Changes in voltage-dependent ion currents during meiosis and first mitosis in eggs of an ascidian

Abstract

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, I Na was lost exponentially, the density of I Ca remained relatively constant, and the amplitudes of both I Cl and membrane surface area fluctuated in time with the cell cycle. I Cl 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 I Cl 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 I Cl 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 I Cl, are tightly correlated with developmental events.