Calcium sensitisers: mechanisms of action and potential usefulness as inotropes

Computational approaches identify a transcriptomic fingerprint of drug-induced structural cardiotoxicity

Structural cardiotoxicity (SCT) presents a high-impact risk that is poorly tolerated in drug discovery unless significant benefit is anticipated. Therefore, we aimed to improve the mechanistic understanding of SCT. First, we combined machine learning methods with a modified calcium transient assay in human-induced pluripotent stem cell-derived cardiomyocytes to identify nine parameters that could predict SCT. Next, we applied transcriptomic profiling to human cardiac microtissues exposed to structural and non-structural cardiotoxins. Fifty-two genes expressed across the three main cell types in the heart (cardiomyocytes, endothelial cells, and fibroblasts) were prioritised in differential expression and network clustering analyses and could be linked to known mechanisms of SCT. This transcriptomic fingerprint may prove useful for generating strategies to mitigate SCT risk in early drug discovery.

Functional and structural differences between skinned and intact muscle preparations

Myofilaments and their associated proteins, which together constitute the sarcomeres, provide the molecular-level basis for contractile function in all muscle types. In intact muscle, sarcomere-level contraction is strongly coupled to other cellular subsystems, in particular the sarcolemmal membrane. Skinned muscle preparations (where the sarcolemma has been removed or permeabilized) are an experimental system designed to probe contractile mechanisms independently of the sarcolemma. Over the last few decades, experiments performed using permeabilized preparations have been invaluable for clarifying the understanding of contractile mechanisms in both skeletal and cardiac muscle. Today, the technique is increasingly harnessed for preclinical and/or pharmacological studies that seek to understand how interventions will impact intact muscle contraction. In this context, intrinsic functional and structural differences between skinned and intact muscle pose a major interpretational challenge. This review first surveys measurements that highlight these differences in terms of the sarcomere structure, passive and active tension generation, and calcium dependence. We then highlight the main practical challenges and caveats faced by experimentalists seeking to emulate the physiological conditions of intact muscle. Gaining an awareness of these complexities is essential for putting experiments in due perspective.

Omecamtiv mecarbil evokes diastolic dysfunction and leads to periodic electromechanical alternans

Omecamtiv mecarbil (OM) is a promising novel drug for improving cardiac contractility. We tested the therapeutic range of OM and identified previously unrecognized side effects. The Ca2+ sensitivity of isometric force production (pCa50) and force at low Ca2+ levels increased with OM concentration in human permeabilized cardiomyocytes. OM (1 µM) slowed the kinetics of contractions and relaxations and evoked an oscillation between normal and reduced intracellular Ca2+ transients, action potential lengths and contractions in isolated canine cardiomyocytes. Echocardiographic studies and left ventricular pressure-volume analyses demonstrated concentration-dependent improvements in cardiac systolic function at OM concentrations of 600-1200 µg/kg in rats. Administration of OM at a concentration of 1200 µg/kg was associated with hypotension, while doses of 600-1200 µg/kg were associated with the following aspects of diastolic dysfunction: decreases in E/A ratio and the maximal rate of diastolic pressure decrement (dP/dtmin) and increases in isovolumic relaxation time, left atrial diameter, the isovolumic relaxation constant Tau, left ventricular end-diastolic pressure and the slope of the end-diastolic pressure-volume relationship. Moreover, OM 1200 µg/kg frequently evoked transient electromechanical alternans in the rat in vivo in which normal systoles were followed by smaller contractions (and T-wave amplitudes) without major differences on the QRS complexes. Besides improving systolic function, OM evoked diastolic dysfunction and pulsus alternans. The narrow therapeutic window for OM may necessitate the monitoring of additional clinical safety parameters in clinical application.

Inodilators in septic shock: should these be used?

