The effects of 2,3-butanedione monoxime on initial heat, tension, and aequorin light output of ferret papillary muscles

Isolation of Atrial and Ventricular Cardiomyocytes for In Vitro Studies

High quality cardiomyocyte isolation is of critical importance for successful studies of myocardial function at the cellular and molecular level. Although previous work has established isolation procedures for various species, it still remains challenging to produce consistently a high yield of viable and healthy cardiomyocytes. The basis for the most successful and reproducible isolation of cardiomyocytes from intact hearts is the Langendorff retrograde perfusion technique. Here, we will illustrate in detail all practical aspects of the enzyme-based Langendorff isolation of rat atrial and ventricular cardiomyocytes. This includes a series of obligatory steps starting from quick aortic cannulation to rinse the heart from blood, short perfusion of the heart with Ca²⁺-free solution to dissociate cells at the level of intercalated discs, followed by longer perfusion with low Ca²⁺-containing enzyme solution in order to disrupt the extracellular matrix network, extraction of the released cardiomyocytes and gentle Ca²⁺ reintroduction to allow cells to return gradually to normal cytosolic Ca²⁺ levels. The average yield of intact viable ventricular myocytes that can be achieved with our protocol is ≈70% (range ≈50-90%). For atrial myocytes, in general, it is slightly (≈10%) lower than for ventricular myocytes. The yield depends on the age of the rat and the degree of cardiac remodeling such that digestion of older and more remodeled hearts (more fibrosis) usually results in lower yields. Isolated atrial and ventricular cardiomyocytes may be employed for studies of cardiomyocyte function (e.g., shortening/contraction, intracellular [Ca²⁺] transients) as well as for biochemical and molecular biological studies (e.g., immunoblotting, PCR).

Hypoxia increases the release of salmon cardiac peptide (sCP) from the heart of rainbow trout (Oncorhynchus mykiss) under constant mechanical load in vitro

Our aim was to study the effects of hypoxia on the release of salmon cardiac peptide (sCP) from an isolated heart ventricle of trout during a constant mechanical load. Trout heart ventricles were studied in vitro. The ventricle was placed in an organ bath at 12 °C in which a constant mechanical load could be imposed on the ventricle while buffer solution was circulating. Ventricles were field-stimulated with a supramaximal voltage pulse at a rate of about 0.3 s⁻¹. Samples of 1 ml were collected at an interval of 10 min for 200 min from the organ bath and assessed with a radioimmunoassay for sCP. After a control period of 20 min, ventricles were exposed to hypoxia produced with N₂ gassing (n = 9) or to hypoxia with 20 mM BDM, a nonselective myosin ATPase inhibitor locking cross-bridges in a pre-power-stroke state inhibiting force production with normal electrical activity (n = 10). In this model and setup, hypoxia stimulated the release of sCP, but the interindividual variation in the response was large. At the end of hypoxia exposure, the concentration of sCP in the organ bath was about sixfold higher than at the start of the exposure (P < 0.05, one-way ANOVA for repeated measurements, followed by Dunnett's multiple comparison test). When BDM was introduced into the bath, the ventricle still secreted sCP but the hypoxic response was smaller than in the experiments without BDM. In the trout heart ventricle, there is a hypoxia-sensitive component in the release mechanism of sCP which is independent of contraction.

The mechanical uncoupler blebbistatin is associated with significant electrophysiological effects in the isolated rabbit heart

Blebbistatin (BS) is a recently discovered inhibitor of the myosin II isoform and has been adopted as the mechanical uncoupler of choice for optical mapping, because previous studies suggest that BS has no significant cardiac electrophysiological effects in a number of species. The aim of this study was to determine whether BS affects cardiac electrophysiology in isolated New Zealand White rabbit hearts. Langendorff-perfused hearts (n= 39) in constant-flow mode had left ventricular monophasic action potential duration (MAPD) measured at apical and basal regions during constant pacing (300 ms cycle length). Standard action potential duration restitution was obtained using the single extrastimulus method with measurement of the maximal restitution slope. Ventricular fibrillation threshold was measured as the minimal current inducing sustained ventricular fibrillation with burst pacing (30 stimuli, at 30 ms intervals). Optical action potentials were recorded using the voltage-sensitive dye di-4-ANEPPS. Measurements were taken at baseline and after 60 min perfusion with BS (5 μm). Blebbistatin significantly prolonged left ventricular apical (mean ± SEM; from 129.9 ± 2.9 to 170.7 ± 4.1 ms, P < 0.001, n= 8) and basal MAPD (from 135.0 ± 2.3 to 163.3 ± 5.6 ms, P < 0.001) and effective refractory period (from 141.3 ± 4.8 to 175.6 ± 3.7 ms, P < 0.001) whilst increasing the maximal slope of restitution (apex, from 0.79 ± 0.09 to 1.57 ± 0.16, P < 0.001; and base, from 0.71 ± 0.06 to 1.44 ± 0.24, P < 0.001) and ventricular fibrillation threshold (from 5.3 ± 1.1 to 17.0 ± 2.9 mA, P < 0.001). In other hearts, blebbistatin significantly prolonged optically recorded action potentials (from 136.5 ± 6.3 to 173.0 ± 7.9 ms, P < 0.05, n= 4). In control experiments, the increase of MAPD with blebbistatin was present whether the hearts were perfused in constant-pressure mode (n= 5) or in unloaded conditions (n= 5). These data show that blebbistatin significantly affects cardiac electrophysiology. Its use in optical mapping studies should be treated with caution.

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.

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.

In the RyR2(R4496C) mouse model of CPVT, β-adrenergic stimulation induces Ca waves by increasing SR Ca content and not by decreasing the threshold for Ca waves

RATIONALE

mutations of the ryanodine receptor (RyR) cause catecholaminergic polymorphic ventricular tachycardia (CPVT). These mutations predispose to the generation of Ca waves and delayed afterdepolarizations during adrenergic stimulation. Ca waves occur when either sarcoplasmic reticulum (SR) Ca content is elevated above a threshold or the threshold is decreased. Which of these occurs in cardiac myocytes expressing CPVT mutations is unknown.

OBJECTIVE

we tested whether the threshold SR Ca content is different between control and CPVT and how it relates to SR Ca content during β-adrenergic stimulation.

METHODS AND RESULTS

ventricular myocytes from the RyR2 R4496C(+/-) mouse model of CPVT and wild-type (WT) controls were voltage-clamped; diastolic SR Ca content was measured and compared with the Ca wave threshold. The results showed the following. (1) In 1 mmol/L [Ca(2+)](o), β-adrenergic stimulation with isoproterenol (1μmol/L) caused Ca waves only in R4496C. (2) SR Ca content and Ca wave threshold in R4496C were lower than those in WT. (3) β-Adrenergic stimulation increased SR Ca content by a similar amount in both R4496C and WT. (4) β-Adrenergic stimulation increased the threshold for Ca waves. (5) During β-adrenergic stimulation in R4496C, but not WT, the increase of SR Ca was sufficient to reach threshold and produce Ca waves.

CONCLUSIONS

in the R4496C CPVT model, the RyR is leaky, and this lowers both SR Ca content and the threshold for waves. β-Adrenergic stimulation produces Ca waves by increasing SR Ca content and not by lowering threshold.

Tropomyosin period 3 is essential for enhancement of isometric tension in thin filament-reconstituted bovine myocardium

Tropomyosin (Tm) consists of 7 quasiequivalent repeats known as "periods," and its specific function may be associated with these periods. To test the hypothesis that either period 2 or 3 promotes force generation by inducing a positive allosteric effect on actin, we reconstituted the thin filament with mutant Tm in which either period 2 (Delta2Tm) or period 3 (Delta3Tm) was deleted. We then studied: isometric tension, stiffness, 6 kinetic constants, and the pCa-tension relationship. N-terminal acetylation of Tm did not cause any differences. The isometric tension in Delta2Tm remained unchanged, and was reduced to approximately 60% in Delta3Tm. Although the kinetic constants underwent small changes, the occupancy of strongly attached cross-bridges was not much different. The Hill factor (cooperativity) did not differ significantly between Delta2Tm (1.79 +/- 0.19) and the control (1.73 +/- 0.21), or Delta3Tm (1.35 +/- 0.22) and the control. In contrast, pCa(50) decreased slightly in Delta2Tm (5.11 +/- 0.07), and increased significantly in Delta3Tm (5.57 +/- 0.09) compared to the control (5.28 +/- 0.04). These results demonstrate that, when ions are present at physiological concentrations in the muscle fiber system, period 3 (but not period 2) is essential for the positive allosteric effect that enhances the interaction between actin and myosin, and increases isometric force of each cross-bridge.

