Ca(2+) current activity decreases during meiotic progression in bovine oocytes

By using the whole cell voltage-clamptechnique, we studied changes in plasma membrane permeability atdifferent meiotic stages of bovine oocytes. Follicular oocytes werematured in vitro and activated by Ca²⁺ ionophore. Oocytesat germinal vesicle (GV), germinal vesicle breakdown (GVBD), metaphaseI (MI), metaphase II (MII), and meiosis exit were used forelectrophysiological recording. By clamping the oocytes at 30 mV, wefound that the L-type voltage-dependent Ca²⁺ channels wereactive at the GV stage and that their activity decreased after the GVBDstage. Furthermore, the resting potential decreased from the GV to theMI stage and increased again at MII. A significant decrease of thesteady-state conductance occurred from the GV to the MI stage, followedby a sharp increase at the MII stage. With the addition of organicL-type Ca²⁺ channel blockers (nifedipine and verapamil), weinhibited the Ca²⁺ currents. However, only in the case ofverapamil was there a decrease of in vitro maturation efficiency. Ourresults suggest that, in addition to the cumulus-oocyte junctions, theplasma membrane channels provide another mode of Ca²⁺ entryinto bovine oocytes during meiosis.