Septic shock involves a complex interaction between abnormal vasodilation, relative and/or absolute hypovolemia, myocardial dysfunction, and altered blood flow distribution to the tissues. Fluid administration, vasopressor support and inotropes, represent fundamental pieces of quantitative resuscitation protocols directed to assist the restoration of impaired tissue perfusion during septic shock. Indeed, current recommendations on sepsis management include the use of inotropes in the case of myocardial dysfunction, as suggested by a low cardiac output, increased filling pressures, or persisting signals of tissue hypoperfusion despite an adequate correction of intravascular volume and mean arterial pressure by fluid administration and vasopressor support. Evidence supporting the use of inotropes in sepsis and septic shock is mainly based on physiological studies. Most of them suggest a beneficial effect of inotropes on macro hemodynamics especially when sepsis coexists with myocardial dysfunction; others, however, have demonstrated variable results on regional splanchnic circulation, while others suggest favorable effects on microvascular distribution independently of its impact on cardiac output. Conversely, impact of inodilators on clinical outcomes in this context has been more controversial. Use of dobutamine has not been consistently related with more favorable clinical results, while systematic administration of levosimendan in sepsis do not prevent the development of multiorgan dysfunction, even in patients with evidence of myocardial dysfunction. Nevertheless, a recent metanalysis of clinical studies suggests that cardiovascular support regimens based on inodilators in sepsis and septic shock could provide some beneficial effect on mortality, while other one corroborated such effect on mortality specially in patients with proved lower cardiac output. Thus, using or not inotropes during sepsis and septic shock remains as controversy matter that deserves more research efforts.

Inotropes and inodilators for acute heart failure: sarcomere active drugs in focus

Acute heart failure (AHF) emerges as a major and growing epidemiological concern with high morbidity and mortality rates. Current therapies in patients with acute heart failure rely on different strategies. Patients with hypotension, hypoperfusion, or shock require inotropic support, whereas diuretics and vasodilators are recommended in patients with systemic or pulmonary congestion. Traditionally inotropic agents, referred to as Ca²⁺ mobilizers load the cardiomyocyte with Ca²⁺ and thereby increase oxygen consumption and risk for arrhythmias. These limitations of traditional inotropes may be avoided by sarcomere targeted agents. Direct activation of the cardiac sarcomere may be achieved by either sensitizing the cardiac myofilaments to Ca²⁺ or activating directly the cardiac myosin. In this review, we focus on sarcomere targeted inotropic agents, emphasizing their mechanisms of action and overview the most relevant clinical considerations.

Berbamine increases myocardial contractility via a Ca2+-independent mechanism

Berbamine (BM), a natural compound derived from Berberis vulgaris L, has been reported to inhibit cardiac contractile function at higher concentrations. Here, we report that BM had concentration-dependent biphasic effects on myocardial contraction in Langendorff-perfused rat hearts, that is, at lower concentrations (30-100 nM), it displayed positive inotropic and lusitropic effects, whereas at a higher concentration of 1 μM, it caused a negative inotropic effect after an initially weak increase. These effects were further confirmed in cardiomyocytes isolated from the left ventricles of rats. Moreover, the increased cell shortening by BM at concentrations from 0.1 to 100 nM was not associated with an alteration of intracellular Ca transients. Consistently, at 30 nM, BM shifted the cell shortening--Ca transient relationship curve induced by cumulative elevation of extracellular Ca concentration to the left. Furthermore, BM significantly increased membrane-bound but not filament-bound protein kinase C epsilon (PKCε) in the isolated hearts and cardiomyocytes. Such a translocation was inhibited by PKCε-specific inhibitor PKCε V1-2 concomitant with the abolishment of the BM-induced increase in contraction. These findings reveal the positive inotropic effect of BM in the myocardium and demonstrate that BM increases myocardial contractility by increasing myofilament Ca sensitivity via a PKCε-dependent signaling pathway.

Cardiac function during mild hypothermia in pigs: increased inotropy at the expense of diastolic dysfunction

AIM

The induction of mild hypothermia (MH; 33 degrees C) has become the guideline therapy to attenuate hypoxic brain injury after out-of-hospital cardiopulmonary resuscitation. While MH exerts a positive inotropic effect in vitro, MH reduces cardiac output in vivo and is thus discussed critically when severe cardiac dysfunction is present in patients. We thus assessed the effect of MH on the function of the normal heart in an in vivo model closely mimicking the clinical setting.

