Differences in cardioprotective efficacy of adrenergic receptor antagonists and Ca2+ channel antagonists in an animal model of dilated cardiomyopathy. Effects on gross morphology, global cardiac function, and twitch force

Cardiovascular Diseases in Pet Birds: Therapeutic Options and Challenges

Cardiovascular disease, including congestive heart failure, pericardial disease, and atherosclerosis, is becoming increasingly better recognized in companion birds. A wide range of medications is available to treat these conditions, including diuretics, vasodilators, positive and negative inotropes, antiarrhythmic agents, and pentoxifylline. This review systematically discusses each of these drug classes and their potential applications in avian species. Although treatment approaches remain largely empirical and extrapolated from small animal and human medicine, the management strategies presented here have the potential to both maintain quality of life and extend survival time for the avian cardiac patient.

Calcium and Heart Failure: How Did We Get Here and Where Are We Going?

The occurrence and prevalence of heart failure remain high in the United States as well as globally. One person dies every 30 s from heart disease. Recognizing the importance of heart failure, clinicians and scientists have sought better therapeutic strategies and even cures for end-stage heart failure. This exploration has resulted in many failed clinical trials testing novel classes of pharmaceutical drugs and even gene therapy. As a result, along the way, there have been paradigm shifts toward and away from differing therapeutic approaches. The continued prevalence of death from heart failure, however, clearly demonstrates that the heart is not simply a pump and instead forces us to consider the complexity of simplicity in the pathophysiology of heart failure and reinforces the need to discover new therapeutic approaches.

Cardiovascular Drugs in Avian, Small Mammal, and Reptile Medicine

Cardiovascular disease, including congestive heart failure, pericardial disease, and atherosclerosis, is becoming increasingly better recognized in companion birds, small mammals, and reptiles. A wide range of medications is available to treat these conditions, including diuretics, vasodilators, positive and negative inotropes, antiarrhythmic agents, and pentoxifylline. This review systematically discusses each of these drug classes and their potential applications in exotic species. Although treatment approaches remain largely empirical and extrapolated from small animal and human medicine, the management strategies presented here have the potential to both maintain quality of life and extend survival time for the exotic cardiac patient.

Intracellular devastation in heart failure

End-stage heart failure is characterized by a number of abnormalities at the cellular level, which include changes in excitation-contraction coupling, alterations in contractile proteins and activation/deactivation of signaling pathways. Even though many of these changes are adaptive to the high workload and stress in heart failure, a significant number of these alterations are deeply deleterious to the cardiac cell. In this article, we will review the changes in calcium cycling that occur in myopathic hearts and how they can be effectively targeted. We will also focus on protein misfolding in the setting of cardiac dysfunction.

Chronic treatment with carvedilol improves ventricular function and reduces myocyte apoptosis in an animal model of heart failure

BACKGROUND

Beta blocker treatment has emerged as an effective treatment modality for heart failure. Interestingly, beta-blockers can activate both pro-apoptotic and anti-apoptotic pathways. Nevertheless, the mechanism for improved cardiac function seen with beta-blocker treatment remains largely unknown. Carvedilol is a non-selective beta-blocker with alpha-receptor blockade and antioxidant properties. We therefore studied the impact of the effects of carvedilol in an animal model of end-stage heart failure.

RESULTS

To test whether chronic treatment with beta-blockade decreases apoptosis, we treated myopathic turkeys with two dosages of carvedilol, 1 mg/kg (DCM1) and 20 mg/kg (DCM20), for four weeks and compared them to non-treated DCM animals (DCM0) and to control turkeys (CON). Echocardiographic measurements showed that non-treated DCM animals had a significantly lower fractional shortening (FS) when compared to CON (68.73 +/- 1.37 vs. 18.76 +/- 0.59%, p < 0.001). Both doses of carvedilol significantly improved FS (33.83 +/- 10.11 and 27.73 +/- 6.18% vs. 18.76 +/- 0.59% for untreated DCM, p < 0.001). DCM left ventricles were characterized by a higher percentage of apoptotic nuclei when compared to CON (5.64 +/- 0.49 vs. 1.72 +/- 0.12%, respectively p < 0.001). Both doses of carvedilol significantly reduced the number of apoptotic nuclei (2.32 +/- 0.23% and 2.36 +/-6% 1 mg and 20 mg/kg respectively).

CONCLUSIONS

Carvedilol improves ventricular function. Furthermore, treatment with carvedilol decreased the incidence of apoptosis in cardiac myocytes from failing hearts at both doses. These data suggest that the inhibition of apoptosis with carvedilol may lead to improvement in ventricular function and may underlie a beneficial effect of beta-blockade independent of heart rate lowering effects.