Methods for studying mechanical control of angiogenesis by the cytoskeleton and extracellular matrix

Mechanical forces that capillary endothelial cells generate in their cytoskeleton and exert on their extracellular matrix adhesions feed back to modulate cell sensitivity to soluble angiogenic factors, and thereby control vascular development. Here we describe various genetic, biochemical, and engineering methods that can be used to study, manipulate, and probe this physical mechanism of developmental control. These techniques are useful as in vitro angiogenesis models and for analyzing the molecular and biophysical basis of vascular control.

Blebbistatin specifically inhibits actin-myosin interaction in mouse cardiac muscle

Blebbistatin is a powerful inhibitor of actin-myosin interaction in isolated contractile proteins. To examine whether blebbistatin acts in a similar manner in the organized contractile system of striated muscle, the effects of blebbistatin on contraction of cardiac tissue from mouse were studied. The contraction of paced intact papillary muscle preparations and shortening of isolated cardiomyocytes were inhibited by blebbistatin with inhibitory constants in the micromolar range (1.3-2.8 muM). The inhibition constants are similar to those previously reported for isolated cardiac myosin subfragments showing that blebbistatin action is similar in filamentous myosin of the cardiac contractile apparatus and isolated proteins. The inhibition was not associated with alterations in action potential duration or decreased influx through L-type Ca(2+) channels. Experiments on permeabilized cardiac muscle preparations showed that the inhibition was not due to alterations in Ca(2+) sensitivity of the contractile filaments. The maximal shortening velocity was not affected by 1 muM blebbistatin. In conclusion, we show that blebbistatin is an inhibitor of the actin-myosin interaction in the organized contractile system of cardiac muscle and that its action is not due to effects on the Ca(2+) influx and activation systems.

A fiber-based ratiometric optical cardiac mapping channel using a diffraction grating and split detector

Optical fiber-based mapping systems are used to record the cardiac action potential (AP) throughout the myocardium. The optical AP contains a contraction-induced motion artifact (MA), which makes it difficult to accurately measure the action potential duration (APD). MA is removed by preventing contraction with electrical-mechanical uncoupling drugs, such as 2,3-butanedione monoxime (BDM). We designed a novel fiber-based ratiometric optical channel using a blue light emitting diode, a diffraction grating, and a split photodetector that can accurately measure the cardiac AP without the need for BDM. The channel was designed based on simulations using the optical design software ZEMAX. The channel has an electrical bandwidth of 150 Hz and an root mean-square dark noise of 742 muV. The channel successfully recorded the cardiac AP from the wall of five rabbit heart preparations without the use of BDM. After 20-point median filtering, the mean signal/noise ratio was 25.3 V/V. The APD measured from the base of a rabbit heart was 134 +/- 8.4 ms, compared to 137.6 +/- 3.3 ms from simultaneous microelectrode recordings. This difference was not statistically significant (p-value = 0.3). The quantity of MA removed was also measured using the motion ratio. The reduction in MA was significant (p-value = 0.0001). This fiber-based system is the first of its kind to enable optical APD measurements in the beating heart wall without the use of BDM.

Use of thin filament reconstituted muscle fibres to probe the mechanism of force generation

The technique of selective removal of the thin filament by gelsolin in bovine cardiac muscle fibres, and reconstitution of the thin filament from isolated proteins is reviewed, and papers that used reconstituted preparations are discussed. By comparing the results obtained in the absence/presence of regulatory proteins tropomyosin (Tm) and troponin (Tn), it is concluded that the role of Tm and Tn in force generation is not only to expose the binding site of actin to myosin, but also to modify actin for better stereospecific and hydrophobic interaction with myosin. This conclusion is further supported by experiments that used a truncated Tm mutant and the temperature study of reconstituted fibres. The conclusion is consistent with the hypothesis that there are three states in the thin filament: blocked state, closed state, and open state. Tm is the major player to produce these effects, with Tn playing the role of Ca2+ sensing and signal transmission mechanism. Experiments that changed the number of negative charges at the N-terminal finger of actin demonstrates that this part of actin is essential to promote the strong interaction between actin and myosin molecules, in addition to the well-known weak interaction that positions the myosin head at the active site of actin prior to force generation.

Effects of mechanical uncouplers, diacetyl monoxime, and cytochalasin-D on the electrophysiology of perfused mouse hearts

Chemical uncouplers diacetyl monoxime (DAM) and cytochalasin D (cyto-D) are used to abolish cardiac contractions in optical studies, yet alter intracellular Ca2+ concentration ([Ca2+]i) handling and vulnerability to arrhythmias in a species-dependent manner. The effects of uncouplers were investigated in perfused mouse hearts labeled with rhod-2/AM or 4-[β-[2-(di- n-butylamino)-6-naphthyl]vinyl]pyridinium (di-4-ANEPPS) to map [Ca2+]i transients (emission wavelength = 585 ± 20 nm) and action potentials (APs) (emission wavelength > 610 nm; excitation wavelength = 530 ± 20 nm). Confocal images showed that rhod-2 is primarily in the cytosol. DAM (15 mM) and cyto-D (5 μM) increased AP durations (APD75 = 20.0 ± 3 to 46.6 ± 5 ms and 39.9 ± 8 ms, respectively, n = 4) and refractory periods (45.14 ± 12.1 to 82.5 ± 3.5 ms and 78 ± 4.24 ms, respectively). Cyto-D reduced conduction velocity by 20% within 5 min and DAM by 10% gradually in 1 h ( n = 5 each). Uncouplers did not alter the direction and gradient of repolarization, which progressed from apex to base in 15 ± 3 ms. Peak systolic [Ca2+]i increased with cyto-D from 743 ± 47 ( n = 8) to 944 ± 17 nM ( n = 3, P = 0.01) but decreased with DAM to 398 ± 44 nM ( n = 3, P < 0.01). Diastolic [Ca2+]i was higher with cyto-D (544 ± 80 nM, n = 3) and lower with DAM (224 ± 31, n = 3) compared with controls (257 ± 30 nM, n = 3). DAM prolonged [Ca2+]i transients at 75% recovery (54.3 ± 5 to 83.6 ± 1.9 ms), whereas cyto-D had no effect (58.6 ± 1.2 ms; n = 3). Burst pacing routinely elicited long-lasting ventricular tachycardia but not fibrillation. Uncouplers flattened the slope of AP restitution kinetic curves and blocked ventricular tachycardia induced by burst pacing.

Modulation of acto-myosin contractility in skeletal muscle myoblasts uncouples growth arrest from differentiation

Cell-substratum interactions trigger key signaling pathways that modulate growth control and tissue-specific gene expression. We have previously shown that abolishing adhesive interactions by suspension culture results in G0 arrest of myoblasts. We report that blocking intracellular transmission of adhesion-dependent signals in adherent cells mimics the absence of adhesive contacts. We investigated the effects of pharmacological inhibitors of acto-myosin contractility on growth and differentiation of C2C12 myogenic cells. ML7 (5-iodonaphthalene-1-sulfonyl homopiperazine) and BDM (2,3, butanedione monoxime) are specific inhibitors of myosin light chain kinase, and myosin heavy chain ATPase, respectively. ML7 and BDM affected cell shape by reducing focal adhesions and stress fibers. Both inhibitors rapidly blocked DNA synthesis in a dose-dependent, reversible fashion. Furthermore, both ML7 and BDM suppressed expression of MyoD and myogenin, induced p27kip1 but not p21cip1, and inhibited differentiation. Thus, as with suspension-arrest, inhibition of acto-myosin contractility in adherent cells led to arrest uncoupled from differentiation. Over-expression of inhibitors of the small GTPase RhoA (dominant negative RhoA and C3 transferase) mimicked the effects of myosin inhibitors. By contrast, wild-type RhoA induced arrest, maintained MyoD and activated myogenin and p21 expression. The Rho effector kinase ROCK did not appear to mediate Rho's effects on MyoD. Thus, ROCK and MLCK play different roles in the myogenic program. Signals regulated by MLCK are critical, since inhibition of MLCK suppressed MyoD expression but inhibition of ROCK did not. Inhibition of contractility suppressed MyoD but did not reduce actin polymer levels. However, actin depolymerization with latrunculin B inhibited MyoD expression. Taken together, our observations indicate that actin polymer status and contractility regulate MyoD expression. We suggest that in myoblasts, the Rho pathway and regulation of acto-myosin contractility may define a control point for conditional uncoupling of differentiation and the cell cycle.

