virginiana plasmodesmata whereas the firm attachment of myosin to actin, promoted by NEM, closes them.

Reduced effects of BAY K 8644 on L-type Ca2+ current in failing human cardiac myocytes are related to abnormal adrenergic regulation

Abnormal L-type Ca(2+) channel (LTCC, also named Cav1.2) density and regulation are important contributors to depressed contractility in failing hearts. The LTCC agonist BAY K 8644 (BAY K) has reduced inotropic effects on failing myocardium. We hypothesized that BAY K effects on the LTCC current (I(CaL)) in failing myocytes would be reduced because of increased basal activity. Since support of the failing heart with a left ventricular assist device (LVAD) improves contractility and adrenergic responses, we further hypothesized that BAY K effects on I(CaL) would be restored in LVAD-supported failing hearts. We tested our hypotheses in human ventricular myocytes (HVMs) isolated from nonfailing (NF), failing (F), and LVAD-supported failing hearts. We found that 1) BAY K had smaller effects on I(CaL) in F HVMs compared with NF HVMs; 2) BAY K had diminished effects on I(CaL) in NF HVM pretreated with isoproterenol (Iso) or dibutyryl cyclic AMP (DBcAMP); 3) BAY K effects on I(CaL) in F HVMs pretreated with acetylcholine (ACh) were normalized; 4) Iso had no effect on NF HVMs pretreated with BAY K; 5) BAY K effects on I(CaL) in LVAD HVMs were similar to those in NF HVMs; 6) BAY K effects were reduced in LVAD HVMs pretreated with Iso or DBcAMP; 7) Iso had no effect on I(CaL) in LVAD HVMs pretreated with BAY K. Collectively, these results suggest that the decreased BAY K effects on LTCC in F HVMs are caused by increased basal channel activity, which should contribute to abnormal contractility reserve.

Action of 2,3-butanedione monoxime on capacitance and electromotility of guinea-pig cochlear outer hair cells

1. Whole-cell patch-clamp recordings were obtained from isolated cochlear outer hair cells (OHCs) while applying 2,3-butanedione monoxime (BDM) by pressure. BDM (5 mM) shifted the range of voltage sensitivity of membrane capacitance and cell length in the hyperpolarised direction by -49.6 +/- 4.0 mV (n = 12; mean +/- S.E.M.), without appreciable effects on membrane conductance. The shift was completely reversible and dose dependent, with a Hill coefficient of 1.8 /- 0.4 and a half-maximal dose of 3.0 +/- 0.8 mM (values +/- S.D). 2. The shift of the capacitance curve was also reproducible in cells whose natural turgor had been removed. BDM had no detectable effect on the capacitance of Deiters' cells, a non-sensory cell type of the organ of Corti. 3. The effect of BDM on membrane capacitance was faster than that of salicylate. At similar saturating concentrations (20 mM), the time constant of the capacitance changes was 1.8 +/- 0.3 s (n = 3) for salicylate and 0.75 +/- 0.06 s (n = 3) for BDM. The recovery periods were 13 +/- 1 s and 1.7 +/- 0.4 s, respectively (means +/- S.E.M.). 4. The effect of BDM, a known inorganic phosphatase, was compared to the effects of okadaic acid, trifluoperazine and W-7, which are commonly used in studies of protein phosphorylation. Incubation of OHCs with okadaic acid (1 microM, 30-60 min) shifted the voltage sensitivity of the membrane capacitance in the hyperpolarised direction. Incubation with trifluoperazine (30 microM) and W-7 (150 microM) shifted it in the opposite, depolarised direction. BDM induced hyperpolarising shifts even in the presence of W-7. 5. Simultaneous measurement of membrane capacitance and intracellular free Ca2+ concentration ([Ca2+]i) showed that BDM action on OHC voltage-dependent capacitance and electromotility is not mediated by changes of [Ca2+]i. 6. Our results suggest that: (a) the effects of BDM are unrelated to its inorganic phosphatase properties, cell turgor conditions or Ca2+ release from intracellular stores; and (b) BDM may target directly the voltage sensor of the OHC membrane motor protein.

