Oximes: 'enzymatic' slow channel antagonists in canine cardiac purkinje fibers?

Modifications of mechanoelectric feedback induced by 2,3-butanedione monoxime and Blebbistatin in Langendorff-perfused rabbit hearts

AIM

Myocardial stretching is an arrhythmogenic factor. Optical techniques and mechanical uncouplers are used to study the mechanoelectric feedback. The aim of this study is to determine whether the mechanical uncouplers 2,3-butanedione monoxime and Blebbistatin hinder or modify the electrophysiological effects of acute mechanical stretch.

METHODS

The ventricular fibrillation (VF) modifications induced by acute mechanical stretch were studied in 27 Langendorff-perfused rabbit hearts using epicardial multiple electrodes and mapping techniques under control conditions (n = 9) and during the perfusion of 2,3-butanedione monoxime (15 mM) (n = 9) or Blebbistatin (10 μm) (n = 9).

RESULTS

In the control series, myocardial stretch increased the complexity of the activation maps and the dominant frequency (DF) of VF from 13.1 ± 2.0 Hz to 19.1 ± 3.1 Hz (P < 0.001, 46% increment). At baseline, the activation maps showed less complexity in both the 2,3-butanedione monoxime and Blebbistatin series, and the DF was lower in the 2,3-butanedione monoxime series (11.4 ± 1.2 Hz; P < 0.05). The accelerating effect of mechanical stretch was abolished under 2,3-butanedione monoxime (maximum DF = 11.7 ± 2.4 Hz, 5% increment, ns vs baseline, P < 0.0001 vs. control series) and reduced under Blebbistatin (maximum DF = 12.9 ± 0.7 Hz, 8% increment, P < 0.01 vs. baseline, P < 0.0001 vs. control series). The variations in complexity of the activation maps under stretch were not significant in the 2,3-butanedione monoxime series and were significantly attenuated under Blebbistatin.

CONCLUSION

The accelerating effect and increased complexity of myocardial activation during VF induced by acute mechanical stretch are abolished under the action of 2,3-butanedione monoxime and reduced under the action of Blebbistatin.

Effects of diacetyl monoxime and cytochalasin D on ventricular fibrillation in swine right ventricles

Whether or not the excitation-contraction (E-C) uncoupler diacetyl monoxime (DAM) and cytochalacin D (Cyto D) alter the ventricular fibrillation (VF) activation patterns is unclear. We recorded single cell action potentials and performed optical mapping in isolated perfused swine right ventricles (RV) at different concentrations of DAM and Cyto D. Increasing the concentration of DAM results in progressively shortened action potential duration (APD) measured to 90% repolarization, reduced the slope of the APD restitition curve, decreased Kolmogorov-Sinai entropy, and reduced the number of VF wave fronts. In all RVs, 15-20 mmol/l DAM converted VF to ventricular tachycardia (VT). The VF could be reinduced after the DAM was washed out. In comparison, Cyto D (10-40 micromol/l) has no effects on APD restitution curve or the dynamics of VF. The effects of DAM on VF are associated with a reduced number of wave fronts and dynamic complexities in VF. These results are compatible with the restitution hypothesis of VF and suggest that DAM may be unsuitable as an E-C uncoupler for optical mapping studies of VF in the swine RVs.

Inhibitory effect of 2,3-butanedione monoxime (BDM) on Na(+)/Ca(2+) exchange current in guinea-pig cardiac ventricular myocytes

