Calcium fluxes in hydrozoan embryos depend, in part, on exocytosis and fluid phase endocytosis

In the hydrozoan Phialidium gregarium, the constitutive calcium influx of c leavage stage embryos in sea water is 1.96 +/-0.75 x 10(-15) moles/embryo/m inute. Treating embryos with 227 mM KCl in seawater briefly increases the c alcium influx more than 100-fold, to 3.9 x 10(-13) mol/embryo/min. About 62 % of the KCl-induced calcium influx is due to calcium flowing through volta ge-sensitive calcium channels. This causes a marked intracellular calcium t ransient and secretion of intracellular vesicles. The other component (appr oximate to 38%) of the calcium influx occurs via fluid phase endocytosis of the extracellular medium (detected using extracellular H-3-sucrose). KCl-treatment of Ca-45 loaded embryos induces a Ca-45 efflux which can reac h peak fractional rates of 0.98/min, during which 55-75% (mean 66%) of the total Ca-45 is lost. The KCl-induced calcium efflux is due, in part, to sec retion because loaded H-3-sucrose is effluxed simultaneously. This pathway may be important for the calcium efflux necessary for longterm calcium home ostasis in cells.