O2 consumption of mechanically unloaded contractions of mouse left ventricular myocardial slices

Left ventricular (LV) myocardial slices were isolated from murine hearts (300 μm thick) and were stimulated at 1 Hz without external load. Mean myocardial slice O2 consumption (MVo2) per minute (mMVo2) without stimulation was 0.97 ± 0.14 ml O2·min−1·100 g LV−1 and mean mMVo2 with stimulation increased to 1.80 ± 0.17 ml O2·min−1·100 g LV−1 in normal Tyrode solution. Mean ΔmVo2 (the mMVo2 with stimulation − the mMVo2 without stimulation) was 0.83 ± 0.12 ml O2·min−1·100 g LV−1. There were no differences between mean mMVo2 with and without stimulation in Ca2+-free solution. The increases in extracellular Ca2+ concentrations up to 14.4 mM did not affect the mMVo2 without stimulation but significantly increased the mMVo2 with stimulation up to 140% of control. The ΔmMVo2 significantly increased up to 190% of the control in a dose-dependent manner. In contrast, the shortening did not increase in a dose-dependent manner. Cyclopiazonic acid (CPA; 30 μM) significantly reduced the ΔmMVo2 to 0.27 ± 0.06 ml O2·min−1·100 g LV−1 (35% of control). The combination of 5 mM 2,3-butanedione monoxime (BDM) and 30 μM CPA did not further decrease ΔmMVo2. Although BDM (3–5 mM) decreased the ΔmMVo2 by 28–30% of control in a dose-independent manner, 3–5 mM BDM decreased shortening in a dose-dependent manner. Our results indicate that the ΔmMVo2 of mouse LV slices during shortening under mechanically unloaded conditions consists of energy expenditure for total Ca2+ handling during excitation-contraction coupling, basal metabolism, but no residual cross-bridge cycling.

Extracellular matrix controls myosin light chain phosphorylation and cell contractility through modulation of cell shape and cytoskeletal prestress

The mechanism by which vascular smooth muscle (VSM) cells modulate their contractility in response to structural cues from extracellular matrix remains poorly understood. When pulmonary VSM cells were cultured on increasing densities of immobilized fibronectin (FN), cell spreading, myosin light chain (MLC) phosphorylation, cytoskeletal prestress (isometric tension in the cell before vasoagonist stimulation), and the active contractile response to the vasoconstrictor endothelin-1 all increased in parallel. In contrast, MLC phosphorylation did not increase when suspended cells were allowed to bind FN-coated microbeads (4.5-microm diameter) or cultured on micrometer-sized (30 x 30 microm) FN islands surrounded by nonadhesive regions that support integrin binding but prevent cell spreading. Cell spreading and MLC phosphorylation also both decreased in parallel when the mechanical compliance of flexible FN substrates was raised. MLC phosphorylation was inhibited independently of cell shape when cytoskeletal prestress was dissipated using a myosin ATPase inhibitor in fully spread cells, whereas it increased to maximal levels when microtubules were disrupted using nocodazole in cells adherent to FN but not in suspended cells. These data demonstrate that changes in cell-extracellular matrix (ECM) interactions modulate smooth muscle cell contractility at the level of biochemical signal transduction and suggest that the mechanism underlying this regulation may involve physical interplay between ECM and the cytoskeleton, such that cell spreading and generation of cytoskeletal tension feed back to promote MLC phosphorylation and further increase tension generation.

The electrophysiological and mechanical effects of 2,3-butane-dione monoxime and cytochalasin-D in the Langendorff perfused rabbit heart

Procedures that reduce contraction are used to facilitate optical measurements of membrane potential, but it is unclear to what extent they affect the excitability of the heart. This study has examined the electrophysiological consequences of a range of extracellular [Ca2+] (0.7-2.5 mmol l(-1)), 2,3-butane-dione monoxime (BDM; 1-20 mmol l(-1)) and cytochalasin-D (Cyto-D; 1-5 micromol l(-1)).

METHODS

Monophasic action potentials (MAPs) were recorded from the basal epicardial surface of the left ventricle of isolated rabbit hearts. Conduction delay (CD) and time to 90% repolarisation of the monophasic action potential (MAPD90) were measured. The effects of BDM and Cyto-D on restitution were studied at a [Ca2+] of 1.9 mmol l(-1). Restitution curves for MAPD90 were generated using a standard S1-S2 protocol.

RESULTS

All manoeuvres decreased left ventricular developed pressure (LVDP): 0.7 mmol l(-1) Ca2+ to 74.0 +/- 6.1%, 20 mmol l(-1) BDM to 4.5 +/- 1.0%, and 5 micromol l(-1) Cyto-D to 12.8 +/- 3.5% of control value. CD decreased from a control value (33.3 +/- 1.0 ms, n= 16) to 93.0 +/- 2.2% in 0.7 mmol l(-1) Ca2+, but increased to 133.7 +/- 10.5% in 20 mmol l(-1) BDM and 127.4 +/- 10.6% in 5 micromol l(-1) Cyto-D. At 350 ms pacing cycle length, MAPD90 (control = 119.6 +/- 1.7 ms n= 16) was prolonged by reduced extracellular [Ca2+]. BDM had no effects on MAPD90 at control pacing rates. Cyto-D caused a significant prolongation (to 115.0 +/- 3.0% of control, n= 6) at the highest concentration studied (5 micromol l(-1)). Both BDM (20 mmol l(-1)) and Cyto-D (3 micromol l(-1)) flattened the restitution curves but neither agent altered maximum MAPD90.

CONCLUSIONS

Extracellular [Ca2+] of 1.9 mmol l(-1) in conjunction with a moderate dose of Cyto-D (3 micromol l(-1)) reduced contractility with minimal effects on action potential duration and conduction at a fixed pacing cycle length. However, both BDM and Cyto-D had pronounced effects on electrical restitution.

Left Ventricular Mechanoenergetics in Small Animals

Studies on left ventricular mechanical work and energetics in rat and mouse hearts are reviewed. First, left ventricular linear end-systolic pressure-volume relation (ESPVR) and curved end-diastolic pressure-volume relation (EDPVR) in canine hearts and left ventricular curved ESPVR and curved EDPVR in rat hearts are reviewed. Second, as an index for total mechanical energy per beat in rat hearts as in canine hearts, a systolic pressure-volume area (PVA) is proposed. By the use of our original system for measuring continuous oxygen consumption for rat left ventricular mechanical work, the linear left ventricular myocardial oxygen consumption per beat (VO2)-PVA relation is obtained as in canine hearts. The slope of VO2-PVA relation (oxygen cost of PVA) indicates a ratio of chemomechanical energy transduction. VO2 intercept (PVA-independent VO2) indicates the summation of oxygen consumption for Ca2+ handling in excitation-contraction coupling and for basal metabolism. An equivalent maximal elastance (eEmax) is proposed as a new left ventricular contractility index based on PVA at the midrange left ventricular volume. The slope of the linear relation between PVA-independent VO2 and eEmax (oxygen cost of eEmax) indicates changes in oxygen consumption for Ca2+ handling in excitation-contraction coupling per unit changes in left ventricular contractility. The key framework of VO2-PVA-eEmax can give us a better understanding for the biology and mechanisms of physiological and various failing rat heart models in terms of mechanical work and energetics.

Directional control of lamellipodia extension by constraining cell shape and orienting cell tractional forces

Directed cell migration is critical for tissue morphogenesis and wound healing, but the mechanism of directional control is poorly understood. Here we show that the direction in which cells extend their leading edge can be controlled by constraining cell shape using micrometer-sized extracellular matrix (ECM) islands. When cultured on square ECM islands in the presence of motility factors, cells preferentially extended lamellipodia, filopodia, and microspikes from their corners. Square cells reoriented their stress fibers and focal adhesions so that tractional forces were concentrated in these corner regions. When cell tension was dissipated, lamellipodia extension ceased. Mechanical interactions between cells and ECM that modulate cytoskeletal tension may therefore play a key role in the control of directional cell motility.