1. The effect of 2,3-butanedione monoxime (BDM), a 'chemical phosphatase', on Na(+)/Ca(2+) exchange current (I(NCX)) was investigated using the whole-cell voltage-clamp technique in single guinea-pig cardiac ventricular myocytes and in CCL39 fibroblast cells expressing canine NCX1. 2. I(NCX) was identified as a current sensitive to KB-R7943, a relatively selective NCX inhibitor, at 140 mM Na(+) and 2 mM Ca(2+) in the external solution and 20 mM Na(+) and 433 nM free Ca(2+) in the pipette solution. 3. In guinea-pig ventricular cells, BDM inhibited I(NCX) in a concentration-dependent manner. The IC(50) value was 2.4 mM with a Hill coefficients of 1. The average time for 50% inhibition by 10 mM BDM was 124+/-31 s (n=5). 4. The effect of BDM was not affected by 1 microM okadaic acid in the pipette solution, indicating that the inhibition was not via activation of okadaic acid-sensitive protein phosphatases. 5. Intracellular trypsin treatment via the pipette solution significantly suppressed the inhibitory effect of BDM, implicating an intracellular site of action of BDM. 6. PAM (pralidoxime), another oxime compound, also inhibited I(NCX) in a manner similar to BDM. 7. Isoprenaline at 50 microM and phorbol 12-myristate 13-acetate (PMA) at 8 microM did not reverse the inhibition of I(NCX) by BDM. 8. BDM inhibited I(NCX) in CCL39 cells expressing NCX1 and in its mutant in which its three major phosphorylatable serine residues were replaced with alanines. 9. We conclude that BDM inhibits I(NCX) but the mechanism of inhibition is not by dephosphorylation of the Na(+)/Ca(2+) exchanger as a 'chemical phosphatase'.

Differential effects of cytochalasin D and 2,3 butanedione monoxime on isometric twitch force and transmembrane action potential in isolated ventricular muscle: implications for optical measurements of cardiac repolarization

INTRODUCTION

2,3-Butanedione monoxime (BDM) has been widely used to inhibit contraction during optical recordings of cardiac membrane voltage changes, even though it markedly abbreviates cardiac action potentials.

METHODS AND RESULTS

We compared the effects of BDM and of the F-actin disrupter cytochalasin D (cyto D) on isometric twitch force and transmembrane action potentials in isolated canine right ventricular trabeculae superfused with Tyrode's solution (2 mmol/L CaCl2, 37 degrees C) and stimulated at 0.5 Hz. BDM at 10 mmol/L and cyto D at 80 micromol/L were equally effective in reducing peak isometric force to 10%+/-3% (n = 6; mean+/-SEM) and 8%+/-1% (n = 8), respectively. Neither agent significantly altered resting tension. While 10 mmol/L BDM markedly shortened the action potential duration at 90% repolarization (APD90) from 198+/-7 msec to 146+/-9 msec (P < 0.001), 80 micromol/L cyto D had no significant effects on APD90 or on any other action potential parameter. The effects of BDM on peak isometric force and APD were completely reversible after 15 minutes of washout, whereas in the cyto D group contractile force continued to be reduced (13%+/-3%) and action potential characteristics did not show significant changes from control values after a 60-minute period of superfusion with cyto D-free Tyrode's solution.

CONCLUSION

We conclude that cyto D should be considered an alternative excitation-contraction uncoupler for optical mapping studies of cardiac repolarization.

Effects of 2,3-butanedione monoxime on atrial-atrioventricular nodal conduction in isolated rabbit heart

INTRODUCTION

2,3-Butanedione monoxime (BDM) has been found to reversibly block cardiac contraction, without blocking electrical conduction. This study characterizes the dose-dependent effects of BDM on the conduction through the atrioventricular node (AVN) of rabbit heart.

METHODS AND RESULTS

Thirteen isolated atrial-AVN preparations were used in control, during and after exposure to 5, 10, and 20 mM BDM. Anterograde and retrograde pacing protocols were used to obtain the Wenckebach cycle length, effective and functional refractory periods of the AVN, index of AVN conduction delay (the area under the AVN conduction curve), as well as index of intra-atrial conduction delay between the AVN inputs. Compared to control, 5 and 10 mM BDM produced either shortening or no effect on all of the above parameters except a slight (6% and 14%, respectively) increase in the intra-atrial delay. At 20 mM, BDM produced a further increase in the intra-atrial delay (up to 50%) as well as in the retrograde AVN conduction delay (up to 16%), while the characteristics of the anterograde conduction were still improved. The effects of perfusion with BDM on these parameters were reversible after washout.

