Suppression of Contraction Raises Calcium Ion Levels in the Heart of Zebrafish Larvae

Zebrafish larvae have emerged as a valuable model for studying heart physiology and pathophysiology, as well as for drug discovery, in part thanks to its transparency, which simplifies microscopy. However, in fluorescence-based optical mapping, the beating of the heart results in motion artifacts. Two approaches have been employed to eliminate heart motion during calcium or voltage mapping in zebrafish larvae: the knockdown of cardiac troponin T2A and the use of myosin inhibitors. However, these methods disrupt the mechano-electric and mechano-mechanic coupling mechanisms. We have used ratiometric genetically encoded biosensors to image calcium in the beating heart of intact zebrafish larvae because ratiometric quantification corrects for motion artifacts. In this study, we found that halting heart motion by genetic means (injection of tnnt2a morpholino) or chemical tools (incubation with para-aminoblebbistatin) leads to bradycardia, and increases calcium levels and the size of the calcium transients, likely by abolishing a feedback mechanism that connects contraction with calcium regulation. These outcomes were not influenced by the calcium-binding domain of the gene-encoded biosensors employed, as biosensors with a modified troponin C (Twitch-4), calmodulin (mCyRFP1-GCaMP6f), or the photoprotein aequorin (GFP-aequorin) all yielded similar results. Cardiac contraction appears to be an important regulator of systolic and diastolic Ca²⁺ levels, and of the heart rate.