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

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

2,3-butanedione monoxime unmasks Ca(2+)-induced NADH formation and inhibits electron transport in rat hearts

We used 2,3-butanedione monoxime (BDM) to suppress work by the perfused rat heart and to investigate the effects of calcium on NADH production and tissue energetics. Hearts were perfused with buffer containing BDM and elevated perfusate calcium to maintain the rates of cardiac work and oxygen consumption at levels similar to those of control perfused hearts. BDM plus calcium hearts displayed higher levels of NADH surface fluorescence, indicating calcium activation of mitochondrial dehydrogenases. These hearts, however, displayed 20% lower phosphocreatine levels. BDM suppressed the rates of state 3 respiration of isolated mitochondria. Uncoupled respiration was suppressed to a lesser degree, and the state 4 respiration rates were not affected. Double-inhibitor experiments with liver mitochondria using BDM and carboxyatractyloside (CAT) were used to identify the site of inhibition. BDM at low levels (0-5 mM) suppressed respiration. In the presence of CAT at levels that inhibit respiration by 60%, low levels of BDM were without effect. Because these effects were not additive, BDM does not inhibit adenine nucleotide transport. This was supported by an assay of adenine nucleotide transport in liver mitochondria. BDM did not inhibit ATP hydrolysis by submitochondrial particles but strongly suppressed reversed electron transport from succinate to NAD(+). Oxidation of NADH by submitochondrial particles was inhibited by BDM but oxidation of succinate was not. We conclude that BDM inhibits electron transport at site 1.

Effects of serine/threonine protein phosphatases on ion channels in excitable membranes

This review deals with the influence of serine/threonine-specific protein phosphatases on the function of ion channels in the plasma membrane of excitable tissues. Particular focus is given to developments of the past decade. Most of the electrophysiological experiments have been performed with protein phosphatase inhibitors. Therefore, a synopsis is required incorporating issues from biochemistry, pharmacology, and electrophysiology. First, we summarize the structural and biochemical properties of protein phosphatase (types 1, 2A, 2B, 2C, and 3-7) catalytic subunits and their regulatory subunits. Then the available pharmacological tools (protein inhibitors, nonprotein inhibitors, and activators) are introduced. The use of these inhibitors is discussed based on their biochemical selectivity and a number of methodological caveats. The next section reviews the effects of these tools on various classes of ion channels (i.e., voltage-gated Ca(2+) and Na(+) channels, various K(+) channels, ligand-gated channels, and anion channels). We delineate in which cases a direct interaction between a protein phosphatase and a given channel has been proven and where a more complex regulation is likely involved. Finally, we present ideas for future research and possible pathophysiological implications.

Effects of the myosin inhibitor 2,3-butanedione monoxime on the physiology of fission yeast

F-actin and associated myosins are thought to take part in a wide range of cellular processes, like motility and contraction, polarized growth, and secretion. The reagent 2,3-butanedione monoxime (BDM) is a well characterized inhibitor of the contraction of vertebrate muscle that reversibly affects myosin function and influences the intracellular concentration of Ca2+. Here we describe the influence of BDM on growth and division of the fission yeast Schizosaccharomyces pombe. At concentrations from 1-30 mM, BDM gradually inhibited formation and growth of S. pombe colonies on agar plates, with a lethal effect at > or = 15 mM. In strains of S. pombe that were blocked by elevated temperature from entry into mitosis, drug treatment reversibly decreased microtubule-independent tip growth and septation, with an IC50 value around 12 mM; nuclear division, on the other hand, was essentially unaffected by up to 15 mM BDM. At 30 mM BDM the secretion of invertase, which required both F-actin and microtubules, was decreased to the same extent as that seen when cytochalasin D was used to disrupt F-actin. However, the actin cytoskeleton was insensitive to up to 10 mM BDM, while the actin patches lost their polar distribution at 20-30 mM BDM. Cells treated with 5-20 mM BDM for 3 hours and then high pressure frozen did not show an accumulation of secretory vesicles. However, 10 mM BDM treatment disorganized the fungal cell wall, resulting in some unusually thick parts lying next to regions were the wall was almost absent. These defects could be rescued by incubating the cells in inhibitors of glucanases. Osmolytic stabilization with sorbitol rescued the effect of 15 mM BDM on colony survival, indicating that the secretion of wall components and/or wall-modifying enzymes may be the principal reason for cell death caused by BDM. Our results are consistent with the hypothesis that BDM influences actin-dependent processes in fission yeast and that actomyosin-dependent motility contributes to the secretory process of tip growth.

