Comparative study of MDL 17043 and MDL 19205, new positive inotropic agents, by use of isolated, blood-perfused dog-heart preparations

Two inotropes with different mechanisms of action: contractile, PDE-inhibitory and direct myofibrillar effects of levosimendan and enoximone

We characterized the Ca2+-sensitizing and phosphodiesterase (PDE)-inhibitory potentials of levosimendan and enoximone to assess their contributions to the positive inotropic effects of these drugs. In guinea pig hearts perfused in the working-heart mode, the maximal increase in cardiac output (55%, P<0.05) was attained at 50 nM levosimendan. The corresponding value for enoximone (36%) was significantly smaller (P<0.05) and was observed at a higher concentration (500 nM). In permeabilized myocyte-sized preparations levosimendan evoked a maximal increase of 55.8+/-8% (mean+/-SEM) in isometric force production via Ca2+ sensitization (pCa 6.2, EC50 8.4 nM). Enoximone up to a concentration of 10 microM failed to influence the isometric force. The PDE-inhibitory effects were probed on the PDE III and PDE IV isoforms. Levosimendan proved to be a 1300-fold more potent and a 90-fold more selective PDE III inhibitor (IC50 for PDE III 1.4 nM, and IC50 for PDE IV 11 microM, selectivity factor approximately 8000) than enoximone (IC50 for PDE III 1.8 microM, and IC50 for PDE IV 160 microM, selectivity factor approximately 90). Hence, our data support the hypothesis that levosimendan exerts positive inotropy via a Ca2+-sensitizing mechanism, whereas enoximone does so via PDE inhibition with a limited PDE III versus PDE IV selectivity.

Cardiovascular actions of OPC-18790: a novel positive inotropic agent with little chronotropic action

OPC-18790 [(+/-)-6-[3-(3,4-dimethoxybenzylamino)-2-hydroxypropoxy]- 2(1H)-quinolinone], a novel positive inotropic agent, was investigated in several in vitro and in vivo experiments to elucidate its cardiovascular effects and its mechanism of action. In isolated blood-perfused dog heart preparations, OPC-18790 increased contractile force at 10 to 1,000 nmol i.a.; increased coronary arterial blood flow at 30 to 1,000 nmol; and decreased sinus rate slightly at 1,000 nmol. Atrio-ventricular nodal conduction was slightly facilitated with OPC-18790 (10 to 1,000 nmol), whereas ventricular automaticity tended to decrease. OPC-18790 (10(-6) to 10(-4) M) increased contractile force in isolated ventricular muscles of dogs, cats, rabbits and guinea pigs but not rats. OPC-18790 increased left ventricular contractile force dose-dependently in anesthetized open-chest dogs and in conscious dogs with slight or no changes in heart rate and blood pressure. The positive inotropic effect of OPC-18790 was not affected by beta-blockade. OPC-18790 (10(-5) to 10(-4) M) prolonged the duration of action potential in guinea pig papillary muscles. Na+, K(+)-ATPase was not inhibited, but peak-III phosphodiesterase (low Km cyclic AMP specific fraction, inhibited by cyclic GMP) was inhibited by OPC-18790 (IC50 = 0.41 x 10(-6) M) in dog myocardium. However, such an inhibitory action of phosphodiesterase can hardly be reconciled with the lack of a positive chronotropic effect shown by OPC-18790. Thus, these results suggest that OPC-18790 may have an additional mechanism. The cardiovascular effects revealed by this study suggest that OPC-18790 may exert a beneficial effect in the treatment of congestive heart failure.

T-1583 and forskolin are similar in their cardiac effects and dissimilar in their vascular effects

The cardiac and coronary vascular effects of T-1583, a selective beta 1-adrenoceptor full agonist, and forskolin, a direct activator of adenylate cyclase, were compared in isolated, blood-perfused papillary muscle, sinoatrial node, and atrioventricular (AV) node preparations of dogs. Both agents were injected intra-arterially. The two agents increased the force of contraction of the paced papillary muscle and the unpaced one, and the rate of automaticity of the latter. They increased sinus rate and accelerated AV nodal conduction. In producing these effects T-1583 was 50 to 80 times more potent than forskolin, indicating that both agents have similar cardiac profiles. At the doses that produced a 50% increase in the force of contraction of the papillary muscle, both agents produced about a 20% increase in sinus rate. Such degrees of force-rate separation were close to those obtained with most new positive inotropic agents with an inhibitory action on cyclic AMP phosphodiesterase. T-1583 differed distinctly from forskolin in that the former increased only slightly coronary blood flow, whereas the latter increased it greatly. Thus, forskolin is more coronary vasodilatory than positively inotropic, and more positively inotropic than positively chronotropic. T-1583 is a more positively inotropic than positively chronotropic and more positively chronotropic than coronary vasodilatory.

