Characterization of embryonic cardiac pacemaker and atrioventricular conduction physiology in Xenopus laevis using noninvasive imaging

Abstract

Congenital heart defects often include altered conduction as well as morphological changes. Model organisms, like the frog Xenopus laevis, offer practical advantages for the study of congenital heart disease. X. laevis embryos are easily obtained free living, and the developing heart is readily visualized. Functional and morphological evidence for a conduction system is available for adult frog hearts, but information on the normal properties of embryonic heart contraction is lacking, especially in intact animals. With the use of fine glass microelectrodes, we were able to obtain cardiac recordings and make standard electrophysiological measurements in 1-wk-old embryos ( stage 46). In addition, a system using digital analysis of video images was adapted for measurement of the standard cardiac intervals and compared with invasive measurements. Video images were obtained of the heart in live, pharmacologically paralyzed, stage 46 X. laevis embryos. Normal values for the timing of the cardiac cycle were established. Intervals determined by video analysis ( n = 53), including the atrial and ventricular cycle lengths (473 ± 10 ms and 464 ± 19 ms, respectively) and the atrioventricular interval (169 ± 5 ms) were not statistically different from those determined by intrathoracic cardiac recordings. We also present the data obtained from embryos treated with standard medications that affect the human conduction system. We conclude that the physiology of embryonic X. laevis cardiac conduction can be noninvasively studied by using digital video imaging. Additionally, we show the response of X. laevis embryonic hearts to chronotropic agents is similar but not identical to the response of the human heart.

Automated high-throughput measurement of body movements and cardiac activity of Xenopus tropicalis tadpoles

“…We validated this methodology and analyzed alterations of cardiac activity that were induced by established drugs. Propranolol is a ß-adrenergic receptor antagonist that slows down heart conduction of mammalian and Xenopus laevis tadpole hearts [30]. In agreement with these results, X. tropicalis tadpoles responded with a decrease of heart beat rates (bradycardia) and showed arrhythmic figures.…”

“…In agreement with these results, X. tropicalis tadpoles responded with a decrease of heart beat rates (bradycardia) and showed arrhythmic figures. In contrast, atropine caused an increase of X. tropicalis heart beat rates (tachycardia) suggesting an interference with the parasympathetic control of cardiac activity [30]. These observations showed that our cardiac activity assay is a useful tool for high-throughput in vivo studies relevant to humans, since characteristic pharmacology responses of the human heart are reproduced in Xenopus.…”

“…Due to the relatively large size of tadpoles, the heart beat rates and buccal pumping rates can be measured by visual examination of the animal or from video recordings [28,29]. Other approaches for measuring cardiac function of tadpoles include microscope video analysis [30,31], microstructural analysis with high-resolution imaging techniques [32][33][34] or microelectrode systems [30,35,36]. However, these methodologies access only behavior or cardiac function, but not both and furthermore, they can be expensive or require time consuming experimentations.…”

Xenopus: An emerging model for studying congenital heart disease

“…The Xenopus model lends itself to studying the epicardium and potential defects in valve formation and conduction systems due to the ease of manipulating gene function and the established techniques of tissue explanting, antisense MO microinjection, lineage tracing, and transgenics. Recent advances in live imaging have enabled researchers to utilize the Xenopus model to visualize cardiac development in real time and to use non-invasive electrical recording, Doppler optical cardiograms, and optical coherence tomography to study heart structure, conduction, and blood flow to determine the role of the epicardium in these processes (Bartlett and others, 2004; Kieserman and others, 2010; Mariampillai and others, 2007; Yelin and others, 2007). …”

Lessons from the lily pad: using Xenopus to understand heart disease

“…Synthetic constructs that fused the engrailed repressor to Nkx2-5[22] resulted in embryos with very small, malformed hearts. Injections of Nkx2-5 transcripts based on some of the early mutations found in human phenocopied not only defects in atrial septal and atrioventricular valve formation but also established that Xenopus tadpoles can be used to evaluate mutations that lead to conduction system anomalies[23,24]. Direct measurement of a tadpole electrocardiogram and histology showing a defective atrial septal development are shown in figure 3.…”