CONCLUSIONS

Aside from its known effect as an electromechanical uncoupler, BDM reversibly altered some of the electrical responses of the AVN. Most of these alterations, however, did not impede but rather improved AVN conduction. Since a dose of 10 mM is sufficient to fully eliminate undesirable motion, BDM should be considered a safe and valuable tool in AVN studies in vitro requiring a mechanically quiescent preparation.

The Ca2+ sensitizer EMD 53998 antagonizes the effect of 2,3-butanedione monoxime on skinned cardiac muscle fibres

The effects of 2,3-butanedione monoxime (BDM) and 5-[1-(3,4-dimethoxybenzoyl)-1,2,3,4-tetrahydro-6-quinolyl]-6-me thy l-3,6-dihydro-2H-1,3,4-thiadiazin-2-one (EMD 53998) on cardiac muscle were studied in skinned muscle fibres from the right ventricle of the porcine heart. BDM decreases the Ca2+ sensitivity (pCa50 for 50% activation) and it exerts a dose-dependent inhibitory effect on force in troponin I (TnI)-depleted (unregulated) cardiac skinned muscle fibres (IC50 approximately 20 mM) thereby mimicking the effect of the TnI inhibitory peptide (cTnI 137-148, corresponding to the cardiac TnI inhibitory region) and that of inorganic phosphate (Pi). This inhibitory action can be antagonized by the calcium-sensitizing cardiotonic thiadiazinone derivative EMD 53998 that increases the IC50 to about 30 mM. In skinned fibres, BDM (10 mM) also increased the ratio of ATPase activity to isometric force (tension cost), whereas EMD 53998 (20 mu M) decreased it. We propose that BDM antagonizes EMD 53998 because both compounds affect the Pi release step of the crossbridge cycle in an antagonistic manner.

Characterization of an ATP-modulated large conductance Ca(2+)-activated K+ channel present in rat cortical neurones

1. Single channel current recordings were used to study the characteristics of a large conductance Ca(2+)-activated K+ (BKCa) channel present in neurones acutely dissociated from the rat motor cortex. Application of ATP to the intracellular surface of excised inside-out patches produced a large, concentration-dependent increase in BKCa channel activity. 2. This ATP-mediated activation was dependent upon the presence of Mg2+ in the intracellular bathing solution and was diminished by the phosphatases 2,3-butanedione monoxime (BDM) or alkaline phosphatase and by the protein kinase inhibitors staurosporine, H-7 and PKI. 3. ADP stimulated BKCa channel activity in a Mg(2+)-dependent manner, an action also inhibited by the concomitant application of PKI or BDM. The effect of ADP was reduced by application of hexokinase and glucose or by application of the adenylate kinase inhibitor Ap5A. 4. Of other nucleotides tested, only CTP consistently activated BKCa channel activity. 5. Using the cell-attached configuration, bath application of forskolin or dibutyryl cAMP stimulated BKCa channel activity. 6. It is concluded that BKCa channel activity in the rat motor cortex is subject to modulation by the activity of a closely associated kinase. The ability of cAMP activators to stimulate BKCa channel activity in the intact cell suggests that this system may be of physiological importance.

The effect of 2,3-butanedione 2-monoxime (BDM) on ventricular trabeculae from the avian heart

2,3-butanedione 2-monoxime (BDM, 3-30 mM) decreased twitch force of intact ventricular trabeculae isolated from 19-day embryonic chick hearts in a dose-dependent manner. The responses to BDM were rapid and reversible. In an attempt to determine the cellular basis for the inhibitory effect of BDM, experiments were carried out on skinned muscle fibres and isolated myocytes. In trabeculae skinned with Triton X-100, BDM depressed maximum calcium activated force (Fmax) with an IC50 of 14 mM. At 3 mM BDM, the proportional decrease in twitch force in intact tissue was similar to that of Fmax in skinned tissue. At higher BDM concentrations (10 and 30 mM), however, the proportional decrease in twitch force was greater than that of Fmax. BDM (up to 10 mM) had no effect on the normalized force-pCa relationship. In saponin-skinned preparations, BDM (3 and 30 mM) released calcium from the fully loaded sarcoplasmic reticulum to a slightly greater extent in the absence of calcium (pCa 8.5) than in the presence of a fixed level of free calcium (pCa 5.5). Whole cell patch clamping of freshly isolated chick myocytes demonstrated that BDM caused a dose-dependent decrease in the T- and L-type calcium current. Therefore, at low BDM concentrations (3 mM), the decrease in twitch force can be ascribed predominantly to depression of the contractile apparatus while, at higher concentrations of BDM, there is an additional inhibitory effect of BDM on excitation-contraction coupling.

