Inotropic actions of BDF 9148 and DPI 201-106 and their enantiomers in guinea-pig, rat and human atria

A sodium channel enhancer, LY341311, increases myocardial contractile performance without increasing heart rate in conscious normal dogs: a comparison with dobutamine

BACKGROUND

Catecholamines and many inotropic agents increase cardiac contractility but also cause excessive myocardial O2 consumption (MVO2). We determined if the novel Na+ channel enhancer LY341311, which increases myocardial contractility independent of beta receptors, can produce significant cardiac inotropic effects compared with dobutamine but at lower oxygen cost in conscious dogs.

METHODS AND RESULTS

Mongrel dogs were chronically instrumented for measurement of arterial pressure, left ventricular (LV) pressure and internal diameter, coronary blood flow, and arterial and coronary sinus O2 content. Both LY341311 and dobutamine produced dose-dependent increases in LV dP/dt, dP/dt/40, fractional shortening, and cardiac stroke work and minute work estimated from the LV pressure-diameter loop. The major difference between LY341311 and dobutamine was an opposing effect on heart rate with LY341311 slightly reducing it but dobutamine markedly increasing it. LY341311 caused a significantly smaller increase in MVO2 than dobutamine (P <.05) and produced similar cardiac inotropic effects, yielding a higher cardiac mechanical efficiency than dobutamine. However, after pacing to match heart rate with dobutamine LY341311 increased MVO2 markedly, approaching the same level as with dobutamine.

CONCLUSIONS

The novel Na+ channel enhancer LY341311 caused significant increases in myocardial contractility and contractile performance without increasing heart rate. It had a beneficial energetic effect on the heart with significantly less O2 cost and improved cardiac mechanical efficiency.

Identification of a cardiac sodium channel insensitive to synthetic modulators

BACKGROUND

DPI 201-106 (DPI) was the first synthetic compound showing cardioselective modulation of voltage-gated sodium channels (VGSCs) resulting in a positive inotropic effect. Currently, the exact mode of action for this class of compounds is not known.

METHODS

Effects of different natural and synthetic sodium channel modulators were investigated in cardiac tissue of several species with conventional electrophysiologic methods.

RESULTS

In electrically driven cardiac tissues, all compounds investigated increased force of contraction (FC) and action potential duration (APD) with increasing concentrations except for DPI in cattle trabecular muscle, which demonstrated no effect. Interestingly, calculation of EC50 levels at 30% repolarization demonstrates that natural VGSC-ligands were highly potent in prolonging the APD in cattle whereas no positive trends could be obtained for DPI and SDZ 211-939 (SDZ) in cattle.

CONCLUSIONS

These results demonstrate that the binding site for DPI and SDZ is distinct from sites 2 or 3 of the VGSC alpha-subunit. Moreover, this is the first time that these compounds show no effect or even shortening of APD. This finding will enable the characterization of the mode of action and probably the binding site for synthetic VGSC-modulators.

Effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes

BDF 9198 (a congener of DPI 201 - 106 and BDF 9148) was found to be a positive inotrope on guinea-pig isolated ventricular muscle strips. The effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes were studied using the whole cell patch clamp method. In normal external solution, at 37 degrees C, action potential duration at 50% repolarization (APD(50)) was 167.4+/-8.36 ms (n=37). BDF 9198 produced a concentration-dependent increase in APD(50) (no significant increase at 1x10(-10) M; and APD(50) values of 273.03+/-35.8 ms at 1x10(-9) M; n=6, P<0.01 and 694.7+/-86.3 ms at 1x10(-7) M; P<0.001, n=7). At higher concentrations in the range tested, BDF 9198 also induced early and delayed and after-depolarizations. Qualitative measurements of I(Na) with physiological [Na](o) showed prolongation of the current by BDF 9198, and the appearance of transient oscillatory inward currents at high concentrations. Quantitative recording conditions for I(Na) were established using low external [Na] and by making measurements at room temperature. The current - voltage relation, activation parameters and time-course of I(Na) were similar before and after a partial blocking dose of Tetrodotoxin (TTX, 1 microM), despite a 2 fold difference in current amplitude. This suggests that voltage-clamp during flow of I(Na) was adequately maintained under our conditions. Selective measurements of I(Na) at room temperature showed that BDF 9198 induced a concentration-dependent, sustained component of I(Na) (I(Late)) and caused a slight left-ward shift in the current - voltage relation for peak current. The drug-induced I(Late) showed a similar voltage dependence to peak current in the presence of BDF 9198. Both peak current and I(Late) were abolished by 30 microM TTX and were sensitive to external [Na]. Inactivation of control I(Na) during a 200 ms test pulse to -30 mV followed a bi-exponential time-course. In addition to inducing a sustained current component, BDF 9198 left the magnitude of the fast inactivation time-constant unchanged, but increased the magnitude of the slow inactivation time-constant. Additional experiments with a longer pulse (1 s) raised the possibility that in the presence of BDF 9198, I(Na) inactivation may be comprised of more than two phases. No significant effects of 1x10(-6) M BDF 9198 were observed on the L-type calcium current, or delayed and inward rectifying potassium currents measured at 37 degrees C. It is concluded that the prolongation of APD(50) by BDF 9198 resulted from selective modulation of I(Na). Reduced current inactivation induced a persistent I(Na), increasing the net depolarizing current during the action potential. This action of the drug indicates a potential for 'QT prolongation' of the ECG. The observation of after-depolarizations suggests a potential for proarrhythmia at some drug concentrations.

Investigations of the mechanism of the positive inotropic action of BDF 9148: comparison with DPI 201-106 and the enantiomers

The electromechanical and biochemical activities of the positive inotropic compounds BDF 9148 and DPI 201-106 were compared in guinea-pig myocardic preparations. Additionally, the properties of the BDF 9148 enantiomers were studied to compare their positive inotropic effects. In guinea pig papillary muscles, BDF 9148 exerted a concentration-dependent increase of force of contraction with a 50% effective concentration (EC50) value of 0.6 microM, compared with 1.3 microM for DPI 201-106. Like that of DPI, the inotropic effect of BDF 9148 was abolished by treatment with tetrodotoxin (TTX) but not affected by treatment with carbachol. Likewise, pretreatment of the papillary muscles with propranolol, cimetidine, and histamine did not affect the contractile effects of BDF 9148. In the left atria, both agents had a positive inotropic effect with an EC50 of 0.2 microM for BDF and 0.8 microM for DPI. Incubation of single concentrations of the respective drugs for a period of 90 min with guinea pig papillary muscles resulted in slightly differing parameters of isometric contraction. In contrast to DPI, BDF 9148 prolonged the contraction time transiently. Time to peak force was not markedly influenced by either drug. The functional refractory period was prolonged by both drugs to a similar extent. At 10 microM, BDF 9148 showed a biphasic effect on the action potential duration (APD) most evident at APD90, whereas DPI prolonged APD90 progressively until the 90 min. The positive inotropic effect of BDF 9148 could be demonstrated by the (S-), whereas the (R+)-enantiomer was without effect. Neither DPI nor BDF 9148 increased myocardial cyclic adenosine monophosphate (cAMP) in isolated rat cardiomyocytes and guinea pig papillary muscles. Additionally, neither BDF 9148 nor DPI showed an inhibitory effect on the guinea pig myocardic Na+/K(+)-ATP'ase activity in the concentration range with a positive inotropic effect in the guinea-pig papillary muscle.

Veratridine augments and BDF 9148 attenuates the spontaneous contractile force of the rat portal vein

Veratridine and BDF 9148 both increase the force of myocardial contraction but their effects on the contractility of vascular smooth muscle are largely unknown. We have examined the effects of the drugs on the contractile force of the rat portal vein. The force responses of the rat portal vein were not altered by tetrodotoxin (< or = 30 microM) but were augmented by Bay K 8644 (10 nM-10 microM), and attenuated by nifedipine (10 nM-10 micro M). Veratridine slightly reduced the force of spontaneous contractions at concentrations of 0.1-1 microM, then augmented the force at concentrations up to 100 microM. The reduction was prevented by 30 nM tetrodotoxin; a higher concentration of 3 microM prevented the effect of veratridine. Pretreatment with nifedipine at 30 nM reduced the maximum augmentation observed with veratridine. BDF 9148 (0.3-300 microM) attenuated the force and this attenuation was increased in the presence of tetrodotoxin at 3 microM but reversed to an augmentation in the presence of Bay K 8644 at 10 microM. The augmentation produced by BDF 9148 in the presence of Bay K 8644 was reduced by tetrodotoxin at 30 nM. These results suggest that veratridine predominantly opens sodium channels which in turn open voltage-dependent calcium channels to augment the force responses of the rat portal vein, but attenuate force in the presence of sodium-channel blockade with tetrodotoxin. BDF 9148 predominantly closes calcium channels to attenuate the force of the rat portal vein but in the presence of maximum calcium-channel opening with Bay K 8644 augments force probably by opening sodium channels.

