Arrhythmogenic potential of positive inotropic agents

The clinical use of positive inotropic agents has been associated with increased mortality, with proarrhythmia speculated to be a contributing factor. This study compares the arrhythmogenic potential of six positive inotropic agents representing different mechanistic classes: the β-adrenergic agonist dobutamine, the adenylyl cyclase activator forskolin, the phosphodiesterase-III inhibitor milrinone, the cardiac glycoside ouabain, and the sodium channel agonists DPI 201-106 and BDF 9148. These agents were studied in dogs with anterior myocardial infarction using lower and higher dose i. v. regimens targeted to elicit 20–40% and 70–90% increases in LV+dP/dt, respectively. Precipitation of new ventricular arrhythmia by programmed ventricular stimulation was observed in all treatment groups. Incidences of new arrhythmia were comparable in the lower dose regimens, ranging from 16.7% (3/18 animals with BDF 9148) to 31.6% (6/19 animals with DPI 201-106), and in the higher dose regimens, ranging from 10.0% (1/10 animals with milrinone) to 27.7% (5/18 animals with DPI 201-106). The overall incidence of new ventricular arrhythmia ranged from 27.3% (3/11 animals with ouabain) to 47.4% (9/19 animals with DPI 201-106). No differences were observed in underlying infarct size or time from infarction to electrophysiologic study between subgroups of animals in which new arrhythmias were precipitated vs. those remaining non-responsive in any treatment group. The positive inotropic agents tested displayed diverse total group effects on heart rate, electrocardiographic intervals including QTc and ventricular refractoriness. Within individual treatment comparisons revealed a general but not universal pattern of greater ventricular refractory period values in newly inducible vs. non-inducible subgroups in the DPI 201-106, BDF 9148 and ouabain (low and high dose); milrinone and dobutamine (high dose) treatment groups. These findings indicate that regardless of underlying cellular mechanisms of action, the six positive inotropic agents tested all displayed comparable proarrhythmic potentials unrelated to underlying infarct size and time from infarction. This observation suggests the general shared property of increased myocardial contractility, potentially adversely affecting myocardial oxygen balance, myocardial perfusion and electrical stability in the setting of previous myocardial infarction, to be a common underlying cause for arrhythmogenesis. Additionally, alterations in ventricular refractoriness and repolarization may contribute significantly to proarrhythmia with some positive inotropic interventions. Received: 20 July 1999, Returned for 1. revision: 16 September 1999, 1. Revision received: 26 October 1999, Returned for 2. revision: 24 November 1999, 2. Revision received: 22 December 1999, Accepted: 6 January 2000     

Studies on the cellular mechanisms of action of positive and negative inotropic agents

Summary We have reviewed the mechanism by which drugs that elevate cyclic AMP level modify myocardial contractility. We have presented preliminary evidence about the mechanism by which muscarinic agonists antagonize the effects of these drugs. Finally, we suggest that the protein phosphorylation experiments, particularly if done in dispersed myocytes, could be an efficient and cost-effective method of screening drugs which may act by elevating intracellular levels of cyclic AMP.     

Presence of negative inotropic agents in canine plasma during positive end-expiratory pressure.

Application of positive end-expiratory pressure (PEEP) will reduce cardiac output (CO). Humoral mediation of this event by circulating negative inotropic agents was examined using a rat papillary muscle bioassay. Twenty-seven dogs were anesthetized with an iv pentobarbital infusion. Plasma was obtained before and after 30 minutes of PEEP. The plasma was oxygenated in a small (4.5-ml) papillary muscle chamber using a diffusion membrane. An average PO2 of 416 mm Hg was achieved. PEEP plasma reduced developed tension (Tpd) from 2.16 +/- 1.0 to 1.90 +/- 1.05 g (P less than 0.0001). A fall in Tpd was observed whether or not CO was maintained constant with fluid infusion. Resting tension was unchanged. The percent reduction in Tpd correlated with the fall in CO (r = 0.63, P less than 0.01) when fluid was not infused to maintain CO. Reapplication of control plasma restored Tpd. Barbiturate levels in anesthetized dogs rose from 17.3 to 19.4 microns/ml during PEEP (P less than 0.1). Addition of pentobarbital to normal plasma led to a slight decrease in Tpd only when the concentration exceeded 99 microns/ml. In three experiments on ex vivo perfused hearts, application of PEEP led to lowering of peak systolic pressure (PSP) within 5 minutes. Removal of PEEP restored PSP in a similar time. The results support the hypothesis that the decline in CO with PEEP is mediated in part by a circulating negative inotropic agent.     