Cardiac and coronary vasodilator profile of pimobendan, a new cardiotonic drug, revealed by use of isolated, blood-perfused dog heart preparations

Cardiac and coronary vasodilator effects of pimobendan (UD-CG 115 BS) were assessed in isolated, blood-perfused papillary muscle, sinoatrial (SA) node, and atrioventricular (AV) node preparations of dogs. Pimobendan was administered intra-arterially. In paced papillary muscle preparations, the drug increased the force of contraction in a dose-dependent manner. In SA node preparations, the drug produced an increase in sinus rate. When the drug was injected into the artery supplying the AV node, the drug produced a decrease in AV conduction time by accelerating AV nodal conduction in AV node preparations. In spontaneously beating papillary muscle preparations, the drug increased the rate of ventricular automaticity. No ventricular arrhythmias were produced by the drug. In all preparations, the drug increased (coronary) blood flow. The order of effectiveness of pimobendan on the above cardiovascular variables was as follows: ventricular muscle contraction much greater than coronary blood flow greater than SA nodal automaticity greater than AV nodal conduction greater than ventricular automaticity. The results indicate that pimobendan is relatively specific for force and its cardiovascular profile is very similar to those of phosphodiesterase inhibitors. Therefore, it is likely that this cardiovascular profile would be determined by its inhibitory action on phosphodiesterase.

Comparative study of cardiovascular profiles of milrinone and amrinone by use of isolated, blood-perfused dog heart preparations

The cardiac and coronary vasodilator effects of milrinone and amrinone were compared in isolated, blood-perfused papillary muscle and sinoatrial (SA) node and atrioventricular (AV) node preparations of dogs. Milrinone (0.3-100 nmol) and amrinone (0.01-3 mumol) were administered intra-arterially. Both drugs increased the force of contraction of paced and unpaced papillary muscles and the rate of automaticity of the latter; they increased sinus rate and accelerated AV nodal conduction. However, both drugs were not homogeneously effective on cardiac variables but affected them in the following order: The force of contraction of the ventricular muscle greater than SA nodal automaticity divided by AV nodal conduction greater than ventricular automaticity. In producing these cardiac effects, milrinone was 30-60 times more potent than amrinone. Both drugs increased (coronary) blood flow in all preparations. In this respect milrinone was about ten times more potent than amrinone. As a result, milrinone can be characterized as having almost equal cardiotonic and coronary vasodilatory effects, whereas amrinone is more coronary vasodilatory than cardiotonic. These differences in cardiovascular profile may contribute to their differential salutary mechanisms in the treatment of heart failure. Both drugs induced neither AV nodal tachycardia nor ventricular arrhythmia.

Cardiac versus coronary vasodilator actions of isobutylmethylxanthine in dogs: comparison with theophylline

Cardiac and coronary vasodilator effects of isobutylmethylxanthine (IBMX) and theophylline were compared in isolated, blood-perfused papillary muscle, sino-atrial (SA) node and atrioventricular (AV) node preparations of dogs. IBMX (1 nmol-1 mumol) and theophylline (30 nmol-10 mumol) were injected intra-arterially. In paced papillary muscle preparations, both agents increased the force of contraction. In spontaneously beating papillary muscle preparations, both agents increased the rate of ventricular automaticity. In SA node preparations, both agents increased sinus rate. In AV node preparations, both agents decreased AV conduction time only when they were injected into the artery supplying the AV node. In all preparations, both agents increased coronary blood flow. However, both agents were not homogeneously effective on these cardiovascular variables, but showed selectivity in the following order: Ventricular muscle contraction not equal to coronary blood flow greater than SA nodal automaticity not equal to AV nodal conduction greater than ventricular automaticity. These results indicate that IBMX and theophylline have almost an identical profile in their cardiac and coronary vasodilator effects. The two agents were different in that IBMX was 40-50 times as potent as theophylline in producing these effects. Thus, both agents appeared to express their cardiovascular profiles mainly through inhibition of cyclic AMP phosphodiesterase, although theophylline has been claimed to have additional actions.