The Harry M. Zweig Memorial Fund for Equine Research


Ionic Basis of Repolarization in Equine Heart:
Relevance to Atrial Fibrillation and Acquired LQTS

Dr. Robert F. Gilmour

The equine heart exhibits a variety of different rates and rhythms that may be considered normal, depending on the animal's level of activity (e.g, rest, sleep, or exercise). In adult horses at rest, a regular heart rate of 26-50 beats/minute is most frequently detected, with each beat originating in the sinus node pacemaker region of the heart. During exercise, however, the heart rate typically exceeds 200 beats/minute. Compared to other companion and performance animals, the horse experiences greater variation in heart rate and rhythm in response to fluctuations in the tone of the autonomic nervous system (the part of the nervous system that governs involuntary functions and the "fight or flight response"). The heightened response of the equine heart to changes in autonomic tone is reflected by the wide range of normal heart rates in the horse. However, abnormal cardiac rhythms (arrhythmias) in horses also occur in conjunction with activation of the autonomic nervous system, such as occurs during intense exercise or endurance training, metabolic derangements, and inflammatory diseases. In fact, cardiac arrhythmias are more common in the horse than in other domestic species. Some of these abnormal rhythms, such as a wandering pacemaker, are benign and cause no overt symptoms, whereas other arrhythmias, such as atrial fibrillation, can profoundly affect athletic performance. More severe rhythm disturbances, including ventricular fibrillation and torsades de pointes, are life threatening.

In the horse, as in other species, electrical impulses generated by an individual heart cell trigger the contraction of that cell. Consequently, coordinated activation of cardiac ionic currents and the electrical impulses they mediate is critical for ensuring properly timed and sequenced contraction of the various chambers of the heart. Stereotyped patterns of contraction are necessary for maximizing cardiac output, in that they optimize the coupling between filling of the cardiac chambers and subsequent ejection of blood into the vessels that carry blood (and oxygen) to muscles and other organs. Disruption of the orderly sequence of cardiac activation, as occurs during a cardiac arrhythmia, impairs the contractile performance of the heart, thereby reducing output and the supply of blood to peripheral tissues, as well as to the heart itself. As a consequence, cardiac arrhythmias typically are associated with fatigue (poor muscle perfusion) and symptoms of disorientation (poor brain perfusion). Although it has been suspected for many years that abnormalities of ionic currents underlie the development of cardiac arrhythmias, an exact correspondence between aberrant behavior of a specific ionic current and the development of a particular arrhythmia has been difficult to establish. Nevertheless, recent advances in the ability to detect and characterize cardiac ionic currents have stimulated renewed interest in linking ionic currents with the development of arrhythmias.

Of the many types of ionic currents in the heart, outward potassium currents play a particularly important role in sustaining the complex patterns of electrical activity that exist in different regions of the heart under different physiological conditions. Potassium movement from the inside of the cardiac cell to the outside via specific potassium channels is in large part responsible for the duration of the cellular electrical response and, therefore, for the duration of the contractile response. Moreover, potassium channels are the receptors for many of the drugs commonly used to treat cardiac arrhythmias (e.g., quinidine), as well as the source of toxicity for non-cardiac drugs that are known to occasionally produce arrhythmias (e.g., cisapride and erythromycin).

In humans, small companion animals and laboratory rodents a great deal of information exists regarding the molecular nature of potassium channels in different regions of the heart. This information is available as the result of basic laboratory research carried out over the past 5-10 years. Interestingly, there is tremendous species variation in the nature of potassium channels. Thus, a major lesson learned from a decade of intense research is that one cannot generalize from one species to another with respect to potassium channels and repolarizing cardiac currents. In human medicine, new information about the molecular structure and biophysical properties of human cardiac potassium channels, obtained largely from cardiac explants, has been used to address questions such as: why do some drugs work better on atrial than ventricular arrhythmias?; why do arrhythmias such as atrial fibrillation become refractory to treatment over time?; why do some non-cardiac drugs (e.g., terfanadine, ketaconozole, erythromycin, cisapride) cause lethal ventricular arrhythmias? These questions also pertain to the horse, where answers will require direct knowledge of the potassium channels in the equine heart. Unfortunately, at present we have no information regarding which of the potassium channels identified in other species are present in the equine heart and how they may, individually or in concert, control the duration of the cardiac electrical response.