New mechanisms for positive inotropic agents: Focus on the discovery and development of imazodan

Summary Intense efforts during the last decade to identify a useful positive inotropic agent to replace digitalis for the treatment of congestive heart failure have led to the discovery of several dozen potential substitutes, of which a number are currently undergoing clinical trials. In addition to producing a variety of new therapeutic entities, research in this area has also yielded valuable new information regarding the fundamental events that regulate calcium homeostatis and contractile function in the cardiac cell. For example, several of these new inotropic agents, including the calcium-channel stimulator BAY-k 8644, the sodium-channel stimulator DPI-201-186, and the sodium-calcium exchange inhibitor dichlorobenzamil, have provided considerable insight into the role of sodium and calcium in regulating contractility and the molecular events that mediate potential-dependent ion channels. Likewise, the discovery and development of agents like imazodan, amrinone, enoximone, and other selective type III phosphodiesterase inhibitors have provided new information regarding multiple molecular forms of cyclic nucleotide phosphodiesterase, compartmentation of cyclic AMP, and the importance of soluble vs. membrane-bound phosphodiesterases.     

Independence of the positive inotropic effect of ouabain from the inhibition of the heart Na+/K+ pump.

Isolated left atria from guinea pigs were stimulated at 3.3 Hz and bathed at 30 degrees C in Tyrode's solution containing 6 mM KCl. After equilibration, this solution was replaced by a low-K solution or by Tyrode's solution containing ouabain or dihydroouabain. These treatments evoked an increase in the contractility of the atria. The time to peak increase was about 30 min, and the inotropic effect was sustained for at least 40 min. After 30 min, 42K was added to the bathing solution in order to estimate the activity of the Na+/K+ pump. A linear relationship was observed between the degree of inhibition of the Na+/K+ pump and the increase in systolic tension. The regression line was the same for low-K solutions and dihydroouabain but not for ouabain. For a given degree of inhibition of the pump, ouabain evoked a higher increase in contractility. These findings indicate that inhibition of the Na+/K+ pump can be the only mechanism responsible for the positive inotropic effect of dihydroouabain but cannot be the sole mechanism for that of ouabain.     

Reversibility of inotropic effects of ouabain on canine atrial and ventricular tissues

The experiments reported here were designed to quantitate reversibility of ouabain-induced inotropy in atrial and ventricular trabeculae of the dog heart. At 30 degrees C, 1 Hz, 2 mM Ca2+ and 5.9 mM K+, the positive inotropic effects, measured after a 1 h exposure to a single priming concentration of 0.5 microM ouabain, decreased after the removal of the drug with a half time (t1/2) of washout of 9.52 +/- 2.72 h in atrial and 7.64 +/- 1.58 h in ventricular trabeculae. When toxicity occurred it was of three types: transient toxicity, toxicity resembling a 'fast washout,' and severe toxicity characterized by profound and persistent negative inotropy and contracture. Use of the classical glycoside bioassay (Hatcher's digitalis titration) during and after washout, provided a semiquantitative estimation of ouabain remaining in the trabeculae; after a 4 h washout of the drug, the trabeculae were challenged by the addition of one-half of the initial concentration of ouabain (0.25 microM). Trabeculae, which had responded to ouabain with only positive inotropic effects and no toxicity, still contained substantial concentrations of ouabain. Trabeculae which reacted clearly with toxicity to the priming dose of ouabain, showed a significant loss of contractile force and development of contracture. The fastest ouabain washout we observed was 3.1 h.     

Effects of diphenylhydantoin on coronary resistance and myocardial contractile force before and after ouabain

These studies suggest that the direct local effect of DPH is to relax coronary smooth muscle and depress myocardial muscle. Further, the results indicate that neither the vascular nor myocardial effects of DPH are mediated through a stimulating action of the drug on the cell membrane Na⁺, K⁺ ATPase. In contrast, the studies support the hypothesis that the vasodilator but not the cardiac depressant action of potassium results from a stimulating action of this ion on the sarcolemmal sodium-potassium pump.     