Multiple effects of 2,3-butanedione monoxime

2,3-Butanedione monoxime, also known as diacetyl monoxime, is a nucleophilic agent which dephosphorylates acetylcholinesterase poisoned with organophosphates. This "chemical phosphatase" activity stimulated studies of the effect of 2,3-butanedione monoxime on phosphorylation-dependent cellular processes. As a result of these studies, we know that the drug affects a number of mechanisms including muscle contraction, ionic current flow and synaptic transmission. Furthermore, it may be used as a component of cardioplegic solutions since it protects cardiac tissue exposed to certain ischaemic conditions. While this MiniReview reveals the diversity of its cellular actions, there continues to be unresolved questions regarding its molecular mechanism.

Effect of 2,3-butanedione monoxime on smooth-muscle contraction of guinea-pig portal vein

Effects of 2,3-butanedione-2-monoxime (BDM) on the contraction of intact and skinned smooth muscles from guinea-pig portal vein were examined. In intact preparations loaded with fura-2, 5-10 mM BDM markedly suppressed Ca2+ transients and force developments induced by 154 mM potassium and by phenylephrine (0.1 mM). On the other hand, in Ca(2+)-free depolarizing solution, BDM did not suppress phenylephrine (0.1 mM)-induced Ca2+ transient and force development. In skinned preparations obtained with Staphylococcus aureus alpha-toxin treatment, BDM did not markedly affect active force development. The above results indicate that BDM suppresses contraction of the portal vein mainly by the inhibition of voltage-dependent cytosolic Ca2+ transients. An additional result suggests that BDM suppresses the force-enhancing effect of alpha 1-adrenergic agents on the contractile elements.

The effect of oximes on the dihydropyridine-sensitive Ca current of isolated guinea-pig ventricular myocytes

Exposure of isolated guinea-pig ventricular myocytes to the uncharged oximes 2,3 butanedionemonoxine (BDM) and norPAM (but not by the charged PAM) results in a dose-dependent reduction of the duration of the action potential. The nifedipine-sensitive Ca current is fully inhibited by BDM (IC(50)5.8 +/- 0.4 mM) and nor PAM but is little affected by PAM. This inhibition is unaltered by the presence of BAY K 8644 but is antagonized by isoprenaline. The effect of isoprenaline is more pronounced when the solution in the patch pipette contains the non-hydrolysable analogue of adenosine 5'-triphosphate, ATP gamma S (the IC50 is increased to 44.0 +/- 5.2 mM). A hastening of the inactivation of the L-type Ca current persists when either 10 mM 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N-tetraacetic acid (BAPTA) or 3 mM ATP gamma S is present in the pipette solution or when BAY K 8644 or isoprenaline are present in the bathing fluid. These results suggest that the inhibition of the Ca current is due to the phosphatase-like activity of the oximes but differs in some respects from previous work where a reduced level of phosphorylation is achieved by the introduction of protein kinase inhibitors or protein phosphatases into the sarcoplasm in guinea-pig myocytes. These differences could be explained if Ca channel availability is regulated by at least two sites of cAMP-dependent phosphorylation with oximes able to rapidly dephosphorylate both sites, while one of these sites is not readily dephosphorylated by the endogenous phosphatases.