METHODS

Ten anaesthetized, female human-sized pigs were acutely catheterized for measurement of pressure-volume loops (conductance catheter), cardiac output (Swan-Ganz catheter) and for vena cava inferior occlusion. Controlled MH (from 37 to 33 degrees C) was induced by a vena cava inferior cooling catheter.

RESULTS

With MH, heart rate (HR) and whole body oxygen consumption decreased, while lactate levels remained normal. Cardiac output, left ventricular (LV) volumes, peak systolic and end-diastolic pressure and dP/dt(max) did not change significantly. Changes in dP/dt(min) and the time constant of isovolumetric relaxation demonstrated impaired active relaxation. In addition, MH prolonged the systolic and shortened the diastolic time interval. Pressure-volume analysis revealed increased end-systolic and end-diastolic stiffness, indicating positive inotropy and reduced end-diastolic distensibility. Positive inotropy was preserved during pacing, while LV end-diastolic pressure increased and diastolic filling was substantially impaired due to delayed LV relaxation.

CONCLUSION

MH negatively affects diastolic function, which, however, is compensated for by decreased spontaneous HR. Positive inotropy and a decrease in whole body oxygen consumption warrant further studies addressing the potential benefit of MH on the acutely failing heart.

Acute heart failure: inotropic agents and their clinical uses

Inotropic agents are indispensable for the improvement of cardiac contractile dysfunction in acute or decompensated heart failure. Clinically available agents, including sympathomimetic amines (dopamine, dobutamine, noradrenaline) and selective phosphodiesterase-3 inhibitors (amrinone, milrinone, olprinone and enoximone) act via cAMP/protein kinase A (PKA)-mediated facilitation of intracellular Ca2+ mobilisation. Phosphodiesterase-3 inhibitors also have a vasodilatory action, which plays a role in improving haemodynamic parameters in certain patients, and are termed inodilators. The available inotropic agents suffer from risks of Ca2+ overload leading to arrhythmias, myocardial cell injury and ultimately, cell death. In addition, they are energetically disadvantageous because of an increase in activation energy and cellular metabolism. Furthermore, they lose their effectiveness under pathophysiological conditions, such as acidosis, stunned myocardium and heart failure. Pimobendan and levosimendan (that act by a combination of an increase in Ca2+ sensitivity and phosphodiesterase-3 inhibition) appear to be more beneficial among existing agents. Novel Ca2+ sensitisers that are under basic research warrant clinical trials to replace available inotropic agents.

Use of the Ca-shortening curve to estimate the myofilament responsiveness to Ca2+ in tetanized rat ventricular myocytes

We previously estimated the myofilament responsiveness to Ca(2+) in isolated intact ventricular myocytes, using the steady-state relationship between cytosolic Ca(2+) concentration ([Ca(2+)](i)) and cell-shortening during tetanus (Ca-L trajectory). This method was useful and easy; however, it could not be used for a high dose of Ca sensitizer because the instantaneous plots after the application of Ca sensitizer did not make a fixed point of shortening (we used 5% shortening). Therefore we must produce another method to investigate Ca(2+) responsiveness. For an estimation of a wider range of the Ca-L trajectory, we fitted the Ca-L trajectory data with the Hill equation to construct the Ca-shortening curve. To fit this curve, we measured the maximal shortening, which was on average 31.6%. The value of [Ca(2+)](i) to produce the half-maximal shortening (Ca(50)) was dose-dependently decreased by EMD57033 (sensitization). Either isoproterenol or 3-isobutyl-1-methylxanthine increased Ca(50) (desensitization) with a concomitant increase in intracellular c-AMP. EMD57439, a selective PDE-III inhibitor, did not significantly increase the c-AMP concentration and produced little change in Ca(50). These results are in agreement with previous reports with skinned or intact multicellular preparations. The Ca-shortening curve constructed in intact cardiac myocytes can be used to estimate the myofibrillar responsiveness to Ca(2+) in a wide range of [Ca(2+)](i).