To address the lack of information regarding potassium channel function in the horse, we propose a series of experiments that will identify the types of potassium channels present in the equine heart, their biophysical properties and their response to agents that may promote or suppress cardiac arrhythmias. We anticipate that this research will generate new, species-specific knowledge concerning the molecular nature of the potassium channels responsible for repolarization of equine atrial and ventricular muscle. These results will be important not only for understanding the normal function of equine cardiac electrical activity, but also for identifying the mechanisms and potential therapy for cardiac arrhythmias in the horse. Many cardiac drugs currently used to treat arrhythmias such as atrial fibrillation have the undesirable side effect of excessively prolonging ventricular repolarization and placing horses at risk of potentially fatal ventricular arrhythmias. Furthermore, many of these drugs have limited efficacy in treating chronic or recurrent atrial fibrillation. Elucidation of a molecular basis for regional differences in equine cardiac repolarization is a necessary first step for the design of selective anti-arrhythmic agents capable of treating atrial fibrillation without changing the ventricular rhythm.

Defining the molecular nature of equine cardiac potassium channels also will help practitioners to anticipate and avoid potentially fatal drug interactions. There is an increasing number of human label drugs being compounded into novel dosage forms for use in equine medicine. For example, post-operative ileus in horses is routinely treated with an intravenous solution made from pulverized tablets containing the prokinetic agent cisapride (PropulsidÒ) dissolved in tartaric acid. In humans, cisapride's ability to block specific cardiac potassium channels has been associated with a high risk of inducing potentially fatal ventricular arrhythmias. Additional clinical factors that predispose human patients to this serious side-effect also have been identified, including female gender, slow resting heart rate, electrolyte disturbances and concurrent therapy with other drugs that block potassium channels or inhibit cisapride metabolism. As a result of knowledge gained from basic and clinical research, physicians can make informed decisions concerning cisapride therapy and can reasonably assess the risk of serious side effects in individual patients who are candidates for drug therapy. Equine veterinarians do not have these important options because of the lack of basic knowledge about equine cardiac potassium channels and the lack of clinical data regarding cardiac electrical activity in equine patients treated with different dosages of cisapride in various clinical settings. The proposed investigation seeks to fill these important gaps in knowledge, so that in the future equine practitioners will be able to determine whether a human label drug like cisapride represents a novel life-saving therapy or a potential "lethal weapon".

To achieve the aims of this project, experiments will be conducted in the basic science laboratories and clinics at Cornell University and Kansas State University. Our approach is to: 1) identify the types of potassium channels present in the equine heart; 2) characterize the properties of the currents that flow through such channels; 3) determine the effects of potassium channel blocking agents on individual currents; 4) correlate drug effects on individual currents with changes in the electrical response of isolated cardiac cells and cardiac electrical activity in intact horses. Initially, the molecular basis of potassium channels expressed in the equine atria and ventricles will be determined by immunoblotting (Western analysis) and reverse transcription polymerase-chain reaction (RT-PCR). The biophysical and pharmacological properties of the potassium currents carried by these channels will then be determined using whole cell patch clamp techniques in isolated equine heart cells. The functional significance of these potassium currents will be assessed not only in vitro by recording action potentials from equine cardiac tissue, but also in vivo by recording ECGs from equine patients receiving cisapride.

In summary, the proposed project will generate new knowledge about the mechanisms responsible for repolarization of the equine heart. The information obtained will be immediately integrated into clinical studies assessing the cardiotoxic potential of the prokinetic agent cisapride in the setting of post-operative ileus. More importantly, the experimental results also will be used to advance our long-term goal of developing more effective and safer therapeutic strategies for treatment of cardiac arrhythmias, especially atrial fibrillation.