Regulation of myocardial contractility 1958-1983: an odyssey

CalTrack: High-Throughput Automated Calcium Transient Analysis in Cardiomyocytes

[Figure: see text].

Myocardial Contractility: Historical and Contemporary Considerations

The term myocardial contractility is thought to have originated more than 125 years ago and has remained and enigma ever since. Although the term is frequently used in textbooks, editorials and contemporary manuscripts its definition remains illusive often being conflated with cardiac performance or inotropy. The absence of a universally accepted definition has led to confusion, disagreement and misconceptions among physiologists, cardiologists and safety pharmacologists regarding its definition particularly in light of new discoveries regarding the load dependent kinetics of cardiac contraction and their translation to cardiac force-velocity and ventricular pressure-volume measurements. Importantly, the Starling interpretation of force development is length-dependent while contractility is length independent. Most historical definitions employ an operational approach and define cardiac contractility in terms of the hearts mechanical properties independent of loading conditions. Literally defined the term contract infers that something has become smaller, shrunk or shortened. The addition of the suffix “ility” implies the quality of this process. The discovery and clinical investigation of small molecules that bind to sarcomeric proteins independently altering force or velocity requires that a modern definition of the term myocardial contractility be developed if the term is to persist. This review reconsiders the historical and contemporary interpretations of the terms cardiac performance and inotropy and recommends a modern definition of myocardial contractility as the preload, afterload and length-independent intrinsic kinetically controlled, chemo-mechanical processes responsible for the development of force and velocity.

Levosimendan beyond inotropy and acute heart failure: Evidence of pleiotropic effects on the heart and other organs: An expert panel position paper

Levosimendan is a positive inotrope with vasodilating properties (inodilator) indicated for decompensated heart failure (HF) patients with low cardiac output. Accumulated evidence supports several pleiotropic effects of levosimendan beyond inotropy, the heart and decompensated HF. Those effects are not readily explained by cardiac function enhancement and seem to be related to additional properties of the drug such as anti-inflammatory, anti-oxidative and anti-apoptotic ones. Mechanistic and proof-of-concept studies are still required to clarify the underlying mechanisms involved, while properly designed clinical trials are warranted to translate preclinical or early-phase clinical data into more robust clinical evidence. The present position paper, derived by a panel of 35 experts in the field of cardiology, cardiac anesthesiology, intensive care medicine, cardiac physiology, and cardiovascular pharmacology from 22 European countries, compiles the existing evidence on the pleiotropic effects of levosimendan, identifies potential novel areas of clinical application and defines the corresponding gaps in evidence and the required research efforts to address those gaps.

The role of phosphodiesterase inhibitors in heart failure

Myocardial contractility is dependent on available intracellular calcium and this can be enhanced by increasing intracellular cyclic adenosine monophosphate. One way of achieving this is by inhibiting the phosphodiesterase III enzyme. Over the last 15 years, a number of new drugs with this mechanism of action have been studied in man and have been found not only to have a positive inotropic action on the heart but also a vasodilating action on peripheral blood vessels. This combination of effects produces favourable haemodynamic improvement in patients with chronic heart failure. While some smaller studies showed that this did translate into an improvement in symptoms and functional capacity, a large well-designed and controlled clinical trial showed that survival was decreased when milrinone was used in target daily doses of 40 mg. For this reason, chronic long-term oral therapy with phosphodiesterase III inhibitors is not currently being actively pursued. They may still have a role as acute short-term therapy in severely ill patients who do not respond adequately to optimal standard drug therapy. Milrinone has been one of the most widely studied drugs in this regard. Even during short-term administration, its use should be closely monitored for any evidence of an increase in ventricular arrhythmias or decrease in ventricular function.

The challenge of cardiomyopathy

The combined clinical and pathophysiologic characteristics and diagnostic features as well as current concepts of pathogenesis, therapy and prevention of the principal forms of cardiomyopathy are reviewed. These include hypertrophic cardiomyopathy, dilated cardiomyopathy, restrictive cardiomyopathy and specific cardiac muscle disease. Emphasis is placed on recent developments and unresolved questions requiring application of newer techniques of molecular biology and genetics and adult myocyte culturing.