Novel suppression of an L-type calcium channel in neurones of murine dorsal root ganglia by 2,3-butanedione monoxime

1. Voltage-activated currents through calcium channels in primary cultures of murine dorsal root ganglion cells (DRG) were studied with the whole-cell and cell-attached patch recording techniques. 2. The chemical phosphatase 2,3-butanedione monoxime (BDM) reversibly reduced the amplitude of L-type calcium current (ICa) in a dose-dependent manner; at a concentration of 20 mM, BDM caused a 47% suppression of ICa. 3. Application of 10 mM-8-bromo-cyclic AMP or 50 microM-isoprenaline onto DRG treated with BDM completely restored ICa to the pre-BDM level. 4. In striking contrast, bath application of Bay K 8644 (0.5-5 microM) had no effect on the BDM-suppressed ICa. As expected, Bay K 8644 alone caused a two- to threefold increase of the maximal ICa and shifted its I-V relationship to the left. Interestingly, if a cell was first exposed to Bay K 8644 further treatment with 20 mM-BDM resulted in 100% suppression of ICa. This suggests that Bay K 8644 changes the conformation of the calcium channel to one which is more sensitive or more accessible to the action of the phosphatase. 5. Pre-treatment of DRG with an activator of protein kinase C, 12-O-tetradecanoyl-phorbol-13-acetate, did not antagonize BDM's effect on ICa. 6. The depressant action of BDM on ICa was distinct from that of nifedipine in that it did not exhibit use dependence. 7. When single calcium channel currents were recorded in cell-attached patches (barium as the charge carrier), bath application of BDM reduced the percentage of time that the channel spent in the open state. 8. Superfusion with 8-bromo-cyclic AMP restored the ensemble macroscopic 'ICa' to the pre-BDM amplitude. This was due to a dramatic enhancement of the frequency of channel openings. 9. We suggest that BDM acts through the cytoplasm to alter cyclic AMP-dependent protein kinase modulation of neuronal L-type calcium channels. The brief, high-frequency openings which 8-bromo-cyclic AMP activates in the presence of BDM may reflect a rapid phosphorylation-dephosphorylation sequence which controls channel gating.

The action of 2,3-butane-dionemonoxime on the inotropic state in guinea-pig myocardium

Isolated papillary muscles from guinea-pig right ventricles were used (temperature 33 degrees C, stimulation frequency 0.5 Hz). Isometric twitch and action potentials were recorded. Upon addition of 2,3-butane-dionemonoxime (BDM) (2 mM) the peak twitch force was reduced from 4.17 +/- 0.4 mN/mm2 to 1.68 +/- 0.3 mN/mm2 (n = 9, P less than 0.001). The time course of the isometric twitch was slightly altered. Time to peak tension (TPT) was reduced by 12.0 +/- 3% (n = 9, P less than 0.001) whereas time to half relaxation (THR) was left unaffected. The rate of rise of force was reduced by 35 +/- 3.2 mN/mm2s i.e. 46 +/- 3%. The action potential duration and amplitude was not significantly changed by the drug. The shape of the curve relating peak twitch force of an extra beat to the preceding test interval, i.e. mechanical restitution, was affected by 2 mM 2,3-butane-dionemonoxime. The curve reached its maximum faster after addition of the drug. Maximum postextrasystolic potentiation (force in response to the prepreceding test interval) was 3.2 +/- 0.4 mN/mm2 in 2 mM 2,3-butane-dionemonoxine and 7.6 +/- 0.7 mN/mm2 in control (n = 6). However the percentage potentiation was very similar in control (82%) and in presence of 2,3-butane-dionemonoxime (91%). Peak twitch force in relation to peak force of the preceding potentiated contraction during decay of postextrasystolic potentiation was analysed. There was a linear relation between the variables, the slope being 0.34 +/- 0.04 in control and 0.30 +/- 0.02 in 2,3-butane-dionemonoxime. This suggests that the drug is without an action on the fraction of calcium recirculating within the cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 2,3-butanedione monoxime on whole-cell Ca2+ channel currents in single cells of the guinea-pig taenia caeci