Rapid electrical stimulation of contraction reduces the density of beta-adrenergic receptors and responsiveness of cultured neonatal rat cardiomyocytes. Possible involvement of microtubule disassembly secondary to mechanical stress

BACKGROUND

Although tachycardia is commonly present in patients with congestive heart failure, its role in the development of congestive heart failure remains unclear. We studied the effect of rapid electrical stimulation of contraction on beta-adrenergic receptor (beta-AR) signal pathway in cultured cardiomyocytes of neonatal rats.

METHODS AND RESULTS

Contraction of cardiomyocytes was induced by electrical stimulation at 50 V with twice the threshold pulse width. beta-ARs were identified by [(3)H]CGP-12177 and [(3)H]dihydroalprenolol. Electrical stimulation reduced cell-surface but not total beta-AR density; the effect was dependent on pacing frequency (a reduction of 11%, 28%, and 18% in cells paced at 2.5, 3. 0, and 3.3 Hz, respectively). This reduction was apparent at 3 hours, in contrast to reduced beta-AR density after exposure to isoproterenol (ISP) for 1 hour. The fraction and inhibition constant of beta-AR binding agonist with high affinity were not affected by rapid electrical stimulation. In cardiomyocytes paced at 3.0 Hz for 24 hours, the response to ISP decreased compared with unpaced cells, 142% versus 204% of baseline with 1 micromol/L ISP, whereas the responses to forskolin or acetylcholine were not different. Treatment of cardiomyocytes with 2,3-butanedione monoxime (10 mmol/L) or taxol (10 micromol/L) inhibited the rapid pacing-induced reduction in beta-AR density.

CONCLUSIONS

Our results suggest that contractile activity is involved in regulation of cardiac function by modulating the beta-AR system independently of hemodynamic and neurohormonal factors. This may help to elucidate the role of mechanical stress in the development of heart failure.

Myosin light chain kinase mediates sarcomere organization during cardiac hypertrophy in vitro

During the development of hypertrophy, cardiac myocytes increase organization of the sarcomere, a highly ordered contractile unit in striated muscle cells. Several hypertrophic agonists, such as angiotensin II, phenylephrine, and endothelin-1, have been shown to promote the sarcomere organization. However, the signaling pathway, which links extracellular stimuli to sarcomere organization, has not been clearly demonstrated. Here, we demonstrate that myosin light chain kinase specifically mediates agonist-induced sarcomere organization during early hypertrophic response. Acute administration of a hypertrophic agonist, phenylephrine, or angiotensin II, causes phosphorylation of myosin light chain 2v both in cultured cardiac myocytes and in the adult heart in vivo. We also show that both sarcomere organization and myosin light chain 2v phosphorylation are dependent on the activation of Ca2+/calmodulin pathway, a known activator of myosin light chain kinase. These results define a new and specific role of myosin light chain kinase in cardiac myocytes, which may provide a rapid adaptive mechanism in response to hypertrophic stimuli.

Cardiovascular effects of subchronically low/high carbon monoxide exposure in rats

This study was designed to determine whether subchronic CO exposure ranging from 15 to 530 ppm induced modifications in the rat cardiovascular system. We investigated the degree of resistance to an in vitro transient ischemia in the hearts exposed in vivo to different CO concentrations for 1-4 weeks. Subchronic CO exposure induced dose and/or time-dependent increases (hematocrit, cardiomegaly and coronary flow). We showed an increase in the ventricular tachycardia (VT) incidence with the passing weeks of exposure, which demonstrated the proarrhythmic activity of CO even in low doses (15 ppm). The contractile recovery decreased owing to a low (50 ppm) or high (530ppm) CO exposure after a 25-min ischemia period. This diminution seems to be dependent on the increased amplitude of ischemic contracture. The present study supports the hypothesis that subchronic CO exposure, even at low levels of CO, can produce cardiovascular changes and could explain the increased risk of myocardial infarction.

Optimizing the oxygen balance during initial reperfusion with 2,3-butanedione monoxime attenuates cardiac reperfusion injury

The effect of 20 mmol/L butanedione monoxime on myocardial ischemia/reperfusion damage was studied in isolated guinea pig hearts. Three groups of hearts (n = 8) were perfused in the Langendorff mode and cardioplegic arrest was induced with St. Thomas Hospital II solution (STS) at 37 degrees C for 50 min. Myocardial oxygen demand, recovery of myocardial function, and creatine kinase release during 30 min of reperfusion were monitored. Preservation of myocardial ultrastructure was determined by electron microscopy. Control (C) hearts underwent cardioplegic arrest and reperfusion without treatment. BDM was added during cardioplegic arrest in BDMSTS hearts, or to the initial (20 min) reperfusate in BDMREP hearts. BDM during initial reperfusion markedly reduced O2 demand and prevented creatine kinase release from cardiac myocytes, resulting in improved recovery of myocardial function and attenuation of myocardial ultrastructural damage after washout of the drug. In contrast, addition of BDM to the cardioplegic solution provided no protection from ischemic or reperfusion injury.

Effects of 2,3-butanedione monoxime on excitation-contraction coupling in frog twitch fibres

10 and 30 mM 2,3-butanedione monoxime (BDM) applied extracellularly to voltage-clamped frog skeletal muscle twitch fibres suppressed both Ca2+ release flux and intramembranous charge movement. Both effects could be clearly separated. The early peak of the Ca2+ release flux was suppressed at every test voltage. The steady level attained at the end of a 100 ms clamp depolarization was relatively spared for lower depolarizing pulses, but was as suppressed as the peak at voltages above -20 mV. The intramembranous charge movement was affected mainly in the I gamma component. The drug had a distinct effect on the kinetics of the intramembranous charge movement current around the threshold for Ca2+ release. The three kinetic components of I gamma were simultaneously affected. For more positive depolarizations where the kinetic effect was not evident, the oxime had no significant effect on the charge moved. Under conditions in which I gamma was absent (i.e. stretched fibres, intracellular solutions containing 6 to 10 mM BAPTA), treatment with 10 mM BDM had a small, not significant suppressive effect on the maximum charge moved (Qmax), while it affected Ca2+ release significantly. When 10 mM BDM was applied in the presence of 0.2 mM tetracaine, the local anaesthetic-resistant Ca2+ release flux was not further suppressed by the oxime.

Real time in situ confocal imaging of calcium wave in the perfused whole heart of the rat

To understand the calcium handling in whole heart having automaticity of the sinus node, we have developed a system of in situ imaging the intracellular calcium ion concentration in the perfused whole heart of the rat. The system consists of a stage-fixed upright microscope equipped with a real-time confocal laser scanning device of a multipinhole type with a water-immersion objective lens for observation. This in situ imaging system rendered observations and analyses of the rapidly changing images of intracellular calcium dynamics possible in the whole rat heart loaded with fluo-3. The scanning was conducted at a video rate of 30 frames per second, and the confocal effects included both X and Y planes. Calcium waves were frequently interrupted by calcium transients from either external electro-stimulation pulses or spontaneous sinus rhythm. Our findings suggest that abnormal calcium waves in minute areas cannot disturb the excitation-contraction coupling in the whole heart if the myocardial cells have orderly end-on-end intercellular electric paths.

Detachment of low-force bridges contributes to the rapid tension transients of skinned rabbit skeletal muscle fibres

1. To probe the cross-bridge cycle and to learn more about the cardioplegic agent BDM (2,3-butanedione monoxime), its effects on the force-velocity properties and tension transients of skinned rabbit muscle fibres were studied at 1-2 degrees C and pH 7.0. 2. Three millimolar BDM decreased isometric force by 50%, velocity by 29%, maximum power by 73%, and stiffness by 25%, so that the relative stiffness (stiffness/force ratio) increased by 50% compared with reference conditions in the absence of BDM. 3. Tension transients obtained under the reference condition (0 BDM) could be represented by three components whose instantaneous stiffness accounted for the initial (Phase 1) force deviation and whose exponential recoveries caused the rapid, partial (Phase 2) force recovery following the step. The fastest component had non-linear extension-force properties that accounted for about half the isometric stiffness and it recovered fully. The two slower components had linear extension-force properties that together accounted for the other half of the sarcomere stiffness. These components recovered only partially following the step, producing the intermediate (T2) level which the force approached during Phase 2. 4. Matching the force transients obtained under test conditions (3 mM BDM) required three alterations: (1) reducing the amplitude of the two slower components by 50%, in proportion to isometric force, (2) adding a non-relaxing component and (3) decreasing the amplitude of the rapidly recovering component by 12.5% so that its relative amplitude (amplitude/isometric force) was increased by 75%. The non-recovering component and the increase in relative amplitude of the rapid component were responsible for the increase in relative stiffness of the fibres produced by BDM. The rapidly recovering component had the same time constant and step-size-dependent recovery rates as the fastest of the three mono-exponential components isolated from the tension transient response under the reference condition. BDM therefore appeared to augment the fastest component of the tension transient under the reference condition. 5. The results suggest that BDM detains cross-bridges in low-force, attached states. Since these bridges are attached, they contribute to sarcomere stiffness. Since they are detained, relaxation or reversal of their immediate responses is probably due to bridge detachment rather than to their undergoing the power stroke. The observation that a portion of the test response matched the fastest component of the reference response when the amplitude of the fastest component was increased suggests that a part of the normal rapid, transient tension recovery following a release step is due to detachment of low-force bridges moved to negative-force positions by the step.