Dual regulation of myofilament Ca2+ sensitivity by levosimendan in normal and acidotic conditions in aequorin-loaded canine ventricular myocardium

Experiments were carried out in canine ventricular trabeculae loaded with aequorin to investigate the effects of levosimendan {(R)-([4-(1,4,5,6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl)phenyl]-hydrazono)-propanedinitrile} on contractile force and Ca(2+) transients in normal and acidotic conditions. The concentration-response curve for the positive inotropic effect (PIE) of levosimendan was bell-shaped, that is, it declined markedly at 10(-4) M after achieving the maximum at 10(-5) M in normal (pH(o)=7.4) and acidotic conditions (pH(o)=6.6). The positive inotropic effect (PIE) of levosimendan up to 10(-5) M was associated with an increase in Ca(2+) transients and a shift of the relationship of Ca(2+) transients and force to the left of that of elevation of [Ca(2+)](o). Levosimendan at 10(-4) M elicited a negative inotropic effect (NIE) in association with a further increase in Ca(2+) transients, and during washout Ca(2+) transients increased further, while the force was abolished before both signals recovered to the control. In acidotic conditions, the relationship of Ca(2+) transients and force during the application of levosimendan in normal conditions was essentially unaltered, whereas the PIE was suppressed due to attenuation of the increase in Ca(2+) transients. In summary, in intact canine ventricular myocardium, levosimendan elicits a dual inotropic effect: at lower concentrations, it induces a PIE by a combination of increases in Ca(2+) transients and Ca(2+) sensitivity, while at higher concentrations it elicits an NIE due to a decrease in Ca(2+) sensitivity. Acidosis inhibits the PIE of levosimendan due to suppression of the increase in Ca(2+) transients in response to the compound.

Dissociation Between Metabolic and Efficiency Effects of Perhexiline in Normoxic Rat Myocardium

The antianginal agent perhexiline inhibits rat cardiac carnitine palmitoyltransferase-1 (CPT-1) and CPT-2, key enzymes for mitochondrial transport of long-chain fatty acids. We tested the hypothesis that perhexiline, in therapeutic concentrations (2 microM), inhibits palmitate oxidation and enhances glucose oxidation in isolated rat cardiomyocytes and in the working rat heart, thereby increasing efficiency of oxygen utilization. In isolated cardiomyocytes, perhexiline (2 microM) exerted no acute effects on palmitate oxidation, but after 48 hours pre-exposure oxidation was inhibited by perhexiline (2 to 10 microM) by 15% to 35% (P < 0.0002). In non-ischemic working rat hearts (3%BSA, 0.4 mM palmitate, 11 mM glucose, 100 microU/mL insulin) perhexiline (2 microM) had no significant acute effect on cardiac efficiency, palmitate or glucose oxidation, but 24 hours pretreatment with transdermal perhexiline increased cardiac work (by 29%, P < 0.05) and cardiac efficiency (by 30%, P < 0.02) without significant effects on palmitate oxidation. The selective CPT-1 inhibitor oxfenicine (2 mM) inhibited palmitate oxidation and enhanced glucose oxidation, but failed to enhance cardiac efficiency. In conclusion, in the non-ischemic working rat heart, perhexiline increases myocardial efficiency by a mechanism(s) that is largely or entirely independent of its effects on CPT. Effects on cardiac efficiency during ischemia, and with changes in fatty acid oxidation after longer perhexiline pretreatment remain to be determined.

The therapeutic potential of novel cardiotonic agents

During the course of treatment of heart failure patients, cardiotonic agents are inevitable for improvement of myocardial dysfunction. Clinically available agents, such as beta-adrenoceptor agonists and selective phosphodiesterase 3 inhibitors, act mainly via cyclic AMP/protein kinase A-mediated facilitation of Ca(2+) mobilisation (upstream mechanism). These agents are associated with the risk of Ca(2+) overload leading to arrhythmias, myocardial cell injury and premature cell death. In addition, they are energetically disadvantageous because of an increase in activation energy and metabolic effects. Cardiac glycosides act also via an upstream mechanism and readily elicit Ca(2+) overload with a narrow safety margin. No currently available agents act primarily via an increase in the myofilament sensitivity to Ca(2+) ions (central and/or downstream mechanisms). Novel Ca(2+) sensitisers under basic research may deserve clinical trials to examine the therapeutic potential to replace currently employed agents in acute and chronic heart failure patients. Molecular mechanisms of action of Ca(2+) sensitisers are divergent. In addition, they show a wide range of discrete pharmacological profiles due to additional actions associated with individual compounds. Therefore, the outcome of clinical trials has to be explained carefully based on these mechanisms of actions.

