A VOLTAGE-GATED CHLORIDE CHANNEL IN ASCIDIAN EMBRYOS MODULATED BY BOTH THE CELL-CYCLE CLOCK AND CELL-VOLUME

1. Eggs of the ascidian Boltenia villosa have an inwardly rectifying C l- current whose amplitude varies by more than 10-fold during each cel l cycle, the largest amplitude being at exit from M-phase. We examined whether this current was also sensitise to changes in cell volume. 2. Cell swelling, produced by direct inflation through a whole-cell reco rding pipette, greatly increased the amplitude of the Cl- current at a ll stages of the cell cycle in activated eggs. Swelling was much less effective in unfertilized eggs. 3. The increase in Cl- current amplitu de continued for 10-20 min after an increase in diameter that was comp lete in 10 s, suggesting the involvement of a second messenger system in the response. 4. Treatment of unfertilized eggs with 6-dimethylamin opurine (DMAP), an inhibitor of cell cycle-dependent protein kinases, increased the amplitude of the Cl- current and its sensitivity to swel ling to levels characteristic of fertilized eggs. 5. Osmotically produ ced swelling also increased Cl- current amplitude in unfertilized eggs . 6. We propose that dephosphorylation renders the Cl- channel functio nal, and that swelling or activation of the egg increases the sensitiv ity of the channel to dephosphorylation, perhaps by disrupting its lin ks to the cytoskeleton.