Effects of L-arginine and N omega-nitro-L-arginine methyl ester on cardiac perfusion and function after 1-day cold preservation of isolated hearts

BACKGROUND

Coronary flow responses to endothelium-dependent (acetylcholine [ACh] or 5-hydroxytryptamine [5-HT]) and endothelium-independent (adenosine [ADE] or nitroprusside [NP]) vasodilators may be altered before and after 1-day hypothermia during the perfusion of arginine vasopressin (AVP), D-arginine (D-ARG), L-arginine (L-ARG), or nitro-L-arginine methyl ester (L-NAME).

METHODS AND RESULTS

Four groups of guinea pig hearts (37.5 degrees C [warm]) were perfused for 6 hours with AVP, L-ARG, L-NAME, or nothing (control). Five heart groups (cold) were perfused with AVP, D-ARG, L-ARG, L-NAME, or nothing (control), but after 2 hours they were perfused at low flow for 22 hours at 3.7 degrees C and again for 3 hours at 37.5 degrees C. ADE, butanedione monoxime, and NP were given for cardioprotection before, during, and after hypothermia. In warm groups, L-ARG did not alter basal flow or ADE, ACh, 5-HT, or NP responses, whereas L-NAME and AVP reduced basal flow and the ADE response, abolished ACh and 5-HT responses, and increased the NP response. In cold groups after hypothermia. L-ARG did not alter basal flow, but L-NAME, AVP, D-ARG, and control reduced flow. In the postcold L-ARG group, ACh increased peak flow, but NP did not increase flow in other cold groups. Effluent L-ARG and L-CIT in the cold control group fell from 64 +/- 9 and 9 +/- 1 micrograms/L at 1 hour to 36 +/- 5 and 5 +/- 1 micrograms/L at 25 hours, respectively. Left ventricular pressure and cardiac efficiency improved more in the postcold L-ARG group than in the postcold D-ARG, AVP, and L-NAME groups.

CONCLUSIONS

Endogenous effluent levels of L-ARG and L-CIT decrease after 24 hours in isolated hearts, whereas perfusion of L-ARG improves cardiac performance, basal coronary flow, and vasodilator responses. In contrast, L-NAME, L-ARG, and AVP limit flow and performance but maintain a partial vasodilatory response to NP. Sustained release of NO may account for improved performance after L-ARG after hypothermia.

Cytoskeletal mechanics in pressure-overload cardiac hypertrophy

We have shown that the cellular contractile dysfunction characteristic of pressure-overload cardiac hypertrophy results not from an abnormality intrinsic to the myofilament portion of the cardiocyte cytoskeleton but rather from an increased density of the microtubule component of the extramyofilament portion of the cardiocyte cytoskeleton. To determine how, in physical terms, this increased microtubule density mechanically overloads the contractile apparatus at the cellular level, we measured cytoskeletal stiffness and apparent viscosity in isolated cardiocytes via magnetic twisting cytometry, a technique by which magnetically induced force is applied directly to the cytoskeleton through integrin-coupled ferromagnetic beads coated with Arg-Gly-Asp (RGD) peptide. Measurements were made in two groups of cardiocytes from cats with right ventricular (RV) hypertrophy induced by pulmonary artery banding: (1) those from the pressure-overloaded RV and (2) those from the normally loaded same-animal control left ventricle (LV). Cytoskeletal stiffness increased almost twofold, from 8.53 +/- 0.77 dyne/cm2 in the normally loaded LV cardiocytes to 16.46 +/- 1.32 dyne/cm2 in the hypertrophied RV cardiocytes. Cytoskeletal apparent viscosity increased almost fourfold, from 20.97 +/- 1.92 poise in the normally loaded LV cardiocytes to 87.85 +/- 6.95 poise in the hypertrophied RV cardiocytes. In addition to these baseline data showing differing stiffness and, especially, apparent viscosity in the two groups of cardiocytes, microtubule depolymerization by colchicine was found to return both the stiffness and the apparent viscosity of the pressure overload-hypertrophied RV cells fully to normal. Conversely, microtubule hyperpolymerization by taxol increased the stiffness and apparent viscosity values of normally loaded LV cardiocytes to the abnormal values given above for pressure-hypertrophied RV cardiocytes. Thus, increased microtubule density constitutes primarily a viscous load on the cardiocyte contractile apparatus in pressure-overload cardiac hypertrophy.

Rapid shortening during relaxation increases activation and improves systolic performance

BACKGROUND

Previous studies in cardiac muscle and isolated heart preparations generally have attributed positive effects of ejection to greater length-dependent activation. However, there have been some reports of an ejection-related increase in contractile function that is independent of end-diastolic volume (EDV) history. The present study was designed to more fully characterize the mechanoenergetic results of the latter effect in the intact ventricle.

METHODS AND RESULTS

A servomotor was used to initiate left ventricular volume reduction (VR) at end systole, with EDV kept constant. Seven isolated, red blood cell-perfused rabbit hearts were studied at constant EDV during isovolumic contraction, slow VR (5.0 +/- 0.9 EDV/s), and rapid VR (26.8 +/- 5.1 EDV/s). Compared with isovolumic beats, VR caused an enhancement in contractility. This effect was greater for rapid VR and required > 50 beats to attain steady state. Rapid VR increased developed pressure by 15% (92.2 +/- 23.7 [mean +/- SD] versus 105.9 +/- 27.6 mm Hg), maximum dP/dt by 17% (1223 +/- 401 versus 1435 +/- 505 mm Hg.s-1), and Emax (slope of the end-systolic pressure-volume relation) by 13% (69.4 +/- 19.9 versus 78.6 +/- 23.0 mm Hg/mL) (all P < .01). Left ventricular oxygen consumption (VO2) was unchanged with slow VR and decreased by 8% with rapid VR (0.0744 +/- 0.0194 versus 0.0683 +/- 0.0141 mL O2.beat-1.100 g-1; P < .05). In separate hearts (n = 8), costs (basal metabolism and excitation-contraction coupling) were estimated by use of 2,3-butanedione monoxime. Compared with control, rapid VR was associated with a 26% increase in nonmechanical VO2 (0.0248 +/- 0.0021 versus 0.0312 +/- 0.0022 mL O2.beat-1.100 g-1; P < .01), consistent with an increase in calcium cycled per beat.

CONCLUSIONS

Ejection after end systole has a positive effect on ventricular performance that cannot be ascribed to length-dependent activation and is likely related to an increase in calcium available for activation. Similarly, an increase in nonmechanical VO2 associated with ejection suggests a positive interaction between myofilament shortening and activator calcium cycling.

Effects of 2,3-butanedione monoxime on cross-bridge kinetics in rat cardiac muscle

The effects of 2,3-butanedione monoxime (BDM) on isometric force and myofibrillar adenosine 5'-triphosphatase (ATPase) activity were studied in skinned cardiac trabeculae from the rat. ATP hydrolysis was enzymatically coupled to the breakdown of reduced nicotinamide adeninedinucleotide (NADH). The NADH concentration was monitored photometrically. Measurements were performed at a sarcomere length of 2.1 microm, 20 degrees C and pH 7.0. Without BDM, isometric force was 45 +/- 3 kN/m2 and the isometric ATPase activity 0.49 +/- 0.04 mM/s (mean +/- SEM, n = 31). Force gradually decreased as a function of [BDM] to 2.8 +/- 0.4% at 100 mM BDM. ATPase activity was also depressed by BDM, but to a lesser extent than force. BDM therefore has a marked effect on myofibrillar tension cost. The rate of tension redevelopment after unloaded shortening decreased from 29 +/- 2 s-1 (n = 10) without BDM to 22 +/- 1 s-1 (n = 5) at 20 mM BDM. These results, modelled in a two- and three-state scheme of cross-bridge interaction, indicate that, in cardiac muscle, BDM not only affects cross-bridge formation but, especially at high concentrations (>/= 20 mM), also causes a marked increase in the apparent rate of cross-bridge detachment.

