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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.<\/jats:p>","DOI":"10.3390\/bios14050219","type":"journal-article","created":{"date-parts":[[2024,4,29]],"date-time":"2024-04-29T08:49:24Z","timestamp":1714380564000},"page":"219","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":5,"title":["Suppression of Contraction Raises Calcium Ion Levels in the Heart of Zebrafish Larvae"],"prefix":"10.3390","volume":"14","author":[{"given":"Antonio","family":"Martinez-Sielva","sequence":"first","affiliation":[{"name":"Physiology and Cell Dynamics Group, Instituto de Biomedicina de la Universidad de Castilla-La Mancha, Facultad de Medicina de Albacete, Universidad de Castilla-La Mancha, C\/Almansa 14, 02006 Albacete, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Manuel","family":"Vicente","sequence":"additional","affiliation":[{"name":"Physiology and Cell Dynamics Group, Instituto de Biomedicina de la Universidad de Castilla-La Mancha, Facultad de Medicina de Albacete, Universidad de Castilla-La Mancha, C\/Almansa 14, 02006 Albacete, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7421-6168","authenticated-orcid":false,"given":"Jussep","family":"Salgado-Almario","sequence":"additional","affiliation":[{"name":"Physiology and Cell Dynamics Group, Instituto de Biomedicina de la Universidad de Castilla-La Mancha, Facultad de Medicina de Albacete, Universidad de Castilla-La Mancha, C\/Almansa 14, 02006 Albacete, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1127-9313","authenticated-orcid":false,"given":"Aar\u00f3n","family":"Garcia-Blazquez","sequence":"additional","affiliation":[{"name":"Physiology and Cell Dynamics Group, Instituto de Biomedicina de la Universidad de Castilla-La Mancha, Facultad de Medicina de Albacete, Universidad de Castilla-La Mancha, C\/Almansa 14, 02006 Albacete, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9430-4803","authenticated-orcid":false,"given":"Beatriz","family":"Domingo","sequence":"additional","affiliation":[{"name":"Physiology and Cell Dynamics Group, Instituto de Biomedicina de la Universidad de Castilla-La Mancha, Facultad de Medicina de Albacete, Universidad de Castilla-La Mancha, C\/Almansa 14, 02006 Albacete, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9460-0642","authenticated-orcid":false,"given":"Juan","family":"Llopis","sequence":"additional","affiliation":[{"name":"Physiology and Cell Dynamics Group, Instituto de Biomedicina de la Universidad de Castilla-La Mancha, Facultad de Medicina de Albacete, Universidad de Castilla-La Mancha, C\/Almansa 14, 02006 Albacete, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2024,4,27]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"748570","DOI":"10.3389\/fphys.2021.748570","article-title":"Seeing the Light: The Use of Zebrafish for Optogenetic Studies of the Heart","volume":"12","author":"Baillie","year":"2021","journal-title":"Front. 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