SR33805, a Ca2+ antagonist with length-dependent Ca2+ -sensitizing properties in cardiac myocytes

1. This study examined the effects of SR33805, a fantofarone derivative with reported strong Ca(2+) -antagonistic properties, on the contractile properties of intact and skinned rat ventricular myocytes. 2. On intact cells loaded with the Ca(2+)-fluorescent indicator Indo-1, the application of low concentrations of SR33805 enhanced the amplitude of unloaded cell shortening and decreased the duration of cell shortening. Amplitude of the Ca(2+) transient was also decreased. 3. These effects were accompanied with a shortening of the action potential and a dose-dependent blockade of L-type calcium current (IC(50)=2.4 x 10(-8) M). 4. On skinned cardiac cells, the application of a low SR33805 concentration (10(-8) M) induced a significant increase in maximal Ca(2+)-activated force at the two-tested sarcomere lengths (SLs), 1.9 and 2.3 microm. 5. The application of a larger dose of SR33805 (10(-6)-10(-5) M) induced a significant leftward shift of the tension-pCa relation that accounts for Ca(2+)-sensitization of the myofilaments, particularly at 2.3 microm SL. 6. In conclusion, despite its strong Ca(2+)-antagonistic properties SR33805 increases cardiac cell contractile activity as a consequence of its Ca(2+)-sensitizing effects. These effects are attributable to both an increase in the maximal Ca(2+)-activated force and a length-dependent Ca(2+)-sensitization.

Mechanisms of action of novel cardiotonic agents

Regulation of myocardial contractility by cardiotonic agents is achieved by an increase in intracellular Ca2+ mobilization (upstream mechanism), an increase in Ca2+ binding affinity to troponin C (central mechanism), or facilitation of the process subsequent to Ca2+ binding to troponin C (downstream mechanism). cAMP mediates the regulation induced by Ca2+ mobilizers such as beta-adrenoceptor agonists and selective phosphodiesterase III inhibitors acting through the upstream mechanism. These agents act likewise on the central mechanism to decrease Ca2+ sensitivity of troponin C in association with the cAMP-mediated phosphorylation of troponin I. In addition to such a well-known action of cAMP, recent experimental findings have revealed that Ca2+ sensitizers, such as levosimendan, OR-1896, and UD-CG 212 Cl, require the cAMP-mediated signaling for induction of Ca2+ sensitizing effect. These agents shift the [Ca2+] -force relationship to the left, but their positive inotropic effect (PIE) is inhibited by carbachol, which suppresses selectively the cAMP-mediated PIE. These findings imply that cAMP may play a crucial role in increasing the myofilament Ca2+ sensitivity by cross-talk with the action of individual cardiotonic agents. No clinically available cardiotonic agents act primarily via Ca2+ sensitization, but the PIE of pimobendan and levosimendan is partly mediated by an increase in myofilament Ca2+ sensitivity. Evidence is accumulating that cardiotonic agents with Ca2+ sensitizing action are more effective than agents that act purely via the upstream mechanism in clinical settings. Further clinical trials are required to establish the effectiveness of Ca2+ sensitizers in long-term therapy for congestive heart failure patients.

