Ca2+ current activity decreases during meiotic progression in bovine oocytes

By using the whole cell voltage-clamp technique, we studied changes in plas ma membrane permeability at different meiotic stages of bovine oocytes. Fol licular oocytes were matured in vitro and activated by Ca2+ ionophore. Oocy tes at germinal vesicle (GV), germinal vesicle breakdown (GVBD), metaphase I (MI), metaphase II (MII), and meiosis exit were used for electrophysiolog ical recording. By clamping the oocytes at -30 mV, we found that the L-type voltage-dependent Ca2+ channels were active at the GV stage and that their activity decreased after the GVBD stage. Furthermore, the resting potentia l decreased from the GV to the MI stage and increased again at MII. A signi ficant decrease of the steady-state conductance occurred from the GV to the MI stage, followed by a sharp increase at the MII stage. With the addition of organic L- type Ca2+ channel blockers (nifedipine and verapamil), we in hibited the Ca2+ currents. However, only in the case of verapamil was there a decrease of in vitro maturation efficiency. Our results suggest that, in addition to the cumulus-oocyte junctions, the plasma membrane channels pro vide another mode of Ca2+ entry into bovine oocytes during meiosis.