Enhancement of ionic currents through voltage-gated channels in the mouse oocyte after fertilization

1. The changes of voltage-gated ion channels in the mouse oocyte after fertilization were investigated under voltage clamp.2. About 60 min after introduction of sperm suspension into the fertilization medium, the amplitude of inward current through Ca(2+)-channels increased, which occurred at anaphase during the second meiotic division. The peak amplitude of the maximum inward current per unit membrane capacity of the oocytes at metaphase was 20+/-3 muA/muF in 50 mM-Sr medium. It was 28+/-8 muA/muF at anaphase, and 32+/-5 muA/muF at telophase. The kinetic properties as well as selectivity among Ca, Sr and Mn ions were not altered after fertilization.3. The outward surge current which was found at the higher membrane potential over +50 mV also increased in amplitude after fertilization, simultaneously with the increase in amplitude of inward current through Ca(2+)-channels. The means and the standard deviations of the surge current per unit membrane capacity at 120 mV were 31+/-8 muA/muF at metaphase, and 48+/-7 muA/muF at telophase. The kinetic properties of the outward surge current were not altered after fertilization.4. Application of colcemid (10(-7) mole/l.) or cytochalasin B (2 x 10(-5) mole/l.) did not prevent the increase in amplitude of both inward current through Ca channels and the outward surge current.5. The membrane currents in N-18 mouse neuroblastoma cells in logarithmic growth phase were examined under voltage clamp. The N-18 neuroblastoma cells possessed the Ca inward current and the delayed outward current. The kinetic properties and the steady-state inactivation of Ca(2+)-channels in N-18 neuroblastoma cells were compared with those in mouse oocytes. It was concluded that they could be regarded as identical between the mouse oocyte and the N-18 neuroblastoma cell.