Beneficial effects of the Ca2+ sensitizer EMD 57033 in exercising pigs with infarction-induced chronic left ventricular dysfunction

1. It is unknown how cardiac stimulation by Ca(2+) sensitization modulates the cardiovascular response to exercise when left ventricular (LV) function is chronically depressed following a myocardial infarction. We therefore investigated the effects of EMD 57033 at rest and during exercise and compared these to those of the mixed Ca(2+)-sensitizer/phosphodiesterase-III inhibitor pimobendan. 2. Pigs were chronically instrumented for measurement of cardiovascular performance. At the time of instrumentation, infarction was produced by coronary artery ligation (MI, n=12). Studies in MI were performed in the awake state, 2 - 3 weeks after infarction. 3. MI were characterized by a lower resting cardiac output (18%), stroke volume (30%) and LVdP/dt(max) (18%), and a doubling of LV end-diastolic pressure, compared to normal pigs (N, n=13). 4. In 11 resting MI, intravenous EMD 57033 (0.2 - 0.8 mg kg(-1) min(-1)) increased LVdP/dt(max) (57+/-5%) and stroke volume (26+/-6%) with no effect on heart rate, LV filling pressure, and myocardial O(2)-consumption, similar to N. 5. In MI, the effects of EMD 57033 (0.4 mg kg(-1) min(-1), IV) on stroke volume and LVdP/dt(max) were maintained during treadmill exercise up to 85% of maximal heart rate, while heart rate was lower compared to control exercise (all P<0.05). In contrast, the effects of EMD57033 gradually waned in N at increasing intensity of exercise. 6. Compared to N, the cardiostimulatory effects of pimobendan (20 microg kg(-1) min(-1), IV) were blunted in MI both at rest and during exercise compared to N. 7. In conclusion, the positive inotropic actions of the Ca(2+) sensitizer EMD 57033 are unmitigated in resting and exercising MI compared to N, while those of the mixed Ca(2+)-sensitizer/phosphodiesterase-III inhibitor pimobendan are blunted.

Localization of regions of troponin I important in deactivation of cardiac myofilaments by acidic pH

Ca2+-activation of cardiac muscle myofilaments is more sensitive to depression by acidic pH than is the case with skeletal myofilaments. We tested the hypothesis that this difference is related to specific regions of the TnI (troponin I) isoforms in these muscles. We exchanged native Tn complex in detergent-extracted fiber bundles from mouse ventricles with Tn containing various combinations of fast (fsTnI) or slow skeletal (ssTnI) complexed with either cardiac TnC (cTnC) or fsTnC, and with cTnC complexed with the following chimeras: (1) fsTnI N-terminal region (fN) plus cTnI inhibitory peptide (cIp) and cTnI C-terminal region (cC); and (2) cTnI N-terminal region (cN)-cIp-fsTnI C-terminal region (fC). We determined the change in half maximal Ca2+(DeltaEC50) for tension activation at pH 7.0 and pH 6.5. Similar DeltaEC50 values were obtained for unextracted controls (5.53+/-0.30 microm), for preparations containing cTnI-cTnC (5.74+/-0.40 microm), and preparations exchanged with cTnI-fsTnC (5.63+/-0.40 microm). However, replacement of cTnI with fsTnI significantly decreased DeltaEC50 to 3.95+/-0.17 microm. Replacement of cTnI with ssTnI also significantly depressed DeltaEC50 to 2.07+/-0.15 microm. Results of studies using the chimeras demonstrated that the C-terminal domains of cTnI and fsTnI are responsible for these differences. This conclusion also fits with data from experiments in which we measured Ca2+-binding to the regulatory site of cTnC in binary complexes containing cTnC with cTnI, fsTnI, or the chimeras. Our results localize a region of TnI important in effects of acidosis on cardiac myofilaments and extend our earlier data indicating that C-terminal regions of cTnI outside the Ip are critical for activation by Ca2+.

Positive inotropic effects of calcium sensitizers on normal and failing cardiac myocytes

Calcium sensitizers increase myocardial contractile function without affecting Ca2+ homeostasis, which might be beneficial in the treatment of patients with heart failure. However, it remains uncertain whether Ca sensitizers induce quantitatively similar inotropic responses in control and failing hearts. To compare their effects in normal versus failing hearts at the cellular level, shortening mechanics and intracellular calcium ([Ca2+]i) transient were simultaneously measured in the left ventricular myocytes isolated from normal dogs (n = 8) and dogs with rapid pacing-induced heart failure (n = 7). CGP 48506 and EMD 57033 exerted a positive inotropic effect in a dose (0.1-3 microM)-dependent manner in both normal and heart failure myocytes. The percent increase of cell shortening magnitude was comparable between the two groups. CGP 48506 and EMD 57033 did not affect the diastolic cell length and resting [Ca2+]i level. They did not affect the duration of [Ca2+]i transient dynamics. Thus Ca2+ sensitizers exerted comparable positive inotropic effects without affecting the rest cell length and rest [Ca2+]i in normal and heart failure myocytes.

