Effect of atrial artificial electrical stimulation on depolarization and repolarization and hemodynamics of the heart ventricle in rainbow trout Oncorhynchus mykiss

Spatial uniformity of action potentials indicates base-to-apex depolarization and repolarization of rainbow trout (Oncorhynchus mykiss) ventricle

The spatial pattern of electrical activation is crucial for a full understanding of fish heart function. However, it remains unclear whether there is regional variation in action potential (AP) morphologies and underlying ion currents. Because the direction of depolarization and spatial differences in the durations of ventricular APs set limits to potential patterns of ventricular repolarization, we determined AP morphologies, underlying ion currents, and ion channel expression in 4 different regions (spongy myocardium; and apex, base, and middle of the compact myocardium), and correlated them with in vivo electrocardiogram (ECG) in rainbow trout (Oncorhynchus mykiss). ECG recorded from 3 leads indicated that the depolarization and repolarization of AP propagate from base-to-apex, and the main depolarization axis of the ventricle is between +90° and +120°. AP shape was uniform across the whole ventricle, and little regional differences were found in density of repolarizing K+ currents or depolarizing Ca2+ and Na+ currents and the underlying transcripts of ion channels, providing compelling evidence for the suggested excitation pattern. The spatial uniformity of AP durations and base-to-apex propagation of activation with a relatively slow velocity of propagation indicates no special ventricular conduction pathway in the trout ventricle like the His-Purkinje system of mammalian hearts. The sequence of repolarization is solely determined by activation time without being affected by regional differences in AP duration.

Electric catfish hearts are not intrinsically immune to electric shocks

High voltage electric shocks cause life threatening cardiac injuries such as sudden cardiac standstill or severe myocardial injury. Here, we analysed the physiology of the heart of the strongly electric catfish (Malapterurus beninensis) that stuns prey with high-voltage shocks but is immune to its own, as well as external, high-voltage shocks. Neither a detailed analysis of the electrocardiogram nor the structure of the heart indicated a specialized cardiac conduction system. Using a suitable perfusion system, we discovered that, despite its immunity in vivo, the explanted heart of electric catfish can readily be activated by external electrical currents and is equally sensitive to electric shock-induced arrhythmias as similar-sized goldfish hearts. The surprise thus is that the electric catfish has a vulnerable heart that requires to be protected by highly efficient but presently unknown means.