Preservation of myocyte contractile function after hypothermic, hyperkalemic cardioplegic arrest with 2, 3-butanedione monoxime

One proposed contributory mechanism for depressed ventricular performance after hypothermic, hyperkalemic cardioplegic arrest is a reduction in myocyte contractile function caused by alterations in intracellular calcium homeostasis. Because 2,3-butanedione monoxime decreases intracellular calcium transients, this study tested the hypothesis that 2,3-butanedione monoxime supplementation of the hyperkalemic cardioplegic solution could preserve isolated myocyte contractile function after hypothermic, hyperkalemic cardioplegic arrest. Myocytes were isolated from the left ventricles of six pigs. Magnitude and velocity of myocyte shortening were measured after 2 hours of incubation under normothermic conditions (37 degrees C, standard medium), hypothermic, hyperkalemic cardioplegic arrest (4 degrees C in Ringer's solution with 20 mEq potassium chloride and 20 mmol/L 2,3-butanedione monoxime). Because beta-adrenergic agonists are commonly employed after cardioplegic arrest, myocyte contractile function was examined in the presence of the beta-agonist isoproterenol (25 nmol/L). Hypothermic, hyperkalemic cardioplegic arrest and rewarming reduced the velocity (32%) and percentage of myocyte shortening (27%, p < 0.05). Supplementation with 2,3 butanedione monoxime normalized myocyte contractile function after hypothermic, hyperkalemic cardioplegic arrest. Although beta-adrenergic stimulation significantly increased myocyte contractile function under normothermic conditions and after hypothermic, hyperkalemic cardioplegic arrest, contractile function of myocytes exposed to beta-agonist after hypothermic, hyperkalemic cardioplegic arrest remained significantly reduced relative to the normothermic control group. Supplementation with 2,3-butanedione monoxime restored beta-adrenergic responsiveness of myocytes after hypothermic, hyperkalemic cardioplegic arrest. Thus, supplementation of a hyperkalemic cardioplegic solution with 2,3-butanedione monoxime had direct and beneficial effects on myocyte contractile function and beta-adrenergic responsiveness after cardioplegic arrest. A potential mechanism for the effects of 2,3-butanedione monoxime includes modulation of intracellular calcium transients or alterations in sensitivity to calcium. Supplementation with 2,3-butanedione monoxime may have clinical utility in improving myocardial contractile function after hypothermic, hyperkalemic cardioplegic arrest.

Myosin is involved in postmitotic cell spreading

We have investigated a role for myosin in postmitotic Potoroo tridactylis kidney (PtK2) cell spreading by inhibitor studies, time-lapse video microscopy, and immunofluorescence. We have also determined the spatial organization and polarity of actin filaments in postmitotic spreading cells. We show that butanedione monoxime (BDM), a known inhibitor of muscle myosin II, inhibits nonmuscle myosin II and myosin V adenosine triphosphatases. BDM reversibly inhibits PtK2 postmitotic cell spreading. Listeria motility is not affected by this drug. Electron microscopy studies show that some actin filaments in spreading edges are part of actin bundles that are also found in long, thin, structures that are connected to spreading edges and substrate (retraction fibers), and that 90% of this actin is oriented with barbed ends in the direction of spreading. The remaining actin in spreading edges has a more random orientation and spatial arrangement. Myosin II is associated with actin polymer in spreading cell edges, but not retraction fibers. Myosin II is excluded from lamellipodia that protrude from the cell edge at the end of spreading. We suggest that spreading involves myosin, possibly myosin II.

Muscle cross-bridges bound to actin are disordered in the presence of 2,3-butanedione monoxime

Electron paramagnetic resonance spectroscopy was used to monitor the orientation of muscle cross-bridges attached to actin in a low force and high stiffness state that may occur before force generation in the actomyosin cycle of interactions. 2,3-butanedione monoxime (BDM) has been shown to act as an uncompetitive inhibitor of the myosin ATPase that stabilizes a myosin.ADP.P(i) complex. Such a complex is thought to attach to actin at the beginning of the powerstroke. Addition of 25 mM BDM decreases tension by 90%, although stiffness remains high, 40-50% of control, showing that cross-bridges are attached to actin but generate little or no force. Active cross-bridge orientation was monitored via electron paramagnetic resonance spectroscopy of a maleimide spin probe rigidly attached to cys-707 (SH-1) on the myosin head. A new labeling procedure was used that showed improved specificity of labeling. In 25 mM BDM, the probes have an almost isotropic angular distribution, indicating that cross-bridges are highly disordered. We conclude that in the pre-powerstroke state stabilized by BDM, cross-bridges are attached to actin, generating little force, with a large portion of the catalytic domain of the myosin heads disordered.

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.

Mechanical determinants of myocardial oxygen consumption

1. Recent developments which attempt to identify the mechanical determinants of myocardial oxygen consumption (mVO2) are considered, with emphasis being placed on the pressure-work and pressure-volume area (PVA) indices. 2. The difficulty of establishing a realistic in vivo basal mVO2 value is explained and the experimental reasons for the controversy over the magnitude of the activation metabolism are outlined. 3. The time varying elastance model of the heart is discussed including some current problems. The evidence for PVA as a satisfactory index of mVO2 under all physiological and pharmacological conditions is examined. Most pharmacological agents alter the intercept of the mVO2:PVA relationship but do not effect its slope: this result is interpreted to mean that the majority of current inotropic agents alter the energetic cost of calcium release/retrieval but not crossbridge efficiency. 4. An attempt is made to establish the likely cost of a cardiac contraction in man. The use of newer technology (i) to estimate mVO2 via positron emission tomography and (ii) to measure the work and potential energy output of the heart per beat with conductance catheters, is explained.

The energy expenditure of actomyosin-ATPase, Ca(2+)-ATPase and Na+,K(+)-ATPase in guinea-pig cardiac ventricular muscle

1. The rate of heat production (heat rate) and isometric twitch tension of ventricular trabeculae isolated from guinea-pig heart were measured at 37 degrees C in order to determine the relative contributions of actomyosin-ATPase, Ca(2+)-ATPase and Na+,K(+)-ATPase to myocardial energy metabolism. 2. The increase in heat rate recorded during isometric contractions at optimal length (contraction-related heat production) was 19.1 +/- 1.2 mW cm-3 at a stimulation rate of 2 Hz. The tension-time integral of individual contractions measured under the same conditions was 147 +/- 15 mM s cm-2. 3. The heat production of the actomyosin-ATPase was determined by inhibiting the contractile proteins with 2,3-butanedione monoxime (BDM). Contraction-related heat production was reduced by 0.219 +/- 0.010 and the isometric tension-time integral was reduced by 0.288 +/- 0.016 in the presence of 1 mM BDM. From these data an estimate of 0.76 for the relative contribution of the actomyosin-ATPase to contraction-related heat production was derived. 4. The heat production related to actomyosin-ATPase plus Ca(2+)-ATPase was studied by blocking Ca2+ influx into the myocardial cells with a solution containing 100 microM Ca2+ and 400 microM Ni2+. In this solution contraction-related heat production was reduced by 0.907 +/- 0.012. Comparison of this value with the component attributable to the actomyosin-ATPase yields an estimate of 0.15 for the relative contribution of the Ca(2+)-ATPase to contraction related heat production. 5. The heat production related to the Na+,K(+)-ATPase in resting preparations was studied by blocking the sodium pump with 400 microM dihydro-ouabain (DHO). DHO produced a transient decrease in heat rate lasting 1-2 min, which was followed by a secondary increase. From the heat transient produced by DHO the heat rate related to the Na+,K(+)-ATPase in the steady state was extrapolated. The relative contribution of the sodium pump to resting heat production was estimated to be 0.17. 6. The heat production related to the Na+,K(+)-ATPase in contracting preparations was studied by first blocking Ca2+ influx with 100 microM Ca2+ and 400 microM Ni2+, and then inhibiting the sodium pump with 400 microM dihydro-ouabain (DHO). The relative contribution of the sodium pump to contraction-related heat production extrapolated from these data was 0.10, which agreed well with the fraction of contraction-related heat production persisting after blockage of actomyosin-ATPase and Ca(2+)-ATPase (0.09).(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial non-uniformities in [Ca2+]i during excitation-contraction coupling in cardiac myocytes

