In dividing embryos,a localized elevation in intracellular Ca2+([Ca2+]i)at the cleavage furrow has been shown to be essential for cytokinesis.However,the underlying mechanisms for generating and maintaining these[Ca2+]i gradients throughout cytokinesis are not fully understood.In the present study,we analyzed the role of inositol 1,4,5-trisphosphate receptors(IP3Rs)and endoplasmic reticulum(ER)distribution in determining the intracellular Ca2+ gradients in early zebrafish blastomeres.Application of the injected Ca2+indicator,Indo-1,showed that during the first cell division a standing Ca2+ gradient was formed～35 min after fertilization,with the[Ca2+]i spatially decaying from 500-600 nmol/L at the cleavage furrow to 106-200 nmol/L around the nucleus.While the IP3R immunohistochemical fluorescence was relatively concentrated in the peri-furrow region,ER labeling was relatively enriched in both peri-furrow and peri-nuclear regions.Numeric simulation suggested that a divergence in the spatial distribution of IP3R and the locations of Ca2+ uptake within the ER was essential for the formation of a standing Ca2+ gradient,and the Ca2+ gradient could only be well-established under an optimal stoichiometry of Ca2+ uptake and release.Indeed,while inhibition of IP3R Ca2+ release blocked the generation of the Ca2+ gradient at a lower[Ca2+]i level,both Ca2+ release stimulation by inositol 1,4,5-trisphosphate(IP3)injection and ER Ca2+ pump inhibition by cyclopiazonic acid also eliminated the Ca2+ gradients at higher[Ca2+]i levels.Our results suggest a dynamic relationship between ER-mediated Ca2+ release and uptake that underlies the maintenance of the perifurrow Ca2+ gradient and is essential for cytokinesis of zebrafish embryos.