ERG potassium channels and T-type calcium channels contribute to the pacemaker and atrioventricular conduction in zebrafish larvae.

AIM: Bradyarrhythmias result from inhibition of automaticity, prolonged repolarization, or slow conduction in the heart. The ERG channels mediate the repolarizing current IKr in the cardiac action potential, whereas T-type calcium channels (TTCC) are involved in the sinoatrial pacemaker and atrioventricular conduction in mammals. Zebrafish have become a valuable research model for human cardiac electrophysiology and disease. Here, we investigate the contribution of ERG channels and TTCCs to the pacemaker and atrioventricular conduction in zebrafish larvae and determine the mechanisms causing atrioventricular block. METHODS: Zebrafish larvae expressing ratiometric fluorescent Ca2+ biosensors in the heart were used to measure Ca2+ levels and rhythm in beating hearts in vivo, concurrently with contraction and hemodynamics. The atrioventricular delay (the time between the start of atrial and ventricular Ca2+ transients) was used to measure impulse conduction velocity and distinguished between slow conduction and prolonged refractoriness as the cause of the conduction block. RESULTS: ERG blockers caused bradycardia and atrioventricular block by prolonging the refractory period in the atrioventricular canal and in working ventricular myocytes. In contrast, inhibition of TTCCs caused bradycardia and second-degree block (Mobitz type I) by slowing atrioventricular conduction. TTCC block did not affect ventricular contractility, despite being highly expressed in cardiomyocytes. Concomitant measurement of Ca2+ levels and ventricular size showed mechano-mechanical coupling: increased preload resulted in a stronger heart contraction in vivo. CONCLUSION: ERG channels and TTCCs influence the heart rate and atrioventricular conduction in zebrafish larvae. The zebrafish lines expressing Ca2+ biosensors in the heart allow us to investigate physiological feedback mechanisms and complex arrhythmias.

Simultaneous imaging of calcium and contraction in the beating heart of zebrafish larvae.

In vivo models of cardiac function maintain the complex relationship of cardiomyocytes with other heart cells, as well as the paracrine and mechanoelectrical feedback mechanisms. We aimed at imaging calcium transients simultaneously with heart contraction in zebrafish larvae. Methods: To image calcium in beating hearts, we generated a zebrafish transgenic line expressing the FRET-based ratiometric biosensor Twitch-4. Since emission ratioing canceled out the motion artifacts, we did not use myosin inhibitors or tnnt2a morpholinos to uncouple contraction from changes in calcium levels. We wrote an analysis program to automatically calculate kinetic parameters of the calcium transients. In addition, the ventricular diameter was determined in the fluorescence images providing a real-time measurement of contraction correlated with calcium. Results: Expression of Twitch-4 did not affect the force of contraction, the size of the heart nor the heart rate in 3- and 5-days post-fertilization (dpf) larvae. Comparison of 3 and 5 dpf larvae showed that calcium levels and transient amplitude were larger at 5 dpf, but the fractional shortening did not change. To validate the model, we evaluated the effect of drugs with known effects on cardiomyocytes. Calcium levels and the force of contraction decreased by the L-type calcium channel blocker nifedipine, whereas they increased with the activator Bay-K 8644. Caffeine induced bradycardia, markedly decreased ventricular diastolic calcium levels, increased the size of the calcium transients, and caused an escape rhythm in some larvae. Conclusions: The Tg(myl7:Twitch-4) line provides a physiological approach to image systolic and diastolic calcium levels in the heart of zebrafish larvae. Since the heart is beating, calcium levels and contraction can be correlated. This line will be a useful tool to address pathophysiological mechanisms in diseases like heart failure and arrhythmia, in cardiotoxicity studies and for drug screening.