Activators of sodium, calcium and potassium channels modulate the cardiac effects of quinidine in vitro

Quinidine (25.5 mumol/l) reduced the beating frequency of isolated right guinea-pig atria, caused a negative inotropic effect in papillary muscles and slightly raised the contractile force of left atria. The functional refractory period of both tissues was prolonged. A 20% increase of the extracellular sodium concentration did not reverse the effects of quinidine. The Na-channel activator BDF 9148 (1 mumol/l) and the Ca-channel agonist Bay-K-8644 (0.5 mumol/l) further increased the contractile force and caused an additional prolongation of the functional refractory period in quinidine-pretreated atria. Only Bay-K-8644 was able to reverse the negative inotropic effect of quinidine in papillary muscles. The influence of Bay-K-8644 on the contractile force in quinidine-pretreated muscles was not attenuated by lemacalim (3 mumol/l), an activator of ATP-dependent potassium channels, but the duration of the functional refractory period was significantly reduced. These results suggest that a combination of a calcium channel activator and a potassium channel opener might be able to improve the treatment of quinidine intoxications.

Positive inotropic responses of the sodium channel modulator BDF 9148 in diseased rat myocardium

1. This study has defined the positive inotropic responses to the sodium channel modulator BDF 9148 in rats with hypertension, thyroid dysfunction, diabetes or dwarfism. Concentration-response curves to BDF 9148 and calcium chloride were determined in isolated left atria and left ventricular papillary muscles. 2. BDF 9148 increased force of contraction in left ventricular papillary muscles in all disease states with maximal responses comparable to calcium chloride. BDF 9148 potency was significantly decreased in muscles from diabetic rats only. 3. BDF 9148 produced similar responses in left atria except from hyperthyroid rats where negative inotropic responses only were measured. This exception confirms that the left atria is an imperfect model for ventricular responsiveness. 4. Thus, the increase in force of contraction in the ventricles as a consequence of sodium channel modulation by BDF 9148 is maintained in these disease states unlike responses to alpha-or beta-adrenoceptor agonists.

The effects of veratridine and BDF 9148 on the action potentials and contractility of the rat right ventricle

1. The effects of veratridine, BDF 9148 and lignocaine on the action potentials and contractile force of the electrically-driven rat right ventricle have been determined. 2. Veratridine at 10(-7)-10(-6) M and BDF 9148 at 10(-7)-10(-5) M had no effect on the threshold potential or amplitude but prolonged the ventricular action potentials. 3. In contractility studies, veratridine at 10(-7)-10(-6) M augmented the cardiac stimulation responses and the augmenting effects with 3 x 10(-7) and 10(-6) M were greater at 2 than 4 Hz. In the presence of veratridine at 3 x 10(-6) M, the ventricle would not pace. 4. BDF 9148 at 10(-7)-10(-5) M augmented the cardiac stimulation responses and the augmenting effects with 10(-7) and 3 x 10(-7) M were greater at 2 than 4 Hz and the effect was maximal at 3 x 10(-7) M and submaximal at 10(-5) M. The effects of BDF 9148 at 10(-5) were not readily reversible. 5. Lignocaine at 10(-4) M had no effect on the ventricular action potential duration but decreased the threshold potential and amplitude and also reduced the cardiac stimulation force responses. In the presence of lignocaine, the augmenting effects of veratridine and BDF 9148 on ventricular force were reduced. 6. In summary this study has shown that BDF 9148 prolongs the action potential and augments the contractile force responses of the rat right ventricle by a lignocaine-sensitive mechanism. BDF 9148 or similar drugs may have potential as positive inotropes in the treatment of heart failure.

