High heart rate associated early repolarization causes J‐waves in both zebra finch and mouse

Abstract

High heart rates are a feature of small endothermic—or warm‐blooded—mammals and birds. In small mammals, the QT interval is short, and local ventricular recordings reveal early repolarization that coincides with the J‐wave on the ECG, a positive deflection following the QRS complex. Early repolarization contributes to short QT‐intervals thereby enabling brief cardiac cycles and high heart rates. We therefore hypothesized high hearts rates associate with early repolarization and J‐waves on the ECG of endothermic birds. We tested this hypothesis by comparing isolated hearts of zebra finches and mice and recorded pseudo‐ECGs and optical action potentials (zebra finch, n = 8; mouse, n = 8). In both species, heart rate exceeded 300 beats per min, and total ventricular activation was fast (QRS < 10 ms). Ventricular activation progressed from the left to the right ventricle in zebra finch, whereas it progressed from apex‐to‐base in mouse. In both species, the early repolarization front followed the activation front, causing a positive J‐wave in the pseudo‐ECG. Inhibition of early repolarization by 4‐aminopyridine reduced J‐wave amplitude in both species. Action potential duration was similar between ventricles in zebra finch, whereas in mouse the left ventricular action potential was longer. Accordingly, late repolarization had opposite directions in zebra finch (left‐right) and mouse (right‐left). This caused a similar direction for the zebra finch J‐wave and T‐wave, whereas in the mouse they were discordant. Our findings demonstrate that early repolarization and the associated J‐wave may have evolved by convergence in association with high heart rates.

Catecholamines are key modulators of ventricular repolarization patterns in the ball python (Python regius)

“…For ventricular repolarization of pythons to be similar to that of mammals, our study suggests that at least three conditions have to be met. First, there should be evolutionary conservation of the electrophysiological processes, and our RNA sequencing demonstrates a substantial evolutionary conservation on the transcript level, including of the major ion-handling channels, in agreement with previous studies ( Olson, 2006 ; Castoe et al, 2013 ; Duan et al, 2017 ; Filatova et al, 2021 ; Offerhaus et al, 2021 ). Second, catecholamines should augment differences in repolarization time between regions within the ventricle, and we find this to be the case in pythons.…”

Basic Research Approaches to Evaluate Cardiac Arrhythmia in Heart Failure and Beyond

“…Further contrasts are evidenced by morphological changes in complexes of the ECG, such as an ambiguous ST segment and an additional J wave. The J wave arises in the mouse (and other rodents) ECG due to the lack of a plateau phase in the action potential, meaning early repolarisation is visible as a positive deflection shortly after the QRS complex (Offerhaus et al, 2021). It is for this reason also that the mouse ECG has a less pronounced T wave.…”

Avian cardiomyocyte architecture and what it reveals about the evolution of the vertebrate heart

“…The anatomical separation of left and right ventricles in birds and mammals allows the elevation of systemic pressure significantly above pulmonary pressure thereby providing the necessary convection for highly aerobic tissues, whilst avoiding the rupture of thin respiratory surfaces [12][13][14]. The presence of a specialized conduction system [15] and a compact atrial and ventricular wall architecture is important for the fast atrioventricular conduction and rapid ventricular repolarization [16,17] of avian and mammalian hearts compared with those of ectothermic vertebrates (independent of temperature) [14,15,18]. Indeed, the convergent evolution of rapid/ early repolarization in birds (zebrafinch) and mammals (mouse) is undoubtably important for achieving the fast heart rates typical of endotherms [17].…”

“…The presence of a specialized conduction system [15] and a compact atrial and ventricular wall architecture is important for the fast atrioventricular conduction and rapid ventricular repolarization [16,17] of avian and mammalian hearts compared with those of ectothermic vertebrates (independent of temperature) [14,15,18]. Indeed, the convergent evolution of rapid/early repolarization in birds (zebrafinch) and mammals (mouse) is undoubtably important for achieving the fast heart rates typical of endotherms [17].

Figure 1Schematic of the vertebrate phylogeny with taxa from left to right as follows: jawless fishes, cartilaginous fishes, teleost fishes, amphibians, mammals, lizards, snakes, turtles, crocodilians, birds.

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