The intracellular calcium ([Ca2+]i) transient in adult rat heart cells was examined using the fluorescent calcium indicator fluo-3 and a laser scanning confocal microscope. We find that the electrically evoked [Ca2+]i transient does not rise at a uniform rate at all points within the cell during the [Ca2+]i transient. These spatial non-uniformities in [Ca2+]i are observed immediately upon depolarization and largely disappear by the time the peak of the [Ca2+]i transient occurs. Importantly, some of the spatial non-uniformity in [Ca2+]i varies randomly in location from beat to beat. Analysis of the spatial character of the non-uniformities suggests that they arise from the stochastic nature of the activation of SR calcium-release channels. The non-uniformities in [Ca2+]i are markedly enhanced by low concentrations of Cd2+, suggesting that activation of L-type calcium channels is the primary source of activator calcium for the calcium transient. In addition, the pattern of calcium release in these conditions was very similar to the spontaneous calcium sparks that are observed under resting conditions and which are due to spontaneous calcium release from the SR. The spatial non-uniformity in the evoked [Ca2+]i transient under normal conditions can be explained by the temporal and spatial summation of a large number of calcium sparks whose activation is a stochastic process. The results are discussed with respect to a stochastic local control model for excitation-contraction (E-C) coupling, and it is proposed that the fundamental unit of E-C coupling consists of one dihydropyridine receptor activating a small group of ryanodine receptors (possibly four) in a square packing model.

Regional contractile blockade at the onset of reperfusion reduces infarct size in the dog heart

An important mechanism of lethal myocardial reperfusion injury is the development of cellular hypercontracture at the onset of reperfusion. Hypercontracture can lead to cytolysis by mutual mechanical disruption of myocardial cells. 2,3-Butanedione monoxime (BDM) inhibits myofibrillar cross-bridge cycling and may therefore reduce infarct size in ischaemic reperfused myocardium. This study investigated whether a temporary presence of BDM protects against myocardial reperfusion injury in an intact-animal preparation. Anaesthetized open-chest dogs (n = 10) underwent 1 h of left anterior descendent artery (LAD) occlusion and received intracoronary BDM (25 mM, n = 5) or vehicle (n = 5) for 65 min starting with an anoxic local infusion 5 min before reperfusion. Infarct size was assessed by triphenyltetrazolium staining after 6 h reperfusion. The infusion of BDM was accompanied by a transient reduction of left ventricular systolic pressure from 84.3 +/- 11.2 mm Hg during occlusion to 66.4 +/- 9.9 mm Hg at 30 min reperfusion (mean +/- SD, P < 0.01 vs. control). LAD-flow and regional wall motion in the area at risk showed no difference between groups. Infarct size (% of area at risk) was reduced from 24.4 +/- 8.7 (control) to 6.6 +/- 2.0% (BDM) (P < 0.01). The results demonstrate that development of necrosis in reperfused myocardium can be greatly reduced by temporary presence of the contractile inhibitor BDM at the onset of reperfusion.

Comparison of the effects of 2,3-butanedione monoxime on force production, myosin light chain phosphorylation and chemical energy usage in intact and permeabilized smooth and skeletal muscles

The primary goal of this study was to determine the utility of 2,3-butanedione monoxime as a tool for determining and separating the chemical energy usage associated with force production from that of force-independent, or 'activation' processes in smooth and skeletal muscles. We determined the effects of 2,3-butanedione monoxime on force production, myosin light chain phosphorylation and high energy phosphate usage in intact and permeabilized smooth (rabbit taenia coli) and skeletal (mouse extensor digitorum longus) muscles. In the intact taenia coli, 2,3-butanedione monoxime depressed the tonic phase of the tetanus, contractures evoked by high potassium (90 mM) and by carbachol (10(-5) M) and the small force response evoked by these agonists after treatment with D-600 (10(-5) M). In the electrically stimulated intact taenia coli 2,3-butanedione monoxime (0-20 mM) caused a proportional inhibition of tetanic force output, myosin light chain phosphorylation and high energy phosphate usage (ED50 approximately 7 mM for all these parameters). At 20 mM 2,3-butanedione monoxime, force and energy usage fell to near zero and the degree of myosin light chain phosphorylation decreased to resting values, indicating a shut-down of both force-dependent and force-independent energy usage at high concentrations of 2,3-butanedione monoxime. In permeabilized taenia coli, 2,3-butanedione monoxime had little or no depressant effects on force production, ATPase activity or calcium sensitivity. 2,3-butanedione monoxime had a very modest inhibitory effect on the in vitro motility of unregulated actin filaments interacting with thiophosphorylated myosin. In solution, 2,3-butanedione monoxime inhibited myosin light chain kinase, but not the phosphatase (SMP-IV). These results suggest that the major effect of 2,3-butanedione monoxime is not on the contractile proteins themselves, but rather on calcium delivery during excitation, thereby reducing the degree of activation of myosin light chain kinase and subsequent activation of myosin by light chain phosphorylation. Thus, 2,3-butanedione monoxime is not useful for the determination of the energetics of activation processes in smooth muscle because of its inhibition of both force-dependent and force-independent processes. In contrast, in the intact mouse extensor digitorum longus, 2,3-butanedione monoxime inhibits tetanic force production (ED50 approximately 2 mM) to a much greater extent than myosin light chain phosphorylation. When 2,3-butanedione monoxime was used to manipulate force production in muscles at L(o), it was found that approximately 60% of the total energy usage was force-independent and the remainder was force-dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Mechanism of force inhibition by 2,3-butanedione monoxime in rat cardiac muscle: roles of [Ca2+]i and cross-bridge kinetics

1. We investigated the mechanism of force inhibition by 2,3-butanedione monoxime (BDM) on rat cardiac trabeculae. [Ca2+]i was measured by iontophoretic injection of fura-2 salt. Isometric force was recorded at an end-systolic sarcomere length of 2.1-2.2 microns. 2. With an external [Ca2+] of 1 mM, peak twitch force was monotonically reduced with increasing [BMD]; at 5 and 20 mM [BDM], force was 35 and 1% of the control force. In contrast, the mean peak [Ca2+]i during transients was only reduced at [BDM] > or = 10 mM. 3. The duration of the twitch was dramatically reduced by BDM in a dose-dependent fashion with no significant change in the time course of the underlying Ca2+ transients. The abbreviation of twitch force duration was much greater than expected for the observed reduction in peak force by this agent. 4. The mechanism of the inhibition of force by BDM was explored by examining the relationship between twitch force and Ca2+ transients at various values of external [Ca2+]. In the presence of BDM, the steepness of the relationship between peak force and peak [Ca2+]i was reduced compared to control conditions. As a result, significant elevation in the [Ca2+]i transient was unable to reverse the reduction in force observed in the presence of BDM. 5. The direct inhibitory effects of BDM on the contractile system were examined using ryanodine tetani in intact trabeculae to measure the steady-state force-[Ca2+]i relationship. In contrast to the effects on twitch force at 5 mM BDM, maximal force was only reduced to 71% of control. Furthermore, the [Ca2+]i required for half-maximal activation (Ca50) was increased while the Hill coefficient was reduced slightly by BDM. 6. BDM dramatically slowed the rate of rise of tetanic force. At maximal activation, the time required to reach 90% maximal force was prolonged by a factor of 3-8 in the presence of 5 mM BDM. This suggests that the observed reduction in twitch force and steady-state force may result from slowed kinetics of cross-bridge attachment, consistent with recent biochemical studies. 7. The contribution of altered cross-bridge kinetics to the effects of BDM was investigated using a co-operative cross-bridge model of the contractile system. Changing the rate constants for cross-bridge attachment in the model to mimic the reported biochemical effects of BDM reproduced the observed effects of BDM.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical stimulation of contractile activity accelerates growth of cultured neonatal cardiocytes

An electrical stimulation system was designed to regulate synchronized contractile activity of neonatal rat cardiocytes and to examine the effects of mechanical contraction on cardiocyte growth. Continuous electrical stimulation at a pulse duration of 5 milliseconds and frequency of 3 Hz resulted in a time-dependent accumulation of cell protein that reached 34% above initial values, as measured by the protein-to-DNA ratio. The growth response did not occur using voltage amplitudes that were subthreshold for contraction and was independent of contraction frequencies set at > or = 0.5 Hz. The RNA-to-DNA ratio increased in parallel to cell protein, indicating that the capacity for protein synthesis was enhanced by contraction. Rates of 28S rRNA synthesis were accelerated twofold in contracting cardiocytes. By comparison, protein and RNA accumulation did not occur in electrically stimulated cardiocytes in which contraction was blocked by either 10 mumol/L verapamil or by 5 mmol/L 2,3-butanedione monoxime, an inhibitor of actomyosin crossbridge cycling. Electrical stimulation of cardiocyte contraction did not enhance alpha-cardiac actin or myosin heavy chain (alpha+beta) mRNA transcript levels relative to 28S rRNA during the period of rapid growth that occurred over the first 48 hours. It is concluded that (1) electrical stimulation of contraction accelerates cardiocyte growth and RNA accumulation, (2) mechanical contraction is involved in regulating the growth of electrically stimulated cardiocytes, and (3) the levels of alpha-actin and myosin heavy chain mRNA increase in proportion to rRNA during the growth of contracting cardiocytes.