Effects of the Anemonia sulcata toxin (ATX II) on intracellular sodium and contractility in rat and guinea-pig myocardium

The effects of the Anemonia sulcata toxin ATX II on action potentials and contractility of isolated papillary muscles and single myocytes from rat and guinea-pig hearts have been studied. ATX II prolonged the action potential in both rat and guinea-pig papillary muscle. Although it produced a positive inotropic effect in guinea-pig papillary muscle, it failed to do so in rat papillary muscle. However, in single rat and guinea-pig ventricular cells, it both prolonged the action potential and had a positive inotropic effect. We suggest that ATX II does not cause a positive inotropic effect in rat papillary muscle, because it induces Ca2+ overload. In single cells the positive inotropic effect was reduced by approximately 50% when the contractions were triggered by voltage clamp pulses of constant duration rather than by action potentials. This suggests that the inotropic effect of ATX II is in part the result of the prolongation of the action potential. The intracellular Na+ activity (a(i)Na) in single ventricular cells was measured with the Na(+)-sensitive fluorescent dye SBFI. After exposure of the cells to ATX II, a(i)Na was increased by a maximum of 1.9 +/- 0.3 and 2.2 +/- 0.3 mM in rat and guinea-pig cells, respectively. It is suggested that the positive inotropic effect of ATX II is also in part the result of the rise in a(i)Na.

Ion channel modulators as potential positive inotropic compound for treatment of heart failure

1. Current positive inotropy therapy of heart failure is associated with major problems: digoxin and the phosphodiesterase inhibitors can cause life-threatening toxicity while beta-adrenoceptor agonists become less effective inotropic compounds as heart failure progresses. A new approach to positive inotropy is ion channel modulation. 2. An increased influx of Na+ during the cardiac action potential, as measured with DPI 201-106 and BDF 9148 which increase the probability of the open state of the Na+ channel, will increase force of contraction. 3. Activation of L-type Ca2+ channels with Bay K 8644 will increase influx of Ca2+ and increase the force of contraction. However the Ca2+ channel activators developed to date have little potential for the treatment of heart failure as they are vasoconstrictors. 4. Blocking cardiac K+ channels is a possible mechanism of positive inotropy. Terikalant inhibits the inward rectifying K+ channel, tedisamil inhibits the transient outward K+ channel and dofetilide is one of the newly developed inhibitors of the slow delayed outward rectifying K+ channel. All these drugs prolong the cardiac action potential to increase Ca2+ entry and force of contraction. 5. Thus drugs which increase Na+ influx or block K+ channels represent exciting possibilities for positive inotropy and the potential of these compounds for the treatment of heart failure needs to be fully evaluated.

The inotropic agents DPI 201-106 and BDF 9148 differentially affect potassium currents of guinea-pig ventricular myocytes

The inotropic agents DPI 201-106 and BDF 9148 increase action potential duration (APD) of heart muscle. This effect can be explained by inhibition of inactivation of sodium current, which is affected by both agents to a similar extent (Ravens et al. 1991, Br J Pharmacol 104:1019-1023). However, as DPI 201-106 prolongs APD of guinea-pig ventricle to a larger extent than BDF 9148, other currents may also be involved. The aim of the present study was to measure the effects of DPI 201-106 and BDF 9148 on the inward rectifier IK1, and the two components of the delayed rectifier, IKs and IKr. The methyl-for-carbonitrile-substituted derivative BDF 8784 was included to study structure-activity relationships. Single-electrode whole-cell voltage-clamp technique was used to measure membrane currents of guinea-pig ventricular myocytes. Only DPI 201-106 reduced IK1 at potentials both negative and positive to the reversal potential. Three microM of DPI 201-106 reduced IKs, whereas 1 microM of BDF 9148 had no effect on this current. These concentrations were equieffective with respect to positive inotropic action (Ravens et al. 1991, Br J Pharmacol 104:1019-1023). BDF 9148 did however block IKs at higher concentrations, as did BDF 8784. It is concluded that block of outward current by DPI 201-106, but insignificant effects of BDF 9148, are responsible for the differential effects of these compounds on APD at equieffective concentrations with respect to inotropy.