In vivo evidence that EMD 57033 restores myocardial responsiveness to intracoronary Ca(2+) in stunned myocardium

Despite ample in vitro evidence that myofilament Ca(2+)-responsiveness of stunned myocardium is decreased, in vivo data are inconclusive. Conversely, while Ca(2+)-sensitizing agents increase myofilament Ca(2+)-responsiveness in vitro, it has been questioned whether this also occurs in vivo. We therefore tested in open-chest anesthetized pigs whether EMD 57033 (the (+) enantiomer of 5-[1-(3,4-dimethoxybenzoyl)-1,2,3, 4-tetrahydro-6-quinolyl]-6-methyl-3,6-dihydro-2H-1,3, 4-thiadiazin-2-one) increases responsiveness to Ca(2+) of non-stunned myocardium and restores function of stunned myocardium by normalizing the responsiveness to Ca(2+). Studies were performed under beta-adrenoceptor blockade to minimize the contribution of the phosphodiesterase-III inhibitory actions of EMD 57033. Consecutive intracoronary Ca(2+) infusions were used to evaluate the contractile response (assessed by the left ventricular end-systolic elastance, E(es)) to added Ca(2+) of non-stunned myocardium and myocardium stunned by 15 min coronary artery occlusion and 30 min reperfusion. In non-stunned propranolol-treated myocardium, the Ca(2+) infusions doubled E(es) (baseline 6.9+/-0.9 mmHg mm(-2), n=8). Following Ca(2+)-washout, subsequent EMD 57033 infusion (0.1 mg kg(-1) min(-1), i.v.) tripled E(es) (P<0.05) and potentiated the Ca(2+)-induced increase in E(es) to 55.7+/-10.0 mmHg mm(-2) (P<0.05). Stunning (n=7) decreased E(es) to 5.3+/-0.6 mmHg mm(-2) (P>0.10) and attenuated the Ca(2+)-induced increase in E(es) (P<0.05). Subsequent infusion of EMD 57033 increased E(es) to 6.8+/-1.8 mmHg mm(-2) (P<0. 05) and restored responsiveness to added Ca(2+). These in vivo findings are consistent with the in vitro observations that myofilament Ca(2+)-responsiveness of stunned myocardium is reduced and that EMD 57033 increases contractility by enhancing myofilament Ca(2+)-responsiveness.

Capacitative Ca2+ entry involves Ca2+ influx factor in rat glioma C6 cells

Capacitative Ca2+ entry exists in rat glioma C6 cells; however, how the information of depletion of Ca2+ in intracellular stores transmits to the plasma membrane is unknown. In the present study, we examined whether Ca2+ influx factor (CIF) causes capacitative Ca2+ entry in C6 cells. CIF was extracted from non-treated (Non-CIF), bombesin-treated (BBS-CIF) and thapsigargin-treated (TG-CIF) C6 cells by a reverse-phase silica cartridge. The addition of BBS-CIF and TG-CIF gradually increased cytoplasmic Ca2+ concentration ([Ca2+]i) but Non-CIF did not increase [Ca2+]i. Neither BBS-CIF nor TG-CIF elevated [Ca2+]i in the absence of extracellular Ca2+. Gd3+ inhibited the increase in [Ca2+]i induced by BBS-CIF and TG-CIF. Genistein abolished an elevation of [Ca2+]i induced by BBS-CIF and TG-CIF. BBS-CIF and TG-CIF did not increase inositol 1,4,5-trisphosphate accumulation. The results suggest that capacitative Ca2+ entry is caused by CIF in rat glioma C6 cells.