Calcium dynamics in the failing heart: restoration by beta-adrenergic receptor blockade

Changes in calcium (Ca2+) regulation contribute to loss of contractile function in dilated cardiomyopathy. Clinical treatment using beta-adrenergic receptor antagonists (beta-blockers) slows deterioration of cardiac function in end-stage heart failure patients; however, the effects of beta-blocker treatment on Ca2+ dynamics in the failing heart are unknown. To address this issue, tropomodulin-overexpressing transgenic (TOT) mice, which suffer from dilated cardiomyopathy, were treated with a nonselective beta-receptor blocker (5 mg. kg-1. day-1 propranolol) for 2 wk. Ca2+ dynamics in isolated cardiomyocytes of TOT mice significantly improved after treatment compared with untreated TOT mice. Frequency-dependent diastolic and Ca2+ transient amplitudes were returned to normal in propranolol-treated TOT mice and but not in untreated TOT mice. Ca2+ kinetic measurements of time to peak and time decay of the caffeine-induced Ca2+ transient to 50% relaxation were also normalized. Immunoblot analysis of untreated TOT heart samples showed a 3.6-fold reduction of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA), whereas Na+/Ca2+ exchanger (NCX) concentrations were increased 2.6-fold relative to nontransgenic samples. Propranolol treatment of TOT mice reversed the alterations in SERCA and NCX protein levels but not potassium channels. Although restoration of Ca2+ dynamics occurred within 2 wk of beta-blockade treatment, evidence of functional improvement in cardiac contractility assessed by echocardiography took 10 wk to materialize. These results demonstrate that beta-adrenergic blockade restores Ca2+ dynamics and normalizes expression of Ca2+-handling proteins, eventually leading to improved hemodynamic function in cardiomyopathic hearts.

Standardization of an isolated pig heart preparation with parabiotic circulation: methodological considerations

In the present study we standardized an experimental model of parabiotic circulation of isolated pig heart. The isolated heart was perfused with arterial blood from a second animal as support and submitted to regional ischemia for 30 min, followed by total ischemia for 90 min and reperfusion for 90 min. Parameters for measurement of ventricular performance using different indices measured directly or indirectly from intraventricular pressure were defined as: maximum peak pressure, final diastolic pressure, pressure developed, first derivative of maximum pressure (dP/dt max), first derivative of minimum pressure (dP/dt min), systolic stress of the left ventricle (sigmas), and maximum elastance of the left ventricle. Isolated hearts subjected to regional and global ischemia presented significant worsening of all measured parameters. Less discriminative parameters were dP/dt max and dP/dt min. Elastance was the most sensitive parameter during the reperfusion period, demonstrating an early loss of ventricular function during reperfusion. The model proved to be stable and reproducible and permitted the study of several variables in the isolated heart, such as ischemia and reperfusion phenomena, the effects of different drugs, surgical interventions, etc. The model introduces an advantage over the classical models which use crystalloid solutions as perfusate, because parabiotic circulation mimics heart surgery with extracorporeal circulation.

Avian cardiology

The field of avian cardiology is continually expanding. Although a great deal of the current knowledge base has been derived from poultry data, research and clinical reports involving companion avian species have been published. This article will present avian cardiovascular anatomy and physiology, history and physical examination considerations in the avian cardiac disease patient, specific diagnostic tools, cardiovascular disease processes, and current therapeutic modalities.

Intracellular calcium and the relationship to contractility in an avian model of heart failure

Global contractile heart failure was induced in turkey poults by furazolidone feeding (700 ppm). Abnormal calcium regulation appears to be a key factor in the pathophysiology of heart failure, but the cellular mechanisms contributing to changes in calcium fluxes have not been clearly defined. Isolated ventricular myocytes from non-failing and failing hearts were therefore used to determine whether the whole heart and ventricular muscle contractile dysfunctions were realized at the single cell level. Whole cell current- and voltage-clamp techniques were used to evaluate action potential configurations and L-type calcium currents, respectively. Intracellular calcium transients were evaluated in isolated myocytes with fura-2 and in isolated left ventricular muscles using aequorin. Action potential durations were prolonged in failing myocytes, which correspond to slowed cytosolic calcium clearing. Calcium current-voltage relationships were normal in failing myocytes; preliminary evidence suggests that depressed transient outward potassium currents contribute to prolonged action potential durations. The number of calcium channels (as measured by radioligand binding) were also similar in non-failing and failing hearts. Isolated ventricular muscles from failing hearts had enhanced inotropic responses, in a dose-dependent fashion, to a calcium channel agonist (Bay K 8644). These data suggest that changes in intracellular calcium mobilization kinetics and longer calcium-myofilament interaction may be able to compensate for contractile failure. We conclude that the relationship between calcium current density and sarcoplasmic reticulum calcium release is a dynamic process that may be altered in the setting of heart failure at higher contraction rates.

