The　sarcoplasmic　reticulum　(SR)　primarily　serves　as　the　intracellular　Ca2+　store　in　cardiac　myocytes,　mediating　cellular　function　under　cardiac　physiology　and　diseases.　However,　the　properties　of　cardiac　SR　Ca2+　have　not　yet　been　fully　determined,　particularly　in　rats　and　mice,　which　are　the　most　commonly　used　experimental　species　in　studies　on　cardiac　physiology　and　diseases.　Here,　we　developed　a　spatially　detailed　numerical　model　to　deduce　Ca2+　movements　inside　the　junctional　SR　(jSR)　cisternae　of　rat　cardiomyocytes.　Our　model　accurately　reproduced　the　jSR　Ca2+　kinetics　of　local　and　global　SR　Ca2+　releases　reported　in　a　recent　experimental　study.　With　this　model,　we　revealed　that　jSR　Ca2+　kinetics　was　mostly　determined　by　the　total　release　flux　via　type　2　ryanodine　receptor　(RyR2)　channels　but　not　by　RyR2　positioning.　Although　Ca2+　diffusion　in　global　SR　was　previously　reported　to　be　slow,　our　simulation　demonstrated　that　Ca2+　diffused　very　quickly　inside　local　jSR　cisternae　and　the　decrease　in　the　diffusion　coefficient　resulted　in　a　significant　reduction　of　jSR　Ca2+　depletion　amplitude.　Intracellular　Ca2+　was　typically　experimentally　detected　with　fluorescence　dye.　Our　simulation　revealed　that　when　the　dynamical　characteristics　of　fluorescence　dye　exerted　a　minimal　effect　on　actual　Ca2+　mobility　inside　jSR,　the　reaction　rate　of　the　dye　with　Ca2+　could　significantly　affect　apparent　jSR　Ca2+　kinetics.　Therefore,　loading　a　chemical　fluorescence　dye　with　fast　kinetics,　such　as　Fluo-5N,　into　SR　is　important　for　Ca2+　measurement　inside　SR.　Overall,　our　model　provides　new　insights　into　deciphering　Ca2+　handling　inside　nanoscopic　jSR　cisternae　in　rat　cardiomyocytes.　©　The　Author(s)　2024.