Changes in Voltage-dependent Ion Currents During Meiosis and First Mitosis in Eggs of an Ascidian

Different patterns of voltage-dependent ion currents are present in mature eggs and in early embryos of the ascidian Boltenia villosa, as if each ion current is regulated in a different manner between fertilization and the early cleavages of embryogenesis. The ion currents appear and/or disappear with precise timing suggesting that they play important roles at specific times during early development. We investigated changes in three voltage-dependent ion currents (an inwardly rectifying chloride current, a calcium current, and a sodium current) and membrane surface area over time between the resumption of meiosis (with fertilization or activation) and the first mitotic cleavage. Using time-lapse video recordings made during whole-cell patch-clamp experiments, we were able to correlate electrophysiological changes with morphological changes and cell cycle related events. Between fertilization and first cleavage, INa was lost exponentially, the density of ICa remained relatively constant, and the amplitudes of both ICl and membrane surface area fluctuated in time with the cell cycle. ICl and surface area increased whenever the cell began dividing--with the polar body extrusions and the formation of the first cleavage furrow. This suggested that the values of ICl and surface area were largest during interphase and smallest during M-phase of each cell cycle. This hypothesis was supported by an experiment in which entry into M-phase was blocked in fertilized eggs by inhibiting protein synthesis. This prevented the decreases of ICl and surface area but allowed the increases to occur normally. Patterns of change in ion currents are current specific and, as is the case with ICl, are tightly correlated with developmental events.