Cytochalasin D as excitation-contraction uncoupler for optically mapping action potentials in wedges of ventricular myocardium

INTRODUCTION

Cytochalasin D in tissue bath superfusate inhibits the contraction of isolated thin trabeculae from canine right ventricle without affecting the intracellular action potential recorded with glass microelectrode. The purpose of this study was to test whether cytochalasin D could also be used to immobilize perfused wedges of ventricular muscle without affecting the action potential duration or propagation, and also to determine the optimal concentration and time duration of drug in the perfusate.

METHODS AND RESULTS

Using a membrane potential sensitive dye, di-4-ANEPPS, and a high-resolution photodiode optical mapping system at a rate of 1,000 frames/sec, we recorded action potentials on the transmural surface of arterially perfused wedges of muscle from the canine left ventricular free wall. We also recorded arterial pulse pressure as a surrogate for tissue contraction. Cytochalasin D at > or = 20 micromol/L in the perfusate for > or = 6 minutes reduced the arterial pulse pressure to approximately one tenth of its initial value and significantly reduced or eliminated motion artifacts in the action potentials. A sustained concentration of 10 micromol/L cytochalasin D in the perfusate prevented contraction from recurring after the tissue was immobilized with an initial concentration of 25 micromol/L. Cytochalasin D had little effect on the action potential duration and on its transmural gradient, and did not slow the transmural velocity of excitation propagation.

CONCLUSION

Cytochalasin D can be used to uncouple excitation and contraction in perfused canine cardiac muscle for the fluorescent-optical mapping of action potentials without affecting action potential duration or slowing transmural propagation.

Effects of 2,3-butanedione monoxime (BDM) on calcium channels expressed in Xenopus oocytes

1. We examine the actions of a chemical phosphatase, 2,3-butanedione monoxime (BDM), on endogenous and expressed Ca2+ channel currents in Xenopus oocytes. In previous studies on L-type Ca2+ channel currents in cardiomyocytes and dorsal root ganglia, the inhibitory effects of BDM were attenuated by activation of protein kinase A. 2. Ba2+ currents (IBa) through a human wild-type L-type Ca2+ channel complex (i.e. halpha1C, alpha2-deltaa and hbeta1b) are inhibited by BDM with an IC50 of 16 mM, with 10 mM producing a 36.1 +/- 2.2 % inhibition. IBa through endogenous oocyte N-type Ca2+ channels, upregulated by exogenous alpha2-deltaa and hbeta1b subunits, are inhibited to a similar degree by BDM. 3. To examine whether the action of BDM is dependent on PKA-dependent phosphorylation, a clone of halpha1C deficient in all five serine PKA consensus sites (halpha1C-SA5) was co-expressed with alpha2-deltaa and the human cardiac hbeta3 subunit, which naturally lacks PKA consensus sites. This complex exhibited a sensitivity to BDM that was similar to the wild-type complex, with 10 mM BDM producing 31.6 +/- 1.5 % inhibition. 4. As limited proteolysis upregulates Ca2+ channels in cardiomyocytes and renders them less sensitive to BDM, experiments were performed with a carboxyl terminus deletion mutant, halpha1C-Delta1633. IBa through this subunit showed a sensitivity to BDM that was similar to the wild-type complex, with 10 mM BDM producing 31.3 +/- 1.4 % inhibition. However, co-expression with alpha2-deltaa and hbeta3 subunits reduced potency, and is reflected by an increased IC50 of 22.7 mM. 5. The actions of BDM were examined on a rat brain rbA-1 Ca2+ channel clone, alpha1A, co-expressed with alpha2-deltab and beta1b subunit homologues from rat brain. BDM inhibited the current through this channel complex to a similar degree to that seen for cardiac wild-type channels, with 10 mM BDM causing a 33.1 +/- 3.5 % inhibition. 6. The effects of BDM were compared at two holding potentials, -80 and -30 mV, using the halpha1C-Delta1633, alpha2-deltaa and hbeta3 subunit combination. At -30 mV BDM is more potent with 10 mM BDM reducing IBa by 39.8 +/- 2.7 %, compared with 20.8 +/- 2.2 % at -80 mV. 7. The data suggest that BDM may not exert its inhibitory action by means of a chemical phosphatase effect, but by channel block. The similar potency observed between alpha1C, alpha1A and endogenous (N-type) channels may help point towards a possible site of action; differences with the carboxyl deletion mutant may help further to define a locus of interaction.