Changing strategies in the management of heart failure

Forty years ago therapy for congestive heart failure was limited largely to the mercurial diuretics and a variety of cardiac glycoside preparations; these were often ineffective, and the common practice of "pushing" digitalis caused serious, sometimes lethal side effects. Today, a more complete understanding of the regulation of cardiac work and pathophysiology of heart failure is having a profound impact on therapeutic strategy for this common condition. Despite more powerful means to augment myocardial contractility and much more effective diuretics, therapy that relies only on inotropic stimulation and diuresis is no longer optimal for the majority of patients with heart failure. Thus, strategies for the therapy of heart failure must take into account new understanding of mechanisms that initiate, perpetuate and exacerbate the hemodynamic and myocardial abnormalities in these patients. Recognition of the detrimental effects of excessive afterload and the importance of relaxation (lusitropic) as well as contraction (inotropic) abnormalities has led to widespread acceptance of vasodilator therapy, which has dramatically improved our ability to alleviate the symptoms of heart failure. Changes that result from altered gene expression in the hypertrophied myocardium of patients with congestive heart failure can give rise to a cardiomyopathy of overload that, although initially compensatory, may hasten death. These and other advances in our understanding of the pathophysiology, biochemistry and molecular biology of heart failure provide a basis for new therapeutic strategies that can slow the progressive myocardial damage that causes many of these patients to die, while at the same time improving well-being in patients with congestive heart failure.

Influence of altered inotropy and lusitropy on ventricular pressure-volume loops

Each cardiac cycle can be characterized by a pressure-volume loop that graphically depicts the external work of the ventricle. The pressure-volume loop is determined to a large extent by end-diastolic and end-systolic conditions, each of which reflects the properties of the heart and circulation at these two critical times in the cardiac cycle. The pressure and volume at end-diastole and end-systole, in turn, are determined by interactions between the heart and the circulation. End-diastolic pressure and volume reflect preload (venous return) and the lusitropic (relaxation) state of the ventricular walls, and end-systolic pressure and volume are determined by the afterload (peripheral and pulmonary resistance) and the inotropic (contractile) state of the myocardium. Alterations in inotropic and lusitropic state lead to predictable changes in the pressure-volume loop that, when combined with knowledge of preload and afterload, can facilitate understanding of the pharmacologic and pathophysiologic responses of the heart.

Compartmentation and functional mechanisms in myocardial failure and myocardial infarction

Changes in compartmentation and specific mechanism in acute myocardial failure due to global ischemia and in regional myocardial ischemia in dog hearts are described. Ischemic failure was produced by periodic arrest of flow to supported heart preparations perfused with a fluorocarbon (FC-43). Sarcolemmal vesicles (SL) prepared from ischemic failing heart preparations exhibited diminished Ca++ binding and phosphorylation. TA-064, a beta-1-agonist partially abolished the reduction in Ca++ binding and phosphorylation of SL vesicles. The addition of cyclic-AMP (cAMP) and of protein kinase (PK) increased phosphorylation of SL vesicles obtained from non failing heart preparations. Combination of cAMP and of PK had the greatest effect. In contrast to myocardial failure, myocardial infarction is known to produce a large variety of specific disturbances in intermediary cardiac metabolism. Apparently in ischemic failing heart preparations, Ca++ binding and phosphorylation by SL are deficient. The results with TA-064 and isoproterenol suggest that phosphorylation of SL may play a role in the positive inotropic effect of beta-1-agonists.

Perspectives and future trends in cellular electrophysiology: implications for the clinician

As a result of single fiber electrophysiologic studies, the clinical approach to the electrical behavior of the heart has improved. Three areas are examined: 1) the electrocardiographic waveform, 2) normal and abnormal cardiac rhythms, and 3) the mechanism of action of antiarrhythmic drugs. In each area, the results of single fiber studies have provided a conceptual framework for diagnostic and therapeutic decisions. These studies have also enabled investigators to test hypotheses formulated from clinical observations. It may be only a slight exaggeration to attribute many of our recent advances in each of the three areas to the development and use of the microelectrode.