Fluctuation of the Ca-sequestering activity of permeabilized sea urchin embryos during the cell cycle

We have followed the sequestration of Ca(2+) by intracellular compartments in sea urchin embryos through the first cell cycles. To gain biochemical access to these compartments, the embryos were permeabilized by brief exposure to an intense electric field. Sequestration was determined as the retention of tracer, (45)Ca, after filtration of aliquots on Millipore filters. The permeabilized cells sequester Ca(2+) at a constant rate for at least 20 min, with the following characteristics: (i) ATP is required. (ii) Sequestration occurs at Ca(2+) levels corresponding to those estimated in vivo. (iii) The Ca(2+) concentration dependence of sequestration and its insensitivity to mitochondrial poisons imply that the activity derives from a single, nonmitochondrial transport system. The Ca(2+)-sequestering activities of embryos that are permeabiized at successive stages of the first cell cycle (one-cell stage) progressively increase to 5 times the initial level. The rate of sequestration is maximal during telophase and, in some populations of zygotes, is nearly as great throughout prophase. Over the course of the second cell cycle (two-cell stage), the activity undergoes a 2-fold oscillation that bears the same temporal relationship to mitosis as the previous fluctuation.