In vivo assessment of the inotropic and toxic effects of oxidized ouabain

Summary Oxidized ouabain, a product of the oxidative cleavage of the rhamnose ring in ouabain has been found to have a higher inotropic toxic ratio in cultured cardiac myocytes.The purpose of our study was to evaluate the efficacy and toxicity of oxidized ouabain in comparison with ouabain in intact animals. Drugs were infused to healthy cats; the positive inotropic effect, and the time-course of development of arrhythmia were followed and recorded until death. Oxidized ouabain was associated with a higher increase in arterial blood pressure, a mean increase of 41±19% as compared with 21±8% in the ouabain group (p<0.10). There were no significant differences in maximal increases of dP/dt or dP/dt/P (65±29%, 28±10% for oxidized ouabain and 49±16%, 27±11% for ouabain, respectively). The mean doses causing persistent arrhythmia (toxic dose) were 93±23 g/kg of oxidized ouabain vs 39±14 g/kg of ouabain. Lethal arrhythmias were produced by 215±46 g/kg of oxidized ouabain and 62±16 g/kg of ouabain. The radio of toxic to lethal doses was 0.62±0.11 for ouabain vs 0.45±0.09 for oxidized ouabain (p<0.05), but the inotropic to toxic dose ratios were not different.We conclude that oxidized ouabain acts similarly to the known cardiac glycosides in doses which produce inotropic effects in cats, has a lower potency as compared to ouabain, and appears to have a more benign course of intoxication.     

Influence of positive inotropic agents on intracellular calcium transients. Part II. Cardiomyopathic hamster hearts

To study the mechanism of dobutamine on end-stage heart failure, we assessed hemodynamic responses, high-energy phosphates (31P-NMR), and free intracellular calcium ([Ca2+]i) transients (surface fluorometry) during perfusion with 10(-6) mol/L dobutamine in Syrian cardiomyopathic hamsters with severe heart failure. These results were compared to perfusion of the heart with 10(-6) mol/L norepinephrine and 10(-6) mol/L isoproterenol. With the positive inotropic agents the rate-pressure product increased immediately (p less than 0.01 with dobutamine, norepinephrine; p less than 0.003 with isoproterenol); after 10 to 15 minutes of perfusion the rate-pressure product remained relatively stable with norepinephrine and isoproterenol but decreased with dobutamine (p = NS vs control values). [Ca2+]i-transients increased significantly in all groups. The end-diastolic [Ca2+]i decreased continuously with norepinephrine and isoproterenol (p less than 0.008; p less than 0.005) but increased during dobutamine by 19%. Alterations in coronary flow, pHi, high-energy phosphates, and the phosphorylation potential were not significantly different among the three catecholamines. In conclusion, in contrast to norepinephrine and isoproterenol, dobutamine depressed myocardial performance and increased end-diastolic [Ca2+]i in late heart failure.     

Direct positive inotropic and vasoconstrictor effects of endothelin

Summary Endothelin, a newly discovered vasoconstrictor peptide, when added to isolated cat papillary muscles, induced a direct positive inotropic effect that was slow in onset but of long duration. The magnitude of the developed force was concentration dependent. Endothelin exerted a marked concentration-dependent vasoconstriction in isolated cat carotid arteries and rabbit aortic rings. In both the carotid arteries and the aortic rings, endothelin induced a similar vasoconstrictor effect in the presence or absence of an intact endothelium. Addition of propyl gallate, a 5-lipoxygenase inhibitor, ibuprofen, a cyclooxygenase inhibitor, or SKF-525A, a cytochrome P450 inhibitor, at 2 µM did not significantly attenuate the ability of endothelin to vasoconstrict aortic rings in the presence or absence of an intact endothelium. These results demonstrate that the vasoconstrictor activity of endothelin operates independently of all three pathways of arachidonic acid metabolism (i.e., lipoxygenase, cyclooxygenase, or cytochrome P450 pathways) and is not dependent upon other endothelium-derived mediators (e.g., endothelium-derived relaxing factor, or eicosanoids) in these preparations. Moreover, endothelin exerts a direct positive inotropic effect in isolated cat ventricular myocardial tissue.Supported by Research Grant No. HL-25575 from the National Heart Lung and Blood Institute of the NIH.Gibbon Fellow of Thomas Jefferson University.Postodctoral Fellow of the Ischemia-Shock Research Institute of Thomas Jefferson University.     

