The activation potential (AP), like the fertilization potential, is a single active response that can be evoked in mature oocytes by increasing the intracellular Ca²⁺ concentration ([Ca²⁺]_i). The initial rise in [Ca²⁺]_i, by Ca²⁺ entry, is further enhanced by a positive-feedback mechanism of Ca²⁺ release from internal stores. This results in a wave of high [Ca²⁺]_i travelling across the cell, which generates: (i) a large Ca²⁺-dependent Cl^– current (I_Cl(Ca)), responsible for the AP, (ii) the exit from meiotic arrest, and (iii) the massive exocitosis of cortical granules (Evans & Florman, 2002). The aim of this work was to determine, simultaneously, the changes in membrane conductance (g_m) and capacitance (C_m) that follow egg activation, since fusion of a large number of cortical granules to the egg membrane and the subsequent membrane retrieval might affect the electrophysiological properties of the cell.

Eggs were obtained by incubating oocytes, isolated from a small piece of ovary, with progesterone (10 µM) for 12 h. Variations in g_m and C_m were simultaneously monitored, under voltage clamp conditions, by continuously applying, at 0.33 Hz, a brief (300 ms) ramp-pulse protocol, similar to that recently described by Schmitt & Koepsell (2002).

Superfusing the egg with the Ca²⁺ ionophore A-23187 (1 µM) elicited changes in g_m and C_m (see Fig. 1). g_m increased earlier and had a faster time course than C_m (Fig. 1A). So, the time to peak was 1.42 ± 0.23 min (mean ± S.E.M., n = 11) for g_m, versus 2.24 ± 0.39 min (same cells) for C_m. The duration of the peak (measured as the time required to return to basal values) was also shorter for g_m (4.09 ± 0.56 min, n = 11) than C_m (10.30 ± 1.25 min, same cells). At the peak, g_m values increased about 5.3-fold over the basal ones while C_m increased 1.6-fold (Fig. 1B). Such an increase in C_m would be expected if 1.06 X 10⁶ cortical granules of 1 µm in diameter were fused to the cell membrane by this time. Interestingly, both C_m and g_m values were smaller than basal ones over 20 min after g_m peak. This suggests that membrane retrieval after fusion of cortical granules is not a selective process, limited to endocytosis of the added membrane. This non-selective retrieval of cell membrane may contribute to the loss of ion channels and other molecules from the egg membrane after fertilization.

This work was supported by grant BFI2001-0756 from MCYT.