1. The inhibitory actions of cadmium (Cd2+), nifedipine and 2,3-butanedione monoxime (BDM) on whole-cell Ca2+ channel currents in single cells of the guinea-pig taenia caeci were investigated using a single-electrode whole-cell voltage-clamp technique. 2. Calcium channel currents were isolated using pipette solutions containing Cs+, tetraethylammonium and ATP (3 mM). Ca2+ or Ba2+ (7.5 mM) in the bathing solution acted as the charge carrier during inward current flow. Ca2+ channel currents in 7.5 mM-Ba2+ (IBa) were recorded at potentials positive to -40 mV, were maximal near 0 mV and reversed near +60 mV. Ca2+ channel activation showed a sigmoidal relationship with potential, which was half-maximal at -13 mV. 3. Both the inward and outward flow of current was depressed and eventually blocked by 0.3-100 microM-Cd2+, 0.1-10 microM-nifedipine and 2-20 mM-BDM. Half-maximal blockade of IBa at 0 mV was achieved with approximately 3 microM-Cd2+, 1 microM-nifedipine and 10 microM-BDM. Steady-state activation curves were not affected by Cd2+ or BDM, but were shifted in the hyperpolarizing direction by nifedipine at concentrations > 1 microM. 4. Calcium channel currents in single cells and K+ contractures in intact strips were both blocked in a voltage-dependent manner. Steady-state inactivation curves (f infinity (V)) for IBa were shifted 20 mV in the hyperpolarizing direction by 0.3 microM-nifedipine and 4 mV by 10 mM-BDM. From these shifts a dissociation binding constant to inactivated Ca2+ channels for nifedipine was estimated as 78 nM, and for BDM, 5 mM. 5. At 10 microM Cd2+ produced a 43 +/- 6% (n = 3) block of the inward current at 0 mV when Ca2+ (7.5 mM) was the charge carrier (ICa), compared with the 36 +/- 3% block of IBa induced by 1 microM-Cd2+, consistent with the suggestion that Ca2+, Ba2+ and Cd2+ compete for the same binding site. In contrast, nifedipine (1 microM) and BDM (10 mM) blocked ICa more effectively than IBa. 6. Bay K 8644 (1.0 microM) increased Ca2+ channel currents two- to fourfold at all potentials due to a shift, of approximately 10 mV in the negative direction, of their activation curve and an equal shift in the positive direction of their inactivation curve. BDM (5-10 mM) could antagonize the action of Bay K 8644, shifting both curves back towards their control.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of diacetyl monoxime (DAM) on slow and fast action potentials of young and old embryonic chick hearts and rabbit hearts

The effects of the negative inotropic agent, diacetyl monoxime (DAM), were studied on cardiac action potentials in young (3 day old) and old (13-18 day old) embryonic chick hearts and in rabbit papillary muscles. DAM (5-10 mM) decreased the action potential (AP) amplitude, the maximum rate of rise (Vmax), and AP duration of slow and fast APs in both young and old embryonic chick hearts. Concomitantly with these changes, the spontaneous beating rate was increased in young hearts. At 20 mM DAM, the maximum diastolic potential (MDP) was decreased. In fast APs of old embryonic chick hearts, the most prominent effect of DAM was a shortening of AP duration, the effects on AP amplitude, Vmax, and resting potential being less. In rabbit papillary muscles, DAM had little effect on the slow AP parameters except for shortening of AP duration. The onset of the DAM effects was rapid, the peak effect being attained by 5 min in all preparations. Reversal of the effects of DAM upon washout were fast, complete recovery being attained within 10 min. We conclude that DAM suppressed the slow Na+ current present in young chick embryonic hearts. The effects of DAM, up to 10 mM, on Vmax of the fast Na+-dependent APs in old embryonic chick hearts was less, and there was no effect on rabbit hearts. DAM suppressed the slow Ca2+-dependent APs in old embryonic chick hearts but not in rabbit papillary muscles, suggesting that the effect of DAM on the Ca2+ slow channels depends on the species.(ABSTRACT TRUNCATED AT 250 WORDS)