Influence of 2,3-butanedione monoxime on heart energy metabolism

The influence of 2,3-butanedione monoxime (BDM) on function and subcellular energy status in isolated perfused guinea pig hearts was examined during ischemia and reperfusion. For this purpose the mitochondrial and extramitochondrial contents of ATP, ADP, creatine phosphate (CrP) and creatine (Cr) were determined after fractionation of freeze-clamped heart tissue in non-aqueous solvents. Furthermore, the inhibitory action of this compound on isolated cardiac mitochondria and the actomyosin-ATPase was studied. BDM in the millimolar range inhibited both the actomyosin-ATPase in skinned-fibers (IC50 22 mM) and the electron transport chain in isolated mitochondria (IC50 28 mM). In normoxia at 35 degrees C the contractile function of isolated guinea pig hearts was completely inhibited and oxygen consumption was markedly reduced (-60%) by 30 mM BDM. The mitochondrial and extramitochondrial contents of adenine nucleotides (sum of ATP + ADP) and total creatine (sum of CrP + Cr) as well as the extramitochondrial ATP/ADP- and CrP/Cr-ratios were decreased. Similar changes, significantly more pronounced, however, were found after 30 min of warm (35 degrees C) ischemia. However, if hearts were exposed to BDM during cold ischemia, extramitochondrial ATP/ADP- and CrP/Cr-ratios were increased compared to BDM-free controls. If hearts were exposed to BDM during ischemia (at 35 degrees C) and were then reperfused BDM-free, ATP/ADP- and CrP/Cr-ratios were decreased. However, if hearts were exposed to BDM during cold ischemia and were then reperfused BDM-free, extramitochondrial ATP/ADP- and CrP/Cr-ratios were unchanged. These results confirm earlier studies on the tissue protective action of BDM but point to the importance of low temperature exposure to BDM for its beneficial effect.

Optical recordings of the effect of electrical stimulation on action potential repolarization and the induction of reentry in two-dimensional perfused rabbit epicardium

BACKGROUND

Prolonged membrane depolarization induced by an electric shock in the heart may produce propagation block leading to repetitive beats. We studied prolonged depolarization and its role in repetitive beats in a thin epicardial layer of endocardially prefrozen arterially perfused rabbit heart.

METHODS AND RESULTS

A laser scanner recorded optical action potentials at 63 sites within a 1-cm2 area on the left ventricle of hearts stained with potentiometric fluorescent dye. Pacing (S1) produced propagation across the myofibers; then, a 3-millisecond shock (S2) given in the S1 refractory period produced an electric field that decreased in strength with distance along the fibers. The S2 strengths at the center of the scanned region (C) were 2.1 +/- 0.2 or 5.6 +/- 0.3 V/cm (mean +/- SD, n = 4). Repetitive beats occurred in 50% of hearts when C was 2.1 V/cm and in 100% of hearts when C was 5.6 V/cm. With each occurrence of repetitive beats, prolonged depolarization of the shocked action potential occurred within 1 mm of the S2 electrode when C was 2.1 V/cm and within 3 mm when C was 5.6 V/cm. Transient block immediately after S2 occurred between tissue with prolonged depolarization (S2 strength, 6 to 9 V/cm) and tissue without prolonged depolarization (S2 strength, 1 to 3 V/cm). Propagation in the scanned region after S2 occurred first on the side of the block distal to the S2 electrode, propagated from the most recovered to the least recovered tissue, and then turned toward the S2 electrode. When C was 5.6 V/cm, reentry by retrograde propagation near the S2 electrode produced repetitive beats. The center of the reentrant circuit exhibited further transient block and small depolarizations associated with the circulating activation.

CONCLUSIONS

Prolonged depolarization occurs where the S2 strength is more than 6 V/cm, block occurs between regions of prolonged depolarization and no prolonged depolarization, and reentry occurs around the block. Shock-induced prolonged depolarization can be proarrhythmic and may account for electrically induced arrhythmias.

Potentiation of sarcoplasmic reticulum Ca2+ release by 2,3-butanedione monoxime in crustacean muscle

The effect of the chemical phosphatase 2,3-butanedione monoxime (BDM) on various aspects of excitation/contraction coupling in crustacean muscle was investigated. Despite having a depressant effect on vertebrate skeletal and cardiac muscle, BDM was a potentiator of contraction in crustacean muscle. At concentrations of 1-3 mM BDM caused an increase of potassium contractures in bundles of fibers isolated from crayfish muscle. At higher concentrations BDM caused oscillatory contractions by itself. In single voltage-clamped cut muscle fibers loaded with rhod-2, BDM (0.5-2 mM) potentiated the magnitude and duration of intracellular Ca2+ transients elicited by depolarization. At the same time BDM did not affect the rate of Ca2+ removal from the myoplasm under conditions where Ca2+ release was blocked by tetracaine. Nor did BDM increase Ca2+ entry; in fact it caused a decrease in the amplitude of the inward Ca2+ current (ICa). In microsomes isolated from lobster muscle, BDM also potentiated Ca2+ release induced by caffeine and at higher concentrations (above 3 mM) induced release by itself. At the same time it had little effect on Ca2+ uptake. These results indicate that BDM potentiates Ca2+ release in crustacean muscle possibly by dephosphorylation of the Ca(2+)-release channel.

Alterations in intracellular calcium and tension of activated ferret papillary muscle in response to step length changes

1. To study the effects of mechanical constraints on the calcium (Ca2+) affinity of cardiac troponin C, we analysed the tension and aequorin light (AL, intracellular Ca2+) transients in response to a step length change in aequorin-injected ferret right ventricular papillary muscles. The muscle preparations were continuously activated with ouabain (10(-4) M) (ouabain contracture) or with high frequency stimuli in the presence of ryanodine (5 microM) (tetanic contraction). 2. The tension transient in response to either the release or stretch was oscillatory: tension decreased rapidly during the release and then increased, after which it lapsed into a new steady level in a series of damped oscillations. The opposite was true for the stretch. The oscillatory responses were conspicuous and less damped in ouabain-activated preparations (oscillation frequency of 2.2-2.3 Hz at 22 degrees and 4.5-4.6 Hz at 30 degrees C) and much more damped in ryanodine-treated preparations. 3. The transient AL response was also oscillatory, the time course of which corresponded to that of the transient tension response. Regardless of the difference in the time course of the transients in two different preparations and at two different temperatures, the increase in AL corresponded to the decrease in tension, likewise the decrease in AL to the increase in tension. 4. The mean level of AL after release was lower than the control level present just prior to the release in ouabain-activated preparations, but the AL after release finally returned to the nearly control level in ryanodine-treated preparations. 5. When the ryanodine-treated muscle was further treated with 2,3-butanedione monoxime (BDM) (20 mM), the tetanic tension decreased remarkably without affecting the AL signal. The tension transient of this preparation was quite similar to that of the resting muscle, which changed in a nearly stepwise fashion; AL was hardly affected by step length changes, as in the resting muscle, in spite of the higher AL level. 6. These results suggest that the Ca2+ affinity of cardiac troponin C is increased with an increase in tension (i.e. the cross-bridge attachment) and decreased with a decrease in tension i.e. the cross-bridge detachment), and that the mean [Ca2+]i is lowered by release, at least in a Ca(2+)-overloaded condition, mainly through the sarcoplasmic reticulum.