Pharmacological, therapeutic and toxicological properties of furazolidone: some recent research

Some of the recent publications on the pharmacological, therapeutic and toxicological properties of the antimicrobial agent furazolidone (FZ) are briefly reviewed. In animals, most of the recently published papers focus on (1) the methodology of measuring the residues of the drug and its metabolites in edible tissues; (2) the carcinogenicity and genotoxicity of FZ; (3) the cellular and molecular basis of FZ-induced cardiomyopthy, and the action of different cardioprotectant drugs thereon; and (4) hormonal effects. In humans, the use of FZ as an anti-ulcer drug and in controlling infectious diseases, especially opportunistic infections in AIDS patients, is described.

Cellular and molecular remodeling in a heart failure model treated with the beta-blocker carteolol

Broad-breasted white turkey poults fed furazolidone developed dilated cardiomyopathy (DCM) characterized by ventricular dilatation, decreased ejection fraction, beta1-receptor density, sarcoplasmic reticulum (SR) Ca2+-ATPase, myofibrillar ATPase activity, and reduced metabolism markers. We investigated the effects of carteolol, a beta-adrenergic blocking agent, by administrating two different dosages (0.01 and 10.0 mg/kg) twice a day for 4 wk to control and DCM turkey poults. At completion of the study there was 59% mortality in the nontreated DCM group, 55% mortality in the group treated with the low dose of carteolol, and 22% mortality in the group treated with the high dose of carteolol. Both treated groups showed a significant decrease in left ventricle size and significant restoration of ejection fraction and left ventricular peak systolic pressure. Carteolol treatment increased beta-adrenergic receptor density, and the high carteolol dose restored SR Ca2+-ATPase and myofibrillar ATPase activities, along with creatine kinase, lactate dehydrogenase, aspartate transaminase, and ATP synthase activities, to normal. These results show that beta-blockade with carteolol improves survival, reverses contractile abnormalities, and induces cellular remodeling in this model of heart failure.

Mechanism of action of beta-blocking agents in heart failure

Antiadrenergic treatment is currently an emerging and very promising approach to the treatment of chronic heart failure. Although the adrenergic nervous system can be pharmacologically inhibited at multiple levels, it is the use of receptor-blocking agents that has generated the most interest and provided the most data for the "proof of concept" of this approach. In part because antiadrenergic treatment of chronic heart failure has developed in an atmosphere in which it was initially considered to be contraindicated (i.e., before Phase III clinical trials could be initiated), a large body of hypothesis-driven basic and clinical investigation was required to define the overall rationale and demonstrate feasibility. This article will review these data and propose a single primary mechanism of action to explain most of the clinical benefits of these agents.

Efficacy of beta blockers in idiopathic dilated cardiomyopathy and ischemic cardiomyopathy

Despite the well-documented benefits of beta blockade in a variety of cardiovascular conditions, the value of beta blockade in congestive heart failure (CHF) is still in question. The concept of neurohormonal blockade in heart failure has, however, brought beta blockade into focus. There is experimental evidence for the value of blocking sympathetic activation in CHF, and increased sympathetic activation may be an etiologic factor for development of CHF. Clinical studies have shown that long-term beta blockade improves both systolic and diastolic function. The effects on exercise tolerance and quality of life seem to differ between beta1-selective and nonselective beta blockers in favor of the beta1-selective blockers. To date, results of all trials reveal a consistent pattern of decreased cardiovascular morbidity. In one trial of metoprolol, fewer heart transplantations were required; such a reduction may have a great impact on healthcare costs associated with heart failure. Improved long-term survival found by one study must be confirmed in additional trials: 3 such survival trials (with metoprolol, bisoprolol, and bucindolol) are now in progress.

The force-interval relationship in human myocardium

The force-interval relationship is an important modulator of contractility in mammalian myocardium. In a number of mammalian species, increasing the frequency of stimulation results in an increase in force of contraction. Over the last 10 years, the effects of atrial pacing have been closely examined in normal human subjects and in patients with dilated cardiomyopathy, and the effects of the stimulation frequency have been investigated in isolated preparations from nonfailing and failing human hearts. An abnormal force-interval relationship in vivo and in vitro has been a consistent finding in patients with dilated cardiomyopathy, whereby an increase in stimulation frequency fails to increase the contractile response. The force-interval relationship of cardiac muscle has been shown to reflect intracellular calcium cycling and sarcoplasmic reticulum function. Therefore, agents that affect excitation-contraction coupling, in particular intracellular calcium mobilization and sarcoplasmic reticulum function, modulate the response of contraction force to stimulation frequency.