Effect of bepridil on intracellular calcium concentration and contraction in cultured rat ventricular myocytes

We studied the effects of a new antiarrhythmic and antianginal agent, bepridil, on the intracellular calcium transient and contraction of cultured neonatal rat ventricular cells, and compared the effects with those caused by an authentic Ca2+ -entry blocker, D600 (methoxyverapamil). The Ca2+ transient was measured by using dual-wavelength microfluorometry of fura-2. The contraction was measured as a shortening of cell aggregates with the use of a video image-analyzing system. Both bepridil (1-30 microM) and D600 (1-30 microM) decreased the peak systolic amplitude of the Ca2+ transient in a concentration- and frequency-dependent manner. Bepridil, but not D600, significantly shortened the half-decay time of the Ca2+ transient and prolonged the time course of the contraction. D600 decreased the contraction in parallel with the decrease in the peak Ca2+ transient, whereas bepridil exerted no significant effect on the contraction. Bepridil (10 microM) induced a leftward shift (to lower amplitude of peak systolic Ca2+ transient) of the relation between the magnitude of contraction and the peak systolic Ca2+ transient, which was obtained by changing external Ca2+ concentration. In contrast, D600 (10 microM) did not affect the relation. The results suggest that the negative inotropic effect of bepridil (caused by its Ca2+ channel-blocking effect) is offset by its simultaneous increase in the sensitivity of contractile protein(s) to intracellular Ca2+, which may be a unique characteristic of this antiarrhythmic agent in a clinical setting.

Calcium sensitivity and the effect of the calcium sensitizing drug pimobendan in the alcoholic isolated rat atrium

We compared the effect of inotropic interventions (isoproterenol and pimobendan) and the relation between Ca2+ and isometric twitch force in atrial muscle from control rats and rats that had consumed alcohol for 2 months. At extracellular Ca2+ concentrations of 1-4 mM, alcohol atria developed less force than the controls. The median effective concentration (EC50) for extracellular Ca2+ was 3.2 +/- 0.01 mM for the alcohol group and 2.8 +/- 0.001 mM for the control group, whereas at maximal Ca2+, developed force was the same in both groups. To test whether the myofilament response to Ca2+ is altered with chronic alcohol consumption, we measured the relation between Ca2+ and force of atrial fiber bundle preparations extracted with Triton X-100. The Ca2+-force relation of alcohol atria (EC50 = 2.4 +/- 0.001 microM) was significantly shifted to the right of that of the control atria (EC50 = 1.94 +/- 0.001 microM Ca2+). Compared with controls, the alcohol atria demonstrated a significant depression in the inotropic effect of the beta-adrenergic agonist isoproterenol over a broad concentration range (10(-9)-10(-6) M). We also tested the effect of pimobendan, an inotropic agent with both phosphodiesterase-inhibiting and myofilament Ca2+-sensitizing actions. Developed force at concentrations of pimobendan <75 microM was similar between groups. However, at concentrations of pimobendan >75 microM, the developed force in alcohol atria was significantly less than control. Our results indicate that 2 months of alcohol consumption is associated with decreases in myofilament Ca2+ sensitivity and altered responsiveness to different inotropic agents.

Reappraisal of the multicellular preparation for the in vitro physiopharmacological evaluation of myocardial performance

In order to evaluate myocardial performance, single cardiomyocytes suffer from technical problems and from the fact that some basic functional properties vanish when one moves down the hierarchic scale from multicellularity to single cells. The isolated papillary muscle has at present proven to be superior to the isolated intact cardiomyocyte. A large number of major intra- and extracellular features required to describe myocardial performance can be derived from analyzing twitch contraction and relaxation in the multicellular isolated papillary muscle. In addition, the present paper illustrates the possibility to differentiate between effects of inotropic interventions on activating Ca2+ and Ca2+ sensitivity in multicellular preparations, from a grid analysis of isometric twitches in a coordinate system of peak rate of force development (+dF/dt; reflecting the time pattern of twitch contraction) versus time to half relaxation (tHR; reflecting the time pattern of twitch relaxation). The abundance of information about myocardial performance that can be derived from the easily accessible multicellular preparation reflects its physiological kinship with the intact ventricle.