Positive inotropic agents. Generalities and classification

The aim of inotropic therapy is to increase the force of myocardial fibre shortening by improving the availability of calcium to the contractile proteins. Digitalis remains the most widely used drug, but its positive inotropic effects are weak and the therapeutic index is low. Dobutamine is the most useful catecholamine because the most cardioselective and it induces the least increase in myocardial oxygen consumption. There are three groups of new inotropic agents: sympathomimetics: pirbuterol and prenalterol are effective in the short term but tolerance is usually observed within a few weeks. Salbutamol and terbutaline have only been assessed in acute studies, phosphodiesterase inhibitors (MDL 17043, MDL 19025) are powerful inotropic agents in the short and medium term. ARL 115 has mainly been studied by parenteral administration, Amrinone has a largely unknown mode of action, but is a very effective positive inotropic drug; its side-effects limit it as age. However, its derivative, milrinone, seems to be more inotropic and less toxic. The new inotropic drugs currently under assessment are active in the short term but their long-term efficacy and side-effects are still little known. The ideal inotropic agent remains to be discovered.     

Mechanism of action of positive inotropic substances

The understanding of the mechanisms of action of positive inotropic substances requires an understanding of the cellular processes of cardiac contraction. The authors then examine the pharmacology of beta-adrenergic receptors and the intracellular functions of cAMP and discuss the action of dopamine, ibopamine, dobutamine, Corwin, amrinone and Vardax. They also summarize the state of the research into the digitalis receptors which revealed the existence of inotropic receptors with high affinity, different from the receptors responsible for inhibition of the Na-K pump. Finally, they discuss the possible role of cardioginine, an endogenous digitalis factor.     

Effect of positive inotropic agents on myosin isozyme population and mechanical activity of cultured rat heart myocytes.

To examine whether catecholamines have a direct effect on myosin heavy chain expression of heart myocytes or whether they act via an altered work load, myocytes from neonatal rat hearts were cultured in thyroid hormone-free media containing various positive inotropic and chronotropic agents. The velocity and frequency of contraction of the myocytes were monitored using an optoelectronic system. After 3-5 days of culture, myosin isozyme populations, cellular cAMP content, and 2-deoxy-D-glucose uptake of the myocytes were determined. Compared with myocytes cultured in the absence of inotropic agents (32.6 +/- 3.5% V1), the proportion of myosin V1 was significantly (p less than 0.05) increased in the case of 1 microM isoproterenol (48.2 +/- 5.9% V1), 1 microM forskolin (57.1 +/- 11.7% V1), and 1 mM dibutyryl cAMP (79.1 +/- 2.0% V1). Dibutyryl cAMP increased V1 to a similar level as 30 nM triiodothyronine did (70.2 +/- 13.0% V1). Only a small increase was observed in myocytes cultured in the presence of 10 microM phenylephrine (40.4 +/- 8.4% V1), 10 microM ouabain (40.6 +/- 11.9% V1), or 10 microM Bay K 8644 (40.7 +/- 11.7% V1). The agents with a marked effect on myosin heavy chain expression resulted in a higher cAMP content; isoproterenol and forskolin also stimulated 2-deoxy-D-glucose uptake. All agents resulted in a higher velocity of contraction; with the exception of ouabain, frequency of contraction was also increased. A change in Ca2+ concentration in the medium from 1.3 to 2.4 mM resulted in a small increase in V1 (40.7 +/- 5.2% V1) but had the same effect on contraction velocity as dibutyryl cAMP did. Furthermore, 10 nM isoproterenol also increased V1 in myocytes that were arrested with 10 microM verapamil. The increase in V1 in the case of dibutyryl cAMP, isoproterenol, and forskolin is thus most probably not a correlate of the increased